Estimation of specific yield of hard-rock aquifers in Upper Cauvery River basin region in India by application of AMBHAS-1D groundwater model

Assessment of specific yield becomes crucial for an effective groundwater management in hard-rock aquifers in semi-arid regions, especially in southern India with its high dependence on groundwater for irrigation. Specific yield is an important parameter influencing water table fluctuations in groundwater and land-surface models as uncertainties associated with specific yield estimation are passed on to recharge estimation. In southern India, contemporary groundwater levels are heavily influenced by groundwater pumping and are spatially heterogeneous. Comparatively homogeneous natural groundwater levels in the upper weathered zone were observed in the earlier decades of 1970s and 1980s because of relatively lower pumping. Specific yield values estimated for the duration prior to 1990 are representative of upper bounds of specific yield values because of shallower water tables, hence we have selected the duration from 1980-1990 for this study. The existing water table fluctuation methodology by Maréchal et al. (2006) and Groundwater resource Estimation Committee (GEC, 2015) estimates specific yield based on net groundwater level decline during dry season corresponding to known or estimated groundwater draft. This methodology is not feasible for zero draft scenarios prevailing during 1980-1990. An alternate approach is required to account for discharge which was more dominant process to affect groundwater fluctuations when they are shallow. A physically based lumped model for unconfined aquifers, AMBHAS-1D is used in this study which is based on Park and Parker (2008) model. The model is applied on monthly groundwater levels at 100 sites tapping into a geologically homogeneous region of granitic gneissic aquifer in the Upper Cauvery River basin of Karnataka, India. Specific yield values are estimated for each of the 100 sites and a specific yield map for the region is prepared. Even though it is granitic gneissic rock in general, we observed a high variability in estimated specific yield of more than 10 orders of magnitude which can be associated with degree of fracturation, long-term rainfall trends, variation of water level and topographic impacts. Major area of Hassan and Mandya districts of Karnataka state have very low estimated specific yield (<=0.05%) indicating poor fracturing in those regions. Clusters of relatively high specific yield (>1%) are observed in south western part of Mysore district, Mysore city and southern part of Tumkur district depicting weathered upper zone.

References:

GEC (2015). Report of the Ground water resource Estimation Committee, Ministry of Water Resources, Govt. of India, New Delhi.

Maréchal, J. C., Dewandel, B., Ahmed, S., Galeazzi, L., & Zaidi, F. K. (2006). Combined estimation of specific yield and natural recharge in a semi-arid groundwater basin with irrigated agriculture. Journal of Hydrology, 329(1–2), 281–293. https://doi.org/10.1016/j.jhydrol.2006.02.022

Park, E., & Parker, J. C. (2008). A simple model for water table fluctuations in response to precipitation. Journal of Hydrology, 356(3–4), 344–349. https://doi.org/10.1016/j.jhydrol.2008.04.022