100 M. Smith Agricultural and Forest Meteorology 103 2000 99–108
1. Introduction
The great challenge for the coming decades will be the task of increasing food production to ensure food
security for the steadily growing world population, par- ticularly in countries with limited water and land re-
sources. While on a global scale water resources are still ample, serious water shortages are developing in
the arid and semi-arid regions. An increasing number of countries face serious water deficiencies as exist-
ing water resources are fully exploited. The depen- dency on water for future development has become a
critical constraint for development. Over 30 arid and semi-arid countries are expected to be ‘water scarce’ by
2025, meaning an annual water availability of less than 1000 m
3
per capita annually, which will slow down de- velopment, threaten food supplies and aggravate rural
poverty. The situation is aggravated by the declining quality
of water and soil resources, caused to a great extent by human activity. There is an urgent need to arrest this
human-induced degradation of water and soil resources and reclaim those that have been already degraded in
order to meet the present and future food requirements and other needs of the human population.
To focus attention on the growing problem of wa- ter scarcity in relation to food production, the World
Food Summit of November 1996, drew attention to the importance of water as a vital resource for future de-
velopment. Within the framework of the UN Agenda 21, FAO is closely cooperating with other agencies of
the United Nations to address this important issue. A global initiative for concerted action is established in
the Global Water Partnership where national and inter- national agencies and institutions from different sectors
are working together to find solutions for a looming global water crisis.
The concepts and options that can lead to a more effective use of water for crop production are reviewed
in this paper. Estimation of crop water use from cli- matic data is an essential element to achieve better wa-
ter use efficiency. The new perceptions included in the revised FAO guidelines on crop evapotranspiration will
contribute to this. Practical examples are presented to demonstrate in which way more effective planning and
management of irrigated and rainfed agriculture can be achieved. The compilation, processing and analy-
sis of agrometeorological information is a key element in this. Further studies to obtain a better insight in the
interaction between climate and water for crop pro- duction need to become a well defined priority area for
agrometeorological activities.
2. Water for crop production
A major part of the developed global water resources is used for food production. The estimated minimum
water requirement per capita is estimated at 1200 m
3
annually of which 55 m
3
for domestic use and 1150 m
3
for food production FAO, 1994. In most countries 60–80 of the total volume of developed water re-
sources is used for agriculture and may reach well over 80 for countries in the arid and semi-arid regions.
Rainfall contributes to an estimated 65 of global food production, while water for irrigation provides the
remaining 35 on 17 of the total agricultural area. Rainfall is, in most parts of the world, for at least part
of the year, insufficient to grow crops and rainfed food production is heavily affected by the annual variations
in precipitation.
Irrigation is an obvious option to increase and sta- bilize crop production. Major investments have been
made in irrigation over the past 30 years by diverting surface water and extracting groundwater. The irrigated
areas in the world have, over a period of 30 years, in- creased by 25 with, in particular, a period of accel-
erated growth during the 1970s and early 1980s FAO, 1993. The expansion rate has slowed down substan-
tially because a major part of the reliable surface wa- ters have already been developed, while groundwater
resources have become over-exploited at an alarming rate.
With water resources becoming scarce, waters of in- ferior quality are increasingly used. Excessive use and
poor management of such irrigation water has had, in some cases, detrimental effects on soil quality, causing
whole areas to be taken out of production or requiring the construction of expensive drainage works.
Defining strategies in planning and management of available water resources in the agricultural sector is to
become a national and global priority. The effective use of water both in irrigated and rainfed crop production
will be one of the main requirements. The options to optimize crop production per unit of water are outlined
further.
M. Smith Agricultural and Forest Meteorology 103 2000 99–108 101
2.1. Options for effective water use Considerable potential still exists to optimize the use
of water for crop production but solutions for a more efficient use of water for crop production will be dif-
ferent for rainfed and irrigated agriculture.
2.2. Water use efficiency in rainfed crop production Rainfed crop production is subject to frequent fluc-
tuations in precipitation. Failing rains will result in droughts and yield deficits, while excessive rains cause
flooding and crop losses. Crop water use need to be op- timized through a more effective use and conservation
of rainwater and by measures to increase crop produc- tion. Traditional cropping systems and genetic charac-
teristics of the local crop types are adapted to minimize drought risks and to maintain minimum production lev-
els under erratic rainfall supplies. Yields and water use efficiency will remain therefore low even in periods
with ample water supply or increased fertility levels.
The strategy to increase crop production under a given water supply will require the introduction of
better yielding varieties combined with a more secured water supply through improved water conservation. The
introduction of measures to conserve rain water or sup- plemental irrigation combined with appropriate plant
nutrition and cultural practices, will lead to increased
Fig. 1. Water use efficiency of rainfed and irrigated crops.
production levels per unit of water with equal water availability. The underlying concepts to improve water
use efficiency are illustrated in Fig. 1. To plan and manage the various options for increas-
ing water use efficiency of rainfed crop production agri- culture, an analysis of the climatic conditions of the
region is essential. Stochastically determined variabil- ity of rainfall and evapotranspiration is required for
simulation and crop forecasting on expected yield im- provements and options for water storage. Examples
of practical procedures to assess crop water use and crop production levels under restricted rainfall and
water supply are given in Paragraph 4.
2.3. Water use efficiency in irrigation Under irrigated conditions, priorities need to be set
for reducing losses of irrigation water and by more ef- fective irrigation management. Considerable amounts
of water diverted for irrigation are not effectively used for crop production. It is estimated that, on average,
only 45 is used by the crop, with an estimated 15 lost in the water conveyance system, 15 in the field
channels and 25 in inefficient field applications FAO, 1994, as illustrated in Fig. 2. Although irrigation wa-
ter losses may be retrieved within the watershed basin through surface or groundwater refill, the net effect will
be a reduced water availability for crop evapotranspi-
102 M. Smith Agricultural and Forest Meteorology 103 2000 99–108
Fig. 2. Average irrigation water losses.
ration, loss of energy height and deterioration of water quality.
Most of the water loss 40 occurs at farm and field level with a direct effect on crop production due
to inadequate water supplies causing water stress or excessive water and resulting in reduced growth and
leaching of plant nutrients. Considerable scope exists for a more accurate and efficient crop water application
by improved field irrigation methods and better crop water management techniques through the introduction
of irrigation scheduling and water supply control.
To introduce an effective crop water supply system adequate information is required on crop water require-
ments as determined by crop and weather conditions. In Paragraph 4 below an example is given how to evaluate,
plan and manage irrigation supplies and field irrigation practices based on crop and weather conditions for a
given irrigation management system.
3. FAO methodologies on crop water requirements
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