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In shallow water table (WT) environments, plant evapotranspiration (ET) can be estimated by using the observed diel fluctuations in WT that normally correspond to the 24-hour ET cycle. It is now clearly known that estimation of ET from diurnal WT fluctuations is highly influenced by drainable porosity (λ) of aquifer, which depends on the WT depth and soil hydraulic properties. This parameter is usually estimated by integrating the equilibrium soil moisture profile above the WT. However, equilibrium (Eq.) conditions above WT are rarely met the field due to the occurrence of dynamic evaporation and root water uptake. In this study, we used a new expression for λ (non Eq-λ) to estimate ET from diurnal WT fluctuations. The new expression takes into account the quasi-steady vertical soil moisture flux (ET) from the WT to estimate λ. Evapotranspiration from shallow WT was estimated during 2010 and 2011 spring seasons for two potato fields in northeast Florida, managed under a conventional WT control system and the results were compared with the ET values from Penman-Monteith method. It was found that the non Eq-λ produced much better estimation of hourly as well as daily ET as compared to the Eq-λ. Using Eq-λ resulted in significant overestimation of ET especially during periods immediately after rainfall events. The non Eq-λ, on the other hand, seemed to improve the estimation significantly, even during periods immediately after rainfall events suggesting its advantage over the Eq-λ. While ET estimation from WT fluctuation has been mostly used in natural areas that are not directly intervened by human activities (e.g. natural wetlands and riparian zones), our study suggests that this method can also be used in relatively smaller, managed shallow water table fields intersected by drainage ditches. |