Tuesday, April 1, 2008 : 10:20 a.m.
Effects of the Temporal Variability of Evapotranspiration on Hydrologic Simulation
Temporal variations in evapotranspiration (ET) influence the transient water balance of a hydrologic system, but can be difficult to quantify. Hence, the degree of sensitivity of a hydrologic system to ET and the availability of simplified approximations is of interest.
Water-balance based hydrologic models were developed and calibrated to field data for five sites in central
Differences in the stochastic character of precipitation and ET time series result in substantial influence of precipitation and relatively small influence of ET on the quantity and timing of water available to the hydrologic system. At each site, the temporal variability of ET was considerably less than that of precipitation over a wide range of time scales (from daily to annual). Additionally, local hydrologic conditions can be an important factor because various points along a flow path in a hydrologic system respond differently to temporal variations in ET.
Both the Hargreaves and Priestley-Taylor equations for potential ET, when applied with annually invariant monthly vegetation coefficients, can be used with a hydrologic model to produce reasonable predictions of water levels and flows. Simulations using the Hargreaves approximation consistently yielded larger absolute and relative errors than the Priestley-Taylor approximation. However, differences between the Hargreaves and Priestley-Taylor simulations generally were much smaller than differences between these simulations and the simulations using actual ET. This suggests that the simpler Hargreaves equation, requiring only temperature data, may be an adequate substitute for the more complex Priestley-Taylor equation, which requires both temperature and net radiation data. These results are likely applicable in similar humid climates but may not be valid in arid climates.
Andy O'Reilly, U.S. Geological Survey Andy is a hydrologist at the U.S. Geological Survey, Florida Integrated Science Center, in Orlando. He received Bachelor and Master of Civil Engineering degrees from Auburn University and is currently pursuing a Ph.D. in Civil Engineering at the University of Central Florida. Andy has 15 years of professional experience in the hydrologic sciences. He has authored publications on a variety of topics, including hydrogeologic effects of land-based application of reclaimed water, characterization and simulation of regional aquifer systems, ground-water recharge and evapotranspiration processes, and ground-water quality impacts of stormwater management practices.