TESTING OF A WATER-LOSS DISTRIBUTION MODEL FOR MOVING SPRINKLER SYSTEMS

Field water balance measurements using monolithic lysimeters were used in validating the Cupid-DPE model for predicting water loss partition ing during sprinkler irrigation from a moving lateral system fitted wi th impact sprinklers and spray nozzles. The model combines equations g overning water droplet evaporation and droplet ballistics with a compr ehensive plant-environment energy balance model. Comparisons indicate good agreement between measured and modeled transpiration, and the mea sured and modeled soil evaporation during the day of irrigation. Total predicted evapotranspiration 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 us e during irrigation. Total measured and predicted evapotranspiration a greed well for the day following irrigation. Predicted soil evaporatio n rates matched well for the period immediately following irrigation, and cumulative soil evaporation was nearly identical to the measured t otal through the end of the next day. During irrigation, the main wate r loss was shifted from transpiration to evaporation of the wetted-can opy. For equal application volumes, the duration of this effect was gr eater using impact sprinklers due to the greater wetted diameter and l ower average application rate compared to spray nozzles. Predicted wat er flux rates during irrigation were up to 50% greater for canopy evap oration than for transpiration rates predicted immediately prior to th e start of irrigation. Canopy evaporation amounted to 69% and 63% of t he total predicted water use during impact and spray irrigation, respe ctively. It also was 0.69 and 0.28 mm greater; respectively, than the predicted transpiration total during this same time span assuming no i rrigation had been applied. About 13 and 5% of the water applied by ov erhead 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 depth applied for the impact and spray cases, respectively, because tr anspiration and soil evaporation would have occurred even without irri gation. Although droplet evaporation represented less than 1% of the t otal water loss for the day using either type of sprinkler irrigation water did influence the energy transfer between the plant-environment and water droplets during flight, on the canopy, and the soil.