Application of Geostatistical METHOD For stimation of Annual Spatial Rainfall in Arid and Semiarid regions of South Eastof iran

Estimation of rainfall is indispensable in many natural resources and agricultural studies. There are
several methods of estimating rainfall among which inteterpolation is helpful in .this regard. In most cases a method is selected and estimation is then made with unknown precision In this study, 3 methods were' used to estimate annual rainfall. These methods were: Thin Plate Smoothing Splines (TPSS), •with ..and without covariable, We~ :Moving Average,(WMA)and .Kriging (ordinary,Cokriging, Log kriging). The data in 167 stations from south east of iran with 22 years of record were used in this study Elevation was .used as covariable and its correlation with rainfall evaluated. Because of size of the area and dispersing stations the 'correlation coefficient was low and unacceptable for all stations. Therefore, the study area was divided into subcatchments, based on watershed catchments. In cases where few stations existed in the catchment or regression coefficient (between elevation and rainfall) for .two stations was the same, 'those subcatchrnents were combined. Finally, 3 units of subcatchment were obtained. Regression coefficient between rainfall and elevation was low in one of the subcatchrnents (R=O.12) while in the other two subcatchments was higher (0.7 and 0.8). The results show that regression coefficient (between rainfall and elevation) must be grater than 0.6 to improve interpolation accuracy when using elevation ascovariable. The results also show that TPSS
method with elevation as covariable is the most precise method in estimating annual rainfall
(MAE=13-38 mm). Kriging (MAE= 29-50 mm) and WMA methods (26-57 mm) stood in second and third order, respectively.