To find out the variation of precipitation in the region (Assam) monthly average rainfall data of 28 stations were taken for a period of 30 years ranging from January 1988 to December 2018. Spatial variation of rainfall of each month is evaluated using Geospatial analyst tool of ArcGIS (version 10.2.2). Five interpolation techniques IDW, GPI, RBF, LPI, Kriging were used to find out the spatial variation of rainfall over the entire region of Assam. The results obtained by all the techniques were cross validated by finding out the errors and efficiency of all the models using the performance measures discussed above. Table 1 shows the performance of all the models. The rank column in the table tells the overall performance of all the interpolation techniques on the basis of performance measures. From the table we can see IDW at p=3 is the most efficient model while 3rd order GPI is least efficient out of the 20 models utilised. Second best model is again from IDW for weight parameter p = 4. The other models which give reliable results are Simple Kriging and RBF with basis function as spline with tension. However, in some cases user is constrained to use a particular technique only, for such cases also table 1 should be used as a guide to choose the best parameter of a particular model. From table we find out GPI gives its best result when the polynomial order is taken as one. Performance of LPI is good when the kernel function is chosen as Gaussian; similarly for Kriging the results are more favourable for simple as compared to that of universal. Fig 2- Fig 6 shows how the rainfall varies spatially in Assam during month of January using 5 models with the best parameters. Obviously as discussed above IDW will represent the phenomenon more accurately compared to remaining four models.
Fig 7 – Fig 9 shows how the errors are distributed spatially while predicting the rainfall. It is observed that the regions where the precipitation is more intense shows relatively higher error compared to the region where precipitation is less intense. It is clearly visible that the districts like Dhemaji, Tinsukia, Lakhimpur and Dibrugarh are receiving higher amount of rainfall and the errors associated is also higher in these regions.