HR: 16:30h
AN: H42M-03    [PDF]
TI: The impact of uncertainty in evapotranspiration on the water balance at a pasture site in Florida
AU: * Sumner, D M
EM: dmsumner@usgs.gov
AF: U. S. Geological Survey, 224 W. Central Pkwy., Suite 1006, Altamonte Springs, FL 32714 United States
AB: The difference between precipitation (P) and evapotranspiration (ET) provides the net atmospheric input of water to watersheds. Precipitation data commonly are available. However, ET data are scarce and coarse estimates of ET routinely are used in hydrologic analyses. Measured ET values over a 986-day period at a pasture site in Florida provided a unique opportunity to evaluate the sufficiency of coarse estimates of ET in hydrologic analyses. A simple, daily hydrologic model was used to evaluate the sensitivity of the flow system to error in ET. The day-to-day variation in measured vegetation coefficients (ratio of actual to potential ET) was replaced within the hydrologic model with a simple, annually-invariant pattern of monthly vegetation coefficients. This approach was proven to be effective in reproducing unbiased estimates of daily water table fluctuations and flow terms and was successful despite the substantial fluctuation in measured daily vegetation coefficient and significant year-to-year variation in measured vegetation coefficient for a given month. Coarser estimates of the vegetation coefficient (a constant, average value of 0.86 and values biased +/-10%) poorly reproduced the flow system. Errors in estimated flow terms increased by tens of percent and substantial biases were introduced to these estimates. Despite the unbiased estimates of ET produced by a constant, average vegetation coefficient, this approach produced biased partitioning of net atmospheric input into recharge (-13% error) and runoff (5% error). The modest +/-10% bias to vegetation coefficients (and therefore to ET) produced a +/-20% error in net atmospheric input through residual error amplification. This error was further amplified in the fraction of net atmospheric input becoming recharge (+35% / -63%) and again in the fraction of recharge that became subsurface flow (+48% / -86%), indicating that use of unbiased estimates of vegetation coefficient is critical to hydrologic simulation. The results suggest that annually-invariant, monthly vegetation coefficients are the appropriate level of coarseness for hydrologic analyses at the study site. Estimates of potential ET, derived either from a simple meteorological station or remote-sensing approaches, combined with these monthly vegetation coefficients are suitable for generation of historical or ongoing ET estimates in environmental settings similar to the study site. The particular vegetation coefficients and acceptable level of coarseness of these coefficients at other environmental settings can be defined by methods outlined in the present study.
DE: 1818 Evapotranspiration
DE: 1829 Groundwater hydrology
DE: 1836 Hydrologic budget (1655)
SC: Hydrology [H]
MN: 2003 Fall Meeting