Direct evaporation from soil under a row crop canopy

A simplified measurement of the soil evaporation (E) component of evapotranspiration (ET) is needed to obtain independent measurements of transpiration (T) and to evaluate the effects of E and T on ET. Our objective in this study was to evaluate the use of small lysimeters placed under a crop canopy to measure the E component. Lysimeters were constructed of rigid PVC pipe sections, 20.3 cm in diameter and 20, 10, or 5 cm long. Water loss from the lysimeters was recorded daily. The water content of the soil surrounding the lysimeters was measured gravimetrically from composite 1-cm-increment cores sampled daily. The results reported are for two drying cycles of 16 and 13 days in July 1975 and 1976. In order for the lysimeters to behave as the surrounding soil, the water content of the lysimeters must be higher than the soil outside to compensate for changes under the natural conditions due to plant uptake, drainage and upward flow. Since the lysimeters depend on a set of compensating factors to directly measure E, estimates of E from them should be used with caution. A better use of the lysimeters would be to establish a relationship between lysimeter E and the surface soil water content and then use surface water content measurements to infer E.