Cumulative distributions of IGA and HUC12 values of variables are used to calculate network metrics for the United States and each of its major river basins (HUC4) and to identify priority areas for maintaining or adding monitoring sites. A variable's determines the values used creating cumulative distributions and the criteria for identifying priority monitoring areas. There are six types of variables: value-value, value-dvalue, mean-area, multicategory, mean-gradient, and logical-length. The cumulative distributions of IGA and HUC12 variables can be viewed by topic for the United States or major river basins .
| Variable Type | Variables | Ranking Value for Cumulative Distribution | Accumulated Value for Cumulative Distribution | Priority Criteria |
|---|---|---|---|---|
| Value-value | Precipitation, evapo-transpiration, withdrawals, major NPDES point discharges, population | Value of variable aggregated spatially from source data for IGA or HUC12 | Value of variable aggregated spatially from source data for IGA or HUC12 | Maintain monitoring sites at boundaries of IGAs in the upper half of the cumulative distribution, add monitoring sites in IGAs with values 2 x mean value |
| Value-dvalue | Median annual mean, minimum, and maximum daily streamflow; mean annual loads of dissolved solids, nitrogen, phosphourous, and suspended sediment | Difference of values between downstream and upstream boundaries of IGA | Difference of values between downstream and upstream boundaries of IGA | Maintain monitoring sites at boundaries of IGAs in the upper half of the cumulative distribution, add monitoring sites in IGAs with values 2 x mean value |
| Mean-area | Fraction of area where minimum temperature is less than 0 C, land cover, federal and tribal land management | Spatial mean value of variable (fraction of area or density) for IGA or HUC12 | Area value for IGA or HUC12 | Maintain monitoring sites at boundaries of IGAs in the upper half of the cumulative distribution, add monitoring sites in IGAs with values 2 x mean value |
| Multicategory | Surficial geology, ecoregions, climate divisions | Spatial mean value of variable (fraction of area or density) for IGA or HUC12 | Area value for IGA or HUC12 | Maintain monitoring sites at boundaries of IGAs that are more than 90% in under-represented category (a cateorgy with less than 25% of its area in IGAs that have more than 90% of their area in the category), add monitoring sites in under-represented categories |
| Gradient | Mean elevation, drainage area, mean basin slope, mean stream slope, hydroperiod, minimum and maximum monthly temperature | Spatial mean or other value of variable for IGA or HUC12 | Area of IGA or HUC12 | Maintain monitoring sites at boundaries of IGAs in under-represented deciles of HUC12 values or range of IGA values (less than 7.5% of total IGA value in the HUC12 or IGA range decile), add monitoring sites where HUC12s are in an under-represented decile |
| Logical-length | Coastal streams, impaired water quality, Wild and Scenic Rivers | Length of stream with a designation aggregated spatially from source data for IGA or HUC12 | Length of stream with a designation aggregated spatially from source data for IGA or HUC12 | Maintain monitoring sites at boundaries of IGAs in the upper half of the cumulative distribution, add monitoring sites in IGAs with values 2 x mean value |
Konrad, C.P., Anderson, S.W., Restivo, D.E., and David, J.E., 2022, Network Analysis of USGS Streamflow Gages, U.S. Geological Survey Data Release, https://doi.org/10.5066/P9C8NYTO.