Comparing deep drainage estimated with transient and steady state assumptions in irrigated vertisols
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Chloride mass balance (steady state or transient state) models are used extensively in Vertisols of Queensland and New South Wales (NSW) in Australia to estimate deep drainage. The aim of this study was to compare deep drainage estimated assuming steady state and transient state conditions with chloride mass balance models in irrigated cotton (Gossypium hirsutum L.)-based farming systems in the lower Namoi Valley of North Western NSW. Drainage was estimated at seven sites, and treatments included rotation crops such as wheat (21-62 mm/year) (Triticum aestivum), sorghum (12-47 mm/year) (Sorghum bicolor) and dolichos (12-21 mm/year) (Lablab purpureus), minimum tillage (62-83 mm/year), where cotton was sown into standing wheat stubble, and conventional tillage where stubble was incorporated (35-78 mm/year). Soil water content was measured with a neutron moisture meter in the 0.2-1.2 m depth. Soil was sampled before sowing and after harvest to a depth of 1.2 m along diagonal transects. The soil chloride concentration was determined by titration with AgNO3. Irrigation water was also analysed for chloride. The deep drainage estimates were compared using regression analysis and students paired t-test. In addition, a paired t-test of the soil chloride concentration before sowing and after harvest was used to determine if the soil chloride flux was either in a steady state or transient state. In 9 out of the 13 data sets (69%), drainage estimated with the models agreed with changes between pre- and post-season soil chloride concentrations. Under frequently irrigated summer crops such as cotton and sorghum and in better structured soils chloride flux reached steady state conditions whereas under partially-irrigated crops or where soil structure was poorer, the chloride flux deviated markedly from steady-state conditions. The latter observation may be due to preferential flow via deep cracks in infrequently irrigated soil. Deep cracking would be due to the more intense shrinking and swelling in partially irrigated soil in comparison with frequently-irrigated crops. Comparison of estimated deep drainage with pre- and post-season soil chloride concentrations showed that the steady state mass balance model best estimated deep drainage under cotton crops which were irrigated more frequently or wheat crops which had better soil structure.