Ground water Recharge Behaviour of Lateritic Aquifers in the Western Ghat Region - India

Abstract

For sustainable development of water resources, it is imperative to make quantitative estimation of the available water resources. It is necessary to maintain the ground water reservoir in a state of dynamic equilibrium over a period of time and the water level fluctuations have to be kept within a particular range over the monsoon and non-monsoon seasons. Ground water is a dynamic system. The total annual replenishable recharge is around 43 M ha m. In spite of the national scenario on the availability of ground water being favorable, there are many areas in the country facing scarcity of water. To assess the ground water potential, a suitable and accurate technique is required for a meaningful and objective analysis. A critical study has been carried out on the different methods of estimating the ground water recharge potential and compared to decide the most suitable technique for practical utility. In the present study, infiltration tests were conducted at 10 well locations. For this purpose, double ring infiltrometer is used. Two infiltrometers are used, one with inner dia 25 cm, outer dia 35 cm and another with inner dia 15 cm, outer dia 30 cm. The infiltration tests were conducted at 2 locations in each site; one location being at the ground surface and the other one is 1 m below the ground surface. It has been observed that the rate of infiltration with respect to change in diameter of infiltrometer doesn’t change much, but the rate of infiltration is more by 2.5 cm/hr on the surface compared to 1 metre below the ground surface level. It clearly indicates that, on the ground surface there will be a small amount of lateral flow in spite of using the double ring infiltrometer. Electrical resistivity methods have been widely used for various hydro-geological applications. In ground water studies, it is used to find the aquifer type, thickness of aquifer and water table depth. Surface electrical resistivity methods have been applied to the shallow, unconfined, alluvial aquifer adjacent to the Haladi River, between Ullure and Kundapur to estimate aquifer thickness and hydraulic conductivity. From the above tests, it is found that in the study area, the aquifer thickness ranges from 12 m to 18 m. Additional testing of aquifer material for specific gravity, grain size analysis, mapping of aerial–aquifer extent andiv analysis of soil properties yielded estimates of hydraulic conductivity 150-250 m/day. The relative high pervious nature of the alluvial aquifer indicated by these analyses assures an adequate ground water-resource potential from well fields located in the alluvial adjoining a reservoir that impounds the Haladi River. To estimate the annual dynamic ground water recharge of the river basin, the monthly average rainfall and the monthly average ground water level fluctuation data of 10 open wells during the period 1991-2005 is used (Dept. of Mines and Geology, Govt. of Karnataka). Zonation technique has been adopted considering the 10 open wells as the nodal point. The area of influence by the each open well has been calculated. Five methods adopted are, 1) Relationship between rainfall and recharge formula (Krishna Rao) 2) Ten year average water level fluctuation 3) Fluctuation between the lowest and highest water levels over ten years 4) Yearly water level fluctuation 5) Fluctuation in monsoon seasons. A critical study is carried out on the different methods of estimating the ground water recharge and compared to decide the most suitable technique for practical utility. Among the five methods used in the above estimation, yearly water level fluctuation gives accurate estimate, which is on line with Central Water Commission (CWC-2006) estimate. By yearly water level fluctuation method the annual yield of Haladi River basin estimated as 288.22 Mm3. The results of this study helps in accurate prediction of ground water potential of any hydrological unit. This in turn may avoid ground water over exploitation and help to restore the eco-systems.