Evaluation of crop water stress index for LEPA irrigated corn

This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zen mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleusto ll) during the 1992 growing season at Bushland, Tex. The effects of six dif ferent irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm inc rements to maintain the soil water in the 100% treatment within 60-80% of t he "plant extractable soil water" using LEPA technology, which wets alterna te furrows only. The 1892 growing season was slightly wetter than normal. T hus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, w ater use, and water use efficiency of fully irrigated corn were 1.246 kg/m( 2), 786 mm, and 1.34 kg/m(3), respectively. CWSI was calculated from measur ements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A "non-water-stress ed baseline" equation for corn was developed using the diurnal infrared can opy temperature measurements as T-c-T-a= 1.06-2.56 VPD, where T-c was the c anopy temperature (degrees C), Ta was the air temperature (degrees C) and V PD was the vapor pressure deficit (kPa). Trends in CWSI values were consist ent with the soil water contents induced by the deficit irrigations. Both t he dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 o r less for corn. The CWSI was useful fur evaluating crop water stress in co rn and should be a valuable tool to assist irrigation decision making toget her with soil water measurements and/or evapotranspiration models.