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An easy-to-follow methodology is developed for the assessment of regional evapotranspiration in Castilla-La Mancha, a semi-arid region of Spain. The methodology is applied to barley crops to monitor the irrigation scheduling over the region, by using remote sensing techniques supplemented by ground measurements. The methodology can be based on either of two models. In the first one, established by Caselles and Delegido,(1) the reference evapotranspiration, ET(o), derives from the expression

ET(o) = AR(g)(T-a)(max) + BR(g) + C

where A, B and C are empirical coefficients, depending on climatic parameters and determined for each region; R(g) is the daily global radiation; and (T-a)(max) is the maximum air temperature. The second model, proposed by Jackson et al.,(2) considers the actual evapotranspiration

ER = R(n) + D (T-a - T-s)

where R(n), is the net radiation, T-a and T-s are the air and crop surface temperatures, respectively, and D is a semi-empirical coefficient. Both methods were compared with the method of Penman (considered standard), and resulted in differences of +/-1 mm d(-1). The developed methodology has been applied to map the reference and the actual evapotranspiration over a 10 x 10 km area, using the thermal-infrared information provided by the AVHRR (advanced very high resolution radiometer) sensor on board the NOAA. (national oceanic atmosphere administration) satellite on a selected date. (C) 1996 Silsoe Research Institute