Elements of the water-cycle

A - Storage in ice and snow B - Precipitation C - Snowmelt runoff to streams D - Infiltration E - Groundwater discharge F - Groundwater storage G - Water storage in oceans H - Evaporation

I - Condensation J - Water storage in the atmosphere K - Evapotranspiration L - Surface runoff M - Streamflow N - Springs O - Freshwater storage

The Water Cycle: Surface Runoff

Surface runoff is precipitation runoff over the landscape (The following is based on the U.S. Department of the Interior's, United States Geological Survey (USGS), The Water Cycle, Surface Runoff).

The oceans are kept full by precipitation, i.e., rainfall, snow, and also by runoff and discharge from rivers and the ground.  Many people most likely have an overly-simplified notion that precipitation falls on land, flows or rushes 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. (United States Geological Survey, The Water Cycle, Surface Runoff).

 Flooding due to runoff

As rain (or melting snow) hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your street or driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain one might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers. This picture gives a graphic example of how surface runoff (here flowing off a road) enters a small creek. The runoff in this case,  gathers sediment, whether mud or sand, depending on the local soil type, and flows over bare soil, which is then deposited sediment into the river (not good for water quality). The runoff that enters such a creek then flows into a river, which then begins its journey back to the sea or 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 (infiltrates) into ground water. Surface runoff can also be diverted by humans for their own uses.

The small creek shown in the picture above will merge with another creek, eventually flowing into a larger river. Thus, this creek is a tributary to a river somewhere downstream, and the water in that river will eventually flow into an ocean. The concept is not that much different from the small capillaries in your body carrying blood to larger arteries, eventually finding its way to your heart, analagous to the ocean.

Meteorological factors which affect runoff:

• Type of precipitation (rain, snow, sleet, etc.)
• Rainfall intensity
• Rainfall amount
• Rainfall duration
• Distribution of rainfall over the drainage basin
• Direction of storm movement
• Precipitation that occurred earlier and resulting soil moisture
• Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season

Physical factors which affect runoff:

• Land use
• Vegetation
• Soil type
• Drainage area
• Basin shape
• Elevation
• Topography, especially the slope of the land
• Drainage network patterns
• Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or delay runoff from continuing downstream

Human activities which can affect runoff

As more and more people inhabit the Earth, and as more development and urbanization occur, more of the natural landscape is replaced by impervious surfaces, such as roads, houses, parking lots, and buildings that reduce infiltration of water into the ground and accelerate runoff to ditches and streams. In addition to increasing imperviousness, removal of vegetation and soil, grading the land surface, and constructing drainage networks increase runoff volumes and shorten runoff time into streams from rainfall and snowmelt. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams.

Urban development and flooding

Urbanization can have a great effect on hydrologic processes, such as surface-runoff patterns. Imagine it this way: in a natural environment, think of the land in the watershed alongside a stream as a sponge (more precisely, as layers of sponges of different porosities) sloping uphill away from the stream. When it rains some water is absorbed into the sponge (infiltration) and some runs off the surface of the sponge into the stream (runoff). Assume a storm lasting one hour occurs and one-half of the rainfall enters the stream and the rest is absorbed by the sponges. Now, gravity is still at play here, so the water in the sponges will start moving in a general downward direction, with most of it seeping out and into the streambanks during the next day or two.



Next, imagine that concrete and asphalts in the guise of streets, roads and buildings have replaced most of the watershed's treed and grassed surface. When that one inch of rainfall occurs, not able to infiltrate these impervious surfaces it either runs-off directly into streams, rivers and lakes and also very quickly.  This rush of water results in flooding as opposed to   the gradual rise and fall in rivers and lakes.  These floods cause roadways and homes accumulate water resulting in losses of property and commuting time.  Indeed, according to the USGS a flood of of even 10 minutes duration is enough to ruin ones basement.

This foregoing is illustrated in the hydrograph above of two creeks: a rural (Newaukum Creek - blue line) and an urban (Mercer Creek - green line) creek in Washington State. If one measures the area under both curves (the total volume of water that flowed by the measurement location for the time period shown on the X axis) in the chart, they might be the same. But in the urban stream, the water at the measurement site rose at a much higher rate and reached a much higher stage (height) than the rural stream did. The tall, steep curve of Mercer Creek demonstrates that much higher streamflows occurred in the urban stream. Moreover, the urban stream stage fell back towards baseflow much quicker, too, indicating that it wasn't receiving much seepage from ground water. The rural Newaukum Creek rose much slower and reached a lower peak, meaning it may not have flooded at all.  It took longer to fall back to baseflow as ground water slowly seeped into the streambanks over the next week.


Note that the numbers to the right of the two creeks' names is the numerical designation, i.e., the numerical name, assigned to these and other streams by the USGS.  The reason for numerically designating streams and rivers is that names change.  Moreover, the numerical designations incorporate a stream's or creek's location.

Finally, a graphical representation of the degree of water infiltartion versus runoff is demonstrated below. Reproduced from http://www.landscapeforlife.org, and the

 

RUNOFF AMOUNTS FROM DIFFERENT LANDSCAPES - As development increases, so do soil compaction and impervious surfaces. Compacted soils, along with driveways, roads, parking lots, rooftops, and other impervious surfaces, make it difficult for rain to infiltrate into the soil, as in a natural setting. As a result, the more impervious surface in a landscape, the less infiltration and the more stormwater runoff it generates. The illustration above shows the percentage of impervious surface and the amount of infiltration and runoff following a 3-inch rainstorm for each kind of landscape.

In a natural landscape, the soil and vegetation absorb precipitation like a sponge. In developed areas, however, much of the land has been paved over, and the soil itself is often compacted and impervious. The amount of rainfall exceeds the land's ability to absorb it, resulting in stormwater runoff.

Rainfall flows from our roofs to gutters and downspouts, over compacted lawns and driveways into roads, and down storm drains. In most older cities this stormwater can overwhelm sanitary sewers, sending raw sewage as well as runoff carrying fertilizers, pesticides, motor oil, and other pollutants into nearby waterways. Runoff also results in less water infiltrating through the soil to replenish groundwater supplies.

A sustainable home landscape is designed to keep stormwater on the property, minimizing damage to waterways and aquatic life.