Assessment of the Impact of Irrigation Management on Soybean Yield and Water Productivity in a Subhumid Environment

As an effect of climate change, a considerable increase in irrigation demand is expected in subhumid regions. This poses a future challenge for agriculture, which often does not meet the requirements for highly efficient irrigation management. Field experiments in combination with modelling approaches can help in the development and evaluation of alternative irrigation strategies. The aim of this study was to parameterize the FAO AquaCrop model for soybeans in order to evaluate irrigation systems and strategies used by farmers in Austria. Moreover, this study aims at proposing a deficit irrigation strategy to increase irrigation water productivity (WPIrrig). Three irrigation methods were investigated: sprinkler, drip, and hose reel boom. Rainfed conditions were also assessed. Data sets from 2018 and 2019 were used for the parameterization of AquaCrop. Based on threshold values for the percentage of readily available water (RAW), full and deficit irrigation strategies were defined and tested for the years 1990–2019. The parameterization results showed good agreement between the simulated and observed canopy cover and yields from the four water management practices, with Nash-Sutcliffe efficiency coefficients between 0.76 and 0.96. Large errors resulted from the soil water content simulations, mainly due to poor sensor performance. It was concluded that a moderate deficit with RAW thresholds of 150% and 140% for the vegetative and reproductive stages, respectively, was an adequate strategy to improve yields and WPIrrig. Conversely, a severe deficit with an irrigation threshold of 180% RAW throughout the season considerably reduced yields, especially in years with high irrigation demand. Moreover, a significant improvement in WPIrrig was observed when drip systems were used compared to sprinklers, due to the higher efficiency of the former. These results can help scheduling irrigation for soybeans and increasing WPIrrig, thereby reducing pressure on limited water resources.