LAUSR

The total available water in the soil root zone (TAWr), which regulates the plant transpiration, is a critical parameter for irrigation management and hydrologic modeling studies. However, the TAWr was not well-investigated in current hydrologic or agricultural research for two reasons: (1) there is no direct measurement method of this parameter; and (2) there is, in general, a large spatial and temporal variability of TAWr. In this study, we propose a framework to improve TAWr estimation by incorporating the crop water stress index (CWSI) from canopy temperature into the Food and Agriculture Organization of the United Nations (FAO) paper 56 water balance model. Field experiments of irrigation management were conducted for maize during the 2012, 2013 and 2015 growing seasons near Greeley, Colorado, USA. The performance of the FAO water balance model with CWSI-determined TAWr was validated using measured soil water deficit. The statistical analyses between modeled and observed soil water deficit indicated that the CWSI-determined TAWr significantly improved the performance of the soil water balance model, with reduction of the mean absolute error (MAE) and root mean squared error (RMSE) by 17 and 20%, respectively, compared with the standard FAO model (with experience estimated TAWr). The proposed procedure may not work under well-watered conditions, because TAWr may not influence the crop transpiration or crop water stress in both daily and seasonal scales under such conditions. The proposed procedure potentially could be applied in other ecosystems and with other crop water stress related measurements, such as surface evapotranspiration from remote sensing methodology.