ANALYSIS OF DEPENDABILITY CRITERIA IN RESERVOIR PLANNING - A CASE STUDY

Abstract

The practice followed in India is to plan a project for a target demand of not more than 75% dependable yield (with associated temporal distribution) and then test the performance of the system so planned to ensure 75% success of the project. Thus the two concepts namely planning for utilization 75% dependable yield and planning to achieve 75% success of the project are two different concepts. Prevalent criteria for irrigation planning have several drawbacks as given below. • Planning is based on annual reliability only. Seasonal reliability is also important, as crops having different economic value to farmers and to the society are grown in different seasons. • Quantum of failure (water deficit), time length of failure, period of failure vis a vis planned growth, crop specific failure (sustenance crop, cash crop), are not reflected in the prevalent criteria. • Region specific characteristics (drought proneness, differences in hydrologic characteristics of catchment and location of storage site) may necessitate adoption of different planning criteria. The comparative of capacity utilisation in six existing reservoirs in India was carried out. Considerable quantity of live storage is available in carry over storage schemes even at minimum annual condition compared to "within year" storage schemes. 'Variability of the annual minimum storages over the years is large and for a large number of years, the near full condition is not achieved as compared to "within year" storage. Yield series of Mahanadi river at Manibhadra dam site has been synthesised and dependability of inflows analysed and compared with previous studies. The purpose was to adopt an appropriate yield series for reservoir simulation study. The estimated annual yields of 70% dependability are within 95% confidence limits of 75% dependable flow. Similarly estimated 65% dependable flow (3894.47 Th. ha.m) is within 95% confidence limits of 75% dependable flow. In other words, water utilisation planning with 70%, 65% or 60% dependable flows (estimated) may in reality have higher dependability upto 80%, 75%, 70% respectively in consideration of 95% iii Ab stract confidence limits. This aspect also needs to be kept in view while planning for dependable utilisation of flows as per prevailing procedure. It is generally thought that dependable yields in hypothetical years of various dependability are too conservative and therefore should not be used in planning. The study shows that it need not necessarily be so. Therefore while planning for dependable water utilisation on the basis of actual monthly flows in 75% dependable year, the coefficient of variability of flows in each month should be taken into account. Dependable' flows of hypothetical years should be used unless estimates happen to be too conservative. It is also observed that annual flow series shows periodicity over the years. It will be useful to carry out analysis of periodicity using upto date data. Synthesised monthly inflows at Manibhadra dam site were used in simulation study. In the project report proposed storage capacity is 580 Th. ha.m and annual irrigation withdrawal is 1650 Th. ha.m. (31.20% of mean annual flow) corresponding to 75% annual dependability. Annual reliability, time reliability, Kharif season reliability, Rabi season reliability at different annual withdrawal levels (with given monthly distribution) and storage capacities have been worked out. Seasonal reliability appears to be a better criteria as compared to annual reliability as reliable irrigation water supply is crucial for irrigated agriculture in rabi season and may not be so crucial in Kharif season. It is suggested that in case of Manibhadra storage scheme, design irrigation areas could be different for Kharif season crops and Rabi season crops with Kharif design area being more and having lesser reliability (say 67%) and Rabi season area being less'but higher reliability (79.17%). Cost and benefit have been evaluated as function of reservoir capacity and annual irrigation withdrawals. Storage, withdrawal reliability relationship were converted to cost, benefit, reliability relations. Effect of increase in height of reservoir on economic parameters has been evaluated for (i) fixed annual withdrawal and for (ii) fixed annual reliability of 75%. Long term simulation study is preferred over Gould's probability matrix method in case of Manibhadra reservoir.