Evapotranspiration Estimation from Diurnal Water Table Fluctuations: Implementing Drainable and Fillable Porosity in the White Method

Abstract

Diurnal water table fluctuations (DWTFs), normally observed in shallow unconfined aquifers, are commonly used to estimate groundwater evapotranspiration (ETg) by applying the soil water balance pioneered by W.N. White in 1932. A key element of White‐based methods is the drainable porosity (or specific yield), a soil water storage parameter that significantly depends on both the vadose‐zone soil moisture fluxes and water table (WT) elevation. However, it has traditionally been treated as either a constant or a function of the WT under hydrostatic soil moisture conditions. Recent research has shown that, at any given shallow WT position, vadose zone fluxes cause the drainable porosity to behave hysteretically, hence requiring estimation of two distinct parameters, drainable (λd) and fillable porosity (λf), during ETg estimation from DWTFs. We present a White‐based method to estimate ETg that implements separate λd and λf parameters and hence accounts for both WT elevation and unsaturated zone moisture fluxes. The modified method not only improves the ETg estimates but also extend the applicability of the White method to periods during rainfall, unlike most previous implementations, which have been limited to omitting such periods. The modified method is demonstrated here for estimation of ETg from two crop fields in Northeast Florida for two growing seasons, each approximately 50 d, in 2010 and 2011, and the ETg estimates are compared with the standard Penman–Monteith method. The modified method significantly improved ETg estimates compared with the hydrostatic λd based method, reducing the root mean square error by 94 and 96% at hourly and daily resolutions, respectively.