Evaluating water application efficiency of low and mid elevation spray application under changing weather conditions

Over half of the irrigated land in the US, 11.5 million ha, is irrigated with center pivot and linear move systems. Because of this, minor changes in the operation efficiency of these systems can have large impact on overall water conservation. The objective of this study was to evaluate the water application efficiency (WAE) of low and of mid elevation spray application (LESA and MESA) using catch can test and drainage lysimeters, and develop governing equations based on the weather variables. A three-year (2015–2017) field study was conducted at the Washington State University Research and Extension Center, near Prosser. Catch cans were used to collect the fraction of total irrigation-water applied that reached the ground surface as WAE and drainage lysimeters to measure the overall water loss (OAWL) and wind drift and evaporation losses (WDEL), (WDEL = 100-WAE). Air temperature (Ta), relative humidity (RH), short-wave global irradiance (Rg), wind speed (WS), and calculated vapor pressure deficit (VPD) were used as the input weather variables to mixed modeling technique. Results showed that on average 21% more irrigation-water reached the ground with LESA than with MESA systems. Lysimetric measurements showed on average a 16% efficiency difference between MESA and LESA. The monthly WAE differences between MESA and LESA increased from 12 to 30% during the hot summer months and thereafter decreased, from 30 to 9%. The warmer and drier year of 2015 had the highest annual average values of WDEL of 17% for LESA and 19% for MESA. Results indicated a relatively constant WAE for LESA regardless of weather conditions. Mixed modelling showed that VPD was the only significant predictor (P