In the short term, heavy rainfall can lead to an increase in surface water, which can infiltrate into the ground and accumulate as groundwater, thus raising groundwater levels. However, in the long term, large amounts of precipitation may increase the flow and velocity of surface water by raising the level of rivers and lakes. This may result in groundwater sources being taken away and groundwater levels falling as a result. In addition, heavy rainfall may cause flooding and other natural disasters, such as landslides and mudslides, which may disrupt the groundwater system and cause groundwater levels to rise or fall.
The effects of rainy days on groundwater levels can therefore be complex and variable, depending on multiple factors and time scales.