Unimpaired runoff

Unimpaired runoff, also known as full natural flow, is a hydrology term that is used to describe the natural runoff of a watershed or waterbody that would have occurred prior to anthropogenic or human influences on the watershed. Flow readings from river gauges are influenced by upstream diversions, impoundments, and many other alternations of the land that drains into a watershed or of alternatives of a river channel itself. Engineers estimate unimparied or natural runoff by estimating all of the effects of human "impairments" to flow and then removing these effects. Since these calculations involve many assumptions, they tend to be more accurate for either smaller watersheds or when expressed as longer period averages.

Unimpaired runoff is important for legal and scientific reasons. Since human development continues to alter watersheds, unimparied runoff provide fixed frames of references for flow rates. The reason unimpaired runoff is important is because long-term hydrologic records are often used to develop relationships between precipitation, runoff, and water supply. By removing changes in the timing between precipitation and runoff due to human influences, the long-term relationships will be more useful.

Calculating unimparied runoff is also extremely important in identifying long-term climate change impacts. By subtracting the known human influences on a long-term hydrologic record, the records may still show signs of a long-term change. These long-term signals are sometimes described as long-term climate change. It is still possible that the long-term climate signal is caused by larger scale antropogenic sources.

Examples of Use

Unimpaired runoff calculations are used extensively in western United States states such as California for water resources management applications, particularly in the calculation of water year classifications and river indexes.

The following is a list of some examples of use of unimpaired runoff calculations:

• Sacramento Valley 40-30-30 Index (California)

References

• California Cooperative Snow Survey
• NOAA Colorado River Water Supply Outlook