The study focuses attention on the use of inverse modeling technique for estimation of rainfall-recharge in a coastal aquifer. MODINV (Modular Inverse Model), which is software for parameter optimization of 3-D groundwater flow model, MODFLOW is applied to the Central Godavari delta in Andhra Pradesh to estimate the distributed rainfall-recharge during monsoon season.
The model is formulated based on the available information on the physical and hydrogeological framework of the study area. A number of recharge zones are defined in the model to take care of the spatial variation of recharge. The model is calibrated for the transient conditions during non-monsoon period of 1985 when the recharge takes place solely due to return flow of applied irrigation, the quantity of which is estimated before hand with the available data.
The calibrated model is then used to optimize the recharge during different stress periods of the monsoon season of 1985. The recharge from rainfall is computed by subtracting the estimated quantities of recharge due to return flow from the total model recharge. On a distributed basis, the rainfall-recharge coefficient in the lower, middle and upper reaches of the study delta is found to vary from 0.11 to 0.25.
The study concludes that –
- The inverse modeling technique is a viable distributed approach for estimating the rainfall-recharge.
- The distributed values of rainfall-recharge coefficient in the lower, middle and upper reaches of the study delta are found to vary from 0.11 to 0.25 during the monsoon period of 1985.
- The results of the study were also analyzed to compute the recharge coefficient on a lumped basis. The recharge coefficient for the study area as a whole is found to be 0.1717.
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