Estimating Diurnal Variation of Soil Heat Flux for Alfalfa and Grass

Soil heat flux (G) is an important component of evapotranspiration (ET) modeling, especially for estimating hourly ET values. Meteorological and agronomic measurements were made at Kimberly, Idaho, with the purpose of establishing relationships to estimate the diurnal variations of soil heat flux for alfalfa and grass. For alfalfa, the relationship between G and net radiation (Rn) presented pronounced hysteresis effects, which decreased with increasing plant cover. Empirical equations to estimate diurnal variation of G from Rn and plant height (h) were derived. For low cover conditions, including radiometric surface temperature (Tc) improved the performance of the model. For grass, the hysteresis effects were not as pronounce as for alfalfa. Acceptable relationships were found between G and Rn, and also between G and Tc (r 2 ˜ 0.90) for plant heights between 0.1-0.2 m. An improved model (r 2 = 0.94) was obtained by including both Rn and Tc in the regression model.