Effects of agricultural management on measurements, prediction, and partitioning of evapotranspiration in irrigated grasslands

Irrigation is an important component of the hydrologic cycle in agricultural ecosystems, affecting both quantity and quality of surface and ground water. Well-managed irrigation involves balancing irrigation with water consumption by evaporation and transpiration (collectively evapotranspiration), maximizing ecosystem water-use efficiency and minimizing drainage. Here we compare rates of actual crop evapotranspiration (ETC) measured by eddy covariance with reference evapotranspiration (ET0) calculated from meteorological variables for two irrigated ryegrass systems in central South Island, New Zealand between June 2011 and March 2013. The sites were similar in climate, but contrasted in management: one grazed by dairy cattle and the other harvested annually for seed. Over the first year of measurements, cumulative ETC was very similar at the two sites, totalling 791 and 819 mm for the dairy pasture and seed crop respectively, although temporal patterns of partitioning of ETC amongst evaporation and transpiration differed as a result of management activities. Responses of ETC to global radiation, temperature and vapour pressure deficit were all similar during active growing season periods. Differences between the two sites were observed at the end of the second measurement season, when irrigation was ceased in the seed crop prior to final harvest and ETC was reduced compared to ET0. As a result, cumulative ETC was 13% greater for the dairy pasture at the end of the study period.