Estimating Crop ET: using CIMIS and new ET studies

The surface renewal (SR) method has shown promise to provide estimates of H regardless of the stability conditions and without the need for wind speed profile measurements. Because it does not rely on flux gradient theory, the fetch requirement is less, and there are fewer problems with measurement on slopes. When located in or near a plant canopy, air parcels heat or cool as sensible heat exchanges with canopy elements. Under unstable conditions, cool air sweeps into a canopy from above and the air parcels are gradually heated by the canopy elements. Therefore, temperature plots drop sharply as cold air enters the canopy and they show a slow rise as the air is heated. Then the warmed air ejects from the canopy as cool air again sweeps in from above. Under stable atmospheric conditions, the pattern is reversed as heat is transferred from warmer air to a cooler canopy. Temperature plots show ramp-like characteristics and the mean amplitude (a) and inverse ramp frequency duration (dr) can be quantified using structure function analysis. In this paper, we discuss several examples where the SR method was used to estimate ETa and to determine Kc factors.