ESTIMATING SOIL HEAT FLUX FOR ALFALFA AND CLIPPED TALL FESCUE GRASS

Soil heat flux (G) is an important component of evapotranspiration (ET) modeling, especially for estimating ET values for hourly or shorter periods. In this study, meteorological and agronomic measurements were made at Kimberly, Idaho, with the purpose of establishing empirical relationships to estimate G for alfalfa and clipped tall fescue grass. For both plant surfaces, good linear correlation was found for most days between the averages of the 20-min net radiation (Rn) and G values for a given day. However, when the soil surface was wet, after rain or irrigation, the relationship was subject to hysteresis problems. The linear relationship between G and Rn for alfalfa also changed with plant canopy height (h), and an equation was derived to estimate G from Rn and h (r2 = 0.88). This equation fitted measured G data much better than two other commonly used models (Allen et al., 1996; Clothier et al., 1986). For tall fescue grass, h did not affect the relationship between Rn and G, as the grass was clipped weekly resulting in a narrow range of h (0.09 to 0.19 m). A linear equation to estimate G as a function of Rn (r2 = 0.91) was derived for clipped tall fescue grass, which was found to fit measured data equally well as the model proposed by Allen et al. (1998), but that uses a single equation for both daytime and nighttime instead of two separate equations.