Submitted to: Transactions of the ASAE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/27/1995
Publication Date: N/A
Citation: Interpretive Summary: The use of center pivot sprinkler systems is rapidly expanding in the Great Plains of the U.S. Irrigation application methods have changed from high- pressure, high-angle impact sprinklers on the pipeline to lower-pressure spray heads on drops below the pipeline or lower-angle, lower-pressure impacts on the pipeline. This study investigated the component water application losses for pipeline impact sprinklers compared with spray heads. The impact sprinklers had a much lower application rate, but they had a much longer period of wetting. A small part (around 1%) of the applied water was predicted to evaporate in the air. The transpiration of the wetted canopy is greatly reduced and partly offsets the evaporation from the wetted canopy. The evaporation from the wetted canopy and the transpiration exceeded transpiration from a "dry" canopy (but one well supplied with water). A model was described that predicted these factors. Basically, the model agreed with the field observations and followed the dramatic reduction in transpiration when the canopy was wetted until the canopy surface dried. Soil evaporation was predicted to be slightly greater for the spray method due to its shorter period of wetting. The model can be used to study application losses with a wider range of conditions or situations that can be physically measured in the field.
Technical Abstract: Field water balance measurements using monolithic lysimeters were used in validating the Cupid-DVP model for predicting water loss partitioning during sprinkler irrigation from a moving lateral system fitted with impact and spray sprinklers. The model combines equations governing water droplet evaporation and droplet ballistics with a comprehensive plant-environment energy balance model. Comparisons indicate good agreement between measure and modeled transpiration, soil evaporation, and total evapotranspiration rates during the day of irrigation. Soil evaporation rates were under- predicted for the day following irrigation. During irrigation, the main water loss was shifted from transpiration to evaporation from the wetted canopy. For equal application volumes, the duration of this effect was greater using impact sprinklers compared with spray heads. Predicted water flux rates during irrigation were slightly greater for canopy evaporation than for transpiration immediately prior to the start of irrigation. Although droplet evaporation represented less than 1 percent of the total water loss for the day using either type of sprinkler method, irrigation water did influence the energy transfer between the plant-environment and water droplets during flight, on the canopy, and the soil.