Comparing the water use metrics of just-in-case, just-in-time and justified irrigation strategies using a scenario-based tool

Efficient water use is an important outcome of effective irrigation scheduling strategies. We developed a physically-based hydrology tool IrriSET (Irrigation Strategy Evaluation Tool) to simulate the performance of different scheduling strategies against three water use metrics - amount of irrigation used, drainage resulted from irrigation, and duration root zone soil water (crop soil water reservoir) was held within optimum pasture growth conditions during the irrigation season. IrriSET was tested on an irrigated dairy farm in Canterbury, New Zealand, simulating the daily soil water balance using observed and estimated data on rainfall, evaporation, root zone soil water holding capacity, irrigation supply reliability, irrigation infrastructure limitations (how quickly and how much irrigation could be applied per event), and irrigation application efficiency (designed versus actual application) from eighteen irrigation seasons for three scheduling strategies – ‘just-in-case', ‘just-in-time', and ‘justified’. Just-in-case, a rostered, supply-based irrigation strategy, resulted in the most irrigation applied per season and drainage generated, and the least duration of time soil water held in the optimum pasture growth condition. Soil water demand-based just-in-time, and demand- and weather-forecast based justified irrigation strategies resulted in similar water use metrics, though the latter consistently and marginally outperformed the former. Even during irrigation seasons with below normal rainfall and average irrigation supply conditions, though the demand and supply-based strategies used similar irrigation amounts, the latter held soil water in the optimal growth range longer than the former. A strategy that proactively manages irrigation based on supply (irrigation water availability and forecast rainfall) and demand (soil water condition) has been shown to be environmentally sustainable and positively influential of soil water availability within root zone, and thus productivity. •During below-normal rainfall seasons, supply- and demand-based scheduling strategies used similar irrigation amounts.•Supply-based irrigation strategy consistently generated more drainage than demand-based strategies.•On-farm, plant available water influenced the choice of irrigation scheduling strategy.•Off-farm, supply reliability and rainfall influenced the choice of irrigation scheduling strategy.