HYDROLOGICAL STUDIES OF DISTURBED MOUNTAINOUS WATERSHEDS

Abstract

In this research work, some currently used hydrologic models have been studied with the objective to modify them so that they can account for the hydrological processes of disturbed, mountaineous, small watersheds of the himalayan region (Chapter-I). The literature survey conducted during the study (Chapter-II) revealed that in case of mountainous watersheds there are two extreme ends of runoff generation mechanisms viz, the Hortonian overland flow and the subsurface stormflow. On the other hand, some researchers (Freeze, 1980; Beven, 1986, 1991) believe that the channel flows need be simulated through saturation excess runoff, interflow and groundwater flow mechanisms. Three hydrologic models viz. the time-area, variable source area and physiographically distributed models have been used to study the hydrologic behaviour of disturbed, mountainous, small watersheds. The description of two such Watersheds is given in chapter-Ill alongwith availability of data. The availability of meteorologic (i.e. 25 storm events of Jhandoo-Nala and 5 storm events of Bhaintan watershed) and hydrologic data have been discussed. The descriptions of the proposed (above mentioned) models are given in Chapter-IV. It was found that the Time-Area model did not produce satisfactory results if the time of concentration was computed using empirical relationships (i.e. Kirpich formula etc.). However, it produced better results when the time of concentration is computed using the concepts of S-hydrograph (chapter-V). The proposed Variable Source Area model gave quite satisfactory results. It simulated runoff through four components namely the direct flow, the saturated area flow, the interflow and IV the groundwater flow. Three nonlinear reservoirs have been used for the conceptual representation of the runoff mechanism for each of these components of flow. The relationships of variable source area extent' with API, rainfall intensities, interflow, baseflow and saturated flows which were arrived at in this study may be of practical use. The relationship of runoff factor with baseflow may help in determining the runoff volume. In the proposed Distributed Physiographic model (Chapter-V) the watershed is divided into tributary and main channel subwatersheds. The runoff process for each of these subwatersheds is conceptually taken care of with the help of two nonlinear reservoirs. The upper nonlinear reservoir provides an output which is termed as 'surface supply ' (Ss). The lower nonlinear reservoir receives its input through infiltration. Its output is termed as groundwater supply (Sg). These two components (viz. the Ss and Sg) form the total supply (St) to the channel in the form of lateral inflow. The kinematic wave theory is applied for routing of flows through the channel reaches. An implicit finite difference scheme is used for routing flows to the outlet. At confluences, the concept of continuity is used for flow synthesis. The model has produced satisfactory results (Chapter-V). It has the capability of taking into account the changes in hydrologic behaviour due to soil conservation treatments in different parts of the watershed under consideration. For the proposed Varible Source Area model, as well as for the Distributed Physiographic model detailed sensitivity analyses have also been carried out. In the last Chapter (Chapter-VI), summary of the work is presented and the results have been discussed.