Predicting and controlling water logging and groundwater flow in sloping duplex soils in western Australia

Agricultural Water Management 53 (2002) 57–81

Predicting and controlling water logging and groundwater flow in sloping duplex soils in western Australia T.J. Hattona,*, G.A. Bartlea, R.P. Silbersteina, R.B. Salamaa, G. Hodgsona, P.R. Wardb, P. Lamberta, D.R. Williamsona a

CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia CSIRO Plant Industry, Private Bag No. 5, Wembley, WA 6913, Australia

b

Abstract Water logging and groundwater recharge were studied at a site in southwestern Australia characterised by sloping duplex soils in a Mediterranean environment. The specific objectives of the study were: (a) to determine the effectiveness of land management systems involving trees, shallow interceptor drains, and perennial pasture in reducing water logging and recharge risk; and (b) to predict water logging risk at the plot and catchment scale. We found that properties inherent in the site (soil hydraulics, topography, surface dams) had a larger control over seasonal water logging than differences in vegetation cover, with large variations in water logging and recharge over a relatively small area. Such variability would be difficult to capture in any detail using a process model of water logging. The tree/drain systems had local effects on water logging control, but this was mostly due to the direct effects of the trees, which provided localised discharge from deeper groundwater systems. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Water logging; Duplex soil; Western Australia

1. Introduction Changes associated with the clearance of approximately 20 million ha of native vegetation and its replacement with annual cropping systems in Western Australia are profound and significant. In the first instance, it generated an agricultural industry now worth several billions of dollars each year. That industry is now under great threat due to more subtle changes associated with vegetation clearance, particularly hydrological * Corresponding author. Tel.: þ61-8-9333-6208; fax: þ61-8-9387-8211. E-mail address: [email protected] (T.J. Hatton).

0378-3774/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 0 1 ) 0 0 1 5 6 - 1

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changes. Land salinisation and water logging cost rural producers in the order of hundreds of millions of dollars, and this is expected to greatly increase as the area of salinity-affected land expands from the current 10% to more than 30% over the coming decades (George et al., 1997). Hatton and Nulsen (1999) reviewed the nature of ecohydrological changes impacting on agriculture, and concluded that only a production system that substantially mimics the water use behaviour of the original ecosystem will restore the hydrological imbalance in the absence of massive engineering intervention (e.g. drainage, groundwater pumping and saline disposal). Water logging is a widespread phenomenon in the cropping region of Western Australia, and in particular on duplex soils (sandy topsoil over clayey subsoil) in which water perches during the growing season. Duplex soils occupy an estimated 60% of the WA Wheatbelt, and are more common in the medium to high rainfall areas in the southern and western regions (an area of expansion of cropping). According to McFarlane and Cox (1992), about 60% of the sloping duplex soils which make up three-quarters of the main landform in the Upper Great Southern waterlog to the degree where crops are potentially affected. McFarlane et al. (1992) estimated that water logging was more widespread than previously recognised, with annual losses in wheat production in the western region alone to be up to US$ 100 million. The long-term costs due to related problems of dryland salinity are even greater. The irony of crop loss due to excess water during the growing season in an environment in which primary productivity is ultimately limited by the availability of water is obvious and intriguing, and suggests the opportunity for improvements through management. Conceptual hydrological and soil physical models that explain the development of water logging are reasonably well developed. In particular, we can distinguish between water logging associated with seasonal perching from water logging associated with the intersection of a more perennial (regional) aquifer and the land surface. The latter phenomenon is more directly associated with salinisation, although water excess involved in the former may contribute to the recharge of deeper groundwater systems causing water logging and salinisation off-site (George and Conacher, 1993). The distinction between these two water logging phenomena is the primary dichotomy to be made in diagnosing cause. The methods to map those portions of the landscape subject to groundwater discharge water logging are relatively advanced. In the western region, however, the majority of current water logging is caused by seasonal perching on duplex soils and is locally far more difficult to predict, even with detailed physical measurement. The research challenge is to assess the likely water logging risk to crops with or without management intervention. More specifically, we would like to define the information necessary to make such assessments or predictions of water logging risk in association with the variety of land management options available to mitigate this risk. The key (perched) water logging research in Western Australia is summarised by McFarlane and Cox (1992) and Cox and McFarlane (1995). The first and foremost challenge to water logging assessment is the extreme variability in its intensity; Cox (1988) reported three-fold variation in water logging intensity over distances of less than 10 m. Such variation is likely due to corresponding variation in the saturated conductivity of the subsoil upon which water perches (Lehman and Ahuja, 1985; Eastham

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et al., 1999a), and in Western Australia this has been attributed to the lingering and variable effects of woody roots of the original vegetation. Cox and McFarlane (1990) summarise the factors controlling the extent and variability of perched water logging: excess rainfall, poor external drainage (runoff), poor internal drainage (percolation, interflow), and low soil water storage capacity. In turn, these factors are associated with climate, soil type and landform. However, Cox and McFarlane (1990) acknowledge that water logging variability is high both between years and within paddocks, and thus difficult to predict. Two contrasting approaches to water logging control are (a) a change in vegetation cover or management, and (b) enhanced drainage. The former involves the incorporation of vegetation that uses more water over the year (perennials like trees or lucerne); Hatton and Nulsen (1999) review and discuss the effectiveness and attributes of such revegetation. In Western Australia, drainage options developed to control water logging have to date focussed on shallow drains aimed at intercepting overland flow and soil interflow and quickly moving it off-site. Cox (1988) provided the best assessment of the hydrological effectiveness of interceptor drains in the western region. He found that such drainage had substantial positive effects on water logging relief for 25–40 m downslope of the drains in wet years without any indication of over-drainage impacts on crop production in dry years. Interestingly, the fraction of rainfall intercepted by the drains was of the order of only a few percent at the Narrogin site, but up to 19% at the Mt. Barker site in the wettest year. Salerian and McFarlane (1987) demonstrated the cost-effectiveness of open interceptor drains calculated over a 20-year period. There is also a belief that the removal of surface and shallow soil water via drains reduces the recharge of groundwater systems associated with salinisation (Cox and McFarlane, 1990). The overall aim of this project was to understand water logging at two different scales: the plot and catchment (Fig. 1). In this paper we examine water logging over an 11 ha site (‘‘plot scale’’) under annual pasture and lucerne. This site was also modified by the addition of interception drains planted with a narrow belt of trees; these drains and associated tree lines form the upper and lower boundaries of the experimental area. A full description of these treatments is in Hodgson et al. (2002). Our assessment of water logging, and its potential control, across the 639 ha catchment in which the above smaller plot is nested is given in a companion paper (Silberstein et al., 2002). Some specific questions addressed by the study:  To what degree is water logging resulting from lateral redistribution of surface runoff and shallow interflow?  To what degree is water logging controlled by landscape characteristics?  To what degree can deeper-rooted perennials such as lucerne control water logging and groundwater recharge?  To what degree do the shallow interception drain/tree system control water logging and groundwater recharge? A combination of measurements over the period 1995–1998 of soil moisture profiles, site physical descriptions, groundwater monitoring, evaporation, and runoff were used to address the above questions. These field assessments were complemented by complex physical modelling examining the sensitivity of the site to factors that potentially control water logging.

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Fig. 1. Topog element network (O’Loughlin, 1986) for the ‘Ucarro’ catchment, with 2 m contour intervals. Local plot experiment was located in a mid-to-upper slope position in the northeastern portion of the catchment (outlined).

2. Site physiography and geology ‘Ucarro’ (338450 S, 1178270 E) occupies the northern part of the divide between the Blackwood/Arthur river system to the north and the Frankland/Gordon river system to the south. The topography ranges from 300 to 400 m, with actively eroding streams, frequent

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rock outcrops, moderate slopes and valley fill in the northern area. Hodgson et al. (2002) provide a fuller description of the climate, geology, geomorphology and soils. These authors also describe in detail the land and water management systems associated with the catchment. The key treatments for water logging evaluated at the site were (a) the incorporation of perennial pasture, and (b) the construction of shallow interceptor drains lined with a 6 m wide tree belt on the downslope side of the drain. In the case of the local experimental plot, the lower drain discharged into a small surface dam directly downslope of the northern half of the plot. Two groundwater systems operate in the catchment: a shallow transient—perched (winter) system on the ‘B’ or ‘C’ soil horizons; and a perennial—groundwater system at 4–6 m depth. The terms perched and groundwater aquifer are used to define the two systems, respectively. Soil hydraulic data from George (1992) and Silberstein et al. (1999) were used to estimate the specific yield (0.05) used for these systems. Average annual rainfall for the area is 483 mm, of which 362 mm falls in the May–October (winter/spring) period.

3. Monitoring, measurement and analysis Shallow groundwater piezometers (36 piezometers) were installed in May 1995 using spiral augers (see Hodgson et al. (2002), for description and layout). Data loggers were installed in 23 selected piezometers in September 1995 to record hourly water level measurements. Periodic manual water levels were taken in the other piezometers. In March 1996, 20 deep groundwater piezometers were installed. Data loggers were installed in 14 of these piezometers in June 1996. To monitor soil moisture change, 44 neutron moisture meter tubes were installed to 2 m depth (see Hodgson et al. (2002), for design and layout). In 1996, nine neutron access tubes were installed to 6 m to explore the deeper soil profile. Data and calibrations are reported in Lambert et al. (1999). V-notch weirs were installed in the drains and gauged. The volume of water flowing through the drain was calculated from the water height behind the weirs, recorded at 15 min intervals. The depth of water flowing into the drain from the experimental area was calculated from an estimate of the plot area contributing to flow through each weir. The contributing area was assumed to extend from the weir, at right angles to the contours, up the slope to the next drain. A tipping bucket rain gauge was installed in 1996 in the centre of the plot. Calibrations were made to ensure accuracy for falls up to 20 mm. A weather station recording rainfall, radiation, relative humidity, temperature, wind run and atmospheric pressure was installed nearby in the catchment. Airborne radiometric data were obtained in 1996, and a composite index was mapped as an indicator of site heterogeneity. An interpolated groundwater salinity map was developed based on electrical conductivities of water samples taken from deep piezometers within or near the plot. A bromide tracing experiment was performed to quantify the lateral and vertical drainage fluxes in the perched system. A contour trench (4 m  0:5 m) was excavated

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0.1 m into the ‘A’ horizon. Sampling wells were installed into the ‘A’, ‘B1’ and ‘B2’ soil horizons as well as into the deeper aquifer. These nested sets of wells were established in two replicated transects beginning at the trench and then at 2, 5, 11 and 15 m downslope. Prior to the experiment, the wells were pumped and sampled for background levels of bromide. Soils were also sampled over 10 cm sections at each nested well site and analysed for bromide. On 30 August 1998, 900 l of water with a bromide concentration of 500 ppm was applied; all applied water drained out of the trench within 12 h. Well and soil samples were taken every 4 h over the first 2 days following application, and then weekly thereafter until 5 October 1999. Additional sampling of the test site as well as the dam was carried in October 2000. 3.1. Biophysical modelling The pasture experiment was also simulated using the dynamic version of the model (Topog_Dynamic) as described by Silberstein et al. (1999). In this exercise, the growth, water use and root development of the two pasture plots were simulated, along with the accompanying soil moisture fluxes. Results of the model comparison with field data are given by Silberstein et al. (1999). The comparison of measured and modelled water storage in the two experimental plots showed that the model reproduces the water storage in the ‘A’ and ‘B’ horizons well.

4. Results 4.1. Spatial variation in soils A composite map of the potassium, uranium and thorium signatures for the plot revealed substantial variation in soil properties at the local scale, including systematic differences between the lucerne and clover pasture treatment areas. (For brevity only the uranium signature is shown in Fig. 2.) 4.2. Soil water Ward et al. (2001) presented the moisture content measurements for the two plots which showed that there were systematic differences in the soil water holding capacities between clover and lucerne; these treatments were not established on identical soils. In general, there is more soil water holding capacity in the lucerne plot than the clover plot (see later discussion of Fig. 5). Secondly, soil water to 2 m depth under the trees is much lower than under the other vegetation at all times. 4.3. Groundwater level patterns in the plots Conceptual diagrams of the cross-sectional profiles of land surface, perched water, the permanent piezometric surface and the geological basement for the clover and lucerne plots appear in Fig. 3a and b. Most importantly, the deeper groundwater system does not

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Fig. 2. Map of radioactive uranium signature, as measured with airborne radiometrics, across a 5 ha portion of the catchment. The northern half of this plot comprised the lucerne treatment, while the southern half was annual clover pasture.

influence water levels in the shallow transient system anywhere within this plot. Note that the lucerne plot flattens out near the lower drain; this is proximal to the dam into which the drain below the lucerne treatment discharges. The shallow piezometers tapping the perched aquifer show a delayed response to rainfall events. Response to rainfall events takes place after a minimum of 30 days (hole 35) to a maximum of 78 days (hole 23). Water levels rose about 1.0 m from their initial levels following the development of perching after a minimum of 40 mm of rainfall. The water levels dropped to their original level quickly after rainfall ended. During the rainy season most of the piezometers upstream of the drains record water levels between 0.1 and 0.5 m from the surface whereas the piezometric levels downstream of the banks were deeper. Estimates of recharge to the perched aquifer were carried out using a combination of methods. An average rate of drainage of 0.0013 m per day was established from the average

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Fig. 3. Cross-section of study area hillslope under (a) annual clover and (b) lucerne. Note that at no time does the deeper piezometric surface intersect the shallow, transient perched system. Also note the flatter groundwater gradient toward the bottom of the lucerne hillslope.

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Table 1 Recharge to the perched aquifer estimated from well hydrographs, based on average drainage of 1.3 mm per day (established by long term rate of recession of hydrographs) Location

Recharge 1996 (mm)

Recharge 1997 (mm)

Pz2 Pz6 Pz7 Pz8 Pz11 Pz12 Pz13 Pz15 Pz21 Pz22 Pz23 Pz26 Pz28 Pz31 Pz32 Pz34 Pz35 Pz36

153 158 169 152 162 120 152 159 166 29 40 130 0.0 165 122 182 275 196

78 74 140 91 81 63 49 0.0 91 0.0 0.0 127 0.0 153 6.5 215 191 139

recession occurring in the hydrographs, this average rate of drainage was used to calculate the drainage reaching the perched aquifer. The values ranged from a minimum of 29 mm to a high of 275 mm in 1996 (Table 1). The drainage figures for 1997 were much lower. Cumulative water level rises in the perched aquifer indicates that more than 50% of the rain falling in the plots reach the perched aquifer. The apparent recharge rate under the trees and downslope of the drains is much lower; this reflects the elimination of any deep drainage under the trees as well as a net interception of lateral flow from upslope. The water levels in the perched aquifer were always higher than the water levels in the groundwater aquifer. The difference in head was about 5 m at the eastern bank to about 3.0 m at the western bank. The difference in head decreases near the dam to about 2.0 m. The Cox and McFarlane (1995) water logging index was calculated for each of the shallow piezometers for each year of the experiment (Table 2) and maps prepared showing the areas of high SEW30 occurrence (Fig. 4). The most salient feature of these results is the huge variation from year to year and within a treatment. For example, in 1996 values in the clover plot ranged from 0 to 204, while in the same year they ranged from 0 to 631 in the lucerne. Nevertheless, some generalities emerge. First, no water logging was ever recorded under the trees. Second, in the dry year of 1997, water logging was restricted to the lower lucerne plot. Finally, the lucerne plot showed more water logging by this measure than the clover plots in every year, but this response was dominated by the influence of the piezometers in the lower (flatter) portion of the lucerne plot. Also note that relative values of SEW30 vary considerably from year to year and do not necessarily follow rainfall. For example, there was slightly more rainfall in 1996 than 1995, and the SEW30 at bore 36s rose dramatically in that time, but SEW30 was much less at well 35s, and the same value at 32s (Table 2).

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Table 2 SEW30 water logging index of Cox and McFarlane (1995) for experimental plot at ‘Ucarro’a Piezo

Location

1995 (August–December) annual rainfall ¼ 398 mm

1996 annual rainfall ¼ 402 mm

1997 annual rainfall ¼ 323 mm

6s 7s 8s 11s 12s 13s 21s

Lower Lower Lower Lower Lower Lower Lower

38 17 32 5.5 25 11 8

204 170 80 29 9 0

0 0 0 0 0 0 0

Mean, S.E. 15s 20s

Upper clover plot Upper clover plot

19.5, 4.7 25 0

82.0, 35.3 124 0

0 0

Mean, S.E.

12.5, 12.5

62.0, 62.0

0.0, 0.0

Clover mean, S.E. 30s 31s 32s 35s 36s

77.0, 30.6

Lower Lower Lower Lower Lower

17.9, 4.3 15 270 117 197 49

112 117 40 631

0.0, 0.0 0 251 0 74 37

Mean, S.E. 23s 28s

Upper lucerne plot Upper lucerne plot

129.6, 46.9 0 0

225, 136.5 0 0

72.4, 46.7 0 0

0.0, 0.0

0.0, 0.0

0.0, 0.0

92.6, 40.2 0 0 0 0 0

150.0, 98.4 0 0 0 0 0

51.7, 34.9 0 0 0 0 0

0.0, 0.0 0 0

0.0, 0.0 0 0

0.0, 0.0 0 0

0.0, 0.0

0.0, 0.0

0.0, 0.0

clover clover clover clover clover clover clover

plot plot plot plot plot plot plot

lucerne lucerne lucerne lucerne lucerne

plot plot plot plot plot

Mean, S.E. Lucerne mean, S.E. 9s 10s 22s 33s 34s

Below Below Below Below Below

Mean, S.E. 2s 26s

Above tree line Above tree line

tree tree tree tree tree

Mean, S.E.

line line line line line

Tree mean, S.E.

0.0, 0.0 0.0, 0.0 0.0, 0.0 Pn 1995 values are for a partial year. SEW30 ¼ i¼1 ðTi Þ where Ti ¼ 30  Xi for Xi < 30 cm, Ti ¼ 0 for Xi 30 cm. Xi ¼ mean depth of watertable below soil surface (cm). a

4.4. Lateral perched flow The groundwater water levels in the perched aquifer and the groundwater aquifer are smoothed replicas of the topographic surface. The groundwater in the perched aquifer has a gradient of 0.0267 in the lucerne plot and 0.044 in the clover plot. The lateral

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Fig. 4. Contours for (a) 1995, (b) 1996, and (c) 1997 of the SEW30 water logging index of Cox and McFarlane (1995) prepared from field observations in the shallow wells.

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Fig. 4. (Continued ).

flow from the perched aquifer was calculated using Darcy’s equation (Q ¼ TIW, T ¼ 5 m2 per day, I ¼ 0:267 for lucerne to 0.044 under clover and W ¼ 170 m). The lateral flow in the perched aquifer ranged from 8.0 mm under the lucerne plot to 15 mm under the clover plot. The drain weirs gauged only 0.7 and 2.5 mm for the clover and lucerne plots, respectively, in 1996, and only 0.5 and 1.6 mm, respectively, in 1997 (Table 3). The dam into which the drains flow accumulated a third to a half of its capacity in each year of the study, or 800–1250 m3 of water. Even discounting wet season evaporation, direct annual rainfall accounts for about 500 m3. Gauged flow from the drains could account for less than 100 m3 of the required 300–750 m3 of inflow. 4.5. Tracer experiment Background bromide levels in the soil and groundwater were 0.5–1.6 ppm. During the 35 days of monitoring, 69 mm of rainfall fell, with falls of 13 and 9 mm on 3 and 4 September, respectively. No added bromide was detected in the deeper groundwater system at any time during the experiment. Detectable bromide appeared in the ‘A’ and ‘B1’ soil horizons 2 m downslope of the trench at day 20; none appeared at the 5 m sampling site or beyond during the experiment. The recent sampling (October 2000) also showed that no bromide was detected in the lower horizons further downstream from the injection site. Bromide levels in the dam were higher than the background levels (Table 4).

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Table 3 Groundwater recharge estimates for experimental plot, ‘Ucarro’, 1996a Piezometer

Trees

Lucerne

Recharge (mm)

% of rainfall

3d p19d p19m p49d p32d p23d p35d p35m p38d 8d p43d

29 52 37 53

7.3 13.0 9.3 13.2

Mean (mm) S.E.

42.7 5.9

10.7 1.4

Annual pasture

Recharge (mm)

% of rainfall

33 14 31 37 14

8.3 3.5 7.6 9.3 3.4

25.8 4.9

6.4 1.2

Recharge (mm)

% of rainfall

133 78

33.1 19.4

106 27.5

26.3 6.9

a

Estimates based on inverse hydrographic method with a specific yield of 0.05. Annual rainfall for 1996 was 402 mm.

The spatial variability of perched water logging due to topographic redistribution was estimated by application of the O’Loughlin (1986) wetness index using the steady-state Topog model (Fig. 5). Based on measured saturated conductivities, water logging according to this index should be greatest at the bottom of the lucerne plot, where terrain flattens. Table 4 Groundwater recharge estimates for experimental plot, ‘Ucarro’, 1997a Piezometer

Trees

Lucerne

Recharge (mm)

% of rainfall

3d p19d p19m p40d p49d p32d p35d p35m p38d p38m 8d p43d

8 25 11 32 17

2.6 7.7 3.4 9.9 5.3

Mean (mm) S.E.

18.6 4.4

5.8 1.4

Annual pasture

Recharge (mm)

% of rainfall

15 22 25 13 10

8.3 6.7 7.7 3.9 3.2

17.0 2.8

6.0 1.0

Recharge (mm)

% of rainfall

38.8 17.5

12.0 5.4

28.1 2.8

8.7 1.0

a Estimates based on inverse hydrographic method with a specific yield of 0.05. Annual rainfall for 1997 was 323 mm.

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Fig. 5. Topog steady-state wetness index (radiation-weighted) for the shallow, perched system, ‘Ucarro’. Note the expected degree of saturation near the lower drain.

4.6. Biophysical modelling The dynamic modelling simulates the moisture storage under the two pastures and shows lucerne drying out the profile significantly more than clover, especially in the second and third years. This fairly closely emulates the detailed measurements of Ward et al. (2001) (Fig. 6). The ability of lucerne to send roots deeper than clover is simulated by the dynamic model (Fig. 7) and the effect on soil saturation at depth is illustrated by the modelled drying front penetration in Fig. 8. By the third year this is quite significant, and over the long term is likely to result in a reduction in recharge to the deeper groundwater, reducing groundwater pressures further down the catchment. However,

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despite this drying out of the deeper profile, there is very little effect on the water logging within the ‘A’ horizon. The shallow soil gets saturated to near the surface under both pastures (Fig. 8), and this is also shown by the soil moisture data of Ward et al. (2001). This occurs because even though the subsoil is unsaturated its hydraulic conductivity is so low that water still perches on top of it. There is insufficient evaporative demand in winter to prevent the subsurface ‘‘ponding’’ to shallower than 30 cm that causes the water logging. Topog_Dynamic also simulates the SEW30 water logging index of Cox and McFarlane which measures the period of water perching to less than 30 cm from the surface, and correlates well with yield reduction. Modelling the SEW30 index (Fig. 9) is complicated by the fact that we cannot realistically represent the diversity of soil distribution and subsurface structure without an extreme data collection effort. However, while the actual numerical comparison may be difficult we can make a meaningful comparison in the pattern of water logging. The model results do show a similar pattern to the observations, with the worst affected areas being low in the lucerne plot, to the far north end of the lucerne plot and the south end of the clover plot. Simulations with the pastures

Fig. 6. Soil moisture storage under (a) clover and (b) lucerne for the ‘A’ horizon (0–0.45 m) and to 1.2 m, as measured by the TDR probes and extrapolated for each plot on the basis of neutron moisture meters as described by Ward et al. (2001), with modelled moisture storage from the dynamic model simulation.

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Fig. 6. (Continued ).

reversed—that is with lucerne simulated on the clover plot and vice versa—produced similar patterns, indicating that the site is dominated by the topography of the surface and perching horizon, rather than the ability of individual pastures to dry out the profile.

5. Discussion The outstanding feature of these results is the immense complexity and variability of the hydrological processes occurring at a site only 5 ha in extent. These qualities extend to the duration and extent of transient water logging, soil moisture behaviour, and groundwater recharge. Silberstein et al. (1999) concluded that this site was too complex and poorly understood to capture in a physical hydrological process model with typical assumptions made for such systems and with model parameters derived from field measurement. In this paper we show that even empirical interpretations of the soil and groundwater balances are problematic. The spatial variability of perched water logging can be explained to some degree by topography. O’Loughlin (1986) wetness index shows that water logging should be greatest at the bottom of the lucerne plot, where terrain flattens, and this was also the

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Fig. 7. Simulated root development over time under (a) clover and (b) lucerne by the Topog_Dynamic model. The data are presented as contours of root density over time. The annual cycle of root growth and decline is clearly displayed for both pastures, leading to complete death in the case of the annual clover, and simply a decline in root density for lucerne. The progressive deepening of roots from 1 year to the next is clearly displayed for lucerne leading to the drying out of the profile (Fig. 8).

conclusion form the dynamic modelling. Such locations should incur both run-on of surface water and high watertables due to the break of slope. Eastham et al. (1999a) attributed much of the large spatial variability in water logging and depth of the perched layer to local lateral redistribution due to slope and concavity; this is reflected in our

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Fig. 8. Simulated soil moisture saturation under (a) clover and (b) lucerne through time. The annual wetting and drying cycle is visible for both profiles, and shows that in the ‘A’ horizon there is little to separate the two plots. However, there is significantly more drying out at depth under the lucerne as the roots develop from one season to the next.

results as well. Further, the higher groundwater salinity at the pronounced break of slope below the northern half of the plot also indicates inherently less flow than elsewhere. Topography does not wholly explain the immense variations in the data generally, however. Cox (1988) and Cox and McFarlane (1995) suggested that in such duplex soils, the high spatial variability of the saturated conductivity of the subsoil translates to similar

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Fig. 9. SEW30 water logging index in 1996 as simulated by Topog_Dynamic.

variability in perching, and this sensitivity was borne out in Silberstein et al. (1999) modelling of this site. The behaviour of the perched aquifer system responsible for water logging indicates a very wide variation in the shallow recharge rates controlled almost entirely by hydrogeomorphic features and drainage management in the plots. The maximum recharge took place in the piezometers upstream of the lower drain and the dam. The piezometers downstream of the drain through to the mid-slope portions of the hillslope received lower recharge. Hydrographic analysis showed that more than 50–70% of the rainfall infiltrates to the perched aquifer, with higher rates of accumulation particularly in the lower parts of the plots. The waterlogged areas in the two plots extends in a north-south direction parallel to the western drain, with more waterlogged areas in the lucerne plot than in the clover plot. These waterlogged areas are caused by accumulated lateral flow as well as likely backwater effects of the dam. Most of the recorded differences in water levels are attributed to topography, structures and management (drains and trees) and not to the water use of the main crops (lucerne and clover). The findings are consistent with those of Ward et al. (2001), who found only subtle water balance differences between annual pasture and lucerne at this site during the wet season. The extra evapotranspiration from the lucerne pasture during the warmer months leads to a drying out of the subsoil, but this has little effect on the occurrence of

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water logging in the ‘A’ horizon during winter. These results are also consistent with Eastham et al. (1999b) and Eastham and Gregory (1999), who found that differences in the local water balance were more attributable to site than to crop performance or treatment. The dam is filled mainly with the subsurface water that seeps into and underneath the drains. The backwater effect of the dam causes the saturation of the area upstream of the dam and the lateral drains to a distance where the groundwater level resumes it normal gradient following the surface. Nested piezometers installed and sealed at the ‘A’, ‘B1’ and ‘C’ horizons indicate that short-term water logging can occur at the ‘A’ and ‘B’ horizons in response to intense rainfall events. The major perched water logging system that persists and accumulates is developed on the ‘B2’ or ‘C’ horizon. In a wet year or even in a dry year in some landscape positions this will persist within 30 cm or even at the surface for some time. Topography, geology and structures together with the effect of the drains and the backwater effect of the dam and the large variations in soil profile are the main causes of the water logging. While lucerne proved capable of reducing deep drainage as compared to clover, due to the nature of the lucerne plot the overall effect is minimised. The break of slope and backwater effect of the dam retard interflow on the lucerne hillslope relative to the clover hillslope, and thus create localised, more intense recharge in the vicinity of the drains. The variation in apparent recharge in the deeper piezometers was also surprising, given the expected differences in treatments. Eastham and Gregory (1999) also found a twofold variation in recharge on a duplex soil in this region, and their estimates of mean recharge were of a similar order under annual crops as the rates reported herein. Our expectation was that recharge under trees and lucerne would be negligible compared with annual pasture (e.g. the generality made by Hatton and Nulsen, 1999). For the site studied in this paper, Ward et al. (2001) used soil physical methods to infer the water balance for the year 1996 under annual clover or lucerne, and found deep drainage at 1.2 m depth under clover of 80 mm as compared with 30 mm under lucerne. We find similar recharge values based on piezometry under lucerne, but perhaps somewhat less recharge under annual pasture. Factors other than land cover seem to play an important role. The proximity of a piezometer to a geological lineament (e.g. piezometer 8), proximity to a drain, or within the potential backwater effect of the dam below the lucerne plot (e.g. piezometers 31 and 32) seemed to control the rates of seasonal water level change as much as land cover did. It is worth noting that drilling in the clover plot where underlain by a granitic dome revealed no permanent water table at all. Perhaps the greatest conundrum arising from observations on the local water balance is the failure to account for the changes in storage in the dam in terms of the gauged surface inputs. Clearly, a flux of the order of 6–20 mm per year into the dam is unaccounted for in our gauging. How does it get there? Clearly, if it is shallow interflow, the drains are not eliminating it. Subsurface lateral flow can occur with any amount of rain and even in unsaturated soil. In any soil with defined sloping horizons, there will be a flow component parallel with the lowest permeability layer. This flow can cause moisture accumulation in concave parts of the landscape even to saturation. It is postulated that the horizontal flow component will

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be proportional to the vertical component and to the slope. This phenomenon will increase the spatial variability in soil water content, water logging and groundwater recharge. Although very little flow (<2.0 mm) was measured in the drains, the results from this work indicate that the seasonal lateral flow at the A/B or B/C boundaries probably ranges between 15 and 20 mm, which is roughly consistent with the results from the tracer test. Due to the soil’s duplex nature, water accumulates at the interface zone and flows laterally towards the drains, mainly as subsurface flow. As the lower parts of the plot near the drains (and the dam) are saturated for longer periods of time than the upper parts, this suggests that recharge to the main aquifer is taking place mainly along these areas. The fact that little flow is measured in the drains is due to the subsurface lateral flow being recharged to the groundwater aquifer at these localised sites. Eastham et al. (1999a) also found greater deep drainage (groundwater recharge) it local sites where interflow in the perched system accumulated. There is significant redistribution of water within the plots, although little water leaves via overland flow, most of the water leaves the plots as shallow throughflow and deeper groundwater fluxes. Does this phenomenon shed an alternative light on the relative water balances under annual pasture and lucerne? Although it has been shown (Ward et al., 2001; Silberstein et al., 1999) that lucerne achieves greater annual water use than clover through its extended growing season and deeper rooting pattern, the rate of annual recharge to the groundwater aquifer is still 10– 30 mm. The estimate of the recharge to the groundwater aquifer under clover based on piezometry is less certain but it is expected to be higher and in the range of 20–80 mm. However, because the lateral flow from the clover plot would have been much higher than from the lucerne plot, the higher water use by the lucerne was overridden by a higher lateral flow under the clover. While we expected recharge under the tree/drain system to be lower than under lucerne, the results show that apparent recharge under trees is 30– 50 mm. This is not due to deep drainage under the trees but rather the accumulation of lateral flow from upslope. Under steady state, the average horizontal flux (Q) is proportional to the slope of the hydraulic head surface. In a concave landscape (e.g. the lucerne hillslope), the incoming slope is steeper than the outgoing one. Therefore, the incoming flux is higher than the outgoing one, leading to moisture accumulation in low-lying (‘‘concave’’) parts. Based on this relationship the groundwater discharge from the perched aquifer in the lucerne plot would be smaller compared to the clover plot due to the difference in slope. The fact that the soil water in the lower part of the clover plot transect decreased rapidly towards the lower drain, and at a much lower rate in the lucerne plot, indicates that the water is discharging into (or underneath) the drain in the clover plot while it is inhibited by the back water effect and low slope in the lucerne plot. In this regard, it is important to distinguish the effects of the trees from that of the drains. At least on this plot, the drains did not remove any significant amount of water. While drains may flow more substantially on other parts of the property, they do not function as perhaps anticipated locally. This observation is in accord with results reported by Cox (1988) at Narrogin, where there was large spatial variation in the performance and flow of shallow interceptor drains. Given recent research by Ellis et al. (1999) that

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indicates that the hydrological effect of a belt of trees in this climatic zone can extend tens of metres away from the edge of the trees, even in the absence of lateral flow, then it is likely that these belts are implicated in groundwater trends. With lateral flow, the impact of tree belts would be even greater. The observation that groundwater levels are falling in and above the plot, regardless of vegetation cover, is significant. Two alternative hypotheses present themselves, both based on the likely fact that the steeper portions of the catchment, particularly those with shallow regolith as found on the plot, has reach equilibrium with recharge under annual agriculture. The first hypothesis is based on the observation that rainfall during the 4 years of observation was substantially lower than the long-term average, and that this explains the falling trends. The alternative is that the planting of tree belts, even at such wide spacings, has sufficiently reduced deep drainage and intercepted additional groundwater from upslope to cause falling trends across the whole plot. While even piezometer 43 is falling, and it is upslope of the upper tree/drain system that defines the plot, there is in fact a row of trees planted approximately 100 m above that well on the property boundary. The groundwater modelling results support the idea that the tree belts have the potential to lower groundwater levels. Whereas the groundwater is not involved in water logging on the plots, and therefore probably not in the steeper portions of the catchment, it is certainly at or near the land surface farther down the catchment and is causing secondary salinisation. It appears that in the flatter portions of the catchment, groundwater is not falling and may in fact still be rising. We can approximately map the area of high watertables and salinity risk by applying relatively simple methods based on variables derived from elevation, but more detailed modelling indicates that valley bottoms will remain at risk to water logging and salinisation even with widespread adoption of control measures upslope. The pattern of water logging simulated by both the dynamic and steady-state Topog models was also similar to the observed in the field. Both these models indicate that topography is a dominating influence in determining the degree of water logging at this site, and the dynamic modelling indicates that pasture type plays a relatively minor role, although it may have a long-term benefit. However, a high spatial resolution of soil properties data would be needed to delineate this further.

6. Conclusions In ‘Ucarro’, more than 50–70% of the rainfall reaches the perched aquifer; some of the infiltrating water is later used by the crops or evaporated directly from the soil surface. The residual moves as lateral flow in a seasonal perched system in the upper soil or in a perennial groundwater system in the deeper regolith, and accumulates in the concave areas of the landscape, at the break of slope and in (and under) the drains. The local accumulation of water by lateral flow causes concave parts of the landscape to become waterlogged. Due to the saturation upslope and under the drains, outflow from the drains occurs as subsurface flow. Pasture type shows little or no influence over shallow soil water content or winter water logging resulting from the perched system. The higher water soil content and water

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logging in the lucerne plot as compared to the annual clover plot is caused mainly by the subdued hillslope gradient and differences in soil hydraulic properties in the lucerne plot. Although the soil water content under the trees is consistently lower than the either pasture type, water accumulates from upslope of the drains and subsequently percolates to the deeper system under the trees. Thus, while there is no deep drainage from under the trees per se, there are still rises in the deeper watertable, which the trees may intercept to some degree. The experimental plot is part of a landscape unit in which hydrological behaviour is spatially complex over short distances and dominated by variations in a host of physical factors that overwhelm any easy influence through management. This same complexity limits the feasibility of fully capturing the hydrological behaviour and the influence of management with physics-based modelling. Nevertheless, the intensive monitoring at the site has yielded some robust conclusions about how this steeper hydrogeomorphic component of the catchment operates: 1. There is significant redistribution of water within the plots, but little net water leaves via overland flow or shallow throughflow. Topography and very large variations in soil profiles drive this internal redistribution across the site. 2. Two groundwater systems operate: a transient (winter) system perched on the B or C soil horizons; and a more perennial groundwater system at 4–6 m depth. The perched system is wholly responsible for water logging and drains downward into the deeper system. 3. The deeper system exhibits annual rises and falls and with a long-term fall. Lateral discharge downslope is apparently sufficient to accommodate annual recharge. This deeper system is largely confined where measured. This system is associated with land degradation further down in the catchment, but does not cause any problems on the plot. The fluxes of water into and through the two groundwater systems shows very large spatial variability. In some years, there is no water logging at this site regardless of land cover. The effects of management on the hydrological behaviour of this upland component of the catchment can be summarised as follows: 1. The surface drains discharge little water from the perched system in this particular landscape unit. The effect they have on reducing water logging in combination with the row of trees is quite local. This is similar to results by Cox and McFarlane (1990) for the Narrogin site. Similarly, the effect may be larger in a wetter year. 2. Trees maintain a drier condition in the soil (and hence less deep drainage) than annual pasture or lucerne at all times of the year, as well as intercept and discharge groundwater from upslope. The trees associated with the drains do not intercept the all of the deeper groundwater as it moves downslope. The 30–80 mm of annual recharge to the groundwater system entirely discharges from the hydrogeomorphic unit annually (there is currently a long-term decline in groundwater levels); a fraction of this amount is intercepted along with the shallow throughflow to a total of perhaps 50 mm per year. 3. Replacing annual pasture with lucerne significantly reduces groundwater recharge over most of the hillslope, but topographic (slope, concavity) and hydrological factors (backwater effects from dams) can limit this benefit.

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4. Lucerne did not eliminate groundwater recharge after three years, even given substantially below-average rainfall in the final year of the study. 5. Pasture type shows little influence over winter water logging resulting from the perched system, and thus we would again expect no significant carry-over effect on water logging in a season following with crops. 6. At a local scale, it is not possible to predict the extent and duration of perched water logging due to the immense variability of the soil hydraulic properties that we know control it. At a broader scale, a simple topography-based index of likely degree of saturation can identify areas prone to water logging.

Acknowledgements This research was supported by the Grains Research and Development Corporation (Projects CSO154 and CQ4), CSIRO Land and Water, CSIRO Centre for Mediterranean Agricultural Research, and Agriculture Western Australia. We are grateful to the Rundle Family (‘Ucarro’) for their assistance, generosity, and patience over the course of this study. References Cox, J.W., 1988. Seepage interceptor drainage of duplex soils in southwestern Australia. Ph.D. Thesis, University of Western Australia, Perth. Cox, J.W., McFarlane, D.J., 1990. The causes of water logging. Western Aust. J. Agric. 2, 58–61. Cox, J.W., McFarlane, D.J., 1995. The causes of water logging in shallow soils and their drainage in southwestern Australia. J. Hydrol. 167, 175–194. Eastham, J., Gregory, P.J., 1999. The influence of crop management on the water balance of lupin and wheat crops on a layered in a Mediterranean climate. Plant and Soil 219, 1–13. Eastham, J., Gregory, P.J., Williamson, D.R., 1999a. Lateral and vertical fluxes of water associated with a perched water table in a duplex soil. Aust. J. Soil Res. 38, 3–13. Eastham, J., Gregory, P.J., Williamson, D.R., Watson, G.D., 1999b. The influence of early sowing of wheat and lupin crops on evapotranspiration and evaporation from the soil surface in a Mediterranean climate. Agric. Water Manage. 42, 205–218. Ellis, T., Hatton, T.J., Nuberg, I., 1999. A simple method for estimating recharge from low rainfall agroforestry systems. In: Proceedings of the 2nd Inter-Regional Conference on Water-Environment Emerging Technologies for Sustainable Land use and Water Management (Environwater99), Lausanne, Switzerland, 1–3 September 1999. pp. 4–10. George, R.J., 1992. Hydraulic properties of groundwater systems in the saprolite and sediments of the wheatbelt. Western Aust. J. Hydrol. 130, 251–278. George, R.J., Conacher, A.J., 1993. Interactions between perched and saprolite aquifers on a small, salt-affected and deeply weathered hillslope. Earth Surf. Processes and Landforms 18, 91–108. George, R.J., McFarlane, D.J., Nulsen, R.A., 1997. Salinity threatens the viability of agriculture and ecosystems in Western Australia. Hydrogeol. J. 5, 6–21. Hatton, T.J., Nulsen, R.A., 1999. Towards achieving functional ecosystem mimicry with respect to water cycling in southern Australian agriculture. Agrofor. Syst. 45, 203–214. Hodgson, G., Bartle, G.A., Silberstein, R.P., Hatton, T.J., Ward, B.H., 2002. Measuring and monitoring the effects of agroforestry and drainage in the ‘Ucarro’ sub-catchment. Agric. Water Manage. 53, 39–56. Lambert, P., Bartle, G., Williamson, D.R., Hatton, T.J., 1999. A method to identify the risk of crop loss due to water logging on duplex soils. Initial analysis of neutron moisture meter data, CSIRO Land and Water Technical Report 9/99.

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