Some precipitation runs overland by natural drainage channels where it eventually enters rivers, lakes and oceans. Harvesting water on the earth's surface may be done only before it evaporates. Surface runoff, for example, can be intercepted and stored in impoundments for later use. Water shortages in many areas of the world can be alleviated by harvesting surface runoff water. The following figures illustrate practical devices and systems used to harvest water.
Terraces are effective in harvesting surface runoff for cultivating rice and other crops.
Gabions are filled with rock and placed across small valleys to act as barrages to retard runoff. Runoff water collected behind the barrage seeps into the ground. Resulting high soil moisture allows farmers to plant crops behind the barrage after rainy season. These roof catchment devices may provide a family with enough drinking water to last through the dry season. Small ponds provide opportunities for agricultural diversification.