The Rainfall/Water-Table Level Relationship of an Unconfined Aquifer

The soil parameters that determine the infiltration rates of a shallow unconfined coastal aquifer due to rainfall were obtained using the recursive least squares method. The parameters were predicted by minimizing the mean square error between the estimated and measured water-table levels in a bore hole. The parameters λ_t , a drainage factor, β_t , a parameter which accounts for external disturbances, and α_t , a set of parameters which determine the infiltration rates due to rainfall, were all treated as time-dependent variables. Their variation is traced by introducing a "forgetting factor" in the algorithm used to determine the parameters. The parameters were determined using rainfall and water-table levels for the year 1979 at a bore hole. The parameters β_t and α_t varied considerably over the period of time and λ_t remained constant. From the parameters obtained, it appears that most of the recharge due to rainfall takes place within the first two days of the event. After two days there is very little variation in the water-table level due to rainfall although there is a slight drop on the third, fourth and fifth day, presumably due to the escape of entrapped air beneath the water-table level.