EFFECTS OF CROP RESIDUE ON SOIL AND PLANT WATER EVAPORATION IN A DRYLAND COTTON SYSTEM

Dryland agricultural cropping systems emphasize sustaining crop yields with limited use of fertilizer while conserving both rain water and t he soil. Conservation of these resources may be achieved with manageme nt systems that retain residues at the soil surface simultaneously mod ifying both its energy and water balance. A conservation practice used with cotton grown on erodible soils of the Texas High Plains is to pl ant cotton into chemically terminated wheat residues. In this study, t he partitioning of daily and seasonal evapotranspiration (E(t)) into s oil and plant water evaporation was compared for a conventional and a terminated-wheat cotton crop using the numerical model ENWATBAL. The m odel was configured to account for the effects of residue on the radia tive fluxes and by introducing an additional resistance to latent and sensible heat fluxes derived from measurements of wind speed and vapor conductance from a soil covered with wheat-stubble. Our results showe d that seasonal E(t) was similar in both systems and that cumulative s oil water evaporation was 50% of E(t) in conventional cotton and 31% o f E(t) in the wheat-stubble cotton. Calculated values of E(t) were in agreement with measured values. The main benefit of the wheat residues was to suppress soil water evaporation by intercepting irradiance ear ly in the growing season when the crop leaf area index (LAI) was low. In semiarid regions LAI of dryland cotton seldom exceeds 2 and residue s can improve water conservation. Measured soil temperatures showed th at early in the season residues reduced temperature at 0.1 m depth by as much as 5 degrees C and that differences between systems diminished with depth and over time. Residues increased lint yield per unit of E (t) while not modifying seasonal E(t) and reducing cumulative soil wat er evaporation.