ROLE OF TRANSPIRATION SUPPRESSION BY EVAPORATION OF INTERCEPTED WATER IN IMPROVING IRRIGATION EFFICIENCY

Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (I-gross), as well as airborne droplets and ponded water at the soil surface evaporate before use by the crop. However, evaporation of applied water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the irrigated area as compared to a non-irrigated area can be subtracted from I-gross irrigation application losses for a reduced, or net, interception (I-net) loss. This study was conducted to determine the extent in which transpiration suppression due to microclimatic modification resulting from evaporation of plant-intercepted water and/or of applied water can reduce total sprinkler irrigation application losses of impact sprinkler and low energy precision application (LEPA) irrigation systems. Fully irrigated corn (Zea Mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (Tr) was measured using heat balance sap flow gauges. During and following an impact sprinkler irrigation, within-canopy vapor pressure deficit and canopy temperature declined sharply due to canopy-intercepted water and microclimatic modification from evaporation. For an average day time impact irrigation application of 21 mm, estimated average I-gross loss was 10.7%, but the resulting suppression of measured Tr by 50% or more during the irrigation reduced I-gross loss by 3.9%. On days of high solar radiation, continued transpiration suppression following the irrigation reduced I-gross loss an additional 1.2%. Further 4-6% reductions in I-gross losses were predicted when aerodynamic and canopy resistances were considered. Irrigation water applied only at the soil surface by LEPA irrigation had little effect on the microclimate within the canopy and consequently on Tr or ET, or irrigation application efficiency.