Catchment Salt and Water Balance Effects of Irrigation with Groundwater in Rainfed Areas

Salinisation of land and rivers is a problem of national importance in Australia. Appropriate land management options to alleviate salinisation should be chosen with knowledge of the effects of land management on stream flow, stream salinity, stream salt load and land productivity. The Management of Catchment Salinisation (MCS) modelling approach has been described in earlier work (Daamen & Hoxley, 2003). It links a one-dimensional soil water model with a groundwater model to investigate the effects of management options in study areas of approximately 50 km2. The one dimensional model is used to characterise the annual soil water balance as a function of underlying aquifer potential for all required combinations of soil, vegetation and groundwater salinity. It includes the effect of salt accumulation on plant water use. A groundwater model is then used to estimate the depth to watertable across the study area that reflects the topography, hydrogeology and the distribution of vegetation. The MCS model is used to investigate the potential effects of future land use scenarios on catchment salt and water balance. Land use scenarios that have been considered include: forest plantations, revegetation with native trees and shrubs, and development of small areas of crops (10 to 20 ha) irrigated with groundwater. This paper focuses on the development of small crop areas irrigated with groundwater and investigates the sustainability of these schemes. It also compares the reduction of catchment salt load export under irrigation development with the reduction under afforestation. The MCS model is used to compare these land management options in the Gardiner Creek catchment within the area managed by the Goulburn-Broken Catchment Management Authority in Victoria. The reduction in catchment salt load export due to the introduction of irrigated vines is calculated from model outputs to be 1.01 t/y per hectare of irrigated vines. The salt load reduction of planting trees is calculated to be 0.50 t/y per hectare of trees. Therefore the salt load reductions caused by an increase in the area of irrigated grapes is up to twice as effective as planting trees on a land area basis. A rough cost estimate for the installation of groundwater pumps is $20,000 per pump. In contrast, planting or seeding areas to native shrubs/trees is estimated at $1000/ha. Thus in the modelled example the development of 200 ha of native trees would cost $200,000 and the installation of 8 groundwater pumps to irrigate 100 ha would cost $160,000. These are costs that may be subsidised by the Goulburn Broken Catchment Management Authority to achieve a similar salt load benefit. Thus groundwater based irrigation enterprises are likely to provide a more cost effective return on public investment for salt load reduction than revegetation where suitable groundwater reserves are available.