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This study was performed to test three methods based on the FAO-56 "dual" crop coefficient approach to estimate actual evapotranspiration (AET) for winter wheat under different irrigation treatments in the semi-arid region of Tensift Al Haouz, Marrakech (center of Morocco). The three methods differ in the calculation of the basal crop coefficient (K-cb) and the fraction of soil surface covered by vegetation (f(c)). The first approach strictly follows the FAO-56 procedure, with K-cb given in the FAO-56 tables and f(c) calculated from K-cb (No-Calibration method). The second method uses local K-cb and f(c) values estimated from field measurements (Local-Calibration method) and the last approach uses a remotely-sensed vegetation index to estimate K-cb and f(c) (NDVI-Calibration method). The analysis was performed on three fields using actual (AET) measured by Eddy Correlation systems. It was shown that the Local-Calibration approach gave best results. Accurate estimates of K-cb and f(c) were necessary for FAO-56 "dual" crop coefficient application. The locally derived K-cb for winter wheat taken at initial, mid-season, and maturity crop growth were 0.15, 0.90 and 0.23, respectively. The K-cb value at the mid-season stage was found to be considerably less than that suggested by the FAO-56.

Similarity between the seasonal pattern of normalized difference vegetation index (NDVI) and K-cb showed potential for modelling NDVI into a K-cb. The obtained relationships between K-cb and NDVI, and between f(c) and NDVI could be easily incorporated within the FAO-56 "dual" crop coefficient model and, thereby, provide a means to apply remotely sensed observation for real-time wheat irrigation scheduling.

The results obtained were very acceptable especially when the soil evaporation is negligible. Therefore, the K-cb-NDVI relationship employed in the FAO-56 "dual" crop coefficient model holds great potential for estimating crop water requirements on an operational basis and consumption at a regional scale. (c) 2006 Elsevier B.V. All rights reserved.