ESTIMATING EVAPOTRANSPIRATION IN URBAN ENVIRONMENTS

Abstract

Quantifying evapotranspiration (ET) rates in urban environments is paramount for understanding and modeling other hydrologic fluxes such as runoff and recharge. Large impermeable fractions of land in urban areas may only experience evaporation following rainfall events. Pervious fractions are believed to support most of the ET burden. Point measurements of land-cover ET in pervious areas can provide better estimates of the overall urban ET budget. These can be made by examining changes in the total soil moisture above the seasonal low water table or the ET extinction depth. Soil moisture can be determined by summing the average soil moisture content measured at various depths with capacitance sensors on a vertical probe. Graphs of total soil moisture above the water table for a riparian area in west-central Florida display two distinct slopes, a flattened slope during the overnight period and a steeper slope between approximately 9:00 am and 6:00 pm. The overnight slope is believed to correspond to a process removing moisture continually from the soil, such as gravity drainage. The daylight portion of the slope corresponds to ET plus the continuing downward gravity drainage. Daylight ET is the cause of the difference between the two slopes. Different plant communities exhibit measurably different ET rates and can be estimated using this methodology.