Table 2.
Variables used in the variability approach
| Variables | Category | Units | Weighting factor | Reference |
|---|---|---|---|---|
| Range in predicted N and P concentrations in streams | Water quality | mg/L of N or P (logarithmically transformed) | 0.5 for both N and P | (Ator 2019, Robertson and Saad 2019, Wise et al. 2019, Wise 2019, Hoos and Roland 2019) |
| Minimum predicted N and P concentrations in streams | Water quality | mg/L of N or P (logarithmically transformed) | 0.5 for both N and P | (Ator, 2019; Hoos and Roland 2019; Robertson and Saad 2019; Wise 2019; Wise et al. 2019) |
| Range in N and P from manure and fertilizer application rates | Factor | kg/km2/year | 0.5 for both N and P | (Wieczorek et al. 2018) |
| Range in N and P from wastewater treatment plants per unit area | Factor | kg/km2/year as N or P | 0.5 for both N and P | (Wieczorek et al. 2018) |
| Range in N from septic systems per unit area | Factor | kg/km2 | 1 | (LaMotte 2018) |
| Range in tile drain density | Factor | Fraction of HUC08 | 1 | (Valayamkunnath et al. 2020) |
| Range in runoff rates | Factor | m/year | 1 | (Wieczorek et al. 2018) |
| Range in predicted nitrate concentrations in shallow groundwater | Factor | mg/L as N | 0.5 | (Nolan and Hitt 2006) |
| Range in predicted nitrate concentrations in groundwater at the depth of public supply | Factor | mg/L as N | 0.5 | (Ransom et al. 2022) |
| Mean percent residual in N and P predictions | Accuracy | kg/km2/year logarithmically transformed | 1 for both N and P | (Ator 2019, Robertson and Saad 2019, Wise et al. 2019, Wise 2019, Hoos and Roland 2019) |
| Total population served by public and domestic supplies per unit area | Receptor | People/km2 | 1 | (Johnson et al. 2019, Johnson et al. 2022, Johnson et al. 2021) |
| Percent waterbodies in basin | Receptor | % open water | 1 | (Dewitz and U.S. Geological Survey 2021) |
| Mean biotic index in basin | Receptor | Likelihood stream is not in good biological condition | 1 | (Hill et al. 2017) |
| N and P delivered (yield) to downstream receiving waters | Receptor | kg/km2/year | 0.5 for both N and P | (Ator 2019, Robertson and Saad 2019, Wise et al. 2019, Wise 2019, Hoos and Roland 2019) |
All ranges were computed from the 25th and 75th percentile of the HUC08 values. Note that the N and P variables were included in the ranking approach as separate variables. See supplemental information and Qi et al. (2023) for more details