Water balance study of Karuvannur river basin

Abstract

Water balance techniques are a way of solving important theoretical and practical hydrological problems. By using the water balance approach, it is possible to make a quantitative evaluation of the water resources and to assess any changes that might occur through the influence of man's activities. The broad objective of this project was to make a quanti-tative evaluation of the water resources of the Karuvannur River Basin. The ultimate objective was to study the monthly water balance for the basin in order to estimate the balance available for ground water recharge or depletion during the period 1976 to 1985. Monthly rainfall data at all rain gauge stations in the basin were collected. The mean monthly rainfall in the basin for each month of the period 1976 to 1985 was calculated using Thiessen polygon method. Irrigation water was identified as the other input into the basin. Data regarding the amount of water released for irrigation from the Peechi Reservoir was collected. Due to lack of data, contribution from other sources was not taken into account* Surface and subsurface in flow and out flow, into and from the basin, were assumed to be negligible.

Data regarding total runoff from the basin during each month of the above period was. also collected. The amount of water used up in meeting evapotranspiration requirements was estimated. • Daily rainfall data at Ollukkara for a period of 20 years was collected and a frequency analysis was done. The result has been presented graphically. Equations for predicting annual runoff from the basin were developed. It was found that a high degree of correlation exists between annual runoff and annual rainfall values. For each month during the period 1976 to 1985, the balance water, available for ground water accretion or to be met from the ground water reservoir, (unaccounted water) was estimated. It was found that accretion to ground water occurs mainly in June, July and August, Some recharge may occur in the months of May, September, October and November also.

♦ When the water balance is calculated on an annual basis, it was found

that the balance available for ground water accre¬tion is negative in 1976, 1984 and 1985. Low rainfall had been recorded during these three years. The values of the balance water available for ground water accretion or depletion (unaccounted water) when cumulated gave very high values. This may be because of unaccounted factors such as subsurface runoff from the basin, unaccounted portion of surface runoff from the basin etc. These factors could not be accounted for due to lack of data.

The study underlines the necessity for using the available water resources most judiciously. It emphasises the need for adopting appropriate rain water harvesting methods for reducing runoff during the dry months. Optimum development of water resources can be achieved only through the conjunctive use of surface and ground waters.