Groundwater levels vary with place to place with each state. Your argument doesn't hold true for all TN's cauvery districts. For example,if you take Dharmapuri, salem districts, the landscape is highly rocky and uneven, hence the groundwater quantity in these districts is less. But due to the then TN govt's populist move, compulsory rain harvesting tanks in each household and establishments has become a boon for TN. But denying cauvery water citing groundwater level as a reason is atrocious as the extent of cultivated area between these two states vary enormously.
