DETERMINATION OF ANNUAL RUN -OFF OR FLOW OF A CATCHMENT, BASED ON THE AREA OF CATCHMENT AND THE ANNUAL RAINFALL, DERIVED AS A RESULT OF THEIR OBSERVATION.

There are a number of formulae and methods, which are generally based on the area of catchment and the annual rainfall, and are derived as a result of their observations from certain catchments indifferent parts of the country and their suitability is limited only to those catchments. Determination of Annual rainfall or flow off ol a catchment. There are so many variable factors affecting the run offs from different catchments that it is almost impossible to get correct results from any of the formulae. The results of one catchment cannot be applied to another. Therefore these formulae and tables must be used with caution if at all applied. The rational method given for drainage may be used by having maximum rainfall of a year and the minimum rainfall of another year of a place has been found very variable. According to Binnie, the maximum annual rainfall of a place is 1.51 times the average annual rainfall, and the minimum annual rainfall is 0.60 years of the average annual rainfall. Average annual rainfall is the mean of 35 years. The year in which the rainfall is less than the average annual rainfall is called a bad year and the year in which the average annual rainfall is more than the average annual rainfall, is called a good year. Out of the total rainwater, the runoff in an average catchment is only about 30 to 40 percent and out of this total run -off the average possible may be as low as ten percent and as high as 80 percent, with about 40 to 50 percent as average. Study of rain fall is essential for taking maximum and minimum as well as average annual rainfall of the catchments, required for many concerns of the design &Scope of structures.