Runoff coefficient determination based on radiative index

Runoff coefficient (RC) determination based on Radiative Index (RI) was carried out based on available information from 46 watersheds in various regions of Thailand. It is aimed to employ the derived functions for predicting runoff coefficients of any ungage watershed in Thailand. Correlation between RC and RI of each region, using year-by-year data without watershed size class separation, is rather low. Correlation of using year-by-year data of all regions was also low. The negative trend of this relationship was however found in the same manner as that found in the temperate zone. The correlation coefficient (r) of RC and RI was higher when judicious adjustment was employed to eliminate variation cause by error; but was not high enough to be accept. Accordingly, the statistic method called "Outlier" was applied. The result indicate a little lower correlation coefficient (r), but more reliable prediction could be obtained. The correlation of RC and RI was improved when the long-term average was applied to analyze their relationships. The exponential model is the best fit among developed models. The relationship between RC and RI for small watersheds is better than the large one. Correlation between RC and RI for watershed area less than 500 square km was the highest and the best fit is represented by exponential model. The model for determining RC based on RI for any size of watershed in Thailand can be given as RC=0.9194.e*[-0.5031.RI), r=-0.6335, R*[2)=0.4013 and F ratio =28.1516** and for the better predicting runoff coefficient of small watershed not exceed 1,000 square km, the suggested model is: RC=0.9782.e*[-0.4446.RI), r=-0.7086, R*[2)=0.5021 and F ratio =20.1676**. The reliability of models are not quite good to predict runoff coefficient as indicated by the R*[2). Therefore the modified RCRI model was developed for more reliable prediction. The modified models were derived as: RC= 1.8719.e*[-0.3149.RI-0.1720.In(DA), r=0.7142, R*[2)=0.5101 F ratio=21.3477** for generalized model, and RC= 0.9620.e*[-0.2777.RI-o.00096.DA, r=0.7562, R*[2)=0.5719 F ratio = 12.6913** for small watershed model. Application of modified RCRI model for predicting effect of deforestation on streamflow indicated increasing amount of streamflow with decreasing forest area and depending upon degree of remaining forest area in a watershed and the annual rainfall. The magnitude of increasing trend of runoff is, however, still a question to redetermine.