A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District - Final Draft

Environmental degradation associated with shallow saline watertables is a major threat to the sustainability of agricultural industry throughout the Murray-Darling Basin. Located in the western part of the Murray Valley of NSW, the Wakool Irrigation District has experienced a history of water table rise, including likely contribut ions from widespread flooding. The community is interested in scientific evidence quantifying the impact of flooding on the shallow groundwater, in order to target management actions to control water table rise and salinity in this area. This study estimates the spatial and temporal impact of flooding on shallow groundwater for the Wakool Irrigation District through an extensive GIS analysis based on a large amount of piezometric data monitored over many years. By compiling the piezometric data into a GIS database and analyzing the data in a GIS application, we are able to quantify the net recharge caused by flooding and to visualize the spatial extent of the impact of flooding on the shallow water table reflected by water table change. The results show that flooding has a significant impact on the shallow groundwater. The floods during the record wet period of 1973-75 caused a net recharge of around 116x103 ML (0.52ML/ha in average) at the stage when water table rise reached its maximum value around December 1975. Apart from the magnitude of flooding, the amount of the net recharge caused by a single flood event is also related to the initial water table before the flood, which affects the shallow groundwater storage capacity. The higher the initial water table is, the less the shallow groundwater storage capacity will be, and consequently there will be less room for the net recharge, as shown during the 1973-75 floods. More frequent flooding such as the one experienced in 1981, whose recurrence interval is estimated as around 1 in 10 years, could result in 42.68x103 ML or an average of 0.19ML/ha net recharge at the stage around maximum water table mound, given the initial average water table depth being at 4.28m. There are strong connections between the local rainfall, flood, and water table change, suggesting that the floods happened in this area are normally due to both upstream and local rainfall. The major flood recharge areas within the Wakool area are mainly located along the Edward – Niemur River system. The groundwater recession following a flooding is affected by a number of factors, such as the initial water table depth, the climate conditions, the management actions, and etc.