Spatial modelling of groundwater discharge patterns to predict floodplain salinisation and impacts on vegetation health Rebecca Doble, Craig Simmons, Ian Jolly, Glen Walker

CSIRO Land and Water Technical Report No. 1/04 January 2004

Spatial modelling of groundwater discharge patterns to predict floodplain salinisation and impacts on vegetation health Rebecca Doble, Craig Simmons, Ian Jolly, Glen Walker

CSIRO Land and Water Technical Report No. 1/04 January 2004

© 2004 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water.

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ISSN 1446-6171

Cover Photograph

Floodplain seepage, River Murray, South Australia Photographer: Rebecca Doble © 2004 CSIRO

ABSTRACT

The quasi-three-dimensional groundwater model MODFLOW 96 was used to simulate regional groundwater flow through a floodplain on the lower River Murray in South Australia. This report examines the relationship between the proportions of evapotranspiration (ET), seepage and baseflow and the floodplain geometric properties and elevation. The distribution of groundwater flux to the atmosphere is shown to be a good indicator of the rate of salinisation on the floodplain and an improvement on traditional measure of depth to groundwater. Inclusion of elevation data showed the effects of microtopography on groundwater discharge. Floodplain elevation above river level and floodplain width had the most significant effects on groundwater evapotranspiration. Results from the MODFLOW model were compared with an analytical two-dimensional, flat-floodplain cross-sectional model. Although the slice model was found not to perform well for small-scale spatial variation of discharge, the total volumes of evapotranspiration and seepage were predicted with good accuracy on a floodplain scale.

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INTRODUCTION

Background

In arid and semi-arid regions, floodplains are ecologically and hydrologically important parts of a catchment. Due to their proximity to rivers, floodplains and their associated wetlands have higher biodiversity value than surrounding areas. They provide habitat for many aquatic and riparian species, including vegetation, birds and mammals, and when inundated by floods, are breeding grounds for many native fish species. Hydraulically, floodplains often receive groundwater flowing from the catchment. This water either flows directly to the river or is lost through evapotranspiration. Evapotranspiration concentrates any salts present, increasing the salinity of the groundwater and floodplain soils.

The spatial variability of evapotranspiration from floodplains has implications for a number of water resource issues. In regions where aquifers have been depleted by over pumping and unsustainable water use, such as in south-western USA, falling watertables in floodplains has led to dieback of riparian vegetation, leaving bare ground open to invasion from species such as Tamarix (Horton, 1972). The effect of the increased evapotranspiration from Tamarix communities is significant in water balance calculations and can lead to substantial losses from catchments (Horton, 1972; van Hylckama, 1970; Gay and Fritschen, 1979).

In semi-arid areas where saline regional groundwater discharges to streams, such as the lower reaches of the River Murray in South Australia (Figure 1), rising floodplain water tables and changed flow regimes have led to the dieback of riparian vegetation (Margules and Partners et al., 1990). Highly saline water tables are raised due to the higher river levels and adjacent irrigation developments and salt is concentrated in the topsoil through evapotranspiration (Jolly, 1996; Bone and Davies, 1992; Eldridge et al., 1993; Jolly et al., 1993). At least 25% (26 000 hectares) of floodplain in the South Australian section of the River Murray is affected by salinity, and this has been predicted to increase to 30-50% in the next 50 years (MDBMC, 1999). The hydrology, hydrogeology, geomorphology and vegetation of these floodplains are complex, leading to high spatial variability in evapotranspiration.

As result of the salinisation of the lower River Murray and its associated floodplains, policies that require modelling of irrigation developments to predict future salinity impacts and potential changes in vegetation health and biodiversity are being developed in order to improve management and planning (DWR, 2001). The current models of floodplain solute transport processes, however, provide only a coarse prediction of where groundwater discharge and therefore salt stores will occur, and do not assess the magnitude of the impact on vegetation health and the ecological value of floodplains.

Modelling groundwater discharge and salt accumulation

Surface elevation is a key parameter in determining where salt accumulation will occur, as the greatest rates of groundwater discharge by evapotranspiration will be located in low lying areas where the depth to watertable is a minimum. However, soil type (Talsma, 1963; Warrick, 1988), proximity to the river and vegetation coverage and type (Holland, 2002) will also affect the extent of salt accumulation. The episodic flushing of salt from the soil profile by floods further complicates the establishment of salt stores. Salt that has accumulated in the soil profile is released to the river through bank storage mixing, and recharge to the floodplain through flooding and episodic rainfall events. Floods large enough to cover most of a floodplain on the lower River Murray occur on average every eleven years, but due to the highly variable and episodic nature of the Australian climate, the actual time between flood events may be much longer than this.

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To determine a threshold for salinisation due to steady state evaporation of saline groundwater, Warrick (1988) proposed that the limiting rate of upward flow in a soil was inversely proportional to the depth to the watertable but varied according to soil properties, as described by:

nnadAq −=lim (1)

where: qlim is the maximum steady state moisture flux through the soil;

d is the depth to groundwater from the soil surface

a and n are constants, and are dependent on soil characteristics; and

( ) ( )cscn

nA n nπ π= ⎡ ⎤⎣ ⎦ (2)

From (1), the maximum moisture flux or diffuse discharge from a soil can be determined for varying groundwater depths. A decrease in groundwater depth will lead to higher evaporation rates, and therefore greater concentration of salts in the surface soil. In Australia, a threshold groundwater depth of 2 m has been historically used to indicate areas with a risk of salinisation (Barnett et al., 1996; MDBMC, 1999). This value does not take into account varying soil types and can give a false impression that some areas are not at risk when they simply take longer than surrounding low-lying areas to become salinized. The watertable depth at which salt accumulation occurs in soils may vary from less than 1 m to over 6 m (Talsma, 1963; Peck, 1978). The science of predicting both dryland and irrigation salinity using groundwater discharge rates has already been established in soil physics, but is less frequently described in detail by groundwater hydrologists

MODFLOW 96 (McDonald and Harbaugh, 1996), and most other models where discharge is dependent on depth to groundwater, assumes a linear evapotranspiration function. Evapotranspiration varies from a maximum rate specified, occurring when the watertable is at or higher than a specified elevation - usually the soil surface, to zero, when the watertable is at some depth below the surface, such that:

maxqq = d≤0

max 1ext

dq q

d

⎛ ⎞= −⎜ ⎟

⎝ ⎠ 0<d<dext (3)

0=q d≥dext

where: q is the surface discharge rate, or evapotranspiration rate;

qmax is the maximum surface discharge rate;

d is the depth to groundwater from the specified evapotranspiration elevation, usually the ground surface;

dext is the extinction depth, below which there is no further discharge from the groundwater.

As the relationship for evapotranspiration with depth is non linear (1), the linear nature of the relationship used in many current groundwater flow models, may cause discharge to be under-predicted where the watertable is deeper, and over-predicted where it is shallow.

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MODFLOW 2000 includes the module ETS1 that represents evapotranspiration as a piecewise linear function (Banta, 2000), but this module is slow to be incorporated into many pre- and post- processors.

The geometry of floodplains has also been found to affect the distribution of groundwater flow to a river or water body. Cherkauer and McKereghan (1991) studied the distortion of groundwater equipotential lines around embayments of Green Bay and Lake Michigan. They showed that flow was concentrated by up to 500% around the embayments due to the steeper hydraulic gradients that occurred there. They also demonstrated that the degree of amplification was directly proportional to horizontal and vertical penetration of the embayments. Similarly, groundwater baseflow is focused around the outside of river meanders as they intercept the flow path and reduce the distance of flow (Linderfelt and Turner, 2001). The degree of curvature of meander or sinuosity, and hydrologic conditions will therefore affect the distribution of groundwater flowing to a river.

Depending on the degree of meander (wavelength, magnitude and distance from the river to the cliffs) the hydraulic head from the cliffs to the river will become convex for converging flow lines, and concave for diverging lines. Convergence of flow should decrease the average depth to groundwater, therefore increasing the rate of groundwater discharge to the surface or evapotranspiration. However, larger floodplain widths associated with diverging flow zones lead to longer groundwater residence times and therefore a higher total evapotranspiration. This should enhance discharge as the length of flow at which the groundwater table is above the extinction depth, and therefore subject to evapotranspiration, is increased. Similarly, the larger surface area of diverging flow zones is also likely to affect the total flux of groundwater by increasing the surface area available for discharge to take place.

In addition to geometric effects, microtopography, or small-scale variation in elevation, defines the impacts of a shallow saline watertable on vegetation and on the frequency of flood inundation. As ecosystem communities are themselves linked with microtopography, it also defines the impact on biodiversity in the region (Cramer and Hobbs, 2002). Cramer and Hobbs (2002) also note that there is a lack of fine-scale hydrological modelling that considers the impact of salinity on ecosystem structure.

Complex, three-dimensional numerical models are potentially able to model groundwater discharge on a scale detailed enough to predict spatial variation of salt accumulation and therefore vegetation health. However, due to the small cell sizes required in such models, they are typically restricted to very small areas. In addition, three-dimensional models with increasing complexity take longer to run, require more parameter inputs which are largely unknown, and may therefore produce more errors associated with calibration than simpler models (Walker et al., 2002).

Two-dimensional slice models of floodplains allow a clearer comparison between hydrogeologic parameters and groundwater discharge. They also provide a simple means to conduct sensitivity analyses that require numerous repetitions to be performed quickly. Slice models are easier to operate, but there are questions as to whether they provide a good representation of the spatial variation of salt accumulation if there is significant flow in the third dimension due to convergence or divergence of flow lines associated with river meanders.

This report aims to:

• Use a quasi-three-dimensional groundwater flow model to predict groundwater discharge, and therefore areas of salt accumulation in a semi-arid floodplain.

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• Determine the influence of floodplain geometry and elevation on the spatial patterns of groundwater discharge is examined.

• Compare the results with those of a two-dimensional analytical slice model in order to determine when two-dimensional approximations to the floodplain salt accumulation problem are useful.

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SITE DESCRIPTION

The site selected for the study was Clarks Floodplain adjacent to the Bookpurnong Irrigation Area (Figure 1), located on the lower reaches of the River Murray in South Australia (34°21’S, 140°37’E). The field site is located within the semi-arid inland of Australia, with rainfall varying between 200 and 300 mm/yr and potential evaporation of approximately 1800 mm/yr. The site is typical of floodplains on the lower River Murray, which meanders as a single channel across relatively flat terrain within a river valley of alluvial deposits.

Figure 1. Clarks Floodplain on the lower River Murray, South Australia (34°21’S, 140°37’E) showing irrigation areas and flow zones (numbered 1-11).

The hydrogeology of Clarks Floodplain is typical of the eastern part of the lower River Murray, consisting of a layer of Coonambidgal Clay overlying and partially confining the sands of the Monoman Formation (Jolly and Walker, 1995; Jarwal et al., 1996). The Coonambidgal Clay layer ranges from 3 to 7 m thick, while the Monoman Formation is approximately 7 m thick in this area. The cliffs adjacent the floodplains consist of a layer of Woorinen sands over Blanchtown Clay, each approximately 2 m thick (not represented in the model), overlying a layer of Upper Loxton Sands up to 35 m in depth. The whole area is underlain by the Lower Loxton Sands, which acts as an aquitard basement to the shallow aquifer that encompasses the Monoman Formation and Upper Loxton Sands. Groundwater salinity in the Upper Loxton Sands and Monoman Formation is in excess of 35 000 mg/L, while irrigation recharge salinity is typically 5 000 mg/L.

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The Bookpurnong Irrigation District comprises more than 1,500 ha of irrigated land predominantly planted with citrus, vines, and stone fruit (Ingerson and Telfer, 2001). Major irrigation commenced in the mid-1960s, and has been increasing since that time (PPK Environment and Infrastructure, 1998). Excess recharge from the irrigation area has led to the formation of a groundwater mound, which displaces saline groundwater towards the floodplain and has led to increased waterlogging and salinisation on the floodplain, and groundwater seepage at the break of slope adjacent to the cliffs (Telfer and Overton, 1999). Aerial photographs show that extensive degradation has taken place between 1972 and 1998, with most of the vegetation health decline occurring at the back and centre of the floodplain. This is associated with the seepage and presence of a saline backwater that is thought to intercept groundwater and concentrate salt further (Figure 2). Black box (Eucalyptus largiflorens) and red gum (E. camaldulensis) tree communities have been most affected by the salinisation of the floodplain.

Clarks Floodplain is situated below a lock (Figure 2), and therefore the occurrence of vegetation health decline due to weir-induced water logging is minimal. This report therefore focuses specifically on irrigation-induced salinity. Groundwater flow around the lock does occur, but has little effect on Clarks Floodplain.

Figure 2. Vegetation health on the floodplain is shown in greyscale with black representing dead vegetation, dark grey signifying vegetation in poor health, light grey representing healthy vegetation and white designating no vegetation present. For Clarks Floodplain, vegetation health decline was observed at the back of the floodplain and the saline backwater, and extends out towards the centre of the floodplain. Most of the floodplain peninsula areas support healthy vegetation.

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METHODS

Modelling was undertaken with MODFLOW 96, a modular quasi-three-dimensional finite-difference groundwater model developed by the USGS (McDonald and Harbaugh, 1996). The quasi-three-dimensional model was developed as an idealized representation of Clarks Floodplain, to specifically show the spatial variation in groundwater depth and discharge. The intention of the model was not to produce a close fit with historical data in order to facilitate management decisions, but to compare the results with those of a two-dimensional slice model and with floodplain parameters such as width, elevation and convergence. Modelling was focused on floodplain surface water/groundwater interactions rather than effects of groundwater mound development within the highland.

Whilst some data is available for comparison with the highland groundwater mound from the groundwater flow model, there is limited data for comparison with the groundwater levels on the floodplain itself. Equation 1 indicates that evapotranspiration is sensitive to small changes in soil surface and groundwater elevations, and the five available floodplain boreholes used to test the model did not provide enough information about groundwater head distribution for a full calibration. More significantly, no direct spatial measurements of evapotranspiration, the parameter of interest, were available for the floodplain. To attempt to calibrate the model in the traditional sense would therefore be ineffectual (Konikow and Bredehoeft, 1992). The purpose of the model is comparison of variables rather than accurate quantification, therefore calibration was replaced with ‘confidence testing’, a means of comparing model data against known field data in order to ensure that the model was a good representation of the system and that parameter assignment was reasonable. It is acknowledged that confidence testing is not robust enough to accurately quantify the absolute evapotranspiration for various stresses to the system, but the purpose of this modelling is to examine parameter sensitivities, and therefore further calibration was not justified.

Model parameters and their sources are listed in Tables 1 and 2. The irrigation recharge value was taken from AWE (1999), but varied within a range of 133 to 243 mm/yr to obtain a good match with existing borehole and groundwater elevation data in the highland. Horizontal hydraulic conductivity of the Monoman Sands and Upper Loxton Sands were adjusted together such that the groundwater mound that developed under the irrigation area matched in height and shape, with regional bore data from bores GDN051, GDN052, GDN058, GDN065, BKP06, BKP07, BKP08, BKP09 and BKP10 present in the Upper Loxton Sands and Monoman Sands aquifers (AWE (1999) and Primary Industries and Research South Australia (PIRSA) monitored bores). Confidence testing also required the seepage areas from the modelling to match those found on the site.

As the watertable exceeds the soil surface, groundwater begins to pool, and the evapotranspiration rate approaches that of an open water body. The maximum evapotranspiration rate used in MODFLOW for modelling evapotranspiration was not high enough to accurately predict the rate of seepage. Seepage was denoted by flooded cells, which did not contribute to the discharge from the floodplain. To include the contributions of seepage in the water balance, seepage was modelled using the evapotranspiration function but with a maximum rate equal to the evaporation from an open water body, and an extinction depth of 0.2 m. The seepage extinction depth was kept as small as possible to imply that the water table had reached the surface, but still be large enough that the calculations could be processed by the solver function without numerical instability. Seepage was only modelled using this modified evapotranspiration function on the cliff-side of the saline backwater, and only in cells that were initially saturated before the seepage function was included.

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Table 1. Model parameters a

Parameter Source Range Adopted Value Irrigation recharge * AWE (1999) 133-243 175 mm/yr Dryland recharge AWE (1999) 0.1 0.1 mm/yr Maximum evapotranspiration rate on floodplains

Thorburn (1996) 40-440 110 mm/yr

Extinction depth Thorburn (1996), Talsma (1963) 1-7 2 m / 4 m

Open water evaporation rate

Bureau of Meteorology 1700 – 1800 mm/a 1800 mm/yr

Seepage extinction depth *

- 0.1-0.5 0.2m

River stage upstream of Lock 4

SA Water 13.2 m 13.2 m

River stage downstream of Lock 4

SA Water 9.8 - 10.5 m 10.5 m

Floodplain elevation Overton et al. (1999) 12 m – 15 m 12 m – 15 m

Highland elevation Overton et al. (1999); Armstrong et al. (1999) 35 - 40 m 40 m

a Calibrated parameters are indicated with an asterisk (*).

Table 2. Model parameters a

Formation Layer Height (mAHD)

Horizontal conductivity (m/d)

Vertical conductivity (m/d)

Upper Loxton Sands * 3 to 40 5* 1* Range 1 - 10 0.035 - 1 Coonambidgal Clays 10 to 15 0.1 0.01 Range 0.05 – 0.1 0.005 - 0.01 Monoman Sands * 3 to 10 10* 3* Range 10 - 35 0.35 - 1 Lower Loxton Sands -10 to 3 0.2 0.2 Range 0.05-4 0.005-0.4 a Ranges referenced from Armstrong et al. (1999); AWE (1999), Jolly et al. (1998), Evans and Kellett (1989), Barnett and SA Department of Mines and Energy (1991). Calibrated parameters are indicated with an asterisk (*). mAHD represents the official Australian Height Datum in meters.

To investigate the effect of elevation on salt accumulation, GIS data (Overton et al., 1999) provided floodplain elevation data at vertical intervals of 100 mm (Figure 3). The original data were created on a 30x30 m pixel scale, and were laid over the model cells using a weighted average method. The MODFLOW model was discretised into a regular, square grid containing 34224 cells of dimensions 50x50 m (Figure 4), with constant head cells on the outer boundaries. Constant head elevations were 12.5 m, 13.9 m, 15.2 m and 12.5 mAHD, starting from the north-western corner and progressing clockwise around the corners of the model domain. The river boundary was also simulated using a constant head.

A conceptual model shows the geological layers used as well as recharge and evapotranspiration rates (Figure 5). On the highland, the upper, unconfined aquifer was found in the Upper Loxton Sands (Kh = 5 m d-1), which was underlain by the Lower Loxton Sands (Kh = 0.2). On the floodplain, the Coonambidgal Clay (Kh = 0.1) forms a partially confining layer for the alluvial aquifer found within the Monoman Formation (Kh = 10), which is again underlain by the Lower Loxton Sands. Water from irrigation recharge mixes with regional groundwater and forms a groundwater mound beneath the highland irrigation area. The water table rises under the floodplain, leading to seepage at the break of slope, increased evapotranspiration across the floodplain and higher baseflow to the river.

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Figure 3. Floodplain elevation distribution, showing surface elevation varying between 12 to 15 m. Low lying areas are found at the ends of the floodplain peninsulas and in the centre of the floodplain associated with the saline backwater.

Figure 4. Diagram of the floodplain model showing the highland area, irrigated and non-irrigated, the River Murray, Clarks Floodplain and other floodplain areas. Model discretisation may be inferred from the pixel sizes of the river and irrigation area.

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Figure 5. Cross-section of the conceptual model showing the partially confined Monoman Sands alluvial aquifer and the highland Upper Loxton Sands Formation that form the main groundwater flow path toward the river (not to scale). Note the intersection of the ground surface by the watertable at the break of slope, resulting in seepage.

A steady state model was run for current irrigation conditions, with four scenarios intending to show the effects of changing the extinction depth and including floodplain elevation data on the spatial patterns of groundwater discharge. These scenarios were:

• ET extinction depth of 2 m with flat floodplain elevation of 14 mAHD, the overall average floodplain elevation

• ET extinction depth of 4 m with flat floodplain elevation of 14 mAHD.

• ET extinction depth of 2 m with variable floodplain elevation from the GIS coverage, which varied between 12 and 15 mAHD (Figure 3); and

• ET extinction depth of 4 m with variable floodplain elevation from the GIS coverage, which varied between 12 and 15 mAHD.

An extinction depth of 2 m was used for the remainder of the modelling.

The floodplain was divided into a series of flow zones (Figure 1) in order to compare characteristics of the floodplain within each zone. That is, comparing mean and minimum floodplain elevations, localized elevation at the break of slope, width of floodplain and convergence of the flow zone, with the seepage, evapotranspiration and baseflow to the river. The flow zone ends were defined by the edge of floodplain and river, and the adjoining edges by groundwater flow lines taken from the MODFLOW output. The flow lines were defined as the flow path that groundwater took from a set of equally spaced points along the edge of the floodplain to the river, and were determined by generating a set of equipotential contours with groundwater flow being perpendicular to these and toward the river. This minimized groundwater flow between flow zones. Zonebudget (Harbaugh, 1990) was used to perform water balance calculations for each of these zones.

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Floodplain elevation was represented by the mean and minimum elevations of each of the zones above the height of the river. The river level did not change significantly for the length of the river that bordered Clarks Floodplain, and for the purpose of the study was defined as 10.5 mAHD. Floodplain width was defined as the length of the travel path of a particle starting in the centre of the flow zone at the cliff and finishing at the river at a point defined by the hydraulic gradient. This length was calculated from the floodplain groundwater contours in the same way as the zone boundaries. Calculating the number of cells within each flow zone, and multiplying by the cell area determined floodplain surface area. The convergence or divergence of a flow zone was calculated from the boundary length of the flow zone bordering the river divided by the width of the flow zone at the break of slope. Convergence was therefore dependent on river sinuosity.

Holland et al. (2002) developed a two-dimensional analytical slice model, to determine the proportions of groundwater discharged as seepage, evapotranspiration and baseflow to the river for floodplains on the River Murray. The model provided a close match with results from a two-dimensional MODFLOW model, which incorporated a flat floodplain approximation. The model has been used to predict seepage and salinisation risk on a regional scale for the lower River Murray in the context of setting priorities for salinity management. This report aims to discern whether this and similar two-dimensional models are adequate for use in finer-scale, sub-floodplain investigations.

Using the inflow (Qin), zone width (w), aquifer thickness (b) and average floodplain elevation (hf) from each of the flow zones, and the same hydrogeological parameters from the three-dimensional MODFLOW modelling, the Holland et al. (2002) slice model was used to predict the seepage, evapotranspiration and baseflow for each of the flow zones. Seepage and evapotranspiration results from both slice and 3-D models were compared.

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RESULTS

The term groundwater ‘discharge’ is defined in the remainder of this report by the sum of both evapotranspiration and seepage, that is, the total volume of water discharged from the floodplain to the atmosphere.

Plotting groundwater discharge (in mm/yr) across the floodplain provided significantly more information than the conventional watertable depth threshold of 2 m. Not only could areas at risk of salinisation be identified, but the rate of salinisation was also more quantifiable.

The MODFLOW groundwater flow model predicted discharge of groundwater predominantly from the back of the floodplain, near the break of slope where the depth to groundwater was at a minimum (Figure 6). Vegetation health mapping undertaken by PPK Environment and Infrastructure (1998) shows a concentration of trees with poor health around the backwater on Clarks Floodplain (Figure 1), which corresponded with areas of high groundwater discharge (Figure 6).

The four different scenarios in Figure 5 are modelling results from the combinations of two parameters – extinction depths of either 2 m or 4 m, and a floodplain representation using actual elevation (12 to 15 mAHD) or by a level 14 mAHD surface. The extinction depth of 2 m predicted discharge only in areas near the cliff, while the 4 m extinction depth model predicted discharge across the entire floodplain. While elevation information did not significantly change the coarser-scale distribution of discharge on the floodplain, it provided finer-scale predictions for the rate of discharge and revealed a number of areas of higher discharge on the southern edges of the peninsulas and near the wetland in the centre of the floodplain. This suggests that the model is more sensitive to extinction depth when predicting the total volume of discharge, but fine-scale discharge patterns are provided by detailed elevation information.

Seepage

From the modelling scenario using actual elevation data and an extinction depth of 4 m, seepage was predicted to occur at the break of slope between the floodplain and the highland where the water table intersected the ground surface. Seepage was greatest in the area of the saline backwater, which corresponded to the points of lowest elevation along the cliff. These predictions match with observed seepage at the site.

There was very little correlation between the fraction of inflow from the highland expressed as seepage and the mean floodplain elevation for each of the flow zones. Comparison with the mean elevation of a 100 m wide strip of floodplain at the break of slope where groundwater heads are highest however, gave a significant inverse correlation (R2 = 0.82; Figure 7). Under the hydraulic conditions given by the irrigation scenario that was modelled, seepage was found only in areas with an elevation less than a threshold value of 3.6 m above river level.

Evapotranspiration

Evapotranspiration, normalized by the inflow from the highland evapotranspiration was found to have a positive relationship with the floodplain width (R2 = 0.75; Figure 8). For this correlation, data from only eight of the flow zones were used. The additional three zones did not register any discharge at all, as the inflow was too low to raise the groundwater above the extinction depth, therefore they were therefore excluded from the analysis.

Within each of the flow zones the surface area of floodplain was compared with the zone evapotranspiration, and found not to correlate well (R2 = 0.38). Due to the river meanders,

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the flow zone surface areas were quite polarized, with zones associated with floodplain peninsulas (zones 1, 7 and 8) having very high areas compared with small areas for those cells on narrower parts of the floodplain. As discharge was concentrated at the back of the floodplain, the peninsulas contributed very little to the total discharge, but significantly increased the total zone areas.

The evapotranspiration normalized by inflow from the highland had a less significant negative relationship with the mean elevation above the river level for each of the flow zones (R2 = 0.34; Figure 9). For the current irrigation and hydraulic gradient conditions, evapotranspiration only occurred in flow zones with a mean elevation less than 3.75 m above the river level. Dimensionless evapotranspiration did not correspond at all with minimum flow zone elevation.

Plotting convergence (outflow to river boundary length/inflow boundary length), against evapotranspiration and baseflow gave R2 values of less than 0.3, with no clear correlation. Data appeared scattered and a linear trend line that was plotted for the data was close to horizontal.

Comparison with the Two-Dimensional Slice Model

Figure 10 compares the dimensionless total discharge, that is, evapotranspiration plus seepage divided by the total inflow from the highland, on a zone-by-zone basis for both the three-dimensional MODFLOW model and the two-dimensional slice model outlined in Holland et al. (2002). The Holland et al. (2002) model was found to over predict for the northern part of the floodplain, or flow zones 3-8, and under predict for the southern zones, flow zones 9 and 10.

When the total discharge from each of the flow zones was compared, however, the rates of seepage, evapotranspiration and baseflow as a percentage of inflow from the highland were within 1% of each other (Table 3). Table 3 displays both absolute values of seepage, evapotranspiration and baseflow from the floodplain, and each of these factors normalized against the total inflow from the highland. It compares the total and normalized seepage, evapotranspiration and baseflow for both of the models on an entire floodplain basis.

Disparities exist between the results of two-dimensional flat floodplain and the quasi-three-dimensional variable elevation modelling on a sub-floodplain scale. However the averaging effect, combined with the linear evapotranspiration approximation, smears these disparities so that there is very little difference between sets of results on the total floodplain scale. The extent to which this is a result of averaging rather than coincidence in all cases where evapotranspiration is being modelled warrants further investigation.

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a

b

c

d

Figure 6. Contour maps of evapotranspiration from Clarks Floodplain for (a) varying floodplain elevation, 2 m extinction depth, (b) varying floodplain elevation, 4 m extinction depth, (c) constant floodplain elevation, 2 m extinction depth, (d) constant floodplain elevation, 4 m extinction depth. Note that seepage is occurring at the break of slope between the highland and floodplain. Seepage is assumed for discharge values higher than 150 mm/yr.

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Figure 7. Comparison between seepage flow normalized by highland inflow, and mean elevation above river level at the break of slope. Note the seepage threshold at approximately 3.6 m, above which seepage ceases.

Figure 8. Comparison of evapotranspiration (ET) normalized by highland inflow, and floodplain width for each flow zone.

Figure 9. Comparison of evapotranspiration (ET) normalized by highland inflow, compared with mean height of floodplain above river level for each flow zone. Note that the outlying points ‘a’ and ‘b’ corresponded with flow zones with elevation distributed predominantly at the break of slope and edge of river respectively, leading to higher and lower discharge than predicted.

Figure 10. Comparison of dimensionless discharge for both the MODFLOW modelling and the two-dimensional slice model of Holland et al. (2002).

a

b

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DISCUSSION

Identifying areas of salinisation risk

Representing salinisation risk in areas of shallow saline groundwater with evapotranspiration or groundwater flux rather than depth to groundwater allows the direct comparison of potential risk to areas with different soil types, as they already incorporate the effects of varying capillary fringe depth. Rates of salinisation can be determined once groundwater salinity has been incorporated, rather than limiting the area to the polar threshold classifications of ‘at risk’ or ‘not at risk’. Although groundwater discharge is difficult to measure on an in-situ basis, with bore log information a theoretical depth-discharge curve may be developed, and evapotranspiration inferred from groundwater depth.

The inclusion of surface elevation in groundwater flux modelling allows salinisation to be predicted with greater detail. Comparing Figures 6a and 6c with an extinction depth of 2 m, there is little real distinction between the spatial distribution of discharge in each figure, apart from within flow zone 7 in the vicinity of the backwater. If the extinction depth is increased to 4 m, however, as shown in Figures 6b and 6d, the difference in discharge distribution is more obvious, particularly at the ends of the floodplain peninsulas. In both cases, the modelling results that incorporated elevation data led to discharge being focused toward the depression associated with the backwater, but higher discharge rates from other low-lying areas closer to the river were also identified (Figure 6b). Primarily the extinction depth was the dominant variable, which determined the extent of groundwater discharge on a floodplain scale and gave a coarse representation of its spatial variation (Figures 6c and 6d), while the distribution of surface elevation determined the finer-scale spatial patterns of discharge on a sub-floodplain scale (Figures 6a and 6b).

While the 2 m extinction depth was used as a base rate for the remainder of the modelling as it provided a realistic total evapotranspiration volume, it is acknowledged that this depth did not predict discharge out to the edges of the floodplain. For work that requires an understanding of the spatial variability of evapotranspiration and seepage, the linear depth-discharge relationship used by MODFLOW 96 is not sufficient to predict this information accurately, and a user-defined, site-specific curve is necessary, such as that defined in the ETS1 package of MODFLOW 2000.

Table 2. Comparison of total seepage, evapotranspiration (ET) and baseflow for the two- and three-dimensional models a

Holland et al. (2002) MODFLOW quasi 3-D (m3/d) (% of inflow) (m3/d) (% of inflow) Seepage 111.6 9.3% 106.9 8.7% ET 72.2 6.0% 80.0 6.5% Baseflow 1018.6 84.7% 1046.1 84.8% Total 1202.4 100% 1233.0 100% a The fractions are divided by the inflow from the highland of 1201m3/d.

Dependence of salt accumulation on floodplain elevation

Salt accumulation through evapotranspiration of groundwater is sensitive to depth to watertable (1). In the homogeneous situation, discharge per unit area will be dependent only on the difference in elevation between the soil surface and the groundwater. All other characteristics of the floodplain and hydraulic conditions will affect discharge rates only through changing the depth to watertable.

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If groundwater levels are unknown however, depth to watertable may be approximated by the floodplain elevation above river level in order to predict spatial patterns of discharge, where river level is a base groundwater elevation. In situations where groundwater flow is to the river, this forms a best-case scenario.

There should, therefore, be a strong correlation between evapotranspiration from a zone and its mean elevation, but Figure 7 shows a low R2 value. Most currently used groundwater models including MODFLOW 96 approximate evapotranspiration as a linear function of depth to groundwater. Warrick (1988), Gardner and Fireman (1958) and Thorburn et al. (1992) show that the relationship between depth and evaporation in soils is an inverse power relationship. When using a linear equation, to preserve the total discharge as a true representation of field values, a smaller extinction depth must be used, making the results spatially erroneous. While this function gives an accurate prediction of the total evapotranspiration on the floodplain, it tends to under predict discharge in some areas such as those closer to the river with deeper groundwater.

For each flow zone, areas with low elevation may be distributed differently between the high and low groundwater elevations at the cliff and river edges of the floodplain respectively. Zones with a significant proportion of low-lying land near the cliffs, for example those containing the saline backwater, will have a higher rate of evapotranspiration than those where all the low-lying land is near the river. Topography therefore has a significant role in determining the extent and distribution of evapotranspiration and seepage on a floodplain, particularly in areas where the watertable is shallow.

Dependence of seepage on local elevation

Seepage of groundwater on Clarks Floodplain was a result of the groundwater elevation at the break of slope exceeding the floodplain elevation in that area. Constriction of the aquifer from the unconfined highland into the floodplain aquifer, which is partly confined by the Coonambidgal Clay, also encouraged seepage from the Upper Loxton Sands layer present above the saline backwater. The seepage and mean elevation at the base of the cliff were localized parameters, of a much smaller scale than the floodplain, therefore they correlated well compared with evapotranspiration and the mean regional elevation, which were predicted across the entire floodplain. Apart from changes in hydraulic head, and therefore depth to groundwater, no other geometric factor significantly affected the seepage rate from the floodplain.

In heterogeneous situations, however, seepage is not so simply quantified. On inspection of the bore logs taken from the central, longitudinal transect of the saline backwater, it was found that the Coonambidgal Clay layer extended 2 m deeper into the aquifer than in the surrounding floodplains. This suggested that the backwater was a palaeo-river channel now partially filled with deposited clay. As a result, the Monoman Sand aquifer in this area was reduced in thickness by approximately 30%. It is thought that this geomorphological constriction would further augment seepage by reducing the aquifer capacity as it reached the floodplain. As clay-filled palaeochannels and backwaters are common phenomena in meandering river channels, geomorphological constriction may significantly affect the extent of seepage on floodplains in lower reaches of regulated rivers.

Effects of floodplain width, area and proximity to the river on floodplain evapotranspiration

Wider sections of the floodplain resulted in a longer groundwater travel time from the highlands to the river and gave a greater area for discharge to take place. As the hydraulic head was essentially fixed at both the floodplain edge and the river, longer flow paths reduce the hydraulic gradient and therefore the groundwater velocity. Groundwater with a longer residence time in the floodplain was consequently subject to evapotranspiration for longer

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periods of time. There was therefore a strong correlation between floodplain width and the total discharge from the flow zones.

As shown in Figure 6, the majority of groundwater discharge on floodplains tends to occur at, or close to, the geologic aquifer constriction that occurs as groundwater passes into the river valley as the groundwater inflow exceeds the capacity of the aquifer to transport flow at this point.

Flow zone surface area correlated visually with evapotranspiration, but statistically had a weak R2 value. The low correlation is thought to be because the highest proportion of evapotranspiration is found close to the cliffs. Major increases in floodplain area occur closer to the river as river meanders are incorporated into the flow zone (ie zones 7 and 8 in Figure 1). The lower watertable closer to the river reduces the effect of the floodplain area on evapotranspiration.

Effects of groundwater flow convergence on evapotranspiration

As flow zones diverge, the cross-sectional area in the direction of flow is enlarged, and the flow transport capacity of the aquifer is increased. As a corollary, converging flow zones will have decreasing aquifer carrying capacity toward the floodplain, forcing the water table higher toward the soil surface. Groundwater discharge should therefore be higher in converging flow zones.

Modelling showed that while the lower groundwater level in zones with diverging flow lines, such as those associated with floodplain peninsulas, led to a decrease in the evapotranspiration rate, the longer flow paths allowed a greater groundwater residence time during which evapotranspiration could take place. The effects of longer flow paths and larger floodplain areas in diverging zones led to higher evapotranspiration rates, which overwhelmed any effects of the lower groundwater levels. Due to these competing effects, flow zone convergence and divergence had only a weak influence on overall evapotranspiration.

Effectiveness of 2-D slice models for discharge prediction

On an entire floodplain basis, the two-dimensional, flat floodplain, slice model predicted total discharge in Clarks Floodplain adequately, as shown by the small disparity between the model results in Table 3. On a zone scale, however, the slice model did not perform as favourably. For flow zones that incorporate the backwater and therefore have low elevations at the base of the cliff (zones 7, 8, 9, 10), the Holland et al. (2002) model under predicted evapotranspiration and seepage as it did not take into account the distribution of elevation from the edge of the floodplain to the river. The over-prediction of discharge from flow zone 7 was a result of the depression at the edge of the river. It had no impact on discharge in the MODFLOW model, but in the Holland et al. (2002) model, this low lying region reduced the average elevation of the flow zone, and therefore led to discharge being predicted for the cell. Models without correct elevation distributions variation will not accurately predict seepage in these areas.

Although two-dimensional slice models do not consider convergence and divergence of flow lines, they do take into account floodplain width. Evapotranspiration was more appreciably related to floodplain elevation and width than convergence of flow zones. If calculations of evapotranspiration are sensitive and important, then the increased area for discharge should be taken into account in some way, for example using a discharge function that incorporates the width of the flow zone.

Slice models can provide an accurate prediction of the total amount of evapotranspiration and seepage from a floodplain if they have accurate data for hydrological parameters and

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flow lengths. However, to determine the distribution of this discharge, elevation data are required. In order to predict vegetation health, slice models must take into account fluctuations in floodplain microtopography and the length and width of flow paths, or they will not be adequate for estimating evapotranspiration and seepage, which drive salt accumulation.

The type of groundwater flow models chosen will invariably depend on the scale of the investigation and the questions being examined. Large scale, regional projects involving comparison at an inter-floodplain level do not require the same level of detailed information as a study comparing parameters at an intra-floodplain scale.

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CONCLUSIONS

This report has discussed the importance of geometric and elevation factors in determining the spatial distribution of discharge from a floodplain. It used a quasi-three-dimensional MODFLOW model to confirm the critical parameters required to predict groundwater discharge patterns affecting vegetation health, and compared these results with a two-dimensional slice model.

The key findings from this study are:

• Representing groundwater discharge as a flux rather than by depth to groundwater allows it to be predicted independently of soil type. Inclusion of groundwater salinity data would then allow the rate of salinisation to be predicted as a more appropriate alternative to a threshold ‘risk’ or ‘no risk’ classification.

• To assess the magnitude of the impact on floodplain biodiversity and ecological value, groundwater modelling needs to take into account spatial patterns of groundwater discharge and rates of salinisation on a fine scale.

• Elevation is the most critical parameter in determining both seepage and evapotranspiration from a floodplain. Geomorphic features such as backwaters, lagoons and palaeochannels, which are commonly found in meandering river systems, can produce elevation changes that significantly effect the spatial distribution of discharge.

• Floodplain width is critical in determining the proportion of evapotranspiration from a floodplain. It is therefore imperative that two-dimensional slice models give a good representation of the true groundwater flow path with an accurate measure of length.

• Divergence of flow zones and floodplain width had competing effects on the rate of discharge from the floodplain. While divergence increased the aquifer capacity and allowed the watertable to drop, the greater width of floodplain on the inside of meanders led to an overall increase in discharge.

The use of two-dimensional or three-dimensional groundwater flow models is fundamentally a question of scale. For regional, large-scale studies comparing salt balances on an inter-floodplain basis, slice models such as the Holland et al. (2002) model can give good predictions of discharge provided there is adequate data on elevation and floodplain width. For work that requires small-scale, detailed predictions of groundwater discharge or comparisons on an intra-floodplain level, a three-dimensional or quasi-three-dimensional groundwater flow model with small cell sizes is required.

Further refinement and investigation of various other floodplain case studies will confirm the generality of the relationships developed in this report. The relationship between evapotranspiration and convergence of flow lines, specifically as a function of river meander characteristics, could be clarified using a mathematical approach. Additional work is also required to confirm that the ‘smearing’ effects of averaging elevation and other evapotranspiration parameters across a floodplain are valid in all cases.

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ACKNOWLEDGEMENTS

The first author wishes to acknowledge the support of Land and Water Australia and the Centre for Groundwater Studies. The work was also supported through the project ‘Assessing Current and Future Impacts of Land Management Induced Groundwater Discharge on Floodplain Health’, which is a partnership between the River Murray Catchment Water Management Board (RMCWMB), CSIRO Land and Water and the South Australian Department of Environment and Heritage, and Department for Water, Land and Biodiversity Conservation (DWLBC). The project is being funded through the Natural Heritage Trust and by the project partners, and managed by RMCWMB.

Dr Anthony Barr and Wei Yan are thanked for useful suggestions and comments that improved the manuscript.

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