Preliminary water balance models of the Daly river catchment

Abstract

Water is a controversial topic in agriculture and future development. The clearing of land and the conversion to cropping and irrigation is currently being discussed for the region of the Daly River catchment. Proposals to develop sheep and cattle grazing country into cropping and irrigation have been put forward to the Northern Territory Government. These proposals have forced the Northern Territory Government to produce an Integrated Regional Land Use Plan to assess hydrological, biological, environmental and social impacts on the catchment and the Daly River. To assess the hydrological impacts, computer modelling was conducted in MIKE SHE on the changes in water balance. Three soil column water balance models have been produced and calibrated to simulate different land use scenarios. These scenarios include current conditions, native vegetation, and future agricultural development, improved pasture and irrigated peanuts. The analysed results from the water balance models include evapotranspiration, overland flow, crop water use and groundwater recharge. The computer modelling results were compared for the three land use scenarios. It was found that the canopy density and the root zone depth for the crop type had the greatest influence of the water balance. The canopy density controlled the level of surface exposure affecting evaporation rates. High canopy densities (high leaf area index) led to increased evaporation from the canopy while a decrease in evaporation from ponded water and from the soil. The root zone depth dictated the crops ability to draw moisture from the unsaturated zone. Shallow root depths of improved pasture and irrigated peanuts led to increased recharge, increased soil water storages and a rapid decrease in the transpiration rate (for improved pasture) during the dry season.