SOIL AND PLANT WATER EVAPORATION FROM STRIP-TILLED COTTON - MEASUREMENT AND SIMULATION

It is estimated that 20% of cotton producers in the Texas High Plains use the strip tillage practice of planting cotton (Gossypium hirsutum L.) in residues of chemically terminated winter wheat (Triticum aestiv um L.). The effect of the residue on water use partitioning between so il (E(s)) and crop (E(c)) evaporation has not been reported. This stud y tested the hypothesis that the cumulative evapotranspiration (E(t)), of a crop in a strip tillage system and in conventional tillage are s imilar, and that the residue modifies the components of total E(t) by reducing E(s) and increasing E(c). Winter wheat was planted in the fal l of 1990 and was chemically terminated in the spring of 1991 in an Ol ton soil (fine, mixed, thermic Aridic Paleustoll) at Lubbock, TX. The plot was divided into a wheat-stubble and a conventional treatment. Re sidue on half of the plot was left as stubble and residue on the other half was shredded and incorporated. Both plots ware planted with cott on on 20 May 1991. Measurements included soil water and temperature pr ofiles, soil evaporation, weather, and crop phenology and lint yield. Input values obtained from these data were used in the energy and wate r balance model (ENWATBAL) to calculate the daily and seasonal water l ost to E(s) and E(c) for 100 d. Calculated daily E(s) varied <1 SD fro m measured values. Measured mean total E(t) were similar, i.e., 305 +/ - 20 mm for conventional and 304 +/- 41 mm for cotton in the wheat stu bble. Simulated E(t) was within 10% of the measured value. The ratio o f total E(c) to E(t) was 0.50 in the conventional and 0.69 in the whea t stubble cotton. The more efficient use of E(t) in the wheat stubble increased lint yield by 35%. The E(c) water use efficiency was not mod ified by the wheat stubble and was 3.8 g lint kg(-1) of transpired wat er. The modified version of EMWATBAL was adequate to calculate the dai ly and seasonal water use of a cotton crop in a terminated wheat stubb le, and the results support our hypothesis. The E(t) water use efficie ncy was increased by the wheat residue without additional input of wat er.