Surface Runoff
Feeding the Rivers
Surface runoff is precipitation runoff over the landscape. You may have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. That is "overly simplified" because rivers also gain and lose water to the ground. Still, it is true that much of the water in rivers comes directly from runoff from the land surface, which is defined as surface runoff. When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway and into the bar ditch, then to a creek. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers. The runoff sometimes flows over bare soil and deposits sediment into the river (not good for water quality). Thus begins the run-off's journey to the ocean. As with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is , evaporated, transpired, or soaks into ground water. Surface runoff can also be diverted by humans for their own uses. The primary municipal water source for the City of Blanco is the Blanco River. The secondary source is piped from Canyon Lake - more surface runoff. Back to Water Cycle
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