TR-17 : Simple techniques for forecasting of monsoon rainfall and runoff and application to Mahanadi river at Hirakund

Abstract

Monsoon period (June to Oct.) is a crucial period as nearly 90% of annual rainfall and runoff is received during this period in Indian subcontinent. The water resources planning activities e.g. . reservoir operation etc. are dependent mainly on monsoon behaviour. 75% of India's cultivated area is rainfed making forecasting of monsoon rainfall more important. How efficiently the monsoon behaviour (rainfall as well as runoff) can be predicted as on July Ist, August Ist, Sept. Ist and Oct. Ist is of prime importance for hydrologists and engineers concerned with irrigation and water resources planning.

Though for forecasting of monsoon rainfall and runoff complicated methods are available, in this study the monsoon rainfall and runoff have been forecasted using simple correlation technique. The regression relationships have been developed to correlate monsoon runoff with the total runoff upto the end of June, July, August and September. These regression relationship have been used without updating the parameters and after updating the parameters of regression relationships to forecast the monsoon runoff. The efficiency of the regression relationship in calibration and forecasting has been computed. Similar relationship have also been developed to correlate monsoon rainfall with total rainfall upto the end of June, July, August and September.

The runoff data (1946-82) and rainfall data (1901-79) for Hirakud (Orissa) have been used.

The results based on the analysis of above data indicate that for Hirakud

i) The efficiency of monsoon runoff forecast are 71%, 81% and 98% at the end of July, August and September respectively.

ii) The efficiency in identifying whether the current year is going to be below normal or above normal are 71%, 88%, 94% and 100% at the end of June, July, August and Sept. respectively on the basis of 1968-82 runoff data.

iii) The efficiencies of forecasting monsoon rainfall are .60%, 80% and 90% at the end of July, August and Sept. respectively on the basis of 1951 -79, rainfall data and 52%, 79% and 96% on the basis of 1966-79 rainfall data.

iv) The efficiency of regression relationships in identifying whether the current year is going to be below normal or above normal are 62%, 62%, 83% and 93% at the end of June, July, Aug. and Sept. respectively on the basis of 1951-79 data and 64%, 79%, 93% and 93% on the basis of 1968-79 rainfall data.

The results are based on the analysis of only one site and in order to arrive at definite conclusion it is proposed to carry out analysis for other basins also on the similar line.