Water and salt balances of two shallow groundwater cropping systems using subjective and objective irrigation scheduling

Abstract

Evidence suggests that, in general, subjective rather than objective irrigation scheduling decisions are adopted by farmers. Irrigators have ‘calibrated’ themselves with years of experience to irrigate subjectively according to perceived crop water requirements. This study aimed to determine the associated benefits of objective versus subjective scheduling of two shallow groundwater cropping systems. Weekly measurements included rainfall and irrigation amounts, soil water content, groundwater table depth, artificial drainage volumes, and electrical conductivity of irrigation water, groundwater and drainage water. Simulations of evaporation and transpiration were done with the SWAMP model. Based on soil water and salinity status, matric and osmotic stress during the four cropping seasons is considered unlikely. When rainfall-plus-irrigation was compared to evapotranspiration, objective scheduling resulted in an under-supply of 15%, and rainfall and shallow groundwater served as supplementary water sources. Subjective scheduling did not use rainfall efficiently as a source of water and resulted in an over-supply of 10%. Approximately 50% less salt was leached with objective compared to subjective irrigation scheduling. Under shallow groundwater conditions, irrigating subjectively according to crop water requirement results in excessive irrigation, salt addition and leaching compared to objective scheduling. Farmers can address some of the environmental problems associated with irrigation by adopting objective scheduling and reducing the leaching fraction (< 0.15) of shallow groundwater cropping systems.