The Effect of Grazing On Evapotranspiration and Net Carbon Exchange in Tallgrass Prairie.

The Flint Hills region in eastern Kansas is part of the largest unbroken tracts of native tallgrass in North America and over 1 million cattle are grazed on this ecosystem annually. Removal of aboveground biomass by grazing may affect evapotranspiration (ET) and Net Carbon Exchange (NCE). This study compared ET and NCE measurements, and NCE:ET ratios from adjacent grazed and ungrazed pastures for three years using long-term eddy covariance flux measurements. Eddy covariance towers were used to measure water vapor and CO₂ fluxes from the pastures and detect seasonal and interannual variations.  Evapotranspiration was also compared to reference crop ET (ET_o) to test if pasture-ET could be modeled from local weather station data. Grazing caused only small, 3 to 6 %, reductions in seasonal ET compared with ungrazed pastures despite large differences in vegetative cover. Grazing greatly influenced the magnitude of NCE during the growing season causing an average decrease among years of 33 % among treatments between (May – October). Grazing also caused a 33% decrease in the apparent water use efficiency (NCE/ET). Monthly evapotranspiration from grazed and ungrazed grass pastures was strongly correlated with ET_o, and could be modeled to within ±20 % of ET_o.  In summary, grazing had surprisingly minor effects on ET. It is likely that season ET is governed by precipitation and evaporative demand rather than the size of the plant canopy.  Carbon fluxes where more strongly affected by grazing; additional research is needed on the impacts grazing regime on the carbon balance of rangelands.