0016-7622/2014-84-2-221/$ 1.00 © GEOL. SOC. INDIA Estimation of Specific Yields of Individual Litho-units in a Terrain with Multiple Litho-units: A Water Balance Approach RANJAN KUMAR RAY, ARUNANGSHU MUKHERJEE and RUMI MUKHERJEE Central Ground Water Board, Ministry of Water Resources, Govt. of India, Bhujal Bhawan, Faridabad, Haryana Email: ranjan.ray@lycos.com Abstract: In a terrain with more than one litho-unit, the traditional water balance approach provides a single spatially averaged specific yield. A methodology is proposed here, which can be applied to estimate specific yields of individual litho-units in such terrains. This approach is demonstrated here considering two watersheds, which are covered partly by limestone and partly by sandstone. Watershed wise specific yields were estimated using a traditional water balance method. The specific yields thus obtained are the volume-weighted averages of the specific yields of the individual litho- units in the watersheds. Based on the volumes of aquifers desaturated and the watershed wise specific yield values, a set of two linear equations in two variables was formulated. These linear equations were solved to get the specific yields of the individual litho-units. Specific yields of sandstone (Chandarpur Group) and limestone (Charmuria Formation) units in the study area were thus estimated to be 0.004 and 0.037 respectively. Keywords: Specific yield; Dry season water balance, Chhattisgarh Basin. INTRODUCTION There are several field and laboratory methods for estimation of specific yield (Meinzer, 1923; Meinzer, 1932; Johnson, 1967; Moench, 1994; Boucher et al., 2009; Jie et al., 2011). Each method has its own advantages and limitations. In practice, water balance approach has been used extensively for estimation of specific yield (Saha a nd Agrawal, 2006) mostly because it is based on conventionally available data and provides good approxi- mation of spatially and temporally averaged specific yields. In a watershed covered by a single geological unit, water balance study provides an average specific yield value of the concerned geological unit. However, in a watershed with more than one geological unit, the water balance approach provides only one specific yield value for the entire watershed irrespective of the different lithological units. The objective of this paper is to demonstrate an approach by which specific yields of individual litho-units can be estimated by applying water balance method. The proposed method in described here taking a case study from central India. STUDY AREA DESCRIPTION For application of traditional water balance approach, it is unlikely to get two separate watersheds with uniform geology so that specific yields of individual units could be estimated. On the other hand, it is more likely to get two watersheds where each of them is covered by both the geological units. Two watersheds (Fig.1) in Seonath sub- basin located in Durg district of Chhattisgarh state were identified to demonstrate the applicability of the proposed methodology. These watersheds namely Choraha nala watershed (CNW) and Devrani Jethani nala watershed (DJNW) occupy 95 and 122 km 2 area respectively. The watersheds are covered partly by Chandarpur sandstone and partly by Charmuria limestone (Fig.1). These rocks belong to the Precambrian Chhattisgarh Supergroup of rocks (Das et al., 1992; Mukherjee et al., 2014). Chandarpur sandstone is the bottom most unit of the sequence (Precambrian Chhattisgarh Supergroup) and it unconformably overlies the granitoids. Thickness of Chandarpur sandstone varies from 20 to 90m. Charmuria limestone conformably overlies the Chandarpur sandstone. Thickness of Charmuria limestone varies from 200 to 250m. In terms of ground water potential, Charmuria limestone, owing to the presence of karsts is much more productive in comparison to Chandarpur sandstone, which for large parts is highly silicified. Average annual rainfall in the area is 1142 mm (IMD: India Meteorological Department). Majority of the rainfall is caused by SW monsoon, which commences during mid JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.84, August 2014, pp.221-226