Current hydrogeological prognosis is more frequently performed using mathematical modelling methods. In comparison to the still applied analytical calculations, they possess an advantage in that there is a considerably decreased amount of hydrogeological schematization. But also, in the case of the calculational scheme being identical to the model, the factors deciding the accuracy of the calculation as well as the greatness of the error committed can be indicated using mathematical modelling. Such aims were envisaged for the prognostic calculations of a well, positioned on an interfluwe. Analytical calculations and research simulations on a model were performed for 192 computational variables, in which variable aquifer filtration parameters and varying technical and exploitative conditions of the well were taken into consideration (Table 1). A comparison of the obtained results indicated the correctness of analytical calculations only in the case of a lack of surface recharge of the aquifer due to infiltration by atmospheric precipitation. The consideration of this element of the groundwater balance calculation, for unfavourable filtration conditions (low transmissivity), causes the prognosis of the well discharge to be burdened with an error of about 9% (Table 3), whereas the error in the determination of the height of the piezometric water table can reach even upto several hundred percent. The greatness of the calculated relative error is influenced by the cone of depression of the well as well as its position in relation to two parallel recharge boundaries. Under conditions of high transmissivity of the aquifer, in the range of several hundred m2/d, analytical calculation errors decidedly diminish attaining amounts that are practically meaningless in hydrogeological calculations.