Estimating episodic recharge under different crop/
pasture rotations in the Mallee region. Part 2.
Recharge control by agronomic practices
L. Zhang
a,*, W.R. Dawes
a, T.J. Hatton
b, I.H. Hume
c,
M.G. O'Connell
d, D.C. Mitchell
c, P.L. Milthorp
e, M. Yee
eaCSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia bCSIRO Land and Water, Private Bag PO, Wembley, WA, Australia cNSW Agriculture, P.O. Box 736, Deniliquin, NSW 2710, Australia dAgriculture Victoria, Mallee Research Station, Walpeup, VIC 3507, Australia
eNSW 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 was30% 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)
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
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.
Table 1
The crop/pasture rotations modelled by WAVES
Rotation Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8
Table 2
Annual rainfall and estimates of annual recharge at 1.5 m and 4.0 m depths, using the WAVES model
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 dayÿ1based 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.
Table 3
Results of long-term scenario simulations with recharge rate calculated at 4.0 m depth
Site Hillston Walpeup
Rotation
RT1a RT2b RT3c RT4d RT3de RT4df
Average rainfall (mm) 564 351 351
Rooting depth (m) 1.5 1.5 0.5 0.5 1.0 1.0
Minimum recharge (mm/year) 0 4 8 8 5 7
Maximum recharge (mm/year) 15 34 37 37 10 25
Average recharge (mm/year) 4 13 19 26 7 12
aFallow/oat/wheat/wheat/(lucerne
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
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 <5% of the time (Fig. 2(a and b)). At Walpeup, months with rainfall >75 mm account for 16% 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).
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.
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 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.
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.
in average recharge between fallow and non-fallow rotations is 5 mm yearÿ1with 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 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).
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 yearÿ1and 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.
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.
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