Hourly ET Model Calibration with Eddy Flux and Energy Balance Data

Abstract

The Penman equation for estimating hourly evapotranspiration (ET) of an alfalfa reference crop was calibrated for both daytime and nighttime conditions. Hourly net radiation, sensible heat flux, and soil heat flux were measured in the field for 16 24‐hr periods, and evaporative flux was determined as the residual of the surface energy balance equation. The sensible heat flux measurements were made using a portable eddy correlation system that included a sonic anemometer and a fine‐wire thermocouple. Following calibration, the hourly Penman model was applied to independent weather data from two growing seasons. Since only conventional weather parameters were measured during these two seasons, empirical equations were first developed to relate net radiation to solar radiation, and soil heat flux to solar radiation and air temperature. The resulting hourly ET estimates were summed over 2–4 day intervals and compared to ET data obtained from soil water balance measurements. Good agreement was found between estimated and measured values.