Practical application of the surface renewal method

One of the biggest problems in irrigation management is the lack of good, information on crop evapotranspiration (ETc), which is commonly estimated as the product of reference evapotranspiration (ETo) that accounts for weather variation and a crop coefficient factor (Kc) that adjusts for crop and management factors. Although ETc can be measured using lysimeters, eddy covariance, Bowen ratio, etc., the cost and complexity have discouraged agronomists and engineers from using these methods for long-term, seasonal measurements. Each of the measurement methods has good and bad points as well as limitations; however, the clear lack of good Kc information indicates that they are not widely accepted and used. The surface renewal (SR) method was first presented by Paw U and Brunet in the early 1990s, and it was later refined in several field experiments to eliminate the need for use of a computer to collect the field data. The method involves collecting high frequency temperature data that are analyzed with a structure function to quantify ramp characteristics that are common in the temperature traces. Now the data are easily collected, processed, and archived within a data-logger in the field. Like all methods, SR has its downfalls - the main problem being that it must be calibrated in the field using a sonic anemometer. In this paper, we will discuss our experiences with the practical application of the SR method to determine seasonal Kc curves and possible methods to simplify the calibration requirement.