Water stress based deficit irrigation scheduling using RZWQM2 model for maize in Colorado

Abstract. Water productivity is of increasing importance compared to high yield in areas with large water demand but limited water supply like Colorado USA. In this study, model simulations were carried out to investigate the responses of crop yield and water productivity under various irrigation treatments in a maize (Zea mays L.) field in Colorado in 2008-2010. RZWQM2 (Root Zone Water Quality Model) was used in the simulation and the scenario has been well calibrated in a prior study by Ma et al. (2012). The irrigation was triggered when the model simulated water stress (WS) was lower than a fixed WS threshold (WST) and water amount to apply was calculated as a ratio (K) of the water required to replenish the current soil water content to field capacity. Four days forecast precipitation was detracted based on the previous calculation and the minimum irrigation amount was set as 1cm to remove the unwanted WS on that day. Various WST and K combinations were simulated, with WST and K ranged from 0.3 to 1.0 with an interval of 0.05, and 0.1, respectively. Results showed that the proposed irrigation scheduling method was able to save water compared with the ET-based method conducted in the field while maintain the yield, once WST was above 0.6 for the studying area, which results the increase of irrigation water use efficiency (IWUE). A lower value of K was encouraged in terms of water saving potential, whereas it was set as above 0.6 for convenience in practice. The proposed irrigation scheduling method minimized the deep seepage and soil evaporation, as well as soil water content after crop reproductive stage without reduction in crop yield. Therefore, a higher IWUE is achieved. This study suggests that the irrigation scheduling method based on RZWQM2 simulated water-stress can be used as an irrigation management tool both in full and deficit managements. Further test of this system in a field experiment will be conducted.