Stomatal and Canopy Resistance, Plant Physiological Parameters, Evapotranspiration and other Surface Energy Fluxes of Phragmites-dominated Riparian Plant Community in the Platte River Basin, Nebraska, USA

Effective water balance analyses require the knowledge of water use rates of riparian zones. Riparian zones usually consist of a variety of vegetation species that have specific transpiration rates. This mosaic of vegetation presents a challenge of how to improve the accuracy to estimate the evaporative fluxes of the riparian zone. The challenge is compounded by the fact that riparian zones are often exposed to large fluctuations in environmental forcing throughout the growing season. Our study presents a robust methodology to measure total evapotranspiration (ETa) and to estimate the growing season transpiration (TRP) for individual vegetation species within a riparian zone through extensive field campaigns conducted in 2009 and 2010. With the espousal of the measured data for individual species, including coverage, stomatal resistance, photosynthetic photon flux density, canopy light interception, leaf area index, plant height, and other within and above-canopy biophysical properties, we scaled up stomatal resistance to canopy resistance to quantify TRT rates of individual specie. The evaporation rates from the riparian zone were also quantified. We developed mixed riparian vegetation evapotranspiration crop coefficients and transpiration crop coefficients for individual species. The total ETa from mixed plant community was 679 mm in 2009 and 982 mm in 2010. In 2009, the seasonal total daytime transpiration for phragmites, cottonwood, and peach-leaf willow were 483, 522 and 431 mm, respectively, and the values for 2010 were higher 550, 655 and 496 mm, respectively. The transpiration was influenced by vegetation phenology (especially increase in LAI and leaf aging) and strongly correlated to irradiance. The average evaporation rate of the riparian zone was 0.81 mm/day in 2009 and 1.7 mm/day in 2010. The seasonal total daytime evaporation was 150 mm in 2009 and 312 mm in 2010. The presence of flood water on the surface and warmer seasonal average temperatures contributed to the high evaporation rates observed in the 2010 growing season