A U.S. Geological Survey summary of water year 2020 (October 1, 2019, to September 30, 2020), published in September 2021; some of its data were provisional.
How streamflow is ranked
The summary places water year 2020 among the 91 water years from 1930 to 2020, using observed data from the USGS National Streamgage Network. It starts in 1930 because before then most regions had too few streamgages for meaningful statistics.
- Runoff is the flow in a stream divided by the area of its drainage basin — an equivalent depth of water over the basin. Dividing by area makes flows comparable between places. Runoff for an area and period is computed from every streamgage there with a complete record.
- Ranks. A rank of 1 is the highest annual flow of all years analysed; 91 is the lowest. Ranks fall into five categories by percentile of flow:
| Category | Percentile of flow |
|---|---|
| Much below normal | less than 10 percent |
| Below normal | 10–24 percent |
| Normal | 25–75 percent |
| Above normal | 76–90 percent |
| Much above normal | greater than 90 percent |
States and regions are listed from highest to lowest flow; a record high or low is not shown where ranks tie. In these rankings "States" include the District of Columbia, and Puerto Rico is combined with the Virgin Islands.
The nation
Runoff in the contiguous United States in water year 2020 was 11.10 inches, above the 1930–2020 mean of 9.40 inches. Nationwide, average annual streamflow ranked 10th highest of the 91 years.
Figure 1. Annual runoff in the contiguous United States, water years 1930–2020. Credit: U.S. Geological Survey.
By state
| Annual streamflow | States |
|---|---|
| Record high | Michigan |
| Much above normal | Alabama, Arkansas, Illinois, Kentucky, Minnesota, Mississippi, Missouri, Nebraska, North Carolina, North Dakota, South Dakota, Tennessee, Wisconsin |
| Above normal | District of Columbia, Georgia, Iowa, Ohio, Oklahoma, South Carolina, Virginia, West Virginia |
| Below normal | California, Colorado, Hawaii, Nevada, Oregon |
Figure 2. Statewide streamflow ranks for water year 2020, compared with mean annual streamflow for 1930–2020. Some colours in the key may not appear on the map; for Puerto Rico combined with the Virgin Islands, 77 years of data were available and the rank was adjusted. Credit: U.S. Geological Survey.
By region
The country divides into 21 water-resources regions. Each holds the drainage of a major river, the combined drainage of a series of rivers — the Texas-Gulf region takes in several rivers draining into the Gulf of Mexico — or an island or island group. They give a watershed-based frame for comparing streamflow.
| Annual streamflow | Regions |
|---|---|
| Record high | Tennessee |
| Much above normal | Arkansas-White-Red, Great Lakes, Ohio, Souris-Red-Rainy, Upper Mississippi |
| Above normal | Lower Mississippi, Missouri, South Atlantic-Gulf |
| Below normal | California, Hawaii, Great Basin, Rio Grande, Upper Colorado |
Figure 4. Regional streamflow ranks for water year 2020, compared with mean annual streamflow for 1930–2020. Credit: U.S. Geological Survey.
By season
| Streamflow | Autumn (Oct–Dec 2019) | Winter (Jan–Mar 2020) | Spring (Apr–Jun 2020) | Summer (Jul–Sep 2020) |
|---|---|---|---|---|
| Record high | Michigan, Minnesota, North Dakota, South Dakota, Wisconsin | — | — | — |
| Much above normal | Illinois, Iowa, Nebraska | Alabama, Arkansas, Georgia, Illinois, Indiana, Kansas, Kentucky, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Oklahoma, Puerto Rico and the Virgin Islands, South Carolina, South Dakota, Tennessee, Wisconsin | North Carolina, Virginia, West Virginia | Arkansas, Delaware, District of Columbia, Michigan, North Dakota, Wisconsin |
| Above normal | Alabama, Arkansas, Kansas, Kentucky, Maine, Mississippi, Missouri, Montana, New Mexico, New York, Oklahoma, Vermont, Wyoming | Iowa, North Carolina, North Dakota, Ohio, Vermont, Washington, Wyoming | Arkansas, District of Columbia, Kentucky, Michigan, Missouri, North Dakota, South Carolina, South Dakota, Tennessee, Wisconsin | Alabama, Illinois, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, South Dakota, Tennessee, Virginia |
| Below normal | Hawaii, Puerto Rico and the Virgin Islands, Washington | Delaware, Idaho, Rhode Island | Colorado, Nevada, New Mexico, New York, Oregon, Puerto Rico and the Virgin Islands, Vermont | Massachusetts, New Hampshire, Utah |
| Much below normal | Oregon | California | Hawaii | Colorado, Connecticut, Hawaii, New Mexico, Rhode Island, Vermont |
| Record low | — | — | — | Arizona, Maine |
| Nationwide rank | highest of 91 | 6th highest | 34th highest | 25th highest |
Figure 5. Autumn (October–December 2019) statewide streamflow ranks, compared with mean quarterly streamflow for 1930–2020. Credit: U.S. Geological Survey.
Figure 6. Winter (January–March 2020) statewide streamflow ranks, compared with mean quarterly streamflow for 1930–2020. Credit: U.S. Geological Survey.
Figure 7. Spring (April–June 2020) statewide streamflow ranks, compared with mean quarterly streamflow for 1930–2020. Credit: U.S. Geological Survey.
Figure 8. Summer (July–September 2020) statewide streamflow ranks. The fact sheet's caption compares them with mean annual streamflow for 1930–2020, where the other seasonal maps say quarterly. Credit: U.S. Geological Survey.
High and low flows
If each streamgage's data were independent of the others and normally distributed, in any month about 5 percent of streamgages would report high flow (above the 95th percentile) and about 5 percent low flow (below the 5th percentile).
- High flow was more common than expected in six months of water year 2020: October (10 percent of streamgages), November (8), December (7), January (11), February (10) and May (7).
- Low flow exceeded the expected share in only one month, September 2020 (6 percent).
Figure 9. Percentage of streamgages with high and low monthly streamflow, October 1999–September 2020. Credit: U.S. Geological Survey.
The streamgage network
In water year 2020 the USGS ran more than 8,400 year-round, real-time streamgages nationwide; their real-time data are at https://waterwatch.usgs.gov. The same streamflow information serves water management, flood and drought monitoring, bridge design and recreation.
The network is part of the Groundwater and Streamflow Information Program. The USGS operates it, and its funding comes from the USGS and about 1,400 federal, state, tribal, regional and local partners.
Sources
- Jian, X., Wolock, D.M., Lins, H.F., Henderson, R.J., and Brady, S.J., 2021, Streamflow — Water year 2020: U.S. Geological Survey Fact Sheet 2021–3046. https://doi.org/10.3133/fs20213046
- The figures are taken from the fact sheet's PDF.
- Rewritten in hubnx's own words.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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