Improving irrigation water management for crop production is becoming increasingly important in South Texas as the water supplies shrink and competition with urban centers in the region grows. Crop simulators and evapo-transpiration (Crop-ET) technologies are appealing methods for estimating crop water use and irrigation requirements because of the low investment in time and dollars required by on-site (in-field) measurement of soil and/or crop water status. This paper compares the effectiveness of the CroPMan/EPIC crop simulator and Crop-ET approaches estimating the crop water use for irrigation scheduling of the major field crops (cotton, corn and grain sorghum) from on-farm field verification trials and detailed lysimeter studies in the Texas Lower Rio Grande Valley, Coastal Bend and Winter Garden cropping regions. We discuss the implications of the use of these technologies for improving the effectiveness of irrigation and for reducing irrigation water requirements in South Texas.