13.4 What Happens When It Rains?

When precipitation falls from the sky, several things can happen to it. Some will be intercepted by plants before it reaches the ground; this water can evaporate or can eventually fall to the ground or move down the stems of plants or trees to the ground. Some of the rainfall that reaches the ground can infiltrate into the soil.

The infiltration capacity of the soil refers to the rate at which the soil can absorb water. For dry soils, the infiltration capacity is high due to capillary action pulling the water into the soil. As the soil becomes wetter, the infiltration capacity diminishes, and less water can infiltrate.

Infiltration capacity changes as a function of time throughout a rain event and follows an exponential decay curve that approaches a minimum called the equilibrium infiltration capacity, which represents the lowest rate that the infiltration capacity will reach.

If the precipitation rate does not exceed the equilibrium infiltration capacity, all the precipitation reaching the land surface will infiltrate and there will be no runoff. If the precipitation rate is greater than equilibrium infiltration capacity, but less than the initial capacity, initial infiltration will accept all the precipitation followed by runoff or formation of puddles of water on the surface—called depression storage. If the precipitation rate exceeds the equilibrium infiltration capacity then infiltration will occur at the rate of the initial infiltration capacity, while runoff and depression storage commence immediately.

Water that infiltrates into the subsurface will percolate vertically unless it encounters the water table or some variation in permeability that causes it to move laterally. Layers of low‑permeability material in the subsurface can slow down the vertical percolation of groundwater and cause it to move horizontally towards a stream. This flow of water is called interflow.