Comparison of Lysimeter and Neutron Scatter Techniques for Measuring Evapotranspiration from Semiarid Rangelands

Abstract

Evapotranspiration (ET) calculated from changes in soil water content measured by the neutron scatter method was compared to ET measured by lysimetry. During 1968, a near-average precipitation year (33 cm), the neutron method was effective for determining ET over periods of 4 weeks or longer. Cumulative ET curves as determined by lysimetry and the neutron method were in excellent agreement. In 1969, a year with high precipitation in June and July, reliability of the neutron method was severely limited by deep percolation and possibly by surface runoff. Failure of the neutron method to measure accurately water content near the soil surface following periods of precipitation was the major source of error when percolation and runoff were not factors. Sensitivity of the neutron method during a 30-day drying cycle was equal to that of a hydraulic lysimeter. Upward soil water fluxes were evident and are potential sources of error.