TESTING OF A WATER LOSS DISTRIBUTION MODEL FOR MOVING SPRINKLER SYSTEMS

Abstract

Field water balance measurements using monolithic lysimeters were used in validating the Cupid-DPE modelfor predicting water loss partitioning during sprinkler irrigation from a moving lateral system fitted with impactsprinklers and spray nozzles. The model combines equations governing water droplet evaporation and droplet ballisticswith a comprehensive plant-environment energy balance model. Comparisons indicate good agreement between measuredand modeled transpiration, and the measured and modeled soil evaporation during the day of irrigation. Total predictedevapotranspiration during the day of irrigation was greater than measured totals using the monolithic lysimeters.However, part of this difference was because the lysimeters could not measure water use during irrigation. Totalmeasured and predicted evapotranspiration agreed well for the day following irrigation. Predicted soil evaporation ratesmatched well for the period immediately following irrigation, and cumulative soil evaporation was nearly identical to themeasured total through the end of the next day. During irrigation, the main water loss was shifted from transpiration toevaporation of the wetted-canopy. For equal application volumes, the duration of this effect was greater using impactsprinklers due to the greater wetted diameter and lower average application rate compared to spray nozzles. Predictedwater flux rates during irrigation were up to 50% greater for canopy evaporation than for transpiration rates predictedimmediately prior to the start of irrigation. Canopy evaporation amounted to 69% and 63% of the total predicted wateruse during impact and spray irrigation, respectively. It also was 0.69 and 0.28 mm greater, respectively, than thepredicted transpiration total during this same time span assuming no irrigation had been applied. About 13 and 5% of thewater applied by overhead sprinkling was evaporated or transpired during impact and spray irrigation, respectively.However, the net increase in predicted water loss during irrigation was only 5.8% and 2.4% of the irrigated water depthapplied for the impact and spray cases, respectively, because transpiration and soil evaporation would have occurredeven without irrigation. Although droplet evaporation represented less than 1% of the total water loss for the day usingeither type of sprinkler, irrigation water did influence the energy transfer between the plant-environment and waterdroplets during flight, on the canopy, and the soil.