Ecosystem specific yield for estimating evapotranspiration and groundwater exchange from diel surface water variation

Abstract

The White method, routinely used to estimate phreatophyte transpiration from diel groundwater variation, also provides measures of total evapotranspiration (ET) and groundwater fluxes in surface waters. Such applications remain rare, however, and critically require accurate representation of stage-dependent variation in specific yield (Sy). High-resolution stage data from three Florida swamps were used to evaluate different relationships between Sy and stage (ecosystem specific yield, ESY). A discretized form, ESYD, assumes constant Sy near unity for inundated conditions, applying soil Sy for belowground stage and open water Sy (Sy,OW ≈ 1.0) for aboveground stage. A mixture approach, ESYM, applies a stage-dependent interpolation between Sy,Soil and Sy,OW using stage-area relationships and assumes rapid lateral equilibration between inundated and non-inundated wetland areas. Finally, an empirical formulation, ESYRR, uses measured ratios of rain to rise to estimate stage-specific Sy. All formulations yielded reasonable ET rates (ET ≈ PET) at high stage; ESYD markedly overestimated ET (ET/PET > 3) at intermediate stage, whereas ESYM and ESYRR maintained ET/PET near 1.0. Estimated groundwater fluxes using ESYM and ESYRR correlated well with Darcy-estimated flows, but were larger, likely due to uncertainties in Darcy parameters. Well transects across wetlands documented equal water elevation and diel variation across inundated and non-inundated areas, verifying rapid equilibration that reduces Sy and explaining overestimation by ESYD. However, equilibration area varied within and among wetlands, explaining observed differences between ESYM and ESYRR, and suggesting ESYRR may be preferred. Stage histograms followed the shape of ESYRR, highlighting reciprocal influences of ESY on stage stability. Copyright © 2012 John Wiley & Sons, Ltd.