EFFECT OF EVAPO TRANSPIRATION ON RAINFALL- GENERATED RUNOFF

Abstract

The considerable variation of rainfall and runoff from year to year is part of the natural variability in the climatic system. The management of water resources involves designing and operating to cope with this variability. Estimation of potential evapotranspiration (PET) which is an integrated outcome of the variables describing climate, on hydrology essentially involves projections of climatic changes (e.g. temperature, humidity, mean sea level pressure etc.) is required for water availability computations; estimation of daily, weekly, and monthly flows for multipurpose reservoir operation; scheduling of irrigation projects; preparation of long-term flow forecasts; and many other aspects of water resources planning and management. A general change in surface air temperature might be expected to cause changes in both evaporation and transpiration (or evapotranspiration, ET), hence change in PET. In general, ET is the second largest component of the catchment water balance and PET data. This study presents a Soil Conservation Service Curve Number (SCS-CN) based concept for the assessment of mean PET using long term daily rainfall-runoff data. To this end, the curve numbers (CN) were derived from rainfall-runoff data of three different agro-climatic river basins in India and Nepal for different rain durations and these were correlated with PET derived for respective watersheds using Hargreaves's method. The study reveals quantitatively that as PET increases, CN decreases or S increases and, in turn, the rainfall-generated runoff decreases, and. vice versa. Such a relationship invokes determination of PET from the available CN values, and therefore, it may be quite useful in field application.