Quantification of groundwater recharge in a hard rock terrain of Orissa: a case study Ranu Rani Sethi, A. Kumar and S. P. Sharma ABSTRACT Ranu Rani Sethi (corresponding author) A. Kumar Water Technology Center for Eastern Region, Bhubaneswar, Orissa, India E-mail: ranurani@yahoo.com S. P. Sharma IET, Lucknow, Uttar Pradesh, India A study was carried out to select the best method to estimate groundwater recharge in a hard rock terrain. Various standard empirical methods, soil-moisture balance method, water table fluctuation (WTF) method and commonly adopted norms set by Groundwater Estimation Committee (GEC), Govt of India were used to estimate recharge for the Munijhara watershed in the Nayagarh block of Orissa (India). The empirical formulae gave recharge rates ranging from 13 cm to 32 cm/year with average of 22.4 cm and standard deviation of 5.34, independent of other influencing factors like soil, topography and geology. The soil-moisture balance study indicated that recharge is more dependent on the continuous heavy rainfall total annual volume of rainfall. Recharge was limited at up to 10 mm per day, possibly due to presence of hard rock below the soil surface. The rise in water table depth was 3.45 m to 5.35 m with a mean rise of 4.5 m during the year 2006–2007. Annual groundwater recharge based on the WTF approach varied from 10.3 to 16.85 cm with a mean of 13.5 cm, standard deviation of 1.57 cm and coefficient of variation 11.57%. This recharge accounted for 8 to 14% of rainfall received. With a water budget approach based on GEC norms, recharge was calculated as 17 cm per year. The study showed that the magnitudes of annual groundwater recharge as estimated by the WST method and GEC norms are in conformity with other recent findings in India under the same climate conditions. Based on the results recharge structures could be planned in suitable locations to reduce fallow areas under the watershed. Key words | hard rock, monsoon recharge, recharge, watershed INTRODUCTION Recharge is defined as “the entry into the saturated zone of water made available at the water table surface together with the associated flow away from the water table within the saturated zone’ (Freeze & Cherry 1979). Groundwater recharge is a key component in a hydrologic cycle, which is influenced by climate, geomorphology (including topography, soil and vegetation), and geology of any particular area. Choosing the best technique out of various available methods to quantify recharge for a particular area is a difficult task. Methods like empirical formulae (rainfall analysis), water budget, Water Table Fluctuation (WTF) and soil moisture balance techniques are generally being adopted to quantify the recharge under different agro-climatic conditions. These methods consider temporal and spatial variation in estimating recharge and involve a wide range of complexity and expense. Empirical relationships between rainfall and recharge are developed for different climatic conditions in India (Kumar 1977). But, these are independent of other influen- cing factors of the recharge. The soil moisture balance technique for assessing groundwater recharge was used by Houston (1990) and Howard & Karundu (1992). This method provides periodical estimates of direct recharge (i.e. from the infiltration of rainfall) based on changes in doi: 10.2166/wst.2009.403 1319 Q IWA Publishing 2009 Water Science & Technology—WST | 60.5 | 2009