Evapotranspiration crop coefficients for mixed riparian plant community and transpiration crop coefficients for Common reed, Cottonwood and Peach-leaf willow in the Platte River Basin, Nebraska-USA

Abstract

► Evapotranspiration crop coefficients were developed for riparian vegetation . ► Transpiration coefficients were measured for phragmites , willow, cottonwood. ► Vegetation response to flooding caused interannual variability in coefficients. ► Developed coefficients provided robust riparian ET and transpiration estimates. Application of two-step approach of evapotranspiration ( ET ) crop coefficients ( Kc ) to “approximate” a very complex process of actual evapotranspiration ( ETa ) for field crops has been practiced by water management community. However, the use of Kc , and in particular the concept of growing degree days (GDD) to estimate Kc , have not been sufficiently studied for estimation of evaporative losses from riparian vegetation. Our study is one of the first to develop evapotranspiration crop coefficient ( Kc ET ) curves for mixed riparian vegetation and transpiration ( TRP ) crop coefficients ( Kc TRP ) for individual riparian species as a function GDD through extensive field campaigns conducted in 2009 and 2010 in the Platte River Basin in central Nebraska, USA. Kc TRP values for individual riparian vegetation species [Common reed (Phragmites australis), Cottonwood (Populus deltoids) and Peach-leaf willow (Salix amygdaloides)] were quantified from the TRP rates obtained using scaled-up canopy resistance from measured leaf-level stomatal resistance and reference evapotranspiration. The Kc ET and Kc TRP curves were developed for alfalfa-reference ( Kcr ET and Kcr TRP ) surface. The seasonal average mixed riparian plant community Kcr ET was 0.89 in 2009 and 1.27 in 2010. In 2009, the seasonal average Kcr TRP values for Common reed, Cottonwood and Peach-leaf willow were 0.57, 0.51 and 0.62, respectively. In 2010, the seasonal average Kcr TRP were 0.69, 0.62 and 0.83 for the same species, respectively. In general, TRP crop coefficients had less interannual variability than the Kcr ET . Response of the vegetation to flooding in 2010 played an important role on the interannual variability of Kcr ET values. We demonstrated good performance and reliability of developed GDD-based Kcr TRP curves by using the curves developed for 2009 to predict TRP rates of individual species in 2010. Using the Kcr TRP curves developed during the 2009 season, we were able to predict the TRP rates for Common reed, Cottonwood and Peach-leaf willow in 2010 within 7%, 8% and 13% accuracy, indicating a good performance of the two-step approach proposed in this study for estimating TRP for riparian vegetation. The surface conditions of the riparian ecosystem need to be considered when using the two-step approach to estimate ETa or TRP rates of riparian plant communities. The results of this study provide important water use information and data for riparian vegetation that can be used for more robust hydrologic/water balance analyses.