Application Of 1-D And 3-D Models In A Regional Context

Groundwater levels rose (an average of 10m) (mostly in 1970's and 80's) due to irrigation induced recharge to the groundwater in Coleambally Irrigation Area (CIA). Water table depth rise was a direct result of the net amount of recharge over and above the regional groundwater outflow capacity. Control of net recharge through a range of options is critical to the sustainability of CIA and requires both on-farm and regional management options for the groundwater discharge zones. There are two components to the management of recharge: The on-farm component, and the regional component. A regional groundwater model is used to look at overall targets of net recharge within the CIA. This model sets net recharge targets and is used to identify key processes and examine the effect of regional groundwater pumping on watertables and evaluate scenarios. The on-farm component captures the variability of soils, crops, and management at different parts of the region. This study is aimed at investigating regional groundwater outflow characteristics in CIA on clustered farms basis to help identify Land and Water Management Plan (LWMP) targets for water use efficiency and drainage improvement at farm level linked to the regional targets. The groundwater balance for clusters of farms and the groundwater zones was obtained by calibrating a regional groundwater model (MODFLOW) for the Coleambally area. Calibration of the groundwater model helped to quantify vertical and lateral groundwater outflow capacity on a regional and sub-regional basis. Estimates of vertical recharge were obtained by using one-dimensional water balance model called APSIM. APSIM model was applied to 18 detailed monitoring piezometer sites with known water level records scattered in 5 zones of the CIA (Figure 1) to calculate the drainage values (mm/day) for each zone. APSIM results indicate risk of salinisation in most zones. APSIM results depict only the 1-D groundwater dynamics under non-irrigated areas whereas mixed landuse were represented in the MODFLOW cells. A possible way forward is to carry out APSIM simulations for all landuses in each land management unit. However it would require detailed data for each landuse and soil type which was beyond the scope of this study. Although APSIM fluxes do not directly match the MODFLOW calibrated values, they were useful in providing a cross check on the relative magnitude. The analysis has shown that on a regional basis the vertical leakage capacity of the shallow aquifers (20,000-30,000 ML/year) needs to be matched with the overall recharge to maintain current groundwater levels. Since groundwater levels in some regions e. g. south and west (zones 3 and 4 in Figure 1) are already within the root zone there is a need to initially reduce groundwater recharge to less than the aquifer outflow capacity. These regions have the highest risk of soil salinisation. There is a need to reduce total on farm recharge to less than 0.5 ML/ha using winter cropping options as well as limited rice water use and improved water use efficiency. Drainage and reuse options should be considered for some parts of these zones. [GRAPHICS]