Estimation of Evapotranspiration and Water Budget Components Using Concurrent Soil Moisture and Water Table Monitoring

Abstract

Simultaneous measurements of soil moisture profiles and water table heads, along a flow path, were used to determine evapotranspiration (ET) along with other components of the water budget. The study was conducted at a small-scale (~0.8 Km2) hydrologic monitoring field site in Hillsborough County, Florida, from January 2002 to June 2004. Frequency Domain Reflectometry soil moisture probes, installed in close proximity to water table monitoring wells were used to derive changes in the soil water storage. A one-dimensional transect model was developed; changes in the soil water storage and water table observations served as input to determine all vertical and lateral boundary fluxes along the shallow water table flow plane. Two distinct land cover environments, grassland and an alluvial wetland forest, were investigated in this particular study. The analysis provided temporally variable ET estimates for the two land covers with annual totals averaging 850 mm for grassland, to 1100 mm for the alluvial wetland forest. Quantitative estimates of other components of a water budget, for example, infiltration, interception capture, total rainfall excess, and runoff were also made on a quarterly and annual basis. Novelty of this approach includes ability to resolve ET components and other water budget fluxes that provide useful parameterization and calibration potential for predictive simulation models.

Hydrologic Response of Meadow Restoration the First Year Following Removal of Encroached Conifers

“…We evaluated changes in sub-surface hydrology through statistical comparison of soil moisture and water table depths and duration of water table depth compared to published depth to water table for meadow vegetation. We evaluated the water budget of the meadows following methods in Rahgozar et al [35].…”

“…To quantify the different hydrologic changes from the removal of encroached conifers we calculated different components of a water budget using precipitation, soil moisture, and groundwater depth changes following methods developed by Rahgozar et al [35]. The water budget components were represented by WY.…”

“…Infiltration in the unsaturated soil (I s ) was evaluated for all soil moisture sensors at 2-hour intervals (time duration between measurements). When an increase in soil moisture occurred in a 2-hour interval the change in volumetric soil moisture was converted to a depth (mm) based on the unsaturated depth of soil (difference in surface elevation and water table elevation) [35]. The soil moisture sensors installed at 0.3 m and 0.9 m depth were assumed to be representative of soil <0.6 and >0.6 m depth respectively.…”

“…In a given time step (2 h), depending on the algebraic sum of terms on the right-hand side of (Equation (2)), either soil infiltration or soil evapotranspiration is assumed to be occurring. An inherent assumption made here is that, since small time intervals (2 h) were used for the analysis, either evapotranspiration or infiltration took place in a given time step [35].…”

“…Interception capture (Ic) is the initial abstraction from a rainfall event. If there is no runoff accompanying a given rainfall event, then, theoretically, it can be estimated by subtracting the observed rainfall from the observed infiltration [35]. Interception capture was estimated by selecting isolated precipitation events with intensity less than the hydraulic conductivity of the surface soil layers, occurring after medium to dry antecedent conditions.…”