Abstract: In the process of groundwater numerical simulation, groundwater withdrawal is always treated as certain sink item for long time. However, due to lack of accurate statistic data, there exists uncertainty in determining groundwater withdrawal, especially groundwater withdrawal for agricultural irrigation. The traditional method is to simplify the irrigation as several pumping wells with certain pumping rate. Obviously, it can not reflect the real uncertainty of pumping rate. Monte Carlo method is used to structure the stochastic numerical simulation model to fully consider this uncertainty. Based on the probability theory and mathematical statistics, it is concluded that the pumping rate of the hypothetical wells follows normal distribution. The results demonstrate that the traditional method which ignores the effects of uncertainties can not provide the information of reliability of simulation results. But the stochastic simulation can give necessary results for different reliability demands.

Key words: numerical simulation of groundwater, groundwater withdrawal, MonteCarlo method, reliability analysis