Predictive modelling of management options for the control of dryland salinity in a first-order catchment in the wheatbelt of Western Australia

Journal of Hydrology, 145 (1993) 19-40

19

Elsevier Science Publishers B.V., A m s t e r d a m

[2]

Predictive modelling of management options for the control of dryland salinity in a first-order catchment in the wheatbelt of Western Australia R . B . S a l a m a , D . L a s l e t t a n d P. F a r r i n g t o n

CSIRO, Division of Water Resources, Private Bag, Wembley, W.A, 6014, Australia (Received 19 June 1992; revision accepted 6 October 1992)

ABSTRACT Salama, R.B., Laslett, D. and Farrington, P., 1993. Predictive modelling of management options for the control of dryland salinity in a first-order catchment in the wheatbelt of Western Australia. J. Hydrol., 145: 19~10. A prototype flow model has been developed for Cuballing catchment using MODFLOW. The results of the simulations showed that, in a first-order catchment, the management strategies most likely to arrest salinity were reforestation and pumping. The study confirmed that complete reforestation would arrest groundwater discharge and lead to the restoration of salinized land within the catchment. It also showed that replanting the top 25% of the catchment was sumcient to reverse the salinity trend by reducing groundwater discharge to an acceptable level. Drainage along the main stream line was also effective in reducing groundwater levels in the unconfined aquifer and constrained the discharge in the area adjacent to the stream. The simulations also showed that pumping reduced groundwater pressures in both the unconfined and confined aquifers. A single windmill discharging at a rate ranging from 15 to 25 m 3 day i was sufficient to reduce water levels in the unconfined aquifer by more than 5 m and in the confined aquifer by more than I0 m. However, the pumping well must be correctly located in the catchment. The preferred location is in highly transmissive areas with adequate aquifer thickness and upstream from geological structures restricting groundwater flow. In Western Australia, these areas are usually along drainage lines and within discharge areas.

INTRODUCTION

Salinization is now recognized as an increasing problem of land and stream degradation in dryland agricultural regions of Australia (Williamson, 1990). Extensive investigations in Western Australia and other states showed that clearing of land for agriculture has increased recharge to shallow aquifers and led to rising groundwater levels with increased surface discharge of water and C o r r e s p o n d e n c e to: R.B. Salama, CSIRO, Wembley, W.A. 6014, Australia.

0022-1694/93/$06.00

© 1993

Division o f W a t e r Resources, Private Bag,

Elsevier Science Publishers B.V. All rights reserved

20

R.B. SALAMA ET AL.

salts (Peck and Williamson, 1987). In Western Australia, efforts have concentrated on discharge control using a range of land management schemes such as revegetation (Schofield et al., 1989), crop rotation (Nulsen and Baxter, 1982), drainage (George and Frantom, 1991) and pumping (George and Frantom, 1990), to arrest the present trend of salinization in established areas. The success of such schemes is currently being assessed by long-term monitoring in selected catchments. However, monitoring of this type, is frequently site specific, costly in time and resources and can only provide an answer in the long term. An alternative approach is to use groundwater models to predict the effectiveness of management strategies to control salinity development in a catchment. Barnett (1989) used computer modelling to estimate the effects of biological controls on decreasing groundwater inflows to the River Murray. Simple analytical models (Peck, 1976; Morris and Thomson, 1983; Stewart, 1984; Schofield, 1990a) and a numerical groundwater model (Hookey and Loh, 1985a,b,c) have been developed to assess the impact of revegetation strategies in catchments on groundwater discharge and salinization. However, such models lacked the data to account fully for the hydrogeological complexity of salinity development in farming catchments. Alternative management options were not considered. This paper reports on a study that simulates the behaviour of a first order catchment behaviour using three-dimensional groundwater flow models. It predicts the effectiveness of management options applied to that catchment in reducing groundwater discharge and in ameliorating dryland salinization. SITE DESCRIPTION

The study was carried out in a first-order catchment (Falls Farm) (area 1.75 km 2) located near Cuballing in the southwest of Western Australia (Fig. 1). The surface elevation of the catchment was obtained from a detailed topographic map (scale 1:10 000 with 1 m contour intervals). Geological maps were prepared by photogeological interpretation of aerial photos (scale 1:20 000), from seismic surveys (Kevi, 1984) and from the analysis of the drilling logs of four cored wells and 15 observation wells drilled during the study. The saturated hydraulic conductivity of the top soil was obtained from Sudmeyer (1987) and the hydraulic conductivity of the alluvial channel from an ongoing long-term pumping experiment. Water levels were monitored using Wesdata loggers and conductance probes in 17 observation wells (Salama et al., 1991). Rainfall, streamflow and stream water quality (electrical conductivity (EC) and chloride) have been measured by the Water Authority of Western Australia since 1982 and by CSIRO since 1988 (Salama et al., 1993b).

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

21

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Fig. 1. Locality m a p of Cuballing catchment, showing location of catchment in inset, location of basement highs and associated dykes A and B and piezometers in the catchment, location of existing production well P1 and simulated production well P2. The m a p also shows the existing land use, excluding the plantation, reserve and the rock outcrop area (10% of the catchment). The remaining area of the catchment is used for crops and pasture.

22

R.B. SALAMA ET AL.

Groundwater recharge was estimated by three different techniques to give preclearing and postclearing recharge rates and groundwater discharge was estimated using physical and chemical tracer methods (Salama et al., 1993b). Meteorological data have been collected since 1988 by a weather station (Monitor Sensors). The clearing history of the catchment was obtained from aerial photographs taken at the early stages of clearing and from historical records of farmers. AQUIFER BOUNDARIES AND TYPES OF AQUIFERS Detailed hydrogeological studies together with water level patterns in response to barometric pressures showed that three main aquifers are present (Salama et al., 1992). The eastern parts of the catchment are covered by an unconfined aquifer (layer 1) which is formed mainly of colluvium deposits and weathered bedrock material in the eastern margins of the catchment, grading into alluvial channels in the central part of the catchment (Fig. 2). The western and southern parts of the catchment are covered by a semiconfined aquifer system (layer 2) formed of colluvium B. This in turn is formed mainly from laterite and sedimentary weathered material underlain by highly weathered granite. Layer 2 in the western and southern areas of the catchment is underlain by a confined aquifer which is formed mainly of weathered bedrock material (layer 3). Rock outcrops in the western parts of the catchment separate the aquifer systems of layers 2 and 3 into two main channels (Fig. 3). The northeastern channel is caused by preferential weathering of the bedrock. The depth of weathering in this channel is estimated from seismic surveys (Kevi, 1984) to be about 50 m (I in Fig. 3). The north-south channel which is parallel to the existing drainage system is weathered to a depth of 30 m and infilled by alluvial deposits in the upper 10-20 m (II in Fig. 3). Two sets of dykes form groundwater barriers which restrict the groundwater flow in the north-south channel and as a result, divide the catchment into two compartments. FIELD MEASUREMENTS

Hydraulic conductivity Saturated hydraulic conductivity for the soil horizons (top 5 m) were measured using well permeameters, the values ranging from 0.01 to 5.0 m day -I (Sudmeyer, 1987) with soils on the pediment slopes having higher values than the gravelly mid slopes and soils on the lower and mid-slopes without gravel having the lowest (Sudmeyer, 1987).

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

23

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Fig. 2. Cross-sections A-A' (A) and B-B' (B) in Cuballing catchment showing the lithological pattern and the aquifer distribution. Sections show unconfined aquifers (1) in the alluvial channel in A and in the colluvium deposits of the eastern slopes, semiconfined aquifers (2) mainly in the western slopes and the confined aquifer (3) below the clay.

Transmissivity values, calculated from an ongoing pumping test (18 month) used for discharge enhancement, show that the transmissivity values of the channel sediments and the underlying highly weathered basement rocks vary from 2 to 6 m 2 day ~ while hydraulic conductivity values range from 0.5 to 1.0 m day -~ . Analysis of the pumping test data indicates that the boundaries of the sedimentary channel and highly weathered bedrock are narrow (10-20 m), and that two impermeable barrier boundaries are present; one 127 m north of the pumping well and the other 284 m west of the pumping well. Slug tests conducted in all drilled wells showed that the hydraulic conductivity of the permeable layers (screened) increased from the catchment divide to the valley floor. In layer 2 the hydraulic conductivity varied from 0.01 m day-1 in well 7 to 0.1 m day-I in well 5. In layer 3 the hydraulic conductivity ranged from 0.1 m day-t in well 8 to 1.0 m day-1 in well 4.

24

R.B. S A L A M A

E T AL,

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Regolith thickness in m e t r e s .....

C a t c h m e n t boundary

Fig. 3. Depth to bedrock in Cuballing catchment, showing the preferentially weathered channel along I, and the channel along the stream II. Map prepared from seismic surveys (Kevi, 1984) and data from cored and observation wells.

Recharge and discharge Three methods were used to estimate recharge rates (Salama et al., 1993b). These were: (1) the chloridemethod to estimate preclearing recharge rates (Johnston, 1987); (2) the displacement of soil chloride bulge after clearing to estimate postclearing recharge rates (Salama et al., 1993b); (3) the water level rise method (Johansson, 1987). Preclearing recharge rates ranged from 0.4 to 1.0 mm year -t , while postclearing recharge rates using the displacement method were found to be 10

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

25

mm year ~. Recharge rates estimated from groundwater level fluctuations ranged from 2 to 5 mm year -1 for a specific yield of 1%, and from 10 to 25 mm for a specific yield of 5%. The low postclearing recharge rate can be attributed to the low hydraulic properties of the soil profiles and the aquifer material. Groundwater discharge (seepage) occurs upstream from two main geological structures. The first runs along the existing drainage line and is more prominent upstream of the southern and northern basement highs and dykes (A and B in Fig. 1). The second is along the preferentially weathered and conductive channel that discharges upstream of the northern basement high (A in Fig. 1). Groundwater discharge calculated using Darcy's equation ranged from 40 m 3 day-~ in summer to 60 m 3 day-~ in winter (Salama et al., 1993b). Groundwater discharge using chemical tracers (Br- and C1-) was found to range from 70 to 190 m 3 day -~ (Salama et al., 1993b).

Evapotranspiration Water discharges from the soil, groundwater and vegetation into the atmosphere through evapotranspiration. As well as being affected by climatic factors, the amount of discharge is affected by the depth of the groundwater. Relatively few values of evapotranspiration have been obtained in the wheatbelt of Western Australia. For this study, evaporation measurements of annual crops, pastures, perennial shrubs and trees conducted in a similar environment in the Wallatin Creek catchment in the wheatbelt of Western Australia (Farrington et al., 1992, 1993) have been used. These results show that evaporation rates from crops and pastures range from 1 to 4 mm dayduring winter, while those from shrubs range from 3.0 mm day-1 in summer to 0.5 mm day-~ in winter. Mean annual transpiration rates from trees were estimated to range from 0.4 to 1.4 mm day ~, depending on tree density. MODEL PARAMETERS AND VALUES

A modular three-dimensional finite difference groundwater flow model (MODFLOW, McDonald and Harbaugh, 1988) was applied to the catchment to simulate the different management options for reducing groundwater discharge. Owing to its modular structure it is simple to use and maintain. Recharge, evapotranspiration, drainage, recharge and discharge wells together with river interaction can be readily simulated. The only limitation for this type of simulation is that it takes evapotranspiration (ET) out of a cell if the groundwater water level (table) is above the extinction elevation (surface elevation - extinction depth (ED)) of the cell. ED is defined as the depth

26

R.B. SALAMA ET AL.

below which evapotranspiration from the water table ceases. As a result the model does not take into account ET in the unsaturated zone. This effect is taken into account outside the MODFLOW mode. A term called recharge reduction is introduced, which is the amount of ET withdrawn from the recharge as it infiltrates through the unsaturated zone (Appendix). Each simulation was carried out for a period of 10 years. This time span was selected following several test runs covering 10-50 years. Since trees such as red gum Eucalyptus camaldulensis reach maximum leaf area and hence maximum evapotranspiration within 10 years, and pumps and drains achieve maximum drawdown within 5 years, it was considered that the results of a 10 year simulation would give sufficient time to show how different management options can reduce groundwater discharge.

Grid design The simulation of Cuballing catchment is set on a horizontal cell grid of 17 rows by 15 columns. Each cell is 100 m x 100 m (Fig. 4). Out of a possible 255 cells 199 are used to give a catchment area of 2.0 x 1 0 6 m 2 . The catchment area is discretized into three aquifers. Layer 1 covers the eastern, shallower part of the catchment and consists mainly of an unconfined aquifer with an average depth of 10 m. Layer 2 covers the western and southern areas where the aquifer is mainly semiconfined. It varies in thickness from 13 to 24 m. Layer 3 underlies layer 2 and is separated from it by a confining layer varying in thickness from 4 to 6 m.

Recharge The results of detailed hydrogeological and geomorphological studies in the catchment (Salama et al., 1991, 1992) show that the upper reaches of the catchment, where the aquifers are in contact with the rock outcrop, are the main areas of recharge for the three layers, especially the confined third layer. Two mechanisms of recharge are simulated; that which takes place at the boundaries through the joints, fissures and contact with outcrops with the three layers, and the regular seasonal recharge that takes place across the catchment. At the boundary, recharge has been simulated as a constant flux boundary condition ( using wells), while seasonal recharge has been spatially distributed and assumed to occur in winter over layers 1 and 2 only. Recharge rates vary according to the different land management options; natural vegetation, plantations, crop rotation and fallow. To account for this in the simulation each cell is divided into four main land uses. For bare ground winter evapotranspiration (ET), summer ET, and the extinction depth (depth

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

27

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Fig. 4. Plan view of the catchment showing (A) distribution of layers 1 and 2, drainage line and bedrock barriers, and (B) the vertical cross-section C-C', the thicknesses of the three aquifer layers and the interconnection at the stream line.

28

R.B. SALAMA ET AL.

at which no ET takes place) have been held constant with no recharge reduction (Appendix). Crops are assumed to be cyclic, being planted in winter and harvested in spring. Forests (natural vegetation) have constant winter ET, summer ET, and extinction depth, with a recharge reduction over the entire simulation. The ET characteristics of plantations are considered similar to those of forests. They are assumed to be planted at the beginning of the simulation and grow to maturity after a set 'maturation time' (5 years in this simulation). During the maturation period, winter ET, summer ET, extinction depth and recharge reduction increase linearly with time. After maturation they remain constant. If the cell contains bedrock, summer and winter ET and extinction depth are reduced to zero and the recharge that would have fallen into this cell is distributed equally to the adjacent non-bedrock cells (Appendix).

Hydraulic conductivity Soil hydraulic conductivity values from permeameter testing, and values from the pumping test and slug tests carried out in the catchment were used together with the water level map (using flow net procedures and Darcy's law) to assign hydraulic conductivity values to each grid cell. A trial-and-error method was used to refine the hydraulic data sets within prescribed bounds to produce the water level map of the catchment. A hydraulic conductivity of 0.01 m day -t was assigned to the clayey soils of layer 1 in the eastern boundary of the catchment (Fig. 5). The values gradually increased further westward to reach the highest value of 0.1 m day-~ at the channel sediments. The same pattern was used for layer 2, where high hydraulic conductivity values were assigned to the channel sediments and the preferentially weathered bedrock channel. Transmissivity values for the third layer follow the same trend of layer 2, and range from 1 to 6 m 2 day -~ . Larger hydraulic conductivity values were found to cause the cells to dry out. At the same time decreasing the hydraulic conductivity caused an excessive build up of water levels. Also constant flux boundary recharge had to be simulated (using wells) at the rate of 0.63 mm year -t, 1.17 m m year -t and 4.77 m m year-t for layers 1, 2 and 3, respectively, to maintain observed heads and calculated discharge rates. The ongoing long-term pumping test was used to validate the model parameters. The results of the pumping test show that after 1 year of pumping the drawdown in an observation well 40 m away from the pumping well was 2.4 m. The model results show good agreement with a predicted drawdown of 2.3 m.

29

MANAGEMENT O ~ I O N FOR THE CONTROL OF DRYLAND SALINITY

Layer 1

Layer 2

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Fig. 5, Distribution of hydraulic conductivities for layers 1 and 2 and transmissivities for layer 3 in the catchment. Note the lower hydraulic conductivity and transmissivity along the barriers and the increase in hydraulic conductivity in the aquifer material along the drainage line. RESULTS OF SIMULATIONS

Two types of simulations were carried out. The first simulated the effects of reforestation of the catchment with different percentages of forest cover; ranging from 25%, 50%, 75%, 90% to complete reforestation (Fig. 6). The second simulated the effects of clearing the catchment to its present cover and imposing different management options. The options considered were peripheral plantations along the areas of recharge, complete reforestation, enhanced drainage along the stream line, pumping with one well located upstream of discharge area B (Fig. 1) and pumping with two wells located upstream of discharge areas A and B (Fig. 1). In both cases, those areas not planted by trees were used for crop and or pasture on an equal basis.

Results of reforestation options The first simulation (I) was carried out to estimate preclearing groundwater

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II

R.B. SALAMA ET AL

All F o r e s t Cleared

Present Forest Cover

25% F o r e s t Cover

5O% Forest Cover

75% Forest Cover

Complete Forest Cover

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MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

31

pressures and recharge rates and calibrate the model against historical data from the uncleared areas of the catchment. The simulation was also carried out to find the minimum area that can be planted by trees to arrest the trend of increasing groundwater discharge. The results of the first simulation (Fig. 7) show that 25% forest cover equally distributed between the upper and lower areas of the catchment would reduce the water level in layers 1 and 2 at the discharge zone but the pressures in layer 3 would continue to rise during the simulation period. This is mainly due to the continuous inflow boundary condition simulating boundary recharge (i.e. recharge since clearing). When the rates of boundary recharge are reduced an equivalent drop in pressure Occurs.

Replanting the total area of the catchment has a marked impact on water pressures and heads. The aquifers in layers 1 and 2 nearly dry out and the decreased recharge to layer 3 reduces water levels by 3-5 m. In the case of complete forest cover, aquifers below the remnant vegetation area in the western part of the catchment completely dry out as this area has been under reduced recharge rates prior to simulation. The results also show that the two discharge areas in the catchment are greatly reduced by 25% forest cover (Fig. 6). It was also found that planting trees in the upper areas of the catchment only (peripheral plantation), will reduce water levels by about 2 m in the first and second layers, and by less than 1 m in the third layer (Fig. 8).

Results of engineering options The second simulation (II) shows that enhanced drainage by constructing a 3 m drain along the existing channels between discharge areas A and B (Fig. 1) will reduce the groundwater levels by 5 m in the first and second layers but has no effect on the third layer. Pumping with one well reduces water levels at discharge area B by about 8 m in layer 3 and by about 5 m in layers 1 and 2. An additional well upstream of the northern basement high causes a similar effect in discharge area A and increases the drawdown in the southern area to 10 m in layer 3 and to 6 m in layers 1 and 2 (Fig. 8). Fig. 6. Effect on groundwater levels in the catchment of clearing, reforestation, drainage and pumping following 10 years of simulation. Water level contours are in metres below ground surface. Discharge areas occur where groundwater is more than 2 m below the surface. Simulation I shows the reduction in discharge area from 5% of the catchment for all forest cleared to 0% for complete forest cover. Note the blank rectangular area in complete forest cover, caused by the drying of the cells in the reserve through falling water levels. Simulation II shows the effect of drainage and pumping in reducing the discharge area. Pumping by two wells causes water levels below the reserve to dry up.

32

R.B. SALAMA ET AL.

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Fig. 7. Drawdown in the three layers (along section C - C ' in Fig. 4) after I0 years of simulation (I). With all forest cleared, water levels in the three aquifers approach the surface. Water levels start to fall with 25% forest cover, and with complete forest cover, the water levels in all aquifers are greatly reduced. The high water levels in the aquifer boundaries are an artefact of the modelling.

33

M A N A G E M E N T O P T I O N FOR T H E C O N T R O L OF D R Y L A N D SALINITY

PRESENT FOREST COVER

PERIPHERAL PLANTATIONS

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34

R.B. SALAMA ET AL.

DISCUSSION A N D C O N C L U S I O N S

Management options Of the management options examined, complete reforestation will restore the catchment to preclearing water levels and cause the greatest decrease in water levels in layers 1 and 2. However, pumping is more effective in reducing the water levels in all three layers provided the pump is correctly located in the catchment. Planting 25% of the catchment can be effective in reducing the water levels and reducing the saline groundwater discharge but will not be sufficient to reduce pressures in the third layer at the discharge zone within 10 years because of the palaeoflow generated by the recharge over the past 70 years.

Engineering options Pumping. The results of the simulations show that, in a first-order catchment, the management strategy most likely to immediately arrest salinity is groundwater pumping in areas of discharge. This will result in immediate lowering of water levels and focus the area of discharge. The results show that pumping can reduce groundwater pressures in both the unconfined and confined aquifers. A single windmill discharging at a rate ranging from 15 to 25 m 3 day-t is sufficient to reduce water levels in the unconfined aquifer by more than 5 m and in the confined aquifer by more than 10 m. Field measurements and simulation both show that pumping is able to both rapidly reduce pressures in the confined aquifers and lower the water table over large areas. However, siting of the pumping well must be correctly located in the catchment. Normally the well should be placed in highly transmissive areas with adequate aquifer thickness and upstream from any geological structure restricting groundwater flow. In Western Australia, these areas are usually along drainage lines and within discharge areas.

Drainage. Drainage along the main stream line is effective in reducing groundwater levels in the unconfined aquifer along the drain and will constrain the discharge area near the drain. Engineering options can be applied in areas where the disposal of the saline groundwater will not cause environmental problems, such as terminal lakes or saline streams, or in specially constructed evaporation ponds. A detailed study is now being conducted to assess the best disposal options for saline water in the agricultural catchments of the wheatbelt (Salama, 1990).

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

35

Reforestation options This study confirmed that complete reforestation would reduce groundwater levels in discharge areas, which in turn would reverse the upward groundwater flux. This would enable surface water to infiltrate and leach the salt from the top soil leading to restoration of salinized land within the catchment. The simulation also showed that replanting 25% of the catchment, equally distributed between areas of recharge and discharge, would be sufficient to control groundwater discharge. Studies in higher rainfall areas have also shown that 25-50% of a catchment requires reforesting to control groundwater discharge (Loh, 1988; Schofield, 1990b; Bell et al., 1990). Where to plant trees? The effectiveness of trees in reducing groundwater discharge in a catchment depends on where the trees are planted. Trees planted in recharge areas can reduce recharge into the aquifer through increased interception and evapotranspiration. They can also extract from the water table, provided that the groundwater is accessible to tree roots and sufficiently low in salinity to be tolerated by the roots. Generally, trees cannot extract water from confined aquifers because roots are unable to penetrate the overlying aquitards. In discharge areas, salt-tolerant trees can utilize groundwater from both unconfined and confined aquifers provided the trees can tolerate the salinity content of the water. Hydrogeological sense indicates that trees planted in recharge areas are more effective than in discharge areas in controlling salinity because trees will utilize the excess water before it reaches the aquifer, will lower water tables, and thus keep the salt stored in the soil in its place. Morris and Thomson (1983) also recommended that trees planted in recharge areas are more effective in controlling salinity than in discharge areas because restoration of the hydrologic balance in the catchment depends on lowering the water table rather than continually removing groundwater from the discharge areas. They advocated the importance of identifying areas of high groundwater recharge in a catchment where reforestation would be most effective in reducing recharge. However Bell et al. (1987), showed that stream salinity is not reduced without planting in discharge areas. Planting of trees at the discharge areas will not reduce the recharge in the rest of the catchment; water levels will continue to rise, groundwater flow will increase and salts will be leached from the unsaturated zone to the aquifers. The continuous process of groundwater flow downstream will flush more salts to the discharge areas (Salama et al., 1993b). This will in turn increase the salinity in the discharge area, which will eventually kill trees planted there.

36

R.B. SALAMA ET AL.

Areas of recharge and discharge Even today, when considering the best management strategy to control salinity, two basic questions have still not been resolved in most catchments. The first is how to identify areas of recharge and discharge in a catchment. The second is how to estimate the discharge rate, which causes salinity, and the recharge rate, which has to be controlled. In Cuballing catchment, detailed hydrogeological studies and modelling have answered these questions but financial and logistical constraints will limit the number of catchments where similar studies can be made. Nevertheless, simple methodologies developed in the catchment, which are inexpensive in time and resources, can be applied to other catchments. This involves the steps described below, to give order of magnitude estimates (i.e within those of the model results). For identification of areas of recharge and discharge in a catchment, photogeological interpretation techniques can be used to establish the basin geomorphology, and define lithological units, hydrogeological systems and geological structures in a catchment (Salama et al., 1991, 1993a,b). Estimation of discharge rates from a catchment can be calculated using Darcy's equation (Salama et al., 1993b) used earlier in the simulation study. Transmissivity can be estimated from the hydrogeological expressions of the catchment, using parameters derived from lithological units in the catchment. The width and gradient can be calculated from topographic maps. Provided that the catchment has reached equilibrium status, groundwater discharge calculated by this method can be assumed to equal the recharge rate of the catchment. The recharge can then be partitioned using the criteria defined for the land uses as stated previously in the simulation study, taking into account the outcrop area of the confined aquifer, the areal distribution of the unconfined aquifer and the areas of discharge. Strategies can then be formulated for reducing recharge and/or enhancing discharge. ACKNOWLEDGEMENTS The authors wish to thank Dr. Greg Davis (CSIRO) and John Ruprecht (Water Authority of Western Australia) for their critical comments on earlier drafts of the paper, and Ray Falls for his generosity in making land and facilities available. Financial support of the National Soil Conservation Program is also acknowledged. REFERENCES Barnett, S.R., 1989. The management of groundwater-induced river salinity due to land clearing in the Murray Basin, southeastern Australia. In: A. Sahuquillo, J. Andrev and T.

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

37

O'Donnell (Editors), Groundwater Management: Quantity and quality. Proc. Symp., Benidorm, Spain, 1989. IAHS Publication No. 188, pp. 101-109. Bell, R.W., Loh, I.C. and Borg, H., 1987. The effect of non-valley reforestation on water quality and quantity in the Padbury Reservoir catchment and its regional implications. Surface Water Branch, Water Authority of Western Australia, Rep. No. WS5, 74 pp. Bell, R.W., Schofieid, N.J., Loh, I.C. and Bari, M.A., 1990. Groundwater response to reforestation in the Darling Range of Western Australia. J. Hydrol., 119: 179-200. Farrington, P., Salama, R.B., Bartle, G.A. and Watson, G.D., 1992. Water use of agricultural and native plants in a Western Australian wheatbelt catchment. Agric. Water. Manage., in press. Farrington, P., Bartle, G.A., Watson, G.D. and Salama, R.B., 1993. Long term transpiration of two Eucalypt species in a native woodland estimated by the heat pulse technique. Aust. J. Ecol., submitted. George, R.G. and Frantom, P.W.C., 1990. Using pumps and syphons to control salinity at a saline seep in the Wallatin Creek catchment. Western Australian Department of Agriculture, Division of Resource Management, Tech. Rep. No.91, 32 pp. George, R.G. and Frantom, P.W.C., 1991. Drainage of sandplain seeps for salinity control and stock water. J. Agric. West. Aust. (4th Series), 32: 88-93. Hookey, G.R. and Loh, I.C., 1985a. Groundwater simulation of the effect of catchment clearing and partial reforestation at Maringee farms. Public Works Department, Western Australia Water Resources Branch, Rep. No. WRB 122, 34 pp. Hookey, G.R. and Loh, I.C., 1985b. Groundwater simulation of the effect of catchment clearing and partial reforestation on Maxon Farm, Bataling Creek. Public Works Department, Western Australia Water Resources Branch, Rep. No. WRB 123, 21 pp. Hookey, G.R. and Loh, I.C., 1985c. Groundwater simulation of the effect of catchment clearing and partial reforestation on the Lloyd property. Public Works Department, Western Australia Water Resources Branch, Rep. No. WRB 125, 17 pp. Johansson, P.-O., 1987. Estimation of groundwater recharge in sandy till with two different methods using groundwater level fluctuations. J. Hydrol., 90: 183-198. Johnston, C.D., 1987. Water and solute movement in deeply weathered lateritic soil profiles near Collie, Western Australia. M.Sc. Thesis, Department of Agriculture, University of Western Australia (unpubl.). Kevi, L., 1984. Cuballing catchment, seismic refraction survey. Geological Survey of Western Australia, Geophysical Rep. 1/84, 4 pp. Loh, I.C., 1988. The history of catchment and reservoir management on Wellington reservoir catchment, Western Australia. Surface Water Branch, Water Authority of Western Australia, Rep. No. WS35, 39 pp. McDonald, M.G. and Harbaugh, A.W., 1988. A modular three-dimensional finite-difference groundwater flow model. U.S., Geol. Surv., Open-file Rep. 83-875. Morris, J.D. and Thomson, L.A.J., 1983. The role of trees in dryland salinity control. Proc. R. Soc. Victoria, 95(3): 123-131. Nulsen, R.A. and Baxter, I.N., 1982. The potential of agronomic manipulation for controlling salinity in Western Australia. J.Aust. Inst. Agric. Sci., 48: 222-226. Peck, A.J., 1976. Interactions between vegetation and stream water quality in Australia. In: H.F. Heady, D.H. Falkenborg and J.P. Riley (Editors), Watershed Management on Range and Forest Lands, Proc. 5th Workshop US/Australia Rangelands Panel, Boise, ID, June 1975. Utah Water Res. Lab., Logan, UT, pp.149-155.

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Peck, A.J. and Williamson, D.R., 1987. Effects of forest clearing on groundwater. J. Hydrol., 94: 47-65. Salama, R.B., 1990. Discharge enhancement techniques for the control of salinity. Land Water Res. News, 5: 13-15. Salama, R.B., Farrington, P., Bartle, G.A. and Watson, G.D., 1991. The Institution of Engineers Natl. Conf. Publ. No. 91/22, Vol. 3, Identification of recharge and discharge areas in the wheatbelt of Western Australia using water level patterns in relation to basin geomorphology. Int. Hydr. Water Resour. Symp., Perth, 1991. The Institution of Engineers Natl. Conf. Publ. No. 91/22, Vol. 3, pp. 841-846. Salama, R.B., Farrington, P., Bartle, G,A. and Watson, G.D., 1992. Distribution of recharge and discharge areas in a first-order catchment as interpreted from water level patterns. J. Hydrol., 143: 259-277. Salama, R.B., Farrington, P., Bartle, G.A. and Watson, G.D., 1993a. The role of geological structures and relict channels in the development of dryland salinity in the wheat-belt of Western Australia. Aust. J. Earth Sci., 40: 4546. Salama, R.B., Farrington, P., Bartle, G.A. and Watson, G.D., 1993b. Salinity trends in the wheatbelt of Western Australia: results of water and salt balance studies from Cuballing catchment. J. Hydrol., 145: 41-63. Schofield, N.J., 1990a. Determining reforestation area and distribution of salinity control. Hydrol. Sci. J., 35: 1-19. Schofield, N.J., 1990b. Effects of trees on saline groundwater tables. In: P.R. Scott (Editor), Agroforestry - - Integration of Trees into the Agricultural Landscape. Western Australian Department of Agriculture, Tech. Rep. No. 102, pp. 10-30. Schofield, N.J., Loh, I.C., Scott, P.R., Bartle, J.R., Ritson, P., Bell, R.W., Borg, H., Anson, B. and Moore, R., 1989. Vegetation strategies to reduce stream salinities of water resource catchments in south-west Western Australia. Water Authority of Western Australia, Rep. No. WS 33, 81 pp. Stewart, J.B., 1984. Measurement and prediction of evaporation from forested and agricultural catchments. Agric. Water. Manage., 8: 1-28. Sudmeyer, R.A., 1987. The soils of Falls Farm catchment, Cuballing, Western Australia. Department of Agriculture, Western Australia, Division of Resource Management, Tech. Rep. No. 53, 67 pp. Williamson, D.R., 1990. Salinity - - an old environmental problem. In: Year Book Australia 1990. Aust. Govt. Publishing Service, Canberra, pp. 202-211. APPENDIX

Recharge

F o r each cell S u m m e r net recharge = 0 W i n t e r net recharge = R n R n =

R b --

f ~ R R c -- f f R R f

-- f p R R p ( R w s t Rw,~ t m

1 year)

where Rn is the net recharge for cell in winter (m d a y - I );

Rb is

the m a x i m u m

39

MANAGEMENT OPTION FOR THE CONTROL OF DRYLAND SALINITY

recharge beneath bare ground (m day ~); f¢ is the fraction of cell area used for crops; R R c is the recharge reduction for crops (m day-~);ff is the fraction of cell area covered by forest; RRf is the recharge reduction for forests (m day-I);fp is the fraction of cell area used for plantations; RRp is the recharge reduction for a 'mature' plantation (m d a y - l ) ; Rws is the plantation maturation constant, which is 2 + 2 ws = 3.989 (dimensionless); t is the time in years at the 'end' of the season; t m is the plantation maturation time in years; ws is the ratio of winter time length to summer time length, which is 182/183. Evapo tr ansp ir at ion

For each cell, the m a x i m u m net E T rate at the surface is denoted as E T n. In winter ET, = E L L

+ E T f f f + ETwb(1 - f ~

-ff

--fp) +

ETpfp(Rwst - 1 year) Rwstm

where ETc is the m a x i m u m E T rate at surface for crops in winter (m d a y - l ) ; ETf is the m a x i m u m E T rate at surface for a forest in winter (m d a y - 1); ETwb is the m a x i m u m E T rate at surface for bare ground in winter (m day-J ); ETp is the m a x i m u m E T rate at surface for a 'mature' plantation in winter (m day l). In summer Ern = RfErff

+ grsb(1 - i f -

fp) +

RpETpf(Rwst

-

Rw,tm

1

year)

where ETsb is the m a x i m u m E T rate at surface for bare ground in summer (m day l); Rr is the ratio of summer E T rate to winter E T rate for forests (dimensionless); Rp is the ratio of summer E T rate to winter E T rate (ETp) for plantations (dimensionless). Note: fc = 0 in summer. Extinction depth (ED) below the surface

The E T rate is assumed to be the same as the surface E T as depth increases, until a depth of ED - 0 . 5 m is reached net cell E T (depth) = cell E T (surface) for 0 < depth < ED-0.5 m = (ED-depth) x cell E T (surface) 0.5 for E D - 0 . 5 m < depth < ED = 0 for depth > ED

40

RB. SALAMA ET AL.

During the last 0.5 m the ET rate reduces to zero. Note that this is not the standard method that M O D F L O W uses to model extinction depth. In winter

ED = [ED~LET¢ + EDfffETr + (1 - A

-ff

-fp)EDbETwb

ETpfpEDp(Rwst - l year)]/ET ~ + Rw, Im where ED c is the extinction depth for crops (m); EDp is the extinction depth for a 'mature' plantation (m); ED b is the extinction depth for bare ground (m). In summer

EL)

[RfEDfffETf + (1 - fr - fp)EDbETsb _,_

where EDf is the extinction depth for forests (m).