Residue management for reducing evaporation in relation to soil type and evaporativity

Usual residue-management options are to remove the residue, use it as mulch with or without undercutting or to incorporate it into the soil. While the role of surface mulch in evaporation has been widely studied, the information on the effect on evaporation of mulch with undercutting or residue incorporated into soil, particularly in relation to soil type and evaporativity (E(o)) is lacking. We studied the effect of wheat straw used in various ways on the course of evaporation loss from soil columns with three soils at Ludhiana, India and one soil at Bushland, Texas, USA, under two E(o)'s. Energy-limited evaporation rates under mulch (E(om)) followed the soil-specific relation E(om)/E(o) = a e((bRes+cEo)), where Res is residue rate t/ha and a, b and c are constants; E(o) is expressed in mm/d. In an effort to model the total evaporation (CE) during the energy limited stage 'U' was obtained from appropriate CE versus time curves and (CE-U) was regressed over (t-ti)(0.5) to obtain the slope 'alpha' (Ritchie 1972) for the soil-limited evaporation stage. The observed 'U' was independent of mulch rate and E(o) but was strongly affected by soil type. Values of 'alpha' decreased with increase in mulch rate and decrease in E(o) and coarseness of soil. The otherwise short lived benefit of evaporation reduction with mulch per se, which peaked after a few days was maintained when residue was mixed with soil at the stage when evaporation reduction reached a maximum; this benefit continued for several weeks. Cumulative evaporation values computed from 'U' and 'alpha' agreed closely with the observed values under strarr mulch for loamy sand and clay loam soils and for 'undercut' and 'residue mixed' treatments on all soils regardless of E(o), and for all situations under small E(o). However, for sandy loan and silt loam soils under E(o) of 10 mm/d, the modified square root of the time function of Jalota et al. (1988) gave a better fit.