Multi-Approach Evaprotranspiration Estimates for Irrigation Plot Research

Quantifying evapotranspiration (ET) in plot-study research is a challenge because methods are often limited to water balance approaches and related uncertainties. For example, in many conditions, drainage of water below the soil water measurement zone may not be negligible, leading to errors related to that estimate. Ground-based sensing stations combining soil water content measurements (time-domain reflectometry) and proximal sensing of the crop canopy are being validated against eddy covariance measurements in an alfalfa-grass-hay mix under wheel-line (side-roll) irrigation in Eastern Utah. The proximal sensors include: infrared radiometers for canopy-soil system surface temperature; red and near infrared reflectance sensors; and canopy photography for vegetative cover. Supporting weather measurements are collected also, including wind speed, air temperature, and humidity. Incident solar radiation is measured at a nearby station. The data is sufficient to model ET using the Two-Source Energy Balance Model (TSEB) and reflectance-based crop coefficients. These estimates are then compared to ET computed from the soil water balance. Any discrepancies can then be investigated and reconciled, where possible. The result is an ET estimate at the plot-scale that has a quantifiable level of uncertainty. This is a notable improvement over conventional methods used in irrigation plot research.