EVAPOTRANSPIRATION AND YIELD OF BEANS AS AFFECTED BY MULCH AND IRRIGATION

Mulch (crop residue) has been reported to increase soil water storage and crop yield, and to reduce soil water stress under some conditions. However, few studies have had irrigation, and thus evapotranspiration (ET), as a variable where soil water was controlled. Therefore, a study was conducted to evaluate the effect of mulch on ET, bean yield (Phaseolus vulgaris L.), and water use efficiency (WUE), under a wide range of irrigation levels. Treatments were two cultivars (Viva-Pink and Red-63), two mulch rates (bare, and 7.0 Mg ha-1 of wheat straw), and six irrigation levels (from 0.0 to 288 mm) for 2 yr. Different irrigation levels were achieved by a line source sprinkler. Evapotranspiration was estimated from measurements of irrigation, precipitation, soil water depletion, and estimated drainage. Transpiration (Tr) was estimated from a function of relative yield and relative ET. Dry matter and seed yields were significantly greater for mulched than for bare plots. Mulched plots had a higher WUE (yield/ET) than did bare plots for a given irrigation level but increased as irrigation level increased. Seasonal differences in ET between bare and mulched plots were small. The yield-ET relation for mulch was linear but was distinctly different from bare soil, indicating a different partitioning of ET into soil water evaporation (Es) and Tr. For the conditions of this experiment, mulch reduced Es by about 45 mm, at the same ET, and Tr was increased by 45 mm. However, for the same irrigation level, ET was lower for mulched than for bare plots, indicating that not all of the water saved went to Tr.