ESTIMATION OF INFILTRATION AND DEEP DRAINAGE IN A FURROW-IRRIGATED SODIC DUPLEX SOIL

Infiltration and deep drainage fluxes are difficult to measure directly in slowly permeable soils under furrow-irrigated upland crops such as maize (Zea mays L.). This paper combines a solute mass balance model and a furrow irrigation advance model to provide an estimate of these fluxes, indirectly from simpler measurements. The models were applied to a newly cleared sodic duplex soil from the Burdekin River Irrigation Area, north Queensland, where no field measures of infiltration and deep drainage were available. The study site was sown to consecutive furrow-irrigated crops after clearing. In applying these models, measures of soil and irrigation water chloride, irrigation water applied, furrow geometry and irrigation advance were required. Estimated infiltration and deep drainage decreased with distance down the furrow from 1044 and 98 mm year-1 at 50 m to 966 and 0 mm year-1 at 260 m. In an area that received an application of gypsum (20 t ha-1) prior to planting the second crop, values ranged from 1617 and 200 mm year-1 at 50 m to 1370 and 70 mm year-1 at 260 m. Infiltration did not satisfy the estimated soil water deficit unless gypsum was applied. Where gypsum was applied, infiltration exceeded the soil water deficit and deep drainage increased. Evapo-transpiration rate and maize yield also increased. If correct, these results have local and regional implications for irrigation design and management.