Estimation of runoff generating capacity of a small watershed (original) (raw)

Water is the basic requirement for existence of all forms of life on the earth. Water is abundantly available on the globe as it covers nearly three quarters of the earth surface. More than 97% of the earth's water is found in the seas and oceans, which is salty and unfit for drinking and irrigation. Another 2% is locked up in the Polar Regions in the form of ice and the remaining 1% a meager fraction is trapped in underground reservoirs and remains meager fraction that forms the world's main source of water supply. That work is entitled with Estimation of runoff generating capacity of a small watershed. " is undertaken with the following objectives are: To compute runoff coefficient for the selected watershed, To predict monthly runoff at different percent probability levels and to predict annual runoff at different percent probability levels. This study done on Nagwan Watershed of Damodar Barakar Catchment (Jharkhand, India). The monthly and the annual runoff at different percent probability levels were determined by using Weibull's method. July month has the highest average runoff followed by September and August while October has the lowest value of average runoff. July has higher value of runoff coefficient than June due to higher antecedent moisture present in soil and the annual runoff coefficient for the watershed is 0.38. September month has the highest value (0.47) of runoff coefficient while June has the lowest value (0.18). At 50% probability level, the monthly runoff in September is maximum (= 81.4 mm) followed by August (79.7 mm), July (66.5 mm) and June (18.1 mm) while it is minimum in October (8.7 mm) and the expected annual runoff value is 325.4 mm. Whereas at 75% probability level, the expected monthly runoff is maximum in August (= 43.9 mm) followed by September (39.8 mm) and July (30.3mm) while there is no chance of any runoff in October and expected annual runoff is 209.6 mm. Introduction Water is the basic requirement for existence of all forms of life on the earth. Water is abundantly available on the globe as it covers nearly three quarters of the earth surface. More then 97% of the earth's water is found in the seas and oceans, which is salty and unfit for drinking and irrigation. Another 2% is locked up in the Polar Regions in the form of ice. Of the remaining 1% a meager fraction is trapped in underground reservoirs and remains meager fraction that forms the world's main source of water supply. A watershed is a geographical area that catches the rainwater and drains it to a common point known as outlet. The watershed characteristics such as length, width, area, shape, drainage pattern, soil type, vegetation cover, land use, and hydrologic conditions significantly affect the rainfall-runoff process. A part of the precipitation that infiltrates into the soil moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil. This component of runoff is known variously as interflow. Depending upon the time delay between the infiltration and the outflow, the interflow is sometimes classified into prompt interflow, i.e. the interflow with the least time lag and delayed interflow. Another route for the infiltrated water is to undergo deep percolation and reach the groundwater storage in the soil. Based on time delay between rainfall and runoff, the runoff is classified into two categories: direct runoff and base flow. Direct runoff is that part of the runoff which enters the stream immediately after the rainfall. It includes surface runoff, prompt interflow and rainfall on the surface of the stream. Base flow is the delayed flow that reaches outlet essentially as groundwater flow. The runoff water may be harvested and can be used for various productive purposes such as domestic water supply, supplemental irrigation to the crops, livestock drinking and also for fish farming. Knowing the above importance, the present project work entitled