When the forest trees are cut down, then the number of trees decrease which results in less transpiration. Reduced transpiration causes decrease in the amount of water vapour in the atmosphere resulting in less rainfall.
When there is less rainfall in an area, then less water percolates into the ground. Due to this, the groundwater level also gets lowered. The shortage of surface water (in ponds, lakes, etc.) and groundwater due to persistent low rainfall in an area can lead to droughts.
Deforestation also leads to decrease in the water holding capacity of the soil. The movement of water from the soil surface into the ground is reduced.So, there are floods.