Ground water and surface water under stress

Presented at Ground water and surface water under stress: competition, interaction, solutions: a USCID water management conference on October 25-28, 2006 in Boise, Idaho.Includes bibliographical references.This paper presents a procedure to estimate actual evapotranspiration (ET) using a satellite-derived vegetation index. Actual ET is computed in a traditional manner using the crop coefficient (Kc) and reference ET (ETref) procedure (i.e., ET = Kc x ETref) with Kc estimated from the satellite-based NDVI. This study calibrated relationships between Kc and NDVI using satellite-based ET determined by surface energy balance. This unique approach enables calibration of the Kc vs NDVI equations using large numbers of sampled fields (in this case, more than 3000). Thus the calibration represents a regional average Kc estimate. The study was conducted for alfalfa, beans, sugar beet, corn, potatoes, and small grain crops, which are the major crops in southern Idaho. Estimation accuracy for ET was statistically evaluated. Average error of seasonal ET was within 5 percent of the energy balance (EB) determined ET for most crop types. Error in seasonal ET from individual fields is expected to be within 10 percent. NDVI based ET was compared with lysimeter measurements of ET from grass and sugar beets. The seasonal error of the NDVI based method was only 2 percent for grass and 6 percent for the sugar beets, as compared to lysimeter measurements. Statistical accuracy assessments suggest that NDVI based ET estimation can be a robust, simple and inexpensive tool to estimate ET from irrigated agricultural crops with reasonable good accuracy