Recharge and Perched Aquifers Below Irrigation Areas: A Variable-Saturation Model

Recharge to the water table is often treated as a purely vertical process in which recharge rates and areas are identical to deep drainage rates and areas. However, this may not be appropriate where drainage rates are high and a clay layer is present in the unsaturated zone. The clay layer may reduce the initial vertical flux and lead to the development of perched aquifers.

This issue affects the salinity management of Australia’s River Murray, where irrigation in semi-arid areas has increased deep drainage rates by several orders of magnitude when compared with native vegetation. The resulting rising water table causes saline groundwater to flow into the river, increasing its salinity. Management strategies require estimates of future recharge under different conditions. Irrigation areas in Sunraysia and the Riverland are underlain by the Blanchetown Clay, which lies above the water table and alters recharge rates and locations.

This talk presents results of 1D and 2D finite element models of variably-saturated flow beneath irrigation areas where a clay layer is present. Variations in stratigraphy, sediment properties, and irrigation practices are considered. Results show (i) the conditions under which perched aquifers form and (ii) how recharge rates and areas vary over time. A comparison is made between 1D and 2D results, showing the situations in which 1D assumptions may be inadequate.