Drainage and reduced water inflow are important reasons threatening the existence of wetlands at the present time. Based on the hydrologic criteria published by USDA-SCS in 1991, we conducted a modeling analysis with the field hydrology model-DRAINMOD on the change of wetland hydrology. The analysis focused on the change in water volume due to artificial drainage in humid areas and reduced inflow in arid areas. The model simulation results show that in humid areas with annual precipitation more than 1 000 mm, wetland hydrologic criteria can be easily satisfied with no artificial drainage. When drain spacing is larger than 150 m, half year still have wetland hydrology. For an arid area with annual precipitation less than 250 mm, minimum base inflow required to maintain wetland hydrology in the growing season is 173 mm and 155 mm, respectively, for the beginning and ending months of the growing season, which are 22 and 16 times of the average precipitation of the two months.