Comparison of Field Level and Regional Actual ETc Values Developed from Remote Sensing and Dual Crop Coefficient Procedure

Crop evapotranspiration (ETc) estimates are important for regional water planning as well as irrigation scheduling. Traditional ETc computations utilize published crop coefficients (basal) that are adjusted on a daily basis depending on soil water availability (i.e., dual crop coefficient method). Recent advancements include using remote sensing data such as LandSAT combined with a surface energy balance algorithm (METRIC), allowing crop evapotranspiration to be computed for each pixel throughout images taken during the season. There are limitations and advantages for both methods. Comparisons of soil water balance evapotranspiration values to METRIC values for two scenarios in different regions of California have been made. The comparisons show that when averaged either spatially or temporally, values estimated from the methods show a good relationship. However, there can be significant variability between the two methods when looking at instantaneous values (for a specific day that the LandSAT image was taken). The cause for this can be attributed to the inputs into the dual crop coefficient model. Both methods have advantages and disadvantages. If the user has good input information, both methods can provide accurate evapotranspiration estimates. Work is currently underway to leverage advantages from both methods by coupling them together