ARTICLES Jul 7, 2021

Surface Runoff

Runoff is water running off the surface. When there is rainfall, a portion of it seeps into the ground and replenishes groundwater. However, most of it flows out or runs off. Surface runoff may also be due to the water from snowmelt, stormwater, snowfall etc. In runoff, soil is saturated by water and cannot absorb the water anymore.

Much of the runoff ends up as salt water in oceans. The downhill runoff gathers into streams or creeks, and they reach a river before emptying into the ocean. Most of the water in rivers come from overland runoff. Part of this surface water is also used for human consumption and seeps into the ground in these water bodies recharging the aquifers. However, in seasons when precipitation rates are high(high rainfall or snowmelt), the surface runoff resulting in water being drained into the ocean is high. If the land is forested or there are trees and other vegetation, more water is absorbed into the ground. Urbanisation is causing loss of forestland and impervious surfaces that cause water not to be able to seep into the ground, causing water runoff leading to urban flooding. Runoff is a major cause of soil erosion leading to loss of nutrients in the soil affecting agriculture.

The water can also collect pollutants while being carried downhill. This can pollute waterbodies like rivers and even the groundwater resources when the polluted water seeps into ground subsequently.

In short, major effects of runoff are:

  • Soil erosion – Runoff over land can carry the topsoil and deposit them into rivers and ocean.
  • Pollution of surface and ground water
  • Flooding – When the ground cannot absorb water, it can lead to flooding especially in urban areas with poor vegetation and concrete grounds.
  • Depletion of Groundwater – Surface runoff due to change in land structure can reduce the water table in the area over a time.