On-farm Evaporation Basins in South Eastern Australia: Effects of Irrigation/Drainage System Management and Climate Variability

Adverse downstream environmental impacts of saline subsurface drainage water from irrigation areas have led to increasing pressure to retain drainage effluent on-farm. In the Murrumbidgee Irrigation Area (MIA) of south eastern Australia, this has led to the development of on-farm evaporation basins. The size, design and management of these basins has largely been left to the discretion of the farmer. This paper outlines the use of BASINMAN, a water balance model that links the farm sub-surface drainage system with an evaporation basin situated on farm. In the model, all subsurface drainage water from the farm is pumped to the basin, when the basin is full, subsurface drainage stops until the water level in the basin falls. This is when farm waterlogging is likely to occur. Utilising a 35 year climate record the model was used to determine the factors with greatest impact on farm water tables and the required basin area in such a farm/basin system. Key outputs from the model are daily farm water table depths, and daily basin water depths. When running a simulation using a single climate data set the greatest impact on the farm water table was found to be from parameters that control water entering or leaving the farm/basin system. Irrigation efficiency, which determines the amount of excess water entering the water table and hence needing disposal, is one such parameter, as is regional ground water inflow or outflow, which again affects the amount of water requiring disposal. The effect of extreme rainfall years was also investigated to determine if these periods adversely affected the farm/basin system and what design/management measures were required for such periods. Interestingly there was not a good correlation between annual rainfall and farm waterlogging. Waterlogging predominantly occurred in autumn/winter/spring when evapotranspiration (ET) was low, and increased when late irrigations were applied in autumn, or the water table control depth was shallow. In these circumstances the soil volume has little capacity to store rainfall without causing waterlogging. These results indicate that periods of farm waterlogging are more likely to be due to management than extreme rainfall periods. High basin water levels are most likely to occur during periods of high water tables and are therefore also a result of high rainfall, groundwater inflow and poor irrigation/drainage management. Periods of time when basins are full can be reduced with appropriate basin sighting and good irrigation/drainage management. This way the necessity of releasing water from on farm storage during high rainfall periods can be reduced.