Evapotranspiration by Soil Water Balance Using TDR and Neutron Scattering

by Steven R. Evett, Conservation & Production, Research Lab, Bushland, United States,
Terry A. Howell, Conservation & Production, Research Lab, Bushland, United States,
Jean L. Steiner, Conservation & Production, Research Lab, Bushland, United States,
James L. Cresap, Conservation & Production, Research Lab, Bushland, United States,

Abstract: The soil water balance method of estimating evapotranspiration (ET) has been widely used since it is considerably cheaper than alternative methods such as the use of weighing lysimeters. Neutron scattering (NS) is commonly used to measure soil water content. However, for water balance studies, the method has been criticized as imprecise due to difficulties of measurement near the soil surface. Precision can be improved with destructive soil sampling near the surface but this is incompatible with many cropping and experimental systems. We examined time domain reflectometry (TDR) for measuring near surface soil water contents, combined with NS measurements at deeper depths to achieve a non-destructive estimate of ET. TDR probes were installed at depths of 2, 4, 6, 10, 15, 20 and 30 cm at two locations in a large weighing lysimeter and measured every half hour. Neutron scattering measurements were made at two access tube sites on the lysimeter at depth increments of 20 cm from 10 to 190 cm. For a 16 day period, daily change in soil profile water storage in the top 40 cm of soil, as measured by TDR, averaged 88% of total change in storage measured by the lysimeter. Estimates of ET from TDR based change in storage and precipitation data were inaccurate on many days due to water fluxes through the bottom of the layer measured by TDR. However, the soil water storage, computed by the combined TDR measurements for the surface to 40-cm layer and NS measurements below 40 cm, was within 0.7 mm of that measured by lysimeter whereas change in storage based only on NS was in error by 3.6 mm. A combination of daily NS measurements at depth with TDR measurements near the surface holds potential for accurate daily ET estimation.

Subject Headings: Soil water | Water storage | Water content | Water balance | Measuring instruments | Evapotranspiration | Soil analysis | Surface water

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