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Volume :33 Issue : 124 2007      Add To Cart                                                                    Download

Standardization of Evapotranspiration Model Calculations Using Class A Pan in the Asir Area-Saudi Arabia

Auther : Dr. Mohamed F. BuRoba

     Researchers in climatology use mathematical models and empirical equations for estimating the evapotranspiration (based on crop water requirement), particularly in areas that lack or have sparse meteorological stations. A great deal of climate studies have been criticized for extrapolating many mathematical models without being standardized with actual evapotranspiration measurements.

     This research proposes a method for standardizing the evapotranspiration calculated by more than 12 evapotranspiration models such as Blaney-Criddle model (1945), Thornthwaite Model (1948), Penman Model (1954) and Hargreaves Model (1975). Class A Pan evapotranspiration data (from 1970 to 1997) from 6 meteorological stations in Asir area used. The data used in the proposed equations include measurements of temperature, relative humidity, solar radiation and water vapor pressure and evaporation.

     The calculated results have shown spatial variability in annual and monthly levels. The annual values measured from class A Pan exceed variably similar ones calculated by models of Blaney-Criddle, Thornthwaite, Ivanov, Holdridge, Oliver-Papdakis, Turc, Penman and Hargreaves. The difference between measured and calculated annual evapotranspiration ranges between 240 mm (calculated by Hargreaves model in Tathlith station) and 3330 mm (calculated by Thornthwaites model in Sirat Abidah station).

     The annual values of evapotranspiration calculated by Torence, Garnier, Jensen-Haise and Dalton models, exceed variably the Class A Pan-measured annual values, and differ according to the applied model, from station to another.

     The difference between measured and calculated annual evapotranspiration ranges between 50mm (calculated by Jensen-Haises model in Bisha station) and 3620 mm (calculated by Torences model in Nimas station).

     The monthly evaporation values measured from class A Pan exceed variably the calculated values by 629 monthly value out of 864 value that represent the monthly means calculated by the 12 models in all stations considered by the study. The difference between measured and calculated monthly evapotranspiration ranges between 1-40 mm for a total of 107 values, 80-100 mm for a total of 85 values and with amounts exceed 100 m for a total of 296 values. On the other hand, all the calculated evapotranspiration values using Torence model exceed all the measured values in all stations considered by this study. Therefore, the use of all the examined models requires some modifications particularly from station to another, before being applied in agricultural, meteorological and hydrological fields in the mountainous Asir region of KSA.

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