Reference Watershed Approach
A list of similar, nonimpaired, candidate reference watersheds and data considered most relevant for comparison with the impaired watersheds was assembled. Watershed characteristics evaluated in selecting the reference watershed included mean elevation, land-use distribution, physiographic region, slope, soil erodibility, and VSCI score (Table
2). The 2000 Mid-Atlantic Regional Earth Science Applications Center (RESAC) land-use data (
Goetz et al. 2000) were used when comparing impaired and candidate reference watershed land-use distributions. Soils data used for selecting the candidate reference watersheds came from the USDA Natural Resources Conservation Service Soil Survey Geographic Database (USDA-NRCS SSURGO) (
USDA 2011a).
While no two watersheds are identical, a candidate reference watershed that was in the same eco-region and most closely matched the slope and land-use distribution of each impaired watershed was selected as the corresponding reference watershed. Robinson River was selected as the reference watershed for Taylor Creek, and Upper Opequon Creek and Toms Creek were selected as reference watersheds for Long Meadow Run and Turley Creek, respectively (Table
2 and Fig.
2).
The GWLF model was used to simulate sediment loads for the impaired and reference watersheds for a 15-year period (1986–2000). The meteorological data used for these simulations were obtained from the Chesapeake Bay TMDL program (
USEPA 2010a) and were consistent with the data used in the CBWM in order to limit the variability between the RWA and the DM.
GWLF is a lumped parameter model developed to simulate monthly sediment and nutrient loadings in nongauged watersheds (
Haith and Shoemaker 1987). GWLF simulates surface and subsurface flows, sediment yield and sediment delivery, and dissolved and attached nitrogen and phosphorous loads from rural, urban, and mixed-land-use watersheds. It can also simulate septic system loads and accommodate point source discharge data (
Evans et al. 2003). GWLF assumes that all the sediment generated within a given year exits the watershed during the same year (no net annual sediment deposition). The GWLF model-year runs from April 1st to March 31st (
Borah et al. 2006). GWLF requires that land uses be divided into rural (predominantly pervious areas) and urban (predominantly impervious areas) categories. The sediment available for transport from pervious areas is multiplied by a sediment delivery ratio based on watershed size and a transport capacity based on average daily runoff (
Yagow et al. 2004). Sediment loads from impervious areas are calculated using the Sartor and Boyd (
1972) equation, an exponential function that describes the buildup and wash-off of sediment from impervious surfaces. GWLF estimates sediment loading for a given watershed by aggregating loads from all land-use areas into a single watershed sediment yield. Flow routing and streambank and channel erosion were not considered in the original GWLF model (
Haith 1985). However, to improve the model’s efficiency, researchers at Penn State University developed a regression equation that can be used to calculate streambank and channel erosion (
Evans et al. 2003). This equation was added to the ArcView version of GWLF (AVGWLF,
Evans et al. 2003). AVGWLF was further modified to consider monthly sediment yield by land use and to allow sediment loads from impervious areas to be included in the total sediment load generated by the watershed. The AVGWLF model uses a daily time step to estimate sediment yield, which is then aggregated into a monthly yield. Because GWLF was originally developed for use in ungauged watersheds and is typically not calibrated, the default values suggested by Haith et al. (
1992) were used for this application. However, parameters related to sediment and nutrient transport may be adjusted if necessary (
Qi et al. 2017).
For the RWA modeling, watershed boundaries for the impaired and reference watersheds were delineated using the 30-m National Elevation Dataset (NED) obtained from the NRCS-USDA Geospatial Data Gateway (
USDA 2011b). Soil data were also obtained from the NRCS-USDA Geospatial Data Gateway. The outlet of each impaired watershed was coincident with the downstream limit of the impaired stream segment as defined by VDEQ. Outlets for each referenced watershed were designated as the closest tributary confluence downstream from the corresponding biological monitoring station. The watershed areas, land-use polygons, and flow lengths were determined using ArcGIS 9.3. Soil erodibility factors (SSURGO K-factor), percent slope, and mean elevations were calculated for each land-use category using the zonal statistics function in ArcGIS. Land-use information for the impaired and reference watersheds was derived from the 2000 RESAC land-use dataset (
Goetz et al. 2000). The RESAC land-use classes and their distributions within the impaired and reference watersheds are shown in Table
3.
The Biological Systems Engineering (BSE) department at Virginia Tech uses 12 standardized land-use categories when modeling with GWLF (G. Yagow, personal communication, 2011). The RESAC land-use data categories were aggregated into the land-use categories used by BSE (Table
4). Several of the BSE land-use categories included two or more of the RESAC land uses, while others split some of the RESAC data into two or more BSE land-use categories (e.g., the RESAC land-use category cropland was split between high-till and low-till). The grouping or splitting of land-use data is based, in part, on data from the 2011 National Land Cover Database (NLCD) (
Homer et al. 2015), the Conservation Technology Information Center (
CTIC 2016), the agricultural census (
USDA 2014), and the best professional judgment of the modeler.
Yagow et al. (
2002) created two Microsoft Excel spreadsheets (“WATERSHED” and “LANDUSE”) to facilitate GWLF parameterization. The spreadsheets contain lookup tables with parameter values based on characteristics such as watershed location, soil type, and land-use categories. Use of these spreadsheets provided consistency when parameterizing GWLF.
Often, reference and impaired watersheds are not the same size. The size of a reference watershed is typically adjusted to match the impaired watershed. This area adjustment maintains the land-use distribution in the reference watershed. The area adjustment for the reference watershed is calculated as
where
= area-adjusted reference watershed (ha);
= area of the impaired watershed (ha);
= area of the reference watershed (ha); and
= area of each land use within the reference watershed (ha).
Because the sediment delivery ratio (SDR) and the mean channel depths are both functions of the watershed area, the area adjustment ensures that the SDR and the mean channel depth for the impaired watershed and its companion area-adjusted reference watershed are the same. The TMDL target sediment load for the RWA procedure is the simulated load from the area-adjusted reference watershed. The required sediment reduction is calculated as
where
= required sediment reduction (
);
= area-adjusted reference watershed sediment load (
); and
= impaired watershed sediment load (
).
Disaggregate Method
The disaggregate method used land-use-specific unit-area loads (UALs) from two CBWM model runs (an existing-condition run and the 2010 Chesapeake Bay TMDL target load run) and finer-scale, locally assessed land-use inventories of the impaired watersheds of interest in order to calculate TMDL target sediment loads for the impaired watersheds. The CBWM simulated annual sediment loads for the period 1986–2000 for land-based source categories (i.e., nonpoint source loads) for the 2009 progress and 2010 watershed implementation plan (WIP) simulations for the Chesapeake Bay land-river segments that encompassed the impaired and reference watersheds used in this study (
USEPA 2010b). The 2009 progress scenario (
USEPA 2010b) included pollution controlling best management practices (BMPs) in place in 2009 based on data reported by Virginia to the Chesapeake Bay Program (CBP). The 2010 WIP scenario included those BMPs that were proposed to be implemented by the state to meet the Chesapeake Bay TMDL. Table
5 shows land-use-specific unit-area loads for Albemarle County land-river segment (A51003_to_JL1_6770_6850). The UALs were calculated by dividing the annual sediment load for a given land use by the corresponding area:
where
= unit-area load for a given land use/source category (
);
= simulated annual sediment load for the land use/source category (
); and
= area of the land use/source category within a given land-river segment (ha).
For consistency between the RWA and the DM, the same RESAC land uses used in the DM were used to parameterize the GWLF model. To accomplish this, it was necessary to associate locally assessed RESAC land-use classes with the 24 sediment-generating land-use categories used in the CBWM. The CBWM land-use categories were created from a combination of data, such as NLCD imagery and statistical data from the USDA’s agricultural census (
USDA 2014). The 24 sediment-generating land-use categories from the Phase 5.3 CBWM were associated with the appropriate corresponding land-use categories from the GWLF modeling in the RWA. Using the percentage of each CBWM land-use category associated with each CBWM model run, the areas of the 12 broader GLWF land uses in the impaired and reference watersheds were redistributed, as shown in Table
6. The land-use distribution for the 2010 WIP run may be different from the 2009 progress run because some pollution control measures simulated in the 2010 WIP run required a land-use change in the CBWM.
Taylor Creek watershed spanned two CBWM land-river segments (A51125_JL1_6770_6850 and A51003_to_JL1_6770_6850; Turley Creek and Long Meadow Run both lay within one land-river segment (A51165_PS2_5560_5100). Existing sediment loads were calculated by multiplying the 2009 progress run UALs with corresponding locally assessed land-use areas, and the TMDL target loads were calculated by multiplying the 2010 WIP UALs with corresponding locally assessed land-use areas. For example, if the UAL for high-till w/o manure land use was based on the 2009 progress run, and the total area of high-till w/o manure land use within the watershed was 30 ha, the total existing load generated from that land use is (2.4 times 30) .