Optimising irrigation management for a spatially variable soybean field

During the last few years, water has become a limited resource in the southeastern USA. Most farmers, however, are not using any scheduling methods to optimize their irrigation applications. The objective of this study was to determine the feasibility of spatially variable irrigation management for a typical soybean field in the Coastal Plain area. The selected field (9.94 ha) had previously been delineated into five spatially variable management zones. For each of the five selected management zones, the soil water limits were estimated from particle size distribution data. Subsequently the plant available soil water was derived from these limits. A process oriented crop model, CROPGRO-Soybean, was used to determine optimal irrigation schedules for each zone, based on 25 years of local historical weather data. The resulting sets of irrigation strategies were then applied to the entire field, with special emphasis on the strategies that (1) produced the highest yield, (2) showed the earliest sign of water stress, or (3) had the largest area. The impact of spatially variable irrigation was analyzed by meeting the optimal irrigation requirements for each zone for all years. Total production, water usage by irrigation, and water drained were calculated for the entire field based on simulation results of each management zone. An economic analysis, using a price of $222.40 per 1000 kg for soybean and costs of $1.50, $2.00, and $2.50 per ha-cm for irrigation water, was also conducted. Spatially variable irrigation resulted in the best management option. Among the uniform irrigation strategies, management according to the requirements of the largest zone gave the best response. The differences between the best and worst management option were relatively small, i.e. $16/ha. Although this projected benefit is low for the small field, benefits for large center pivot irrigation systems would be attractive to farmers. The approach used in this study is an excellent method to analyze spatially variable irrigation management strategies under weather uncertainty.