Modelling the long-term potential effects of modern irrigation systems on soil–water and salt balances, and crop-water productivity in semi-arid regions

Abstract Modernization of irrigation systems is considered to improve irrigation efficiency, save water, and increase crop yields in water-scare semi-arid regions. This study conducted a long-term (10-years) simulations evaluating the potential effects of three different irrigation scenarios on soil water and salt balances, and crop water productivity of cotton-wheat cultivation in the Hakra Branch Canal command of Punjab, Pakistan. The physically based agro-hydrological model, Soil–Water-Atmosphere-Plant (SWAP) was applied to simulate the long-term (2007–2017) effects of three irrigation scenarios; (1) current surface irrigation ( baseline reference ) based on local farmers observations, (2) improved precision surface irrigation system ( PSIS ), and (3) a high-efficiency irrigation system ( HEIS ). The HEIS scenario without a leaching fraction (noted as HEIS_noLF ), defined as using sprinkler irrigation to bring the soil back to the field capacity, resulted in about 48% less long-term average irrigation needs (830 mm yr −1 ) as compared to the baseline scenario (1590 mm yr −1 ). This reduction in irrigation, however, resulted into a relatively higher average soil salt build-up (as 35 mg cm −2 ) causing a reduction of 18%—30% in the wheat crop yields. The HEIS scenario with a leaching fraction (noted as HEIS_LF ), with an additional irrigation of 60 mm at the start of crop season followed by an additional 10 mm with each irrigation interval, reduced the average salt build up (as 13 mg cm −2 ) and its adverse effects of the crop yields. However, HEIS_LF scenario resulted in the similar average irrigation amounts (955 mm yr −1 ), soil water and salt balances, crop yields and water productivity values as achieved by the PSIS scenario, defined as a fixed depth of 80 mm surface irrigation at each flexible irrigation intervals. This suggests limited scope for irrigation savings by adopting high-efficiency irrigation systems, such as sprinkler, with marginal quality (> 0.9 dS m −1 ) irrigation waters in semi-arid regions of Pakistan. Application of an appropriate leaching fraction is essential for controlling soil salinity build-up from irrigations marginal and saline groundwater in the study area. This reduces any gain to be made by high-efficiency (such as sprinkler) irrigation systems to save irrigation waters. However, there appears scope of improving surface irrigation (e.g. PSIS ) by reducing irrigation depths (through field levelling) and introducing flexible irrigation scheduling, as compared to the current (baseline) irrigation practices.