Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol42.Issue2.Nov1999:
Agricultural Water Management 42 (1999) 237±249
Estimating episodic recharge under different crop/
pasture rotations in the Mallee region. Part 2.
Recharge control by agronomic practices
L. Zhanga,*, W.R. Dawesa, T.J. Hattonb, I.H. Humec,
M.G. O'Connelld, D.C. Mitchellc, P.L. Milthorpe, M. Yeee
a
CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia
b
CSIRO Land and Water, Private Bag PO, Wembley, WA, Australia
c
NSW Agriculture, P.O. Box 736, Deniliquin, NSW 2710, Australia
d
Agriculture Victoria, Mallee Research Station, Walpeup, VIC 3507, Australia
e
NSW Agriculture, P.O. Box 300, Condobolin, NSW 2877, Australia
Accepted 14 December 1998
Abstract
Much environmental degradation, including salinity in the Mallee region of southeastern
Australia, is associated with the loss of native vegetation and increased recharge. As a result,
various agronomic practices have been proposed to reduce groundwater recharge. This study was
conducted to evaluate the impact of these practices on recharge, in particular episodic recharge. A
biophysically based model (WAVES) was used to estimate recharge rates under some typical crop
and pasture rotations in the region using long-term meteorological data. Results show that: (1)
recharge just below the root zone was episodic and that just 10% of annual recharge events
contributed over 85% of long-term totals. Management options such as incorporating lucerne and
deep-rooted non-fallow rotations can reduce both, mean annual recharge, and the number of
episodic events, but not eliminate recharge completely; (2) winter fallows increased soil-water
storage and some of the additional water was stored in the lower portion of the root zone or below
it. This can increase the risk of recharge to groundwater system; (3) changes in land management
may take a considerable period of time (>10 years) to have any noticeable impacts on recharge; and
(4) recharge under lucerne was 30% of that under medic pasture. # 1999 Elsevier Science B.V.
All rights reserved.
Keywords: Agronomic practices; Episodic recharge; Fallowing; Root zone
* Corresponding author. Tel.: +61-2-62465802; fax: +61-2-62465800
E-mail address: lu.zhang@cbr.clw.csiro.au (L. Zhang)
0378-3774/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 3 4 - 7
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
1. Introduction
Groundwater recharge is a small but important component of the water balance in arid
and semiarid areas, such as the Mallee region of southeastern Australia. Precipitation in
the region is marked by extreme variability and these extreme events are important for
recharge process. Studies have shown that significant recharge can occur in these areas,
even though annual potential evapotranspiration exceeds precipitation (Stephens and
Knowlton, 1986; Barnes et al., 1994). Recharge in the Mallee region is generally
considered to be episodic in nature. Episodic recharge is infrequent significant recharge
events. The word `significant' refers to the relative magnitude of the recharge. It is,
therefore, the distribution of these events that determines the patterns of recharge.
Studies also show that deep-rooted plants (i.e. lucerne, trees) appear to use more water
and, hence, to be more effective in reducing recharge to groundwater systems. It is clear
that vegetation plays an important role in the uptake of infiltration of rainfall that
otherwise would become recharge. As a result, various agronomic practices, such as crop/
pasture rotations, have been proposed in the region to control recharge. While it is true
that better agronomic practices can reduce mean annual recharge, it is anticipated that
they are not likely to affect episodic recharge significantly as a result of rare but very
large rainfall events. This presents a challenge for current salinity control strategies.
There are few investigations addressing the issue of episodic recharge and its
variability for periods of decades, which is the time scale of interest for most management
decisions. It has been recognised that field techniques alone are of limited use because it
is difficult to replicate field measurements under different management options and
maintain the measurements for decades. An alternative is to combine short-term field
measurements with modelling techniques to determine long-term impacts of various
agronomic practices on recharge by considering natural variability in precipitation. In this
study, such an approach was taken to examine the ability of different agronomic practices
to control recharge, especially episodic recharge for periods of several decades. We will
investigate the impact of winter fallows on soil-water storage and recharge. We will also
demonstrate how a biophysically based model (WAVES) was used to evaluate the
effectiveness of different agronomic practices in reducing groundwater recharge in the
Mallee region.
2. Methods
The characteristics of episodic recharge and the ability to control it by agronomic
practices were modelled in four management scenarios. These scenarios compared crop/
pasture rotations typical of farming practices in the NSW and Victorian Mallee. The first
two scenarios compared the effectiveness of annual and perennial pastures in controlling
recharge. Two crop rotations were modelled; the first was an eight-year sequence of
fallow/oat/wheat/wheat/(lucerne 4) (RT1) and the second the same as RT1, except with
four years of medic pasture (RT2) (see Table 1). WAVES was used to simulate 32 years
(1957±1989) of deep drainage beneath these rotations using meteorological data obtained
at Hillston. The soil hydraulic properties and vegetation parameters listed in Tables 2 and
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Table 1
The crop/pasture rotations modelled by WAVES
Rotation
RT1
RT2
RT3
RT4
Year 1
a
F
Fa
Me
Me
Year 2
b
O
Ob
Me
Fa
Year 3
c
W
Wc
Wc
Wc
Year 4
c
W
Wc
Me
Me
Year 5
d
L
Me
Me
Fa
Year 6
d
L
Me
Wc
Wc
Year 7
d
L
Me
Me
Me
Year 8
Ld
Me
Me
Fa
a
Fallow.
Oats.
Wheat.
d
Lucerne.
e
Medic pasture.
b
c
3 of Part 1 of this study (Zhang et al., 1999) were used in the simulations. The effect of
fallowing on recharge was evaluated in two further modelling scenarios in which WAVES
simulated recharge beneath a non-fallow rotation, medic/medic/wheat (RT3), and one
with fallow, medic/fallow/wheat (RT4) (see Table 1). These two scenarios ran for 33
years (1957±1990) using meteorological data measured at Walpeup with soil hydraulic
properties listed in Table 2 and vegetation parameters in Table 3 of Part 1 of this study
(Zhang et al., 1999). To further evaluate the impact of rooting depth on recharge, the
second two scenarios were run using a rooting depth of 1.0 m instead of 0.5 m (RT3d and
RT4d). In these simulations all the model parameters were kept constant, except
maximum rooting depth at Walpeup, which varied from 0.5 to 1.0 m.
3. Results and discussion
3.1. Recharge under different crop/pasture rotations
Groundwater recharge was calculated at four selected plots for the period of 1992 to
1995 using the WAVES model (Table 2). The annual recharge rates at 4 m depth ranged
from 9 to 33 mm per year and showed no obvious relationship to the annual rainfall
(Table 2) or crop/pasture rotation. However, the flux simulated by WAVES at 1.5 m, i.e.
the bottom of the root zone, does show patterns which may be associated with rainfall
patterns or crop development. These differences may also be explained by examining the
processes operating within, and below, the root zone.
Within the root zone, i.e. from the surface down to 1.5 m, infiltrated water flows
downward via gravity, is extracted by plant roots and soil evaporation, and may be held in
storage when the water content is reduced to the point where drainage ceases. Any water
that flow past the bottom of the root zone is lost to the plants, and this water cannot be
controlled by any agronomic management. In the unsaturated zone below the normal
depth of crop roots, water is redistributed by gravitational and diffusion processes. When
the water content is high enough, water will flow downward via gravity and become
recharge to some aquifer.
240
Plot
1992
1993
1994
rainfall
(mm)
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
rainfall
(mm)
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
rainfall
(mm)
hillston
1
10
471
471
24
23
13
27
581
581
10
6
12
33
212
212
Walpeup
3
10
Ð
Ð
Ð
Ð
Ð
Ð
348
348
18
21
10
10
153
153
1995
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
rainfall
(mm)
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
3
0
13
29
365
365
0
0
13
25
11
5
10
10
382
382
30
40
10
10
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Table 2
Annual rainfall and estimates of annual recharge at 1.5 m and 4.0 m depths, using the WAVES model
241
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Table 3
Results of long-term scenario simulations with recharge rate calculated at 4.0 m depth
Site
Rotation
Hillston
RT1
RT2
RT3c
RT4d
RT3de
RT4df
Average rainfall (mm)
Rooting depth (m)
Minimum recharge (mm/year)
Maximum recharge (mm/year)
Average recharge (mm/year)
564
1.5
0
15
4
1.5
4
34
13
351
0.5
8
37
19
0.5
8
37
26
351
1.0
5
10
7
1.0
7
25
12
a
Walpeup
b
a
Fallow/oat/wheat/wheat/(lucerne 4).
Fallow/oat/wheat/wheat/(medic 4).
c
Medic/medic/wheat.
d
Medic/fallow/wheat.
e
Medic/medic/wheat.
f
Medic/fallow/wheat.
b
The highest recharge occurred under Plot 10 at Hillston, while the two cropping systems
at Walpeup showed consistently lower recharge rates. The high recharge rate under Plot
10 at Hillston may well be attributed to the fact that the bottom soil layer of Plot 10 was
about 15% wetter than the other plots at the site. As a result, the unsaturated hydraulic
conductivity for Plot 10 was twice that of Plot 1, despite the lower saturated hydraulic
conductivity. Therefore, the cropping rotations had little impact on recharge at 4 m depth.
These results are consistent with our understanding of the processes controlling recharge,
and the measured soil-moisture data.
We have also included the recharge rate (net water flux) passing 1.5 m depth, this being
the common maximum rooting depth of wheat (Incerti and O'Leary, 1990). The Hillston
results show the water use by crops and lucerne accounted for the initial irrigation, stored
water and rainfall, and eliminated drainage below 1.5 m after four years. For Walpeup,
recharge rate at 1.5 m generally followed annual rainfall, with the crop rotation having
little effect. This may be due to the fact that a much shallower rooting depth was used
(see Zhang et al., 1999). It is known that the root zone acts as a buffer in controlling
recharge. A shallower root zone tends to retain less of infiltrated rainfall, thereby
allowing rapid drainage and less time for plants to use the water.
Unsaturated hydraulic conductivity of the soil at the base of Plot 10 at Hillston was
estimated to be 0.8 mm dayÿ1 based on the Broadbridge±White soil model (Broadbridge
and White, 1988). At that rate, it would take nearly 13 years for water from the surface to
reach the bottom of the soil column. This estimate is likely to be an upper limit, but
places the time scale of recharge control in context.
During the period of the study, the deep soil layers were relatively wet and deep
drainage occurred mainly as a result of antecedent soil moisture content. Therefore,
recharge rates at 4 m depth showed little response to annual rainfall or crop rotations. It
can be argued that for deep soil layers with low hydraulic conductivity it takes a long time
for agronomic practices to have noticeable impacts on recharge. However, over long
enough periods of time the effects of vegetation changes may be significant in terms of
rising groundwater tables.
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
3.2. Fallow soil-water storage
It is generally considered that long winter fallows in the Mallee region increase soil
water storage and therefore increase crop yield (French, 1978; Incerti et al., 1993;
O'Leary and Connor, 1997). This study showed that on average, an additional 22±37 mm
of soil water is stored in this environment due to fallowing and the stored water can be
used to increase crop yield (French, 1978). At Hillston, in 1992, similar soil-water
profiles occurred under each cropping system (Fig. 1(a and b)). By sowing in 1993 after a
long fallow, an additional 25 mm of soil water was stored. At Walpeup, in June 1993, the
soil-water storage was similar between fallow and continuous cropping systems (Fig. 1(c
and d)). By sowing in 1994 following fallowing, an additional 44 mm of water was stored
in the upper 1.5 m soil profile.
These results showed that long fallows do increase soil water storage and, in some
cases, there were associated increases in crop yield. However, it is not clear if the
increases in crop yield were due to increased soil-water storage at sowing. There are other
factors, such as soil nitrogen availability and root diseases, which could affect crop yield
(Incerti et al., 1993; French, 1978). The impact of long fallows on recharge is of concern
because some of the additional soil water after fallows has been observed in the lower
portion of the root zone, or below it, potentially leading to greater recharge to
groundwater systems (Incerti et al., 1993; O'Leary and Connor, 1997). This has
implications for recharge control under dryland conditions (O'Connell et al., 1995).
While it is true that vegetation in this environment is efficient in removing soil water from
the root zone. The control of vegetation on recharge is determined by the time during
which the water remains within the root zone and the ability of the plant roots to extract
it. Therefore, any water stored in the lower part of the root zone or below it will be very
likely to become recharge.
The impact of fallowing on recharge depends on soil hydraulic properties and the
maximum rooting depth of successive crops. The impact, and risk of recharge, is much
greater on sandy than on clay soils because the sands are inherently more conductive,
have lower water holding capacity, and higher infiltration rates. At Walpeup, due to
shallower rooting depth the impact of long fallow on crop yield will be even less obvious
and the risk of recharge to groundwater can be greater. Incerti et al. (1993) argued that the
use of long fallows to increase soil-water storage for crop yield cannot be justified and
should be replaced by more intensive crop/pasture rotations. Results from this study
support their argument.
3.3. Recharge control by agronomic practices
Recharge occurs when there is too much of infiltrated rainfall for plants to use or to be
stored in the root zone. As a result, very large rainfall events can cause significant
recharge, although such events may be highly infrequent. A common approach to access
recharge is to use average values. In high rainfall areas, where the recharge process is
almost continuous, this approach is appropriate. However, recharge in the Mallee region
is episodic and most of the time the process of recharge may be hardly discernible. It can,
therefore, be misleading to use average recharge unless the average is taken over a period
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
243
Fig. 1. Measured soil moisture profiles at Hillston before, and after, a fallowing phase (a,b), and at Walpeup
before, and after, a fallowing phase (c,d).
long enough to contain a statistically significant number of extreme rainfall events (Gee
and Hillel, 1988; Barnes et al., 1994). For this reason, the modelling was carried out over
a period of 30 years, the longest rainfall data series available for the region.
Rainfall at both sites showed large variability. At Hillston, there were three extremely
wet years with annual rainfall exceeding almost twice the mean annual rainfall. Months
with rainfall >150 mm accounted for over 20% of the total rainfall, but occurred in 75 mm account for 16%
244
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Fig. 2. Monthly rainfall and frequency distribution at Hillston (a,b), and at Waleup (c,d) during the period 1957±
1992. The horizontal lines indicate the mean rainfall.
of the total rainfall, but only 5% of the time (Fig. 2(c and d)). It is anticipated that these
large infrequent rainfall events could cause significant episodic recharge.
Annual recharge just below the root zone (i.e. 1.0 m depth) was episodic and showed
significant temporal variations (Fig. 3). At Hillston, recharge at 1.0 m depth under the
lucerne rotation (RT1) occurred less frequently and the magnitude was also smaller
compared to the medic rotation (RT2). It is estimated that 10% of annual recharge events
accounted for 50±75% of the total recharge (Fig. 4(a)). At Walpeup, rooting depth had
significant impact on the episodicity of the recharge. When the rooting depth was small
(i.e. 0.5 m), recharge occurred much more frequently under both, the non-fallow (RT3)
and fallow (RT4) rotations (Fig. 3(b and c)). As shown in Fig. 4(b), 10% of annual
recharge events contributed to 20% of the totals; this proportion increased to 85% by
changing the rooting depth from 0.5 to 1.0 m (Fig. 4(c)). The magnitude of these annual
recharge events was as high as 130 mm yearÿ1. However, it should also be noted that the
recharge rates shown in Fig. 3 are annual values and this may have damped the episodic
nature of the actual recharge process.
The episodic nature of the recharge can be described by the cumulative distribution
function shown in Fig. 4. It is clear that the use of the lucerne and non-fallow rotations
could reduce average recharge, and these management options also make recharge more
episodicÐthat is, recharge occurs much less frequently, although its magnitude can still
be significant (see Fig. 4). Better agronomic practices, therefore, are not likely to control
episodic recharge significantly. For example, annual rainfall in 1973 was 538 mm (i.e.
58% higher than the long-term average) and the annual recharge at 1.0 m depth under the
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
245
Fig. 3. Simulated recharge rates at 1.0 m depth for: (a) Hillston under lucerne rotation (RT1) (&), and medic
rotation (RT2) (&); (b) Waleup under non-fallow (RT3) (&) and fallow rotation (RT4) (&) with rooting depth
of 0.5 m; and (c) Waleup under non-fallow (RT3d) (&) and fallow rotation (RT4d) (&) with rooting depth of
1.0 m.
non-fallow rotation (RT3) even exceed that under the fallow rotation (RT4). This was
because the soil profile under the fallow rotation (RT4) was wetter than that under the
non-fallow rotation (RT3), which led to substantially more surface runoff and, hence, less
infiltration. However, this was only observed to occur under wet years and the fallow
rotations generally produced more recharge at 1.0 m depth. At Walpeup, both the fallow
and non-fallow rotations produced similar annual recharge when the rooting depth was
set to 0.5 m (Fig. 4(b)). This is not surprising because, with such a shallow rooting depth,
most large rainfall events could penetrate the root zone and become recharge. Therefore,
it is necessary to use deep-rooted plants in the area for the purpose of recharge control.
The recharge rate at 4.0 m was much less episodic compared to recharge rate at 1.0 m
(Fig. 5). Pulses of infiltrated rainfall are damped in a diffusion-like process and most of
rainfall events can only penetrate the top few meters of the soil layer. If deep-rooted
plants are growing in the area, such as native forest, it is highly likely the water will be
removed efficiently by the vegetation. Clearing of native vegetation in the region created
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Fig. 4. Frequency distribution of annual recharge at 1.0 m depth for: (a) Hillston under lucerne rotation (RT1)
(- - -) and medic rotation (RT2) (ÐÐÐ); (b) Waleup under non-fallow (RT3) (- - -) and fallow rotation (RT4)
(ÐÐÐ) with rooting depth of 50 cm; (c) Walpeup under non-fallow (RT3d) (- - -) and fallow rotation (RT4d)
(ÐÐÐ) with rooting depth of 100 cm. The vertical line indicates 10% probability.
a new balance between rainfall and recharge, which led to dryland salinisation. However,
it appears to be economically and sociably unrealistic to restore the natural balance by
replanting the native vegetation on a sufficient scale. The only practical way of reducing
recharge is to change agronomic practice. Such a change would increase the episodic
nature of recharge, so that rather than there being a constant drainage of water below the
root zone, significant recharge to groundwater would occur only as a result of large
rainfall events.
The results of the long-term scenario modelling for both, Hillston and Walpeup are
summarised in Table 3. Average recharge at 4 m under the lucerne rotation (RT1) was
30% of that under medic rotation (RT2) and this suggests that on average lucerne offers
better control over recharge than medic pasture. For shallow rooted plants (i.e. 0.5 m
rooting depth), average recharge at 4 m under the fallow rotation was 26 mm yearÿ1
compared to 19 mm yearÿ1 from the non-fallow rotation. This difference in recharge as a
result of fallowing is 7 mm yearÿ1, which equates to 35% extra recharge. The difference
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
247
Fig. 5. Cumulative annual rainfall differences from the mean (&) and annual recharge rates at 400 cm depth
for: (a) Hillston under lucerne rotation (- - -) and medic rotation (ÐÐÐ); (b) Waleup with rooting depth of
50 cm; and (c) Waleup with rooting depth of 100 cm under non-fallow (- - -) and fallow rotation (ÐÐÐ).
in average recharge between fallow and non-fallow rotations is 5 mm yearÿ1 with rooting
depth of 1 m (Table 3). O'Connell et al. (1995) using chloride profile analysis found that
fallowing caused similar increases in recharge at Walpeup.
At Hillston, the recharge rate at 4.0 m depth increased dramatically after 10 years for
the medic rotation (RT2) but not for the lucerne rotation (RT1), which continues to
decrease. The recharge under RT2 appears to respond to the cumulative rainfall anomaly.
At Walpeup, a similar trend was observed for the fallow rotation with shallow rooting
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
depth (RT4). However, an increase in recharge occurred after 20 years with deep-rooted
plants (RT4d) (Fig. 5(c)). It is interesting to note that the non-fallow rotation (RT3d) was
not sensitive to the cumulative rainfall anomaly, but the fallow rotation was (Fig. 5).
These results suggest that changes in agronomic practice (e.g. fallowing, crop rotation)
may take a considerable period of time (>10 years) to have any noticeable impacts on
recharge; the difference in recharge under fallow and non-fallow rotations is significant
(Table 3). It is also shown that deep-rooted plants have better control on recharge, but the
degree of control is modified by soil characteristics and the prevailing weather conditions.
Results from this study showed that the recharge just below the root zone is episodic in
the sense that it occurs infrequently and its magnitude is significant. Given the fact that
plants can only use water in root zone, the effect of current agronomic practices on
episodic recharge is limited. During these large rainfall events, the root zone, generally
considered as a buffer zone, became saturated and significant recharge occurred. As a
result, episodic recharge can substantially reduce the effectiveness of land management
options in controlling recharge. This is more so for sandy soils than for clay soils because
of low water holding capacity and high infiltration rates.
4. Conclusions
Recharge just below the root zone is episodic and 10% of the annual recharge events
accounted for 25±85% of the long-term totals under the Mallee conditions. The
magnitude of these annual recharge events can be up to 130 mm yearÿ1 and it is these
events that contribute largely to groundwater recharge. While it is true that better
management options (i.e. lucerne and deep-rooted non-fallow rotations) can reduce mean
annual recharge by eliminating most of the small recharge events, they are not likely to
eradicate the largest episodic events; they can increase the episodicity of the recharge.
Changes in agronomic management (e.g. fallowing, crop rotation) may take a
considerable period of time (>10 years) to have any noticeable impacts on recharge as
observed in the above-mentioned scenario modelling. It is important to recognise the
long-term impacts of any agronomic practices on recharge because the effects of these
changes may not be apparent for a short period of time, but may then be devastating in
terms of rising groundwater tables. Lucerne appeared to have better control on recharge
than medic pasture and average recharge under lucerne was only 30% of that under medic
pasture.
One of the advantages of physically based models such as WAVES is the ability to
simulate the hydrological effects of various land-management options and to identify key
factors controlling the processes. An attempt was made in this study to evaluate the
effects of winter fallowing in crop rotations on groundwater recharge. The results clearly
showed the long-term benefit of non-fallowing in reducing recharge and this has
implications for dryland salinity control. The traditional practice of winter fallow
significantly increased soil moisture storage, groundwater recharge, and the risk of
salinity. We argue that winter fallowing has contributed to dryland salinity in the Mallee
regions, and more areas could be affected in the future unless improved agronomic
practices are implemented. O'Connell et al. (1995) suggest that fallowing may be
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
249
eliminated, provided its alternative encourages vigorous vegetative growth (e.g.
replacement with grass-free pasture, grain legume, oilseed phase) as reported by
Griffiths and Walsgott (1987) and Incerti et al. (1993).
Acknowledgements
This work was partially funded by the Murray±Darling Basin Commission through its
Natural Resources Management Strategy Investigation and Education Program (Grant
No. M4025). Technical assistance in site maintenance and data collection was provided
by S.D. Blandthorn, A.J. Corbett, S. Wisneske, M.C. Brown, M.J. Ferguson, J.L. Latta,
M.W. Ferguson. We are grateful to Mr. W. Milthorpe for the use of his farm at Hillston.
We are grateful to G.R. Walker, V. Snow and F. Lewis, and Joe Landsberg for comments
on a draft of this paper.
References
Barnes, C.J., Jacobsen, G., Smith, G.D., 1994. The distributed recharge mechanism in the Australian arid zone.
Soil Sci. Soc. Am. J. 58, 31±40.
Broadbridge, P., White, I., 1988. Constant rate rainfall infiltration: a versatile non-linear model. 1. Analytic
solution. Water Resour. Res. 24, 145±154.
French, R.J., 1978. The effect of fallowing on the yield of wheat. I. The effect on soil water storage and nitrate
supply. Aus. J. Agric. Res. 29, 653±668.
Gee, G.W., Hillel, D., 1988. Groundwater recharge in arid regions: review and critique of estimation methods.
Hydrol. Proc. 2, 255±266.
Griffiths and Walsgott, 1987. Water use of wheat in the Victorian Mallee. In: Proceedings of the 4th Australian
Agronomy Conference, p. 296 (Australian Society of Agronomy, Parkville, Vic.).
Incerti, M., O'Leary, G.J., 1990. The rooting depth of wheat in the Victorian Mallee. Aus. J. Exp. Agric. 30,
817±824.
Incerti, M., Sale, P.W.G., O'Leary, G.J., 1993. Cropping practices in the Victorian Mallee. 2. Effect of long
fallows on the water economy and yield of wheat. Aus. J. Exp. Agric. 33, 885±894.
O'Connell, M.G., O'Leary, G.J., Incerti, M., 1995. Potential groundwater recharge from fallowing in north-west
Victoria, Australia. Agric. Water Manage. 29, 37±52.
O'Leary, G.J., Connor, D.J., 1997. Stubble retention and tillage in a semiarid environment: 1. Soil water
accumulation during fallow. Field Crops Research, 52, 209±219.
Stephens, D.B., Knowlton Jr., R., 1986. Soil water movement and recharge through sand at a semi-arid site in
New Mexico. Water Resour. Res. 22, 881±889.
Zhang, L., Hume, I.H., O'Connell, M.G., Mitchell, D.C., Milthorpe, P.L., Yee, M., Dawes, W.R., Hatton, T.J.,
1999. Estimating episodic recharge under different crop/pasture rotations in the Mallee region. 1.
Experiments and model calibration. Agric. Water Manage, 42, 219±235.
Estimating episodic recharge under different crop/
pasture rotations in the Mallee region. Part 2.
Recharge control by agronomic practices
L. Zhanga,*, W.R. Dawesa, T.J. Hattonb, I.H. Humec,
M.G. O'Connelld, D.C. Mitchellc, P.L. Milthorpe, M. Yeee
a
CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia
b
CSIRO Land and Water, Private Bag PO, Wembley, WA, Australia
c
NSW Agriculture, P.O. Box 736, Deniliquin, NSW 2710, Australia
d
Agriculture Victoria, Mallee Research Station, Walpeup, VIC 3507, Australia
e
NSW Agriculture, P.O. Box 300, Condobolin, NSW 2877, Australia
Accepted 14 December 1998
Abstract
Much environmental degradation, including salinity in the Mallee region of southeastern
Australia, is associated with the loss of native vegetation and increased recharge. As a result,
various agronomic practices have been proposed to reduce groundwater recharge. This study was
conducted to evaluate the impact of these practices on recharge, in particular episodic recharge. A
biophysically based model (WAVES) was used to estimate recharge rates under some typical crop
and pasture rotations in the region using long-term meteorological data. Results show that: (1)
recharge just below the root zone was episodic and that just 10% of annual recharge events
contributed over 85% of long-term totals. Management options such as incorporating lucerne and
deep-rooted non-fallow rotations can reduce both, mean annual recharge, and the number of
episodic events, but not eliminate recharge completely; (2) winter fallows increased soil-water
storage and some of the additional water was stored in the lower portion of the root zone or below
it. This can increase the risk of recharge to groundwater system; (3) changes in land management
may take a considerable period of time (>10 years) to have any noticeable impacts on recharge; and
(4) recharge under lucerne was 30% of that under medic pasture. # 1999 Elsevier Science B.V.
All rights reserved.
Keywords: Agronomic practices; Episodic recharge; Fallowing; Root zone
* Corresponding author. Tel.: +61-2-62465802; fax: +61-2-62465800
E-mail address: lu.zhang@cbr.clw.csiro.au (L. Zhang)
0378-3774/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 3 4 - 7
238
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
1. Introduction
Groundwater recharge is a small but important component of the water balance in arid
and semiarid areas, such as the Mallee region of southeastern Australia. Precipitation in
the region is marked by extreme variability and these extreme events are important for
recharge process. Studies have shown that significant recharge can occur in these areas,
even though annual potential evapotranspiration exceeds precipitation (Stephens and
Knowlton, 1986; Barnes et al., 1994). Recharge in the Mallee region is generally
considered to be episodic in nature. Episodic recharge is infrequent significant recharge
events. The word `significant' refers to the relative magnitude of the recharge. It is,
therefore, the distribution of these events that determines the patterns of recharge.
Studies also show that deep-rooted plants (i.e. lucerne, trees) appear to use more water
and, hence, to be more effective in reducing recharge to groundwater systems. It is clear
that vegetation plays an important role in the uptake of infiltration of rainfall that
otherwise would become recharge. As a result, various agronomic practices, such as crop/
pasture rotations, have been proposed in the region to control recharge. While it is true
that better agronomic practices can reduce mean annual recharge, it is anticipated that
they are not likely to affect episodic recharge significantly as a result of rare but very
large rainfall events. This presents a challenge for current salinity control strategies.
There are few investigations addressing the issue of episodic recharge and its
variability for periods of decades, which is the time scale of interest for most management
decisions. It has been recognised that field techniques alone are of limited use because it
is difficult to replicate field measurements under different management options and
maintain the measurements for decades. An alternative is to combine short-term field
measurements with modelling techniques to determine long-term impacts of various
agronomic practices on recharge by considering natural variability in precipitation. In this
study, such an approach was taken to examine the ability of different agronomic practices
to control recharge, especially episodic recharge for periods of several decades. We will
investigate the impact of winter fallows on soil-water storage and recharge. We will also
demonstrate how a biophysically based model (WAVES) was used to evaluate the
effectiveness of different agronomic practices in reducing groundwater recharge in the
Mallee region.
2. Methods
The characteristics of episodic recharge and the ability to control it by agronomic
practices were modelled in four management scenarios. These scenarios compared crop/
pasture rotations typical of farming practices in the NSW and Victorian Mallee. The first
two scenarios compared the effectiveness of annual and perennial pastures in controlling
recharge. Two crop rotations were modelled; the first was an eight-year sequence of
fallow/oat/wheat/wheat/(lucerne 4) (RT1) and the second the same as RT1, except with
four years of medic pasture (RT2) (see Table 1). WAVES was used to simulate 32 years
(1957±1989) of deep drainage beneath these rotations using meteorological data obtained
at Hillston. The soil hydraulic properties and vegetation parameters listed in Tables 2 and
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Table 1
The crop/pasture rotations modelled by WAVES
Rotation
RT1
RT2
RT3
RT4
Year 1
a
F
Fa
Me
Me
Year 2
b
O
Ob
Me
Fa
Year 3
c
W
Wc
Wc
Wc
Year 4
c
W
Wc
Me
Me
Year 5
d
L
Me
Me
Fa
Year 6
d
L
Me
Wc
Wc
Year 7
d
L
Me
Me
Me
Year 8
Ld
Me
Me
Fa
a
Fallow.
Oats.
Wheat.
d
Lucerne.
e
Medic pasture.
b
c
3 of Part 1 of this study (Zhang et al., 1999) were used in the simulations. The effect of
fallowing on recharge was evaluated in two further modelling scenarios in which WAVES
simulated recharge beneath a non-fallow rotation, medic/medic/wheat (RT3), and one
with fallow, medic/fallow/wheat (RT4) (see Table 1). These two scenarios ran for 33
years (1957±1990) using meteorological data measured at Walpeup with soil hydraulic
properties listed in Table 2 and vegetation parameters in Table 3 of Part 1 of this study
(Zhang et al., 1999). To further evaluate the impact of rooting depth on recharge, the
second two scenarios were run using a rooting depth of 1.0 m instead of 0.5 m (RT3d and
RT4d). In these simulations all the model parameters were kept constant, except
maximum rooting depth at Walpeup, which varied from 0.5 to 1.0 m.
3. Results and discussion
3.1. Recharge under different crop/pasture rotations
Groundwater recharge was calculated at four selected plots for the period of 1992 to
1995 using the WAVES model (Table 2). The annual recharge rates at 4 m depth ranged
from 9 to 33 mm per year and showed no obvious relationship to the annual rainfall
(Table 2) or crop/pasture rotation. However, the flux simulated by WAVES at 1.5 m, i.e.
the bottom of the root zone, does show patterns which may be associated with rainfall
patterns or crop development. These differences may also be explained by examining the
processes operating within, and below, the root zone.
Within the root zone, i.e. from the surface down to 1.5 m, infiltrated water flows
downward via gravity, is extracted by plant roots and soil evaporation, and may be held in
storage when the water content is reduced to the point where drainage ceases. Any water
that flow past the bottom of the root zone is lost to the plants, and this water cannot be
controlled by any agronomic management. In the unsaturated zone below the normal
depth of crop roots, water is redistributed by gravitational and diffusion processes. When
the water content is high enough, water will flow downward via gravity and become
recharge to some aquifer.
240
Plot
1992
1993
1994
rainfall
(mm)
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
rainfall
(mm)
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
rainfall
(mm)
hillston
1
10
471
471
24
23
13
27
581
581
10
6
12
33
212
212
Walpeup
3
10
Ð
Ð
Ð
Ð
Ð
Ð
348
348
18
21
10
10
153
153
1995
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
rainfall
(mm)
recharge at
1.5 m (mm)
recharge at
4.0 m (mm)
3
0
13
29
365
365
0
0
13
25
11
5
10
10
382
382
30
40
10
10
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Table 2
Annual rainfall and estimates of annual recharge at 1.5 m and 4.0 m depths, using the WAVES model
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Table 3
Results of long-term scenario simulations with recharge rate calculated at 4.0 m depth
Site
Rotation
Hillston
RT1
RT2
RT3c
RT4d
RT3de
RT4df
Average rainfall (mm)
Rooting depth (m)
Minimum recharge (mm/year)
Maximum recharge (mm/year)
Average recharge (mm/year)
564
1.5
0
15
4
1.5
4
34
13
351
0.5
8
37
19
0.5
8
37
26
351
1.0
5
10
7
1.0
7
25
12
a
Walpeup
b
a
Fallow/oat/wheat/wheat/(lucerne 4).
Fallow/oat/wheat/wheat/(medic 4).
c
Medic/medic/wheat.
d
Medic/fallow/wheat.
e
Medic/medic/wheat.
f
Medic/fallow/wheat.
b
The highest recharge occurred under Plot 10 at Hillston, while the two cropping systems
at Walpeup showed consistently lower recharge rates. The high recharge rate under Plot
10 at Hillston may well be attributed to the fact that the bottom soil layer of Plot 10 was
about 15% wetter than the other plots at the site. As a result, the unsaturated hydraulic
conductivity for Plot 10 was twice that of Plot 1, despite the lower saturated hydraulic
conductivity. Therefore, the cropping rotations had little impact on recharge at 4 m depth.
These results are consistent with our understanding of the processes controlling recharge,
and the measured soil-moisture data.
We have also included the recharge rate (net water flux) passing 1.5 m depth, this being
the common maximum rooting depth of wheat (Incerti and O'Leary, 1990). The Hillston
results show the water use by crops and lucerne accounted for the initial irrigation, stored
water and rainfall, and eliminated drainage below 1.5 m after four years. For Walpeup,
recharge rate at 1.5 m generally followed annual rainfall, with the crop rotation having
little effect. This may be due to the fact that a much shallower rooting depth was used
(see Zhang et al., 1999). It is known that the root zone acts as a buffer in controlling
recharge. A shallower root zone tends to retain less of infiltrated rainfall, thereby
allowing rapid drainage and less time for plants to use the water.
Unsaturated hydraulic conductivity of the soil at the base of Plot 10 at Hillston was
estimated to be 0.8 mm dayÿ1 based on the Broadbridge±White soil model (Broadbridge
and White, 1988). At that rate, it would take nearly 13 years for water from the surface to
reach the bottom of the soil column. This estimate is likely to be an upper limit, but
places the time scale of recharge control in context.
During the period of the study, the deep soil layers were relatively wet and deep
drainage occurred mainly as a result of antecedent soil moisture content. Therefore,
recharge rates at 4 m depth showed little response to annual rainfall or crop rotations. It
can be argued that for deep soil layers with low hydraulic conductivity it takes a long time
for agronomic practices to have noticeable impacts on recharge. However, over long
enough periods of time the effects of vegetation changes may be significant in terms of
rising groundwater tables.
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
3.2. Fallow soil-water storage
It is generally considered that long winter fallows in the Mallee region increase soil
water storage and therefore increase crop yield (French, 1978; Incerti et al., 1993;
O'Leary and Connor, 1997). This study showed that on average, an additional 22±37 mm
of soil water is stored in this environment due to fallowing and the stored water can be
used to increase crop yield (French, 1978). At Hillston, in 1992, similar soil-water
profiles occurred under each cropping system (Fig. 1(a and b)). By sowing in 1993 after a
long fallow, an additional 25 mm of soil water was stored. At Walpeup, in June 1993, the
soil-water storage was similar between fallow and continuous cropping systems (Fig. 1(c
and d)). By sowing in 1994 following fallowing, an additional 44 mm of water was stored
in the upper 1.5 m soil profile.
These results showed that long fallows do increase soil water storage and, in some
cases, there were associated increases in crop yield. However, it is not clear if the
increases in crop yield were due to increased soil-water storage at sowing. There are other
factors, such as soil nitrogen availability and root diseases, which could affect crop yield
(Incerti et al., 1993; French, 1978). The impact of long fallows on recharge is of concern
because some of the additional soil water after fallows has been observed in the lower
portion of the root zone, or below it, potentially leading to greater recharge to
groundwater systems (Incerti et al., 1993; O'Leary and Connor, 1997). This has
implications for recharge control under dryland conditions (O'Connell et al., 1995).
While it is true that vegetation in this environment is efficient in removing soil water from
the root zone. The control of vegetation on recharge is determined by the time during
which the water remains within the root zone and the ability of the plant roots to extract
it. Therefore, any water stored in the lower part of the root zone or below it will be very
likely to become recharge.
The impact of fallowing on recharge depends on soil hydraulic properties and the
maximum rooting depth of successive crops. The impact, and risk of recharge, is much
greater on sandy than on clay soils because the sands are inherently more conductive,
have lower water holding capacity, and higher infiltration rates. At Walpeup, due to
shallower rooting depth the impact of long fallow on crop yield will be even less obvious
and the risk of recharge to groundwater can be greater. Incerti et al. (1993) argued that the
use of long fallows to increase soil-water storage for crop yield cannot be justified and
should be replaced by more intensive crop/pasture rotations. Results from this study
support their argument.
3.3. Recharge control by agronomic practices
Recharge occurs when there is too much of infiltrated rainfall for plants to use or to be
stored in the root zone. As a result, very large rainfall events can cause significant
recharge, although such events may be highly infrequent. A common approach to access
recharge is to use average values. In high rainfall areas, where the recharge process is
almost continuous, this approach is appropriate. However, recharge in the Mallee region
is episodic and most of the time the process of recharge may be hardly discernible. It can,
therefore, be misleading to use average recharge unless the average is taken over a period
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
243
Fig. 1. Measured soil moisture profiles at Hillston before, and after, a fallowing phase (a,b), and at Walpeup
before, and after, a fallowing phase (c,d).
long enough to contain a statistically significant number of extreme rainfall events (Gee
and Hillel, 1988; Barnes et al., 1994). For this reason, the modelling was carried out over
a period of 30 years, the longest rainfall data series available for the region.
Rainfall at both sites showed large variability. At Hillston, there were three extremely
wet years with annual rainfall exceeding almost twice the mean annual rainfall. Months
with rainfall >150 mm accounted for over 20% of the total rainfall, but occurred in 75 mm account for 16%
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Fig. 2. Monthly rainfall and frequency distribution at Hillston (a,b), and at Waleup (c,d) during the period 1957±
1992. The horizontal lines indicate the mean rainfall.
of the total rainfall, but only 5% of the time (Fig. 2(c and d)). It is anticipated that these
large infrequent rainfall events could cause significant episodic recharge.
Annual recharge just below the root zone (i.e. 1.0 m depth) was episodic and showed
significant temporal variations (Fig. 3). At Hillston, recharge at 1.0 m depth under the
lucerne rotation (RT1) occurred less frequently and the magnitude was also smaller
compared to the medic rotation (RT2). It is estimated that 10% of annual recharge events
accounted for 50±75% of the total recharge (Fig. 4(a)). At Walpeup, rooting depth had
significant impact on the episodicity of the recharge. When the rooting depth was small
(i.e. 0.5 m), recharge occurred much more frequently under both, the non-fallow (RT3)
and fallow (RT4) rotations (Fig. 3(b and c)). As shown in Fig. 4(b), 10% of annual
recharge events contributed to 20% of the totals; this proportion increased to 85% by
changing the rooting depth from 0.5 to 1.0 m (Fig. 4(c)). The magnitude of these annual
recharge events was as high as 130 mm yearÿ1. However, it should also be noted that the
recharge rates shown in Fig. 3 are annual values and this may have damped the episodic
nature of the actual recharge process.
The episodic nature of the recharge can be described by the cumulative distribution
function shown in Fig. 4. It is clear that the use of the lucerne and non-fallow rotations
could reduce average recharge, and these management options also make recharge more
episodicÐthat is, recharge occurs much less frequently, although its magnitude can still
be significant (see Fig. 4). Better agronomic practices, therefore, are not likely to control
episodic recharge significantly. For example, annual rainfall in 1973 was 538 mm (i.e.
58% higher than the long-term average) and the annual recharge at 1.0 m depth under the
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
245
Fig. 3. Simulated recharge rates at 1.0 m depth for: (a) Hillston under lucerne rotation (RT1) (&), and medic
rotation (RT2) (&); (b) Waleup under non-fallow (RT3) (&) and fallow rotation (RT4) (&) with rooting depth
of 0.5 m; and (c) Waleup under non-fallow (RT3d) (&) and fallow rotation (RT4d) (&) with rooting depth of
1.0 m.
non-fallow rotation (RT3) even exceed that under the fallow rotation (RT4). This was
because the soil profile under the fallow rotation (RT4) was wetter than that under the
non-fallow rotation (RT3), which led to substantially more surface runoff and, hence, less
infiltration. However, this was only observed to occur under wet years and the fallow
rotations generally produced more recharge at 1.0 m depth. At Walpeup, both the fallow
and non-fallow rotations produced similar annual recharge when the rooting depth was
set to 0.5 m (Fig. 4(b)). This is not surprising because, with such a shallow rooting depth,
most large rainfall events could penetrate the root zone and become recharge. Therefore,
it is necessary to use deep-rooted plants in the area for the purpose of recharge control.
The recharge rate at 4.0 m was much less episodic compared to recharge rate at 1.0 m
(Fig. 5). Pulses of infiltrated rainfall are damped in a diffusion-like process and most of
rainfall events can only penetrate the top few meters of the soil layer. If deep-rooted
plants are growing in the area, such as native forest, it is highly likely the water will be
removed efficiently by the vegetation. Clearing of native vegetation in the region created
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
Fig. 4. Frequency distribution of annual recharge at 1.0 m depth for: (a) Hillston under lucerne rotation (RT1)
(- - -) and medic rotation (RT2) (ÐÐÐ); (b) Waleup under non-fallow (RT3) (- - -) and fallow rotation (RT4)
(ÐÐÐ) with rooting depth of 50 cm; (c) Walpeup under non-fallow (RT3d) (- - -) and fallow rotation (RT4d)
(ÐÐÐ) with rooting depth of 100 cm. The vertical line indicates 10% probability.
a new balance between rainfall and recharge, which led to dryland salinisation. However,
it appears to be economically and sociably unrealistic to restore the natural balance by
replanting the native vegetation on a sufficient scale. The only practical way of reducing
recharge is to change agronomic practice. Such a change would increase the episodic
nature of recharge, so that rather than there being a constant drainage of water below the
root zone, significant recharge to groundwater would occur only as a result of large
rainfall events.
The results of the long-term scenario modelling for both, Hillston and Walpeup are
summarised in Table 3. Average recharge at 4 m under the lucerne rotation (RT1) was
30% of that under medic rotation (RT2) and this suggests that on average lucerne offers
better control over recharge than medic pasture. For shallow rooted plants (i.e. 0.5 m
rooting depth), average recharge at 4 m under the fallow rotation was 26 mm yearÿ1
compared to 19 mm yearÿ1 from the non-fallow rotation. This difference in recharge as a
result of fallowing is 7 mm yearÿ1, which equates to 35% extra recharge. The difference
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
247
Fig. 5. Cumulative annual rainfall differences from the mean (&) and annual recharge rates at 400 cm depth
for: (a) Hillston under lucerne rotation (- - -) and medic rotation (ÐÐÐ); (b) Waleup with rooting depth of
50 cm; and (c) Waleup with rooting depth of 100 cm under non-fallow (- - -) and fallow rotation (ÐÐÐ).
in average recharge between fallow and non-fallow rotations is 5 mm yearÿ1 with rooting
depth of 1 m (Table 3). O'Connell et al. (1995) using chloride profile analysis found that
fallowing caused similar increases in recharge at Walpeup.
At Hillston, the recharge rate at 4.0 m depth increased dramatically after 10 years for
the medic rotation (RT2) but not for the lucerne rotation (RT1), which continues to
decrease. The recharge under RT2 appears to respond to the cumulative rainfall anomaly.
At Walpeup, a similar trend was observed for the fallow rotation with shallow rooting
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L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
depth (RT4). However, an increase in recharge occurred after 20 years with deep-rooted
plants (RT4d) (Fig. 5(c)). It is interesting to note that the non-fallow rotation (RT3d) was
not sensitive to the cumulative rainfall anomaly, but the fallow rotation was (Fig. 5).
These results suggest that changes in agronomic practice (e.g. fallowing, crop rotation)
may take a considerable period of time (>10 years) to have any noticeable impacts on
recharge; the difference in recharge under fallow and non-fallow rotations is significant
(Table 3). It is also shown that deep-rooted plants have better control on recharge, but the
degree of control is modified by soil characteristics and the prevailing weather conditions.
Results from this study showed that the recharge just below the root zone is episodic in
the sense that it occurs infrequently and its magnitude is significant. Given the fact that
plants can only use water in root zone, the effect of current agronomic practices on
episodic recharge is limited. During these large rainfall events, the root zone, generally
considered as a buffer zone, became saturated and significant recharge occurred. As a
result, episodic recharge can substantially reduce the effectiveness of land management
options in controlling recharge. This is more so for sandy soils than for clay soils because
of low water holding capacity and high infiltration rates.
4. Conclusions
Recharge just below the root zone is episodic and 10% of the annual recharge events
accounted for 25±85% of the long-term totals under the Mallee conditions. The
magnitude of these annual recharge events can be up to 130 mm yearÿ1 and it is these
events that contribute largely to groundwater recharge. While it is true that better
management options (i.e. lucerne and deep-rooted non-fallow rotations) can reduce mean
annual recharge by eliminating most of the small recharge events, they are not likely to
eradicate the largest episodic events; they can increase the episodicity of the recharge.
Changes in agronomic management (e.g. fallowing, crop rotation) may take a
considerable period of time (>10 years) to have any noticeable impacts on recharge as
observed in the above-mentioned scenario modelling. It is important to recognise the
long-term impacts of any agronomic practices on recharge because the effects of these
changes may not be apparent for a short period of time, but may then be devastating in
terms of rising groundwater tables. Lucerne appeared to have better control on recharge
than medic pasture and average recharge under lucerne was only 30% of that under medic
pasture.
One of the advantages of physically based models such as WAVES is the ability to
simulate the hydrological effects of various land-management options and to identify key
factors controlling the processes. An attempt was made in this study to evaluate the
effects of winter fallowing in crop rotations on groundwater recharge. The results clearly
showed the long-term benefit of non-fallowing in reducing recharge and this has
implications for dryland salinity control. The traditional practice of winter fallow
significantly increased soil moisture storage, groundwater recharge, and the risk of
salinity. We argue that winter fallowing has contributed to dryland salinity in the Mallee
regions, and more areas could be affected in the future unless improved agronomic
practices are implemented. O'Connell et al. (1995) suggest that fallowing may be
L. Zhang et al. / Agricultural Water Management 42 (1999) 237±249
249
eliminated, provided its alternative encourages vigorous vegetative growth (e.g.
replacement with grass-free pasture, grain legume, oilseed phase) as reported by
Griffiths and Walsgott (1987) and Incerti et al. (1993).
Acknowledgements
This work was partially funded by the Murray±Darling Basin Commission through its
Natural Resources Management Strategy Investigation and Education Program (Grant
No. M4025). Technical assistance in site maintenance and data collection was provided
by S.D. Blandthorn, A.J. Corbett, S. Wisneske, M.C. Brown, M.J. Ferguson, J.L. Latta,
M.W. Ferguson. We are grateful to Mr. W. Milthorpe for the use of his farm at Hillston.
We are grateful to G.R. Walker, V. Snow and F. Lewis, and Joe Landsberg for comments
on a draft of this paper.
References
Barnes, C.J., Jacobsen, G., Smith, G.D., 1994. The distributed recharge mechanism in the Australian arid zone.
Soil Sci. Soc. Am. J. 58, 31±40.
Broadbridge, P., White, I., 1988. Constant rate rainfall infiltration: a versatile non-linear model. 1. Analytic
solution. Water Resour. Res. 24, 145±154.
French, R.J., 1978. The effect of fallowing on the yield of wheat. I. The effect on soil water storage and nitrate
supply. Aus. J. Agric. Res. 29, 653±668.
Gee, G.W., Hillel, D., 1988. Groundwater recharge in arid regions: review and critique of estimation methods.
Hydrol. Proc. 2, 255±266.
Griffiths and Walsgott, 1987. Water use of wheat in the Victorian Mallee. In: Proceedings of the 4th Australian
Agronomy Conference, p. 296 (Australian Society of Agronomy, Parkville, Vic.).
Incerti, M., O'Leary, G.J., 1990. The rooting depth of wheat in the Victorian Mallee. Aus. J. Exp. Agric. 30,
817±824.
Incerti, M., Sale, P.W.G., O'Leary, G.J., 1993. Cropping practices in the Victorian Mallee. 2. Effect of long
fallows on the water economy and yield of wheat. Aus. J. Exp. Agric. 33, 885±894.
O'Connell, M.G., O'Leary, G.J., Incerti, M., 1995. Potential groundwater recharge from fallowing in north-west
Victoria, Australia. Agric. Water Manage. 29, 37±52.
O'Leary, G.J., Connor, D.J., 1997. Stubble retention and tillage in a semiarid environment: 1. Soil water
accumulation during fallow. Field Crops Research, 52, 209±219.
Stephens, D.B., Knowlton Jr., R., 1986. Soil water movement and recharge through sand at a semi-arid site in
New Mexico. Water Resour. Res. 22, 881±889.
Zhang, L., Hume, I.H., O'Connell, M.G., Mitchell, D.C., Milthorpe, P.L., Yee, M., Dawes, W.R., Hatton, T.J.,
1999. Estimating episodic recharge under different crop/pasture rotations in the Mallee region. 1.
Experiments and model calibration. Agric. Water Manage, 42, 219±235.