Spatio-temporal Calibration of Blaney–Criddle Equation in Arid and Semiarid Environment

Estimates of reference evapotranspiration (ET<Subscript>o</Subscript>) are widely used in irrigation engineering to define crop water requirements. A major drawback to application of the FAO Penman-Monteith is the relatively high data demand which unfortunately, for many locations; such meteorological variables are often incomplete and/or not available. Alternatively, the Blaney–Criddle (BC) equation is a simpler method for ET<Subscript>o</Subscript> estimation. In this study, the BC equation was calibrated using three methods: spatially calibration at each station for the whole period (ET<Subscript>o</Subscript>-BC<Subscript>S</Subscript>); two periods calibration (ET<Subscript>o</Subscript>-BC<Subscript>S2</Subscript>); and spatial and temporal calibration at each station for each month (ET<Subscript>o</Subscript>-BC<Subscript>S,T</Subscript>) using twelve stations a cross Jordan. The calibration coefficient of BC equation (a, b) were determined at all stations. The results of the calibration methods showed that: (1) the spatial calibration of BC had the highest RMSE, and ME and Lower R<Superscript>2</Superscript> comparing to spatial and temporal calibration and two periods calibration. (2) Improvement was achieved for the BC equation when considering the spatial and temporal calibration for all months at each station. The values of a were negative for all months of any station. The higher values of a are coincided with cold or low temperature months while the high values coincided with high month temperature. The b values were positive for the whole stations and months. As the a values, it seems that b values had higher values in warm months than the cold one. A relatively good improvement could be obtained using two periods calibration instead of one period. The maps of a and b clearly show that a and b varied considerably in the study area and being aware of the spatial temporal variations of climatologically parameters is important in managing the limited water resources. Knowing the spatial temporal changes of such parameters, accurate calculations of ET<Subscript>o</Subscript> can be achieved which will lead to precise and elevate water resources management in the arid region such as Jordan. Copyright Springer Science+Business Media B.V. 2010