Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol43.Issue2.Mar2000:
Agricultural Water Management 43 (2000) 203±218
Predicting and mapping the future demand for
irrigation water in England and Wales
E.K. Weatherhead, J.W. Knox*
Natural Resources Management Department, School of Agriculture Food and Environment, Cranfield
University, Silsoe, Bedford MK45 4DT, UK
Accepted 2 April 1999
Abstract
A methodology has been developed for predicting the future growth in demand for irrigation in
countries with supplemental irrigation such as England and Wales. This takes into account expected
changes in agricultural policy, technical, market and other factors. The methodology has also been
applied within a geographical information system (GIS) to map the growth. The GIS approach
represents an extension of a procedure developed by Knox et al. (1997) (Knox, J.W., Weatherhead,
E.K., Bradley, R.I., 1997. Agricultural Water Management 33, 1±18).
The total net volumetric actual irrigation water requirements for a `design' dry year (20%
exceedance), are predicted to rise from 168106 m3 in 1996 to 244106 m3 in 2021. Meanwhile,
the total net volumetric theoretical irrigation water requirements for a `design' dry year, are
predicted to rise from 204106 m3 in 1996 to 247106 m3 in 2021.
Maps are presented showing the predicted change in the spatial distribution of irrigation demand
between 1996 and 2021, and the theoretical dry year irrigation demand for all irrigated crops in
2021. The application and limitations of the methodology are discussed. # 2000 Elsevier Science
B.V. All rights reserved.
Keywords: England and Wales; GIS; Irrigation; Water demand; Maps; Prediction
1. Introduction
Recent droughts, rising public demand for mains water supply, and increased environmental protection, have all reduced the availability and reliability of water supplies for
agricultural and horticultural irrigation in England and Wales. In many catchments, water
* Corresponding author. Tel.: +44-1525-863328; fax: +44-1525-863000.
E-mail address: [email protected] (J.W. Knox).
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 5 8 - X
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Fig. 1. Total licensed and abstracted volumes of water for irrigation in England and Wales, 1974±1997.
resources are already over-committed, and additional licences for irrigation abstraction
are unobtainable. Yet demand for irrigation water is also increasing (Weatherhead et al.,
1997). Estimates of both the magnitude and location of that growth are an essential
requirement for strategic water resource planning at national and regional levels.
Abstraction of water for spray irrigation in England and Wales is regulated by the
Environment Agency (EA). The volume of water licensed and actually abstracted are
reported annually. Both figures have increased substantially over the last 25 years
(Fig. 1). The Ministry of Agriculture, Fisheries and Food (MAFF) also publishes
statistics on agricultural irrigation in England and Wales, based on surveys carried out
roughly triennially. These surveys report the areas irrigated and water use, by crop
category, and in total. The areas that farmers reported they were `likely to irrigate in a dry
year' from 1955 to 1995 (the most recent survey) are shown in Fig. 2. Rapid growth
occurred up to 1965, followed by a decline until 1974. Since then the area has again risen
sharply, despite a slight decline in 1995.
In 1980, a major national study by the Advisory Council for Agriculture and
Horticulture (ACAH, 1980) predicted a 150% (4% per annum) growth in the irrigated
area in England and Wales between 1977 and 2000, and an overall increase in water use
of 300% (6% per annum), assuming no restrictions on water availability. However, the
subsequent downturn in the profitability of irrigating grass kept actual growth well below
the projected levels. There have also been many attempts to predict irrigation growth
regionally, notably for Anglian region, where most irrigation occurs (e.g. Roughton and
Clarke, 1978; Anglian Water Authority, 1982; Anglian Water Authority, 1988; National
Rivers Authority, 1990). The predictions are summarised in Fig. 3. Although a variety of
methodologies were adopted, common problems have been the difficulty in extricating
the effects of recent weather, incorporating changes in external economic factors, and
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
205
Fig. 2. Total area `likely to be irrigated in a dry year' in England and Wales, 1955±1995.
Fig. 3. Selected predictions of future demand for irrigation in Anglian region, together with reported actual
abstractions (1976±97).
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
predicting the effects of non-availability of water. Several studies failed to differentiate
between the different irrigated crops; others failed to set sensible limits to the total crop
areas or to the percentages of each that would be irrigated. Many of these studies were
commissioned following periods of drought when interest in irrigation was abnormally
high, and consequently over-estimated the subsequent growth in water demand.
Furthermore, all these studies used data aggregated over large areas, and ignored
variations in demand due to the spatial variation in land use, soils and agroclimate. The
development and application of GIS modelling techniques can now provide an effective
tool for including such spatial variation (e.g. Madsen and Holst, 1990; Jacucci et al.,
1995; Knox and Weatherhead, 1999). This paper describes an improved methodology for
predicting the future growth in demand for irrigation in England and Wales, and its
application within a GIS framework for mapping that growth. Where appropriate datasets
are available, the procedures described are equally applicable in other countries.
2. Definitions of dry year demand
There are various ways of defining irrigation `demand'. In this study, two definitions
are considered, termed actual demand and theoretical demand. Actual demand is based
on the gross depths farmers are applying, as reported in the EA water abstraction and
MAFF Irrigation Survey data. It therefore reflects directly the irrigation practices that
farmers find realistic, and includes the effects of equipment constraints, present water
shortages, scheduling errors, and the farmers' scheduling assumptions on irrigation
losses. In contrast, theoretical demand is based on the calculated agronomic water
requirements of the crops that are irrigated, assuming they are correctly irrigated
following typical scheduling recommendations.
All forecasts presented in this study are for a `design' dry year, defined for the UK as a
year with an irrigation need with a 20% probability of exceedance. In this context, 1990
can be considered as approximating to a `design' dry year, whilst 1995 was marginally
more extreme. It is emphasised that all the predictions are of demand; actual water use
will be reduced by any increased restrictions on water availability for irrigation. All
forecasts are for the eight crop categories defined in the MAFF Agricultural and
Horticultural Cropping Census and MAFF Irrigation Surveys (MAFF, 1996; MAFF,
1997); namely early potatoes, maincrop potatoes, sugar beet, vegetables (grown in the
open), soft fruit, orchard fruit, cereals, and grass. For calculating irrigation needs, carrots
were used to represent vegetables, strawberries for small fruit, and mature apples for
orchard fruit.
3. Methodology
3.1. Overview
In an area where irrigation is supplemental to rainfall, such as the UK, many crops are
not irrigated, and even for the irrigated crop categories, not all farmers irrigate.
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
207
Furthermore, many farmers may apply less than agronomic demand, either because of
equipment or water limitations, or as a deliberate policy to maximise profit. The
methodology required for estimating growth is thus more complex than for an arid area,
where demand is more clearly a function of agronomic demand.
The demand for irrigation water in the UK therefore depends on the area of each crop
grown, the proportion of each crop that is irrigated, and the depth of water to be applied to
each crop. Each of these factors in turn depends on agro-economic and technical
conditions which will inevitably change, as well as the fundamental agronomic and
agroclimatic conditions, which will themselves vary.
The methodology adopted involves three stages. Firstly, the existing dry year baseline
situation and the underlying growth rates (excluding annual weather variation) in each of
the above factors are determined. Secondly, the future agro-economic and technical
conditions must be modelled, and their influence on the growth rates estimated. Finally,
the two must be combined in a way that produces feasible predictions.
The methodology was first applied in a spreadsheet model to predict growth in actual
demand at national level from a 1990 baseline (Weatherhead et al., 1994). Predictions
were also made at county and regional level, but adjusted to maintain consistency with
the national predictions. The methodology has now been updated to a 1995 baseline, and
also applied using a GIS procedure, developed from work by Knox et al. (1997), to map
the future growth in theoretical demand.
3.2. Underlying growth rates
In the UK, the irrigated areas and the volumes of irrigation water applied each year
vary considerably depending on the summer weather. The data published in the MAFF
Irrigation Surveys therefore partly reflect the weather in each census year, and do not
directly show the dry year demand in a particular year or indicate the underlying trends in
dry year demand (Knox et al., 1996). However, Weatherhead et al. (1994) developed a
method for analysing the MAFF Irrigation Survey data using calculated theoretical
irrigation needs (depths) for each crop as the independent climate variable in a multiple
linear regression analysis. The regression results show the underlying growth rates in the
areas irrigated, in the proportion of each crop irrigated and in the depth applied. They also
allow the area and volume figures for any year to be adjusted to simulate `design' dry
year conditions occurring at that time.
Weatherhead et al. (1994) analysed the underlying growth rates across four MAFF
Irrigation Surveys, for 1982, 1984, 1987 and 1990. This procedure has been repeated
incorporating data from the MAFF 1992 and 1995 Irrigation Surveys, to update the
results and improve the statistical validity.
3.3. Future changes
The Manchester University Agricultural Policy Model (Burton, 1992) was used to
predict changes in crop areas, prices and yields, up to the year 2021. Modelling was
based on three world agricultural policy scenarios, namely (i) continuation of the
1992 conditions without Common Agricultural Policy (CAP) reforms; (ii) complete
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Table 1
Future rates of change in the fraction of each crop irrigated and depth of irrigation water applied, based on
Weatherhead et al. (1994)
Crop category
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other crops
Initial % change per annum
Fraction of crop irrigated
Depth of irrigation water applied
2
4
2
3
3
3
ÿ4
ÿ5
1
1
1
0
2
2
2
0
0
1
liberalisation and free trade; and (iii) reform of the CAP under a new General Agreement
on Tariff and Trade (GATT). This third scenario was considered by the authors to be the
`most likely', and is used in this study.
The Manchester model predictions of changes in crop areas are applied directly to the
baseline 1995 crop areas to predict future dry year crop areas. The predicted changes in
crop prices and crop yields are used to help estimate changes in the underlying growth
rates for the proportions of the crop irrigated and the depths of volumes applied. Predicted
changes in market forces, and irrigation technology, cost and effectiveness must also be
taken into account. Applying the changes to the growth rates rather than directly to the
values themselves reflects the inertia and committed investment existing in the system.
This stage inevitably remains partly subjective.
The resulting future rates of change (Table 1) are then applied to the baseline data, to
predict future irrigated proportions and irrigation depths. For declining irrigated fractions,
an ordinary compound rate of decline was assumed. For example, if the initial percentage
change per annum is ÿ5%, then 5% of the remaining irrigated fraction is lost each year.
The irrigated fraction will thus approach zero asymptotically. However, for increasing
irrigated fractions, a compound rate of decline of the remaining unirrigated fraction was
assumed. This ensures that increasing fractions approach unity (i.e. 100%) asymptotically, rather than continue to grow. Similarly, increasing application depths were
calculated by assuming a compound decline in the difference between the initial depth
and a maximum depth, arbitrarily set at double the initial depth (in practice this
maximum ceiling value was never reached and therefore had minimal effect on the
result). The predicted crop areas, predicted proportions irrigated and predicted
application depths are then combined. Irrigated areas and volumetric demands are
calculated for each crop category, and then totalled.
3.4. Predicting future actual dry year demand
Weatherhead et al. (1994) used the MAFF, 1992 Agricultural and Horticultural
Cropping Census data as the baseline for predicting crop areas, and the MAFF, 1990
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
209
Irrigation Survey as the baseline for predicting future irrigated fractions and irrigation
depths. Future rates of change in the proportions irrigated and depths applied were based
on the underlying trends from 1982 to 1990, the predictions for the three policy scenarios
modelled, and the authors assessments of technological changes. A meeting of leading
irrigators, irrigation advisers and others was used to judge whether the rates were
reasonable.
Irrigated areas and volumetric demands were calculated for each crop category,
and in total, up to 2021. High, medium and low predictions were produced for each
of the three world agricultural policy scenarios. The modelling was also carried out
at county level, with the results then adjusted for consistency with national totals.
The adjusted county results were then re-aggregated to Environment Agency (EA)
Regions.
The analysis has been repeated using the most recent MAFF cropping and irrigation
census data. The predicted changes in crop areas were updated from 1992 to a 1995
baseline, and applied to the MAFF 1995 Agricultural and Horticultural Cropping Census
data to estimate future crop areas. The MAFF 1995 Irrigation Survey data on irrigated
areas and volumes applied were adjusted to correspond to a `design' dry year occurring in
1995, using regression factors derived from the 1982±95 underlying growth analysis
(Section 3.2). The fraction of each crop irrigated and average irrigation depth, were then
derived for this 1995 `design' dry year. Future rates of change in the proportions irrigated
and depths applied were based on the underlying trends from 1982 to 1995, the
predictions for the reformed CAP scenario, and the authors assessments of technological
changes.
3.5. Predicting and mapping future theoretical dry year demand
Knox et al. (1997) developed a GIS procedure for mapping the present total theoretical
dry year volumetric demand. For each of the main crops currently irrigated, irrigation
needs (depths) were calculated by soil water balance modelling based on current
agronomic recommendations, and then correlated to national datasets on agroclimate,
soils and irrigation practice using a GIS, to generate irrigation need maps. These maps
take into account the spatial variation in local soil type and climate. By multiplying these
maps with datasets on irrigated land use, irrigation demand maps were produced for each
crop category and in total. This procedure has been extended to predict the future
theoretical dry year demand.
The computerised spatial datasets used in both these studies have been described by
Knox et al. (1997), but are briefly reviewed here. National soils (dominant soil
association and profile available water) and agroclimate (potential soil moisture deficit)
datasets were extracted from Land IS, the Land Information System held by the Soil
Survey and Land Research Centre (SSLRC) (Hallett et al., 1996). The resolution of these
raster (gridded) datasets were 1 and 5 km, respectively. Information on land use was
obtained from raster datasets, based on the MAFF, 1994 Agricultural and Horticultural
Cropping Census for England and Wales, at 2 km resolution. Information on the
proportion of each crop that is irrigated were derived from the MAFF, 1995 Irrigation
Survey, at county level.
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Future changes in crop area were derived from the Manchester University Agricultural
Policy Model predictions for the reformed CAP scenario, but adjusted to a 1994 baseline.
For each crop category and predicted year, the future crop areas were estimated by
multiplying the MAFF reported 1994 crop area in each 2 km pixel by these crop area
change predictions.
Future changes in the fraction of each crop that would be irrigated in each pixel were
estimated using the reformed CAP rates of change, at county level (the highest resolution
of data available) (Table 1). Applying these to the future crop area datasets produces a
series of predicted irrigated area maps, for each crop category, for five year intervals for
2001±2021.
Part of the GIS procedure developed by Knox et al. (1997) included the production of
theoretical irrigation need (depth) maps, for each crop category. For this study, it was
assumed that these theoretical values would not change over time (although agronomic
recommendations could change, of course). The possible impact of climate change is
considered later. Using a GIS overlay procedure, each predicted irrigated area map was
then combined with the corresponding theoretical irrigation need map, to produce a
theoretical demand map, for each crop category. By summing the individual maps, the
total theoretical demand for each predicted year was calculated. Comparing these total
theoretical demand maps allowed the production of maps showing the spatial variation in
the growth of theoretical irrigation demand between 1996 and 2021.
4. Results
4.1. Underlying growth
The underlying growth rates from 1982 to 1995 in the area irrigated, volume applied
and average depth applied, for each crop category, as a percentage of the 1995 value, are
shown in Table 2. The underlying growth rate in the total volume of irrigation water
Table 2
Underlying growth rates in the area irrigated, volume applied and average depth, 1982±95
Crop category
% change per annum on 1995 value
Area irrigated
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other crops
Overall
1
4
ÿ2
ÿ3
ÿ1
3
ÿ7
ÿ5
3
1
r2 values are given in brackets.
(0.31)
(0.94)
(0.92)
(0.59)
(0.70)
(0.93)
(0.70)
(0.88)
(0.38)
(0.87)
Volume applied
4
5
ÿ1
ÿ4
3
4
ÿ4
ÿ3
2
3
(0.54)
(0.96)
(0.93)
(0.75)
(0.75)
(0.96)
(0.63)
(0.93)
(0.75)
(0.94)
Average depth
4
2
0
0
4
2
2
1
ÿ1
2
(0.58)
(0.95)
(0.92)
(0.82)
(0.79)
(0.91)
(0.70)
(0.93)
±
(0.89)
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Table 3
Predicted actual dry year demands (103 m3), by crop category, from 1996±2021
Crop category
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other crops
Total
Predicted actual irrigation demand
1996
2001
2006
2011
2016
2021
9080
73160
21650
1880
4990
27770
10510
7730
11110
167860
10110
83430
23740
2080
7190
35320
8300
6420
12220
188820
11110
90270
25800
2260
8970
41530
6560
5350
13350
205180
12040
94920
27820
2400
10670
47570
5170
4410
14490
219500
12940
97960
29800
2540
12140
53640
4080
3600
15650
232350
13740
99890
31750
2670
13440
59730
3240
2880
16830
244160
applied, was 3% per annum. The results show that irrigation is increasingly concentrated
on the more valuable crops, notably maincrop potatoes, small fruit and vegetables, and
that those crops that are irrigated are being given more water. In contrast, a marked
decline in the irrigation of grass and orchard fruit is apparent.
4.2. Future actual dry year demand
Weatherhead et al. (1994) predicted a `most likely' growth in total volumetric demand,
for a `design' dry year, of 1.7% per annum from 1996 to 2001, and 1% per annum from
2001 to 2021. These values have since been used in national water resource planning (e.g.
National Rivers Authority, 1994).
The updated predicted actual irrigation demand from 1996 to 2021, by crop category,
are shown in Table 3. The total net volumetric actual irrigation water requirements for a
`design' dry year are predicted to rise from 168106 m3 in 1996 to 244106 m3 in 2021.
This represents a national predicted average growth rate in actual volumetric demand for
a dry year of 2.5% from 1996 to 2001, which then declines gradually, with an average of
1.5% from 2001 to 2021.
4.3. Future theoretical dry year demand
The predicted theoretical irrigation demands from 1996 to 2021, by crop category, are
shown in Table 4. The theoretical total volumetric irrigation water requirements for a
`design' dry year are predicted to rise from 204106 m3 in 1994 to 247106 m3 in 2021,
an increase of 21%. A map showing the predicted theoretical total net volumetric
irrigation demand for a `design' dry year, for all irrigated crops in 2021, is shown in
Fig. 4. The likely spatial distribution of the changes in theoretical irrigation demand
between 1996 and 2021 is shown in Fig. 5. These maps confirm that theoretical irrigation
demand, and growth, will continue to be strongly concentrated in Eastern England,
notably around the Fens region, and in parts of North Norfolk and the Suffolk coast. Parts
of Kent, Nottinghamshire and Shropshire also show large increases.
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Table 4
Predicted theoretical dry year demands (103 m3), by crop category, from 1996±2021
Crop category
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other cropsa
Total
a
Predicted theoretical irrigation demands
1996
2001
2006
2011
2016
2021
3290
100290
26590
4360
6840
37380
15670
1340
7830
203590
3510
109490
29110
4420
9350
43730
12700
1100
8540
221940
3710
114140
31800
4410
10920
47630
10280
930
8950
232760
3910
115020
33950
4420
12180
51240
8100
780
9180
238770
4050
114500
36400
4600
12980
54900
5480
620
9340
242860
4200
112630
38930
4560
13570
58100
4570
480
9480
246520
Estimated.
5. Discussion
5.1. Actual or theoretical demand
The use of two different definitions of irrigation demand reflects different possible uses
for the results and gaps in the availability of GIS datasets.
The projections of actual demand are based on current irrigation practices and the
volumes farmers currently abstract, and are therefore of direct interest to planners. The
projection methodology allows for predicted changes in irrigation practices, for example,
due to increased investment in equipment, technology change, higher economic
incentives, and improving irrigation efficiency. These projections are probably reasonably
accurate reflections of the volumes farmers really want in the short to medium term, since
such changes are likely to be slow, but they could be unreliable in the longer term.
The estimated theoretical demands probably overstate demands, since not all farmers
can physically follow the scheduling recommendations, and others may be deliberately
practising partial irrigation. Conversely, however, there is no allowance for losses. It is
tempting to think that theoretical demand is somehow a fixed optimum, but that is
misleading. Changes in equipment and economics can and should alter the
recommendations, and hence the theoretical irrigation need. In particular, the percentage
of the crop irrigated is a function of economic benefits and sometimes water availability.
A GIS based approach to modelling demand allows consideration of local variability in
cropping, soils and climate, and hence the production of irrigation demand maps.
However, high quality spatial datasets are a pre-requisite for any GIS modelling. The base
data for predicting actual demand is only available at county level; this relatively coarse
resolution would limit the accuracy and benefit of predicting actual demands within a
GIS framework. In contrast, most of the datasets required to model theoretical demand
are now available at a reasonable resolution, and GIS techniques are readily applicable.
Using theoretical demand to study variability and comparing with actual demand at
county level gives a useful compromise.
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
213
Fig. 4. Predicted total net theoretical volumetric irrigation demand (m3 kmÿ2) in a `design' dry year in 2021.
5.2. Comparison with previous estimates
This study predicted a higher national growth rate in actual volumetric demand for a
`design' dry year than previous estimates by Weatherhead et al. (1994). However, the
differences in the predicted absolute volumes are not large, because of the revised
baseline. Fig. 6 compares the predictions of total actual dry year demand arising from this
study based on (a) the methodology described, (b) the same methodology but using the
underlying trends from 1982 to 1995 without adjustment, and (c) the previous `most
likely' predictions by Weatherhead et al. (1994). The predicted theoretical demand
estimated in this study is also shown (d). The predicted total actual demands are
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Fig. 5. Predicted change in the spatial distribution of volumetric irrigation demand (m3 kmÿ2) between 1996
and 2021.
remarkably similar, although there are differences between crops and between areas, as
discussed earlier. The actual demand predictions estimated in this study approach the
theoretical demand prediction towards 2021, reflecting the increased depths applied on
the dominant crops.
5.3. Regional and catchment predictions
By using the GIS to delineate administrative regions or hydrological catchments of
interest, the local growth and future irrigation requirements can be estimated. This can
also show the future split between crops in each sub-unit, and hence indicate the timing of
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
215
Fig. 6. Comparison of predictions of future volumetric irrigation demand, from 1990±2021.
demand and the likely take-up of particular water conservation measures such as trickle
(drip) irrigation.
5.4. Influence of climate change
The Intergovernmental Panel on Climate Change (IPCC) reported that `the balance of
evidence suggests there is a discernible human influence on the global climate'
(Department of the Environment, 1996). Indeed the spate of recent droughts experienced
in UK are consistent with a changing climate. However, the likely impacts on UK
irrigation are still far from clear. Recent estimates suggest higher temperatures with only
marginally more summer rainfall in the main UK irrigation areas (Department of the
Environment, 1996). Others show a marginal decrease in summer rainfall, further
increasing potential soil moisture deficits (BHS, 1998). Extrapolating from recent
Institute of Hydrology predictions and past quantitative relationships between climate
variation and summer rainfall, Herrington (1996) estimated an additional 27.5% demand
above current trends in EA Anglian Region by the year 2021. However, as he cautions,
using relationships based on past variation to estimate the effects of change is likely to
give an underestimate. Once the likely effects of climate change are more widely
accepted, farmers can be expected to want to increase system capacity and irrigate more
of their crops.
For calculating theoretical demand, the potential soil moisture deficit (PSMD), crop
adjusted, is used as the climatic indicator within the GIS model. Once reliable climate
change predictions are available for UK, it will be feasible to predict future changes in
PSMD, and hence predict the increased theoretical demand for the crops currently
irrigated. However, it will be much more difficult to model the effects of climate change
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
on cropping pattern and on the economics of irrigating particular crops. Furthermore,
potential reductions in water availability due to climate change may themselves affect the
location of irrigated agriculture.
5.5. Limitations
A number of assumptions were made in this study, and their respective limitations on
the accuracy of the predictions made must be recognised. The limited accuracy of the
MAFF data and the spatial integrity of the datasets used in the GIS has been discussed
previously by Knox et al. (1997). It should also be recognised that integrating models
with GIS can introduce an additional problem, notably the perception that GIS can
`generate information' (Grayson et al., 1993). The sophisticated data handling and
visualisation features of GIS can inadvertently seduce the user into an unrealistic sense of
model accuracy, particularly where datasets with varying data structure (e.g. point, vector,
raster), resolution, and format are incorporated. Output from GIS based analysis should
therefore always be interpreted with caution.
The Manchester University model predictions appear to be reasonably robust with
regard to the main irrigated crops, few of which are subject to subsidy under the CAP.
The reformed CAP scenario still appears to be reasonable for the main irrigated crops.
The subsequent choice of rates of change is partly subjective, and could be a major source
of error. The short term predictions are based on recent trends and current expert opinion.
In the medium to long term there is a risk of missing a new trend, for example, a change
in the profitability of irrigating grass, cereals or sugar beet, a change to drought resistant
varieties of potatoes, or the spread of new crops such as irrigated maize. Further research
is needed to predict the effects of reductions in water availability and increased price on
the economics of irrigation and hence on irrigated cropping patterns, and to predict
whether and how restrictions on new abstraction licences will change the location of
irrigated agriculture.
6. Conclusions
The spatial and temporal growth in the actual and theoretical dry year demand for
irrigation in England and Wales has been predicted, incorporating forecasts of changes in
the crop areas, in the fractions of each crop irrigated and in the irrigation application
depths. The GIS procedure enables these predictions to be estimated and mapped by crop
category, region, or catchment.
This study predicted higher underlying national growth rates than previous recent
work, and greater future increases, although the differences in the predicted absolute
volumes are not large, because of the revised baseline. The underlying volumetric growth
from 1982 to 1995 was found to be 3% per annum. The total net volumetric actual
irrigation water requirements for a `design' dry year (20% exceedance), are predicted to
rise by an average of 2.5% per annum from 1996 to 2001, and by an average of 1.5% per
annum from 2001 to 2021. Meanwhile, the total net volumetric theoretical irrigation
water requirements for a `design' dry year, are predicted to rise slightly less rapidly.
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
217
The results can help identify potential future water resource problem areas, and should
be useful for water resource planning by individual farmers as well as by the government,
regulatory authorities and others. The methodology can be applied in other countries
where appropriate datasets are available.
Acknowledgements
This study formed part of a research contract (OC9219) funded by the Ministry of
Agriculture, Fisheries and Food (MAFF). The authors acknowledge the support of the
Soil Survey and Land Research Centre (SSLRC) for their support in digital data
acquisition.
References
ACAH, 1980. Water for Irrigation: Future needs. Advisory Council for Agriculture and Horticulture in England
and Wales. HMSO, London.
Anglian Water Authority, 1982. Anglian water forecasts of demand for direct water use and for in-river needs.
Anglian Water Authority (AWA), Norwich.
Anglian Water Authority, 1988. Anglian water forecasts of demand for direct water use and for in-river needs.
Operations Directorate (AWA), Norwich.
BHS, 1998. Climate change: water and environmental management in the Midlands. British Hydrological
Society Circulation no. 57, 3±4.
Burton, M.P., 1992. An Agricultural Policy Model for the UK. Avebury.
Department of the Environment, 1996. Review of the Potential Effects of Climate Change in the United
Kingdom. HMSO, London.
Grayson, R.B., Broschl, G., Barling, R.D., Moore, I.D., 1993. Process, scale and constraints to hydrological
modelling in GIS. In: Kovar, K., Nachtnebel, H.P (Eds.), Application of Geographic Information Systems
in Hydrology and Water Resources Management, Proceedings of the HydroGIS 93 Vienna Conference,
Publication no. 211. IAHS, Wallingford, pp. 83±92.
Hallett, S.H., Jones, R.J.A., Keay, C.A., 1996. Environmental information systems developments for planning
sustainable land use. Int. J. Geographical Information Systems 10, 47±64.
Herrington, P., 1996. Climate Change and the Demand for Water. Department of the Environment. HMSO, London.
Jacucci, G., Kabat, P., Verrier, P.J., Teixeira, J.L., Steduto, P., Bertanzon, G., Giannerini, G., Huygen, J.,
Fernando, R.M., Hooijer, A.A., Simons, W., Toller, G., Tziallas, G., Uhrik, C., Yovchev, P., Van den Broek,
B.J., 1995. HYDRA: a decision support model for irrigation water management. In: Pereira, L.S., van den
Broek, B.J., Kabat, P., Allen, R.G. (Eds), Crop-water-simulation models in practice, selected papers of the
2nd Workshop, 15th Congress of the International Commission on Irrigation and Drainage, The Hague,
Netherlands, 1993. Wageningen Press, Netherlands, pp. 315±32.
Knox, J.W., Weatherhead, E.K., Bradley, R.I., 1996. Mapping the spatial distribution of volumetric irrigation
water requirements for maincrop potatoes in England and Wales. Agric. Water Manage. 31, 1±15.
Knox, J.W., Weatherhead, E.K., Bradley, R.I., 1997. Mapping the total volumetric irrigation water requirements
in England and Wales. Agric. Water Manage. 33, 1±18.
Knox, J.W., Weatherhead, E.K., 1999. The application of GIS to irrigation water resource management in
England and Wales. Geographical J. 165(1), 90±98.
Madsen, H.B., Holst, K.A., 1990. Mapping of irrigation need based on computerised soil and climatic data.
Agric. Water Manage. 17, 391±407.
MAFF, 1996. Final Results of the June 1995 Agricultural and Horticultural Census: England and Wales, Regions
and Counties. Ministry of Agriculture, Fisheries and Food, Guildford.
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MAFF, 1997. Survey of Irrigation of Outdoor Crops in 1995. Ministry of Agriculture, Fisheries and Food,
Guildford.
National Rivers Authority, 1990. Forecasts of abstraction demands for water, 1990 series, public water supply
and direct abstraction. Anglian Region NRA, Peterborough.
National Rivers Authority, 1994. Water: Nature's Precious Resource. HMSO, London.
Roughton, J.L., Clarke, K.F., 1978. Evidence for the Advisory Council for Agriculture and Horticulture on the
future water needs of the agriculture and horticulture industries. Anglian Water Authority, Norwich.
Weatherhead, E.K., Place, A.J., Morris, J., Burton, M., 1994. Demand for Irrigation Water. NRA R and D Report
14, HMSO, London.
Weatherhead, E.K., Knox, J.W., Morris, J., Hess, T.M., Bradley, R.I., and Sanders, C.L., 1997. Irrigation
Demand and On-Farm Water Conservation in England and Wales. Final Report to MAFF. Cranfield
University, Bedford.
Predicting and mapping the future demand for
irrigation water in England and Wales
E.K. Weatherhead, J.W. Knox*
Natural Resources Management Department, School of Agriculture Food and Environment, Cranfield
University, Silsoe, Bedford MK45 4DT, UK
Accepted 2 April 1999
Abstract
A methodology has been developed for predicting the future growth in demand for irrigation in
countries with supplemental irrigation such as England and Wales. This takes into account expected
changes in agricultural policy, technical, market and other factors. The methodology has also been
applied within a geographical information system (GIS) to map the growth. The GIS approach
represents an extension of a procedure developed by Knox et al. (1997) (Knox, J.W., Weatherhead,
E.K., Bradley, R.I., 1997. Agricultural Water Management 33, 1±18).
The total net volumetric actual irrigation water requirements for a `design' dry year (20%
exceedance), are predicted to rise from 168106 m3 in 1996 to 244106 m3 in 2021. Meanwhile,
the total net volumetric theoretical irrigation water requirements for a `design' dry year, are
predicted to rise from 204106 m3 in 1996 to 247106 m3 in 2021.
Maps are presented showing the predicted change in the spatial distribution of irrigation demand
between 1996 and 2021, and the theoretical dry year irrigation demand for all irrigated crops in
2021. The application and limitations of the methodology are discussed. # 2000 Elsevier Science
B.V. All rights reserved.
Keywords: England and Wales; GIS; Irrigation; Water demand; Maps; Prediction
1. Introduction
Recent droughts, rising public demand for mains water supply, and increased environmental protection, have all reduced the availability and reliability of water supplies for
agricultural and horticultural irrigation in England and Wales. In many catchments, water
* Corresponding author. Tel.: +44-1525-863328; fax: +44-1525-863000.
E-mail address: [email protected] (J.W. Knox).
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 5 8 - X
204
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Fig. 1. Total licensed and abstracted volumes of water for irrigation in England and Wales, 1974±1997.
resources are already over-committed, and additional licences for irrigation abstraction
are unobtainable. Yet demand for irrigation water is also increasing (Weatherhead et al.,
1997). Estimates of both the magnitude and location of that growth are an essential
requirement for strategic water resource planning at national and regional levels.
Abstraction of water for spray irrigation in England and Wales is regulated by the
Environment Agency (EA). The volume of water licensed and actually abstracted are
reported annually. Both figures have increased substantially over the last 25 years
(Fig. 1). The Ministry of Agriculture, Fisheries and Food (MAFF) also publishes
statistics on agricultural irrigation in England and Wales, based on surveys carried out
roughly triennially. These surveys report the areas irrigated and water use, by crop
category, and in total. The areas that farmers reported they were `likely to irrigate in a dry
year' from 1955 to 1995 (the most recent survey) are shown in Fig. 2. Rapid growth
occurred up to 1965, followed by a decline until 1974. Since then the area has again risen
sharply, despite a slight decline in 1995.
In 1980, a major national study by the Advisory Council for Agriculture and
Horticulture (ACAH, 1980) predicted a 150% (4% per annum) growth in the irrigated
area in England and Wales between 1977 and 2000, and an overall increase in water use
of 300% (6% per annum), assuming no restrictions on water availability. However, the
subsequent downturn in the profitability of irrigating grass kept actual growth well below
the projected levels. There have also been many attempts to predict irrigation growth
regionally, notably for Anglian region, where most irrigation occurs (e.g. Roughton and
Clarke, 1978; Anglian Water Authority, 1982; Anglian Water Authority, 1988; National
Rivers Authority, 1990). The predictions are summarised in Fig. 3. Although a variety of
methodologies were adopted, common problems have been the difficulty in extricating
the effects of recent weather, incorporating changes in external economic factors, and
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
205
Fig. 2. Total area `likely to be irrigated in a dry year' in England and Wales, 1955±1995.
Fig. 3. Selected predictions of future demand for irrigation in Anglian region, together with reported actual
abstractions (1976±97).
206
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
predicting the effects of non-availability of water. Several studies failed to differentiate
between the different irrigated crops; others failed to set sensible limits to the total crop
areas or to the percentages of each that would be irrigated. Many of these studies were
commissioned following periods of drought when interest in irrigation was abnormally
high, and consequently over-estimated the subsequent growth in water demand.
Furthermore, all these studies used data aggregated over large areas, and ignored
variations in demand due to the spatial variation in land use, soils and agroclimate. The
development and application of GIS modelling techniques can now provide an effective
tool for including such spatial variation (e.g. Madsen and Holst, 1990; Jacucci et al.,
1995; Knox and Weatherhead, 1999). This paper describes an improved methodology for
predicting the future growth in demand for irrigation in England and Wales, and its
application within a GIS framework for mapping that growth. Where appropriate datasets
are available, the procedures described are equally applicable in other countries.
2. Definitions of dry year demand
There are various ways of defining irrigation `demand'. In this study, two definitions
are considered, termed actual demand and theoretical demand. Actual demand is based
on the gross depths farmers are applying, as reported in the EA water abstraction and
MAFF Irrigation Survey data. It therefore reflects directly the irrigation practices that
farmers find realistic, and includes the effects of equipment constraints, present water
shortages, scheduling errors, and the farmers' scheduling assumptions on irrigation
losses. In contrast, theoretical demand is based on the calculated agronomic water
requirements of the crops that are irrigated, assuming they are correctly irrigated
following typical scheduling recommendations.
All forecasts presented in this study are for a `design' dry year, defined for the UK as a
year with an irrigation need with a 20% probability of exceedance. In this context, 1990
can be considered as approximating to a `design' dry year, whilst 1995 was marginally
more extreme. It is emphasised that all the predictions are of demand; actual water use
will be reduced by any increased restrictions on water availability for irrigation. All
forecasts are for the eight crop categories defined in the MAFF Agricultural and
Horticultural Cropping Census and MAFF Irrigation Surveys (MAFF, 1996; MAFF,
1997); namely early potatoes, maincrop potatoes, sugar beet, vegetables (grown in the
open), soft fruit, orchard fruit, cereals, and grass. For calculating irrigation needs, carrots
were used to represent vegetables, strawberries for small fruit, and mature apples for
orchard fruit.
3. Methodology
3.1. Overview
In an area where irrigation is supplemental to rainfall, such as the UK, many crops are
not irrigated, and even for the irrigated crop categories, not all farmers irrigate.
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
207
Furthermore, many farmers may apply less than agronomic demand, either because of
equipment or water limitations, or as a deliberate policy to maximise profit. The
methodology required for estimating growth is thus more complex than for an arid area,
where demand is more clearly a function of agronomic demand.
The demand for irrigation water in the UK therefore depends on the area of each crop
grown, the proportion of each crop that is irrigated, and the depth of water to be applied to
each crop. Each of these factors in turn depends on agro-economic and technical
conditions which will inevitably change, as well as the fundamental agronomic and
agroclimatic conditions, which will themselves vary.
The methodology adopted involves three stages. Firstly, the existing dry year baseline
situation and the underlying growth rates (excluding annual weather variation) in each of
the above factors are determined. Secondly, the future agro-economic and technical
conditions must be modelled, and their influence on the growth rates estimated. Finally,
the two must be combined in a way that produces feasible predictions.
The methodology was first applied in a spreadsheet model to predict growth in actual
demand at national level from a 1990 baseline (Weatherhead et al., 1994). Predictions
were also made at county and regional level, but adjusted to maintain consistency with
the national predictions. The methodology has now been updated to a 1995 baseline, and
also applied using a GIS procedure, developed from work by Knox et al. (1997), to map
the future growth in theoretical demand.
3.2. Underlying growth rates
In the UK, the irrigated areas and the volumes of irrigation water applied each year
vary considerably depending on the summer weather. The data published in the MAFF
Irrigation Surveys therefore partly reflect the weather in each census year, and do not
directly show the dry year demand in a particular year or indicate the underlying trends in
dry year demand (Knox et al., 1996). However, Weatherhead et al. (1994) developed a
method for analysing the MAFF Irrigation Survey data using calculated theoretical
irrigation needs (depths) for each crop as the independent climate variable in a multiple
linear regression analysis. The regression results show the underlying growth rates in the
areas irrigated, in the proportion of each crop irrigated and in the depth applied. They also
allow the area and volume figures for any year to be adjusted to simulate `design' dry
year conditions occurring at that time.
Weatherhead et al. (1994) analysed the underlying growth rates across four MAFF
Irrigation Surveys, for 1982, 1984, 1987 and 1990. This procedure has been repeated
incorporating data from the MAFF 1992 and 1995 Irrigation Surveys, to update the
results and improve the statistical validity.
3.3. Future changes
The Manchester University Agricultural Policy Model (Burton, 1992) was used to
predict changes in crop areas, prices and yields, up to the year 2021. Modelling was
based on three world agricultural policy scenarios, namely (i) continuation of the
1992 conditions without Common Agricultural Policy (CAP) reforms; (ii) complete
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Table 1
Future rates of change in the fraction of each crop irrigated and depth of irrigation water applied, based on
Weatherhead et al. (1994)
Crop category
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other crops
Initial % change per annum
Fraction of crop irrigated
Depth of irrigation water applied
2
4
2
3
3
3
ÿ4
ÿ5
1
1
1
0
2
2
2
0
0
1
liberalisation and free trade; and (iii) reform of the CAP under a new General Agreement
on Tariff and Trade (GATT). This third scenario was considered by the authors to be the
`most likely', and is used in this study.
The Manchester model predictions of changes in crop areas are applied directly to the
baseline 1995 crop areas to predict future dry year crop areas. The predicted changes in
crop prices and crop yields are used to help estimate changes in the underlying growth
rates for the proportions of the crop irrigated and the depths of volumes applied. Predicted
changes in market forces, and irrigation technology, cost and effectiveness must also be
taken into account. Applying the changes to the growth rates rather than directly to the
values themselves reflects the inertia and committed investment existing in the system.
This stage inevitably remains partly subjective.
The resulting future rates of change (Table 1) are then applied to the baseline data, to
predict future irrigated proportions and irrigation depths. For declining irrigated fractions,
an ordinary compound rate of decline was assumed. For example, if the initial percentage
change per annum is ÿ5%, then 5% of the remaining irrigated fraction is lost each year.
The irrigated fraction will thus approach zero asymptotically. However, for increasing
irrigated fractions, a compound rate of decline of the remaining unirrigated fraction was
assumed. This ensures that increasing fractions approach unity (i.e. 100%) asymptotically, rather than continue to grow. Similarly, increasing application depths were
calculated by assuming a compound decline in the difference between the initial depth
and a maximum depth, arbitrarily set at double the initial depth (in practice this
maximum ceiling value was never reached and therefore had minimal effect on the
result). The predicted crop areas, predicted proportions irrigated and predicted
application depths are then combined. Irrigated areas and volumetric demands are
calculated for each crop category, and then totalled.
3.4. Predicting future actual dry year demand
Weatherhead et al. (1994) used the MAFF, 1992 Agricultural and Horticultural
Cropping Census data as the baseline for predicting crop areas, and the MAFF, 1990
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
209
Irrigation Survey as the baseline for predicting future irrigated fractions and irrigation
depths. Future rates of change in the proportions irrigated and depths applied were based
on the underlying trends from 1982 to 1990, the predictions for the three policy scenarios
modelled, and the authors assessments of technological changes. A meeting of leading
irrigators, irrigation advisers and others was used to judge whether the rates were
reasonable.
Irrigated areas and volumetric demands were calculated for each crop category,
and in total, up to 2021. High, medium and low predictions were produced for each
of the three world agricultural policy scenarios. The modelling was also carried out
at county level, with the results then adjusted for consistency with national totals.
The adjusted county results were then re-aggregated to Environment Agency (EA)
Regions.
The analysis has been repeated using the most recent MAFF cropping and irrigation
census data. The predicted changes in crop areas were updated from 1992 to a 1995
baseline, and applied to the MAFF 1995 Agricultural and Horticultural Cropping Census
data to estimate future crop areas. The MAFF 1995 Irrigation Survey data on irrigated
areas and volumes applied were adjusted to correspond to a `design' dry year occurring in
1995, using regression factors derived from the 1982±95 underlying growth analysis
(Section 3.2). The fraction of each crop irrigated and average irrigation depth, were then
derived for this 1995 `design' dry year. Future rates of change in the proportions irrigated
and depths applied were based on the underlying trends from 1982 to 1995, the
predictions for the reformed CAP scenario, and the authors assessments of technological
changes.
3.5. Predicting and mapping future theoretical dry year demand
Knox et al. (1997) developed a GIS procedure for mapping the present total theoretical
dry year volumetric demand. For each of the main crops currently irrigated, irrigation
needs (depths) were calculated by soil water balance modelling based on current
agronomic recommendations, and then correlated to national datasets on agroclimate,
soils and irrigation practice using a GIS, to generate irrigation need maps. These maps
take into account the spatial variation in local soil type and climate. By multiplying these
maps with datasets on irrigated land use, irrigation demand maps were produced for each
crop category and in total. This procedure has been extended to predict the future
theoretical dry year demand.
The computerised spatial datasets used in both these studies have been described by
Knox et al. (1997), but are briefly reviewed here. National soils (dominant soil
association and profile available water) and agroclimate (potential soil moisture deficit)
datasets were extracted from Land IS, the Land Information System held by the Soil
Survey and Land Research Centre (SSLRC) (Hallett et al., 1996). The resolution of these
raster (gridded) datasets were 1 and 5 km, respectively. Information on land use was
obtained from raster datasets, based on the MAFF, 1994 Agricultural and Horticultural
Cropping Census for England and Wales, at 2 km resolution. Information on the
proportion of each crop that is irrigated were derived from the MAFF, 1995 Irrigation
Survey, at county level.
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Future changes in crop area were derived from the Manchester University Agricultural
Policy Model predictions for the reformed CAP scenario, but adjusted to a 1994 baseline.
For each crop category and predicted year, the future crop areas were estimated by
multiplying the MAFF reported 1994 crop area in each 2 km pixel by these crop area
change predictions.
Future changes in the fraction of each crop that would be irrigated in each pixel were
estimated using the reformed CAP rates of change, at county level (the highest resolution
of data available) (Table 1). Applying these to the future crop area datasets produces a
series of predicted irrigated area maps, for each crop category, for five year intervals for
2001±2021.
Part of the GIS procedure developed by Knox et al. (1997) included the production of
theoretical irrigation need (depth) maps, for each crop category. For this study, it was
assumed that these theoretical values would not change over time (although agronomic
recommendations could change, of course). The possible impact of climate change is
considered later. Using a GIS overlay procedure, each predicted irrigated area map was
then combined with the corresponding theoretical irrigation need map, to produce a
theoretical demand map, for each crop category. By summing the individual maps, the
total theoretical demand for each predicted year was calculated. Comparing these total
theoretical demand maps allowed the production of maps showing the spatial variation in
the growth of theoretical irrigation demand between 1996 and 2021.
4. Results
4.1. Underlying growth
The underlying growth rates from 1982 to 1995 in the area irrigated, volume applied
and average depth applied, for each crop category, as a percentage of the 1995 value, are
shown in Table 2. The underlying growth rate in the total volume of irrigation water
Table 2
Underlying growth rates in the area irrigated, volume applied and average depth, 1982±95
Crop category
% change per annum on 1995 value
Area irrigated
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other crops
Overall
1
4
ÿ2
ÿ3
ÿ1
3
ÿ7
ÿ5
3
1
r2 values are given in brackets.
(0.31)
(0.94)
(0.92)
(0.59)
(0.70)
(0.93)
(0.70)
(0.88)
(0.38)
(0.87)
Volume applied
4
5
ÿ1
ÿ4
3
4
ÿ4
ÿ3
2
3
(0.54)
(0.96)
(0.93)
(0.75)
(0.75)
(0.96)
(0.63)
(0.93)
(0.75)
(0.94)
Average depth
4
2
0
0
4
2
2
1
ÿ1
2
(0.58)
(0.95)
(0.92)
(0.82)
(0.79)
(0.91)
(0.70)
(0.93)
±
(0.89)
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Table 3
Predicted actual dry year demands (103 m3), by crop category, from 1996±2021
Crop category
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other crops
Total
Predicted actual irrigation demand
1996
2001
2006
2011
2016
2021
9080
73160
21650
1880
4990
27770
10510
7730
11110
167860
10110
83430
23740
2080
7190
35320
8300
6420
12220
188820
11110
90270
25800
2260
8970
41530
6560
5350
13350
205180
12040
94920
27820
2400
10670
47570
5170
4410
14490
219500
12940
97960
29800
2540
12140
53640
4080
3600
15650
232350
13740
99890
31750
2670
13440
59730
3240
2880
16830
244160
applied, was 3% per annum. The results show that irrigation is increasingly concentrated
on the more valuable crops, notably maincrop potatoes, small fruit and vegetables, and
that those crops that are irrigated are being given more water. In contrast, a marked
decline in the irrigation of grass and orchard fruit is apparent.
4.2. Future actual dry year demand
Weatherhead et al. (1994) predicted a `most likely' growth in total volumetric demand,
for a `design' dry year, of 1.7% per annum from 1996 to 2001, and 1% per annum from
2001 to 2021. These values have since been used in national water resource planning (e.g.
National Rivers Authority, 1994).
The updated predicted actual irrigation demand from 1996 to 2021, by crop category,
are shown in Table 3. The total net volumetric actual irrigation water requirements for a
`design' dry year are predicted to rise from 168106 m3 in 1996 to 244106 m3 in 2021.
This represents a national predicted average growth rate in actual volumetric demand for
a dry year of 2.5% from 1996 to 2001, which then declines gradually, with an average of
1.5% from 2001 to 2021.
4.3. Future theoretical dry year demand
The predicted theoretical irrigation demands from 1996 to 2021, by crop category, are
shown in Table 4. The theoretical total volumetric irrigation water requirements for a
`design' dry year are predicted to rise from 204106 m3 in 1994 to 247106 m3 in 2021,
an increase of 21%. A map showing the predicted theoretical total net volumetric
irrigation demand for a `design' dry year, for all irrigated crops in 2021, is shown in
Fig. 4. The likely spatial distribution of the changes in theoretical irrigation demand
between 1996 and 2021 is shown in Fig. 5. These maps confirm that theoretical irrigation
demand, and growth, will continue to be strongly concentrated in Eastern England,
notably around the Fens region, and in parts of North Norfolk and the Suffolk coast. Parts
of Kent, Nottinghamshire and Shropshire also show large increases.
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E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
Table 4
Predicted theoretical dry year demands (103 m3), by crop category, from 1996±2021
Crop category
Early potatoes
Maincrop potatoes
Sugar beet
Orchard fruit
Small fruit
Vegetables
Grass
Cereals
Other cropsa
Total
a
Predicted theoretical irrigation demands
1996
2001
2006
2011
2016
2021
3290
100290
26590
4360
6840
37380
15670
1340
7830
203590
3510
109490
29110
4420
9350
43730
12700
1100
8540
221940
3710
114140
31800
4410
10920
47630
10280
930
8950
232760
3910
115020
33950
4420
12180
51240
8100
780
9180
238770
4050
114500
36400
4600
12980
54900
5480
620
9340
242860
4200
112630
38930
4560
13570
58100
4570
480
9480
246520
Estimated.
5. Discussion
5.1. Actual or theoretical demand
The use of two different definitions of irrigation demand reflects different possible uses
for the results and gaps in the availability of GIS datasets.
The projections of actual demand are based on current irrigation practices and the
volumes farmers currently abstract, and are therefore of direct interest to planners. The
projection methodology allows for predicted changes in irrigation practices, for example,
due to increased investment in equipment, technology change, higher economic
incentives, and improving irrigation efficiency. These projections are probably reasonably
accurate reflections of the volumes farmers really want in the short to medium term, since
such changes are likely to be slow, but they could be unreliable in the longer term.
The estimated theoretical demands probably overstate demands, since not all farmers
can physically follow the scheduling recommendations, and others may be deliberately
practising partial irrigation. Conversely, however, there is no allowance for losses. It is
tempting to think that theoretical demand is somehow a fixed optimum, but that is
misleading. Changes in equipment and economics can and should alter the
recommendations, and hence the theoretical irrigation need. In particular, the percentage
of the crop irrigated is a function of economic benefits and sometimes water availability.
A GIS based approach to modelling demand allows consideration of local variability in
cropping, soils and climate, and hence the production of irrigation demand maps.
However, high quality spatial datasets are a pre-requisite for any GIS modelling. The base
data for predicting actual demand is only available at county level; this relatively coarse
resolution would limit the accuracy and benefit of predicting actual demands within a
GIS framework. In contrast, most of the datasets required to model theoretical demand
are now available at a reasonable resolution, and GIS techniques are readily applicable.
Using theoretical demand to study variability and comparing with actual demand at
county level gives a useful compromise.
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
213
Fig. 4. Predicted total net theoretical volumetric irrigation demand (m3 kmÿ2) in a `design' dry year in 2021.
5.2. Comparison with previous estimates
This study predicted a higher national growth rate in actual volumetric demand for a
`design' dry year than previous estimates by Weatherhead et al. (1994). However, the
differences in the predicted absolute volumes are not large, because of the revised
baseline. Fig. 6 compares the predictions of total actual dry year demand arising from this
study based on (a) the methodology described, (b) the same methodology but using the
underlying trends from 1982 to 1995 without adjustment, and (c) the previous `most
likely' predictions by Weatherhead et al. (1994). The predicted theoretical demand
estimated in this study is also shown (d). The predicted total actual demands are
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Fig. 5. Predicted change in the spatial distribution of volumetric irrigation demand (m3 kmÿ2) between 1996
and 2021.
remarkably similar, although there are differences between crops and between areas, as
discussed earlier. The actual demand predictions estimated in this study approach the
theoretical demand prediction towards 2021, reflecting the increased depths applied on
the dominant crops.
5.3. Regional and catchment predictions
By using the GIS to delineate administrative regions or hydrological catchments of
interest, the local growth and future irrigation requirements can be estimated. This can
also show the future split between crops in each sub-unit, and hence indicate the timing of
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
215
Fig. 6. Comparison of predictions of future volumetric irrigation demand, from 1990±2021.
demand and the likely take-up of particular water conservation measures such as trickle
(drip) irrigation.
5.4. Influence of climate change
The Intergovernmental Panel on Climate Change (IPCC) reported that `the balance of
evidence suggests there is a discernible human influence on the global climate'
(Department of the Environment, 1996). Indeed the spate of recent droughts experienced
in UK are consistent with a changing climate. However, the likely impacts on UK
irrigation are still far from clear. Recent estimates suggest higher temperatures with only
marginally more summer rainfall in the main UK irrigation areas (Department of the
Environment, 1996). Others show a marginal decrease in summer rainfall, further
increasing potential soil moisture deficits (BHS, 1998). Extrapolating from recent
Institute of Hydrology predictions and past quantitative relationships between climate
variation and summer rainfall, Herrington (1996) estimated an additional 27.5% demand
above current trends in EA Anglian Region by the year 2021. However, as he cautions,
using relationships based on past variation to estimate the effects of change is likely to
give an underestimate. Once the likely effects of climate change are more widely
accepted, farmers can be expected to want to increase system capacity and irrigate more
of their crops.
For calculating theoretical demand, the potential soil moisture deficit (PSMD), crop
adjusted, is used as the climatic indicator within the GIS model. Once reliable climate
change predictions are available for UK, it will be feasible to predict future changes in
PSMD, and hence predict the increased theoretical demand for the crops currently
irrigated. However, it will be much more difficult to model the effects of climate change
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on cropping pattern and on the economics of irrigating particular crops. Furthermore,
potential reductions in water availability due to climate change may themselves affect the
location of irrigated agriculture.
5.5. Limitations
A number of assumptions were made in this study, and their respective limitations on
the accuracy of the predictions made must be recognised. The limited accuracy of the
MAFF data and the spatial integrity of the datasets used in the GIS has been discussed
previously by Knox et al. (1997). It should also be recognised that integrating models
with GIS can introduce an additional problem, notably the perception that GIS can
`generate information' (Grayson et al., 1993). The sophisticated data handling and
visualisation features of GIS can inadvertently seduce the user into an unrealistic sense of
model accuracy, particularly where datasets with varying data structure (e.g. point, vector,
raster), resolution, and format are incorporated. Output from GIS based analysis should
therefore always be interpreted with caution.
The Manchester University model predictions appear to be reasonably robust with
regard to the main irrigated crops, few of which are subject to subsidy under the CAP.
The reformed CAP scenario still appears to be reasonable for the main irrigated crops.
The subsequent choice of rates of change is partly subjective, and could be a major source
of error. The short term predictions are based on recent trends and current expert opinion.
In the medium to long term there is a risk of missing a new trend, for example, a change
in the profitability of irrigating grass, cereals or sugar beet, a change to drought resistant
varieties of potatoes, or the spread of new crops such as irrigated maize. Further research
is needed to predict the effects of reductions in water availability and increased price on
the economics of irrigation and hence on irrigated cropping patterns, and to predict
whether and how restrictions on new abstraction licences will change the location of
irrigated agriculture.
6. Conclusions
The spatial and temporal growth in the actual and theoretical dry year demand for
irrigation in England and Wales has been predicted, incorporating forecasts of changes in
the crop areas, in the fractions of each crop irrigated and in the irrigation application
depths. The GIS procedure enables these predictions to be estimated and mapped by crop
category, region, or catchment.
This study predicted higher underlying national growth rates than previous recent
work, and greater future increases, although the differences in the predicted absolute
volumes are not large, because of the revised baseline. The underlying volumetric growth
from 1982 to 1995 was found to be 3% per annum. The total net volumetric actual
irrigation water requirements for a `design' dry year (20% exceedance), are predicted to
rise by an average of 2.5% per annum from 1996 to 2001, and by an average of 1.5% per
annum from 2001 to 2021. Meanwhile, the total net volumetric theoretical irrigation
water requirements for a `design' dry year, are predicted to rise slightly less rapidly.
E.K. Weatherhead, J.W. Knox / Agricultural Water Management 43 (2000) 203±218
217
The results can help identify potential future water resource problem areas, and should
be useful for water resource planning by individual farmers as well as by the government,
regulatory authorities and others. The methodology can be applied in other countries
where appropriate datasets are available.
Acknowledgements
This study formed part of a research contract (OC9219) funded by the Ministry of
Agriculture, Fisheries and Food (MAFF). The authors acknowledge the support of the
Soil Survey and Land Research Centre (SSLRC) for their support in digital data
acquisition.
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