Rainfall-runoff estimation of an ungauged Nanjannad watershed in Nilgiris district of Tamil Nadu using WEPP model

The rainfall and runoff relationship is a key factor in assessing the water availability in the watershed. The amount of water available is estimated for watershed interventions and proper watershed planning. It is difficult to establish the rainfall and runoff relationship in the ungauged hill watersheds. The water erosion prediction project (WEPP) model is well established globally for the estimation of runoff. Hence, an attempt has been made to estimate the runoff and also to derive the relationship between rainfall and runoff in the Nanjannad hill watershed of the Nilgiris district of Tamil Nadu State, India. A climate file is been generated using the CLIGEN software using the observed meteorological data, slope data, soil data and management files are being prepared for accurate results. The monthly and annual rainfall and runoff analysis over the period of 7 years (2015-2021) has shown that the month of May has received the maximum rainfall (1606 mm) and maximum runoff (449 mm). The rainfall variation in the watershed is between 880 mm and 1859 mm. The maximum rainfall (1859 mm) and maximum runoff (519.3 mm) occurred in the year 2021. The statistical analysis reveals a positive correlation (R2 = 0.77) between the rainfall and runoff. The runoff varies from 0.72 Mcm to 2.25 Mcm (16.75% to 27.92%). The maximum runoff obtained is 2.25 Mcm (27.92%) and the average runoff in the watershed is obtained at 1.36 Mcm (21.71%). Hence, it is suggested that, watershed interventions can be planned to manage the 22 to 28% annual rainfall and the rainfall and runoff relationship for watershed planning can be considered as 0.22 to 0.28.