STUDIES ON HYDROMORPHOMETRY AND SNOWMELT RUNOFF USING DATA OF CHENAB CATCHMENT

Abstract

The planning and development of water resources of a region requires proper understanding of the hydrological behaviour of the river basins. The analysis of available information is necessary to decide about water availability at a site or expected design discharge at a site. The pre-cipitation runoff process of a catchment is very complex phenomenon and an unknown large number of climatic and phy-siographical parameters, which vary with both space and time, control this phenomenon. The process is still more complicated, when precipitation includes both rain and snow and the basin physiography is mountainous in character, as in the case of the Himalayas. Besides the rugged topography and limited physical accessibility, the Himalayan basins are also faced with problem of limited data availability. As such, there is a need for development of appropriate procedures for estimating snowmelt runoff . The Himalayan region has the following typical seasons viz(i) snow accumulation season from November to February, (ii). snowmelt season from March to June and (iii) monsoon season from July to October.. In the present study, available data (for 1965-81) of the .Chenab basin (area 22850 sq. km) has been used. This includes daily precipitation, daily temperature and daily discharge at some measurement sites, besides other iii useful information available from topographical maps and satellite images (for 1975.81). The main thrust of the study is on the following specific objectives using data of different sub—basins in the Chenab basin: i) Evaluation of hydromorphometric parameters to quantify. the physiographic characteristics of the basin. ii) Development of regression relationships between the flood characteristics and hydromorphometric parameters. iii) Development of relationship between snowmelt runoff and extent of snow cover at the beginning (i.e. March) and during premonsoon season (i.e. March—June). iv) Development of a suitable model structure for model-ling of daily runoff from snowmelt and rain fall during snowmelt season (March—June) and monsoon season (July—September) for typical conditions of the Chenab basin viz. rugged topography, limited data availability, presence of permanent snow cover etc. The study has accordingly been carried out to achieve(-_'. above mentioned objectives and it involved use of: i) techniques for interpretation of toposheets and satellite images, (ii) techniques of statistical analysis, (iii) regionalisation of relationship between snowmelt run-off and snow cover characteristics, (iv) model formulation, calibration using optimization, testing with independent data, averaging of parameters and determination of overall parameters for entire basin and (v)' studies , for effect of change in melt rate, rain fall ?nd daily temperature pattern. iv A brief account of salient features of these studies and results obtained is given in following paragraphs. (a) From the study of hydromorphometric characteristics, it is found that the hydromorphometric relations exhibit deviations from the widely established and accepted laws of drainage compositions. These variations may not be apparently ascribed to lithological variations. It is interpreted that some of the deviations are related to extra-increase in mean stream length with stream order and could be a result of strong structural influence loading too elongated basin shape. Further,, it is inferred that the phenomenon of recent tecto-nic uplift, has also played an important role in creating the lack of hydromorphometric maturity and geometric simi-larity in the Chenab basin. (b) Using multiple regression analysis approach and judgement criteria of F test and t test, the relationships have been developed for average peak discharge Q2, maximum peak discharge Qmp and average annual flow '('Av in m /sec as *u _ dependent variables and drainage are A (km2), channel slope Sc () , drainage density Dd (km) and length of main stream channel Lc(km) as independent variables. The four relationships have been selected in the study which relate flow/flood parameters Qp, Qmp and QAv with A, Lc, A/Scud (Hickok et al parameter) . u (c) The Landsat MSS data mainly for the months of March-June for the years 1975-1981 were used for snow cover area mapping and the corresponding direct runoff (tot=~l runoff minus base flow) for premonsoon season (March-June) were computed for different sub-basins in the Ghenab_basin. Graphical and regression relationships have been developed between this direct runoff assumed as snowmelt runoff and snow cover area at the beginning of melt season. Good fit is indicated in these relationships. (d) Realising the need for dealing with typical condi-tions of the Himalayan basins viz, mountainous physiography, limited accessibility, permanent and temporary snow covers, limited data base etc. simple model structure with reason-able physical base, limited data requirements and capability of using remotely sensed data for snow cover area has been adopted. The snow cover depletion relationship between snow covered area and accumulated generated runoff was established for the sub-basins. For the parameter n of this relation-ship, relation was established with parameter (A/ScVDd)' Also regional relationships were established between total seasonal snowmelt runoff and snow covered area and between rate of increase of melt rate and weighted mean elevation for sub-basins.