Assessing the Difference between Soil and Water Assessment Tool (SWAT) Simulated Pre-Development and Observed Developed Loading Regimes
The purpose of this research was to assess the difference between Soil and Water Assessment Tool (SWAT) simulated pre-development and contemporary developed loading regimes in a mixed-land-use watershed of the central United States (US). Native land cover based on soil characteristics was used to simulate pre-development loading regimes using The Soil and Water Assessment Tool (SWAT). Loading targets were calculated for each major element of a pre-development loading regime. Simulated
... Simulated pre-development conditions were associated with increased retention and decreased export of sediment and nutrients when compared to observed developed conditions. Differences between simulated pre-development and observed developed maximum daily yields (loads per unit area) of suspended sediment (SS), total phosphorus (TP), and total inorganic nitrogen (TIN) ranged from 35.7 to 59.6 Mg km −2 (SS); 23.3 to 52.5 kg km −2 (TP); and, 113.2 to 200.8 kg km −2 (TIN), respectively. Average annual maximum daily load was less during simulated pre-development conditions when compared to observed developed conditions by ranges of 1307 to 6452 Mg day −1 (SS), 0.8 to 5.4 kg day −1 (TP), and 4.9 to 26.9 kg day −1 (TIN), respectively. Hydrologic modeling results indicated that the differences in annual maximum daily load were causally linked to land use and land cover influence on sediment and nutrient loading. The differences between SWAT simulated pre-development and observed contemporary loading regimes from this study point to a need for practical loading targets that support contemporary management and integrated flow and pollutant loading regimes. of increasing agricultural and urban development and are associated wide-spread impairment to receiving waters [8, 9] , there is a need for regional management planning approaches focused on restoration of impaired waters to mitigate future socioeconomic and environmental losses. In response to wide-spread impairment of receiving waters, seminal hydroecological research from Poff et al., 1997, outlined a regional management planning approach to restore the magnitude, frequency, duration, timing and rate of change of flows prior to human disturbance (i.e., a natural flow regime)  . The impetus to restore natural flow regimes was based on previous works that showed that all elements across the continuum of a flow regime are important for ecosystem function    . However, the restoration of natural flow regimes alone may not restore ecological health  . A combined approach that integrates flow, sediment, and nutrient transport processes may be important in developed watersheds. This approach would restore the magnitude, timing, frequency, duration, and rate of change of flow-mediated sediment and nutrient delivery prior to human disturbance (i.e., pre-development loading regimes). Such an approach would promote aquatic ecosystem health, protect ecosystem services, and secure water resources for use in developed watersheds with alterations to flow, sediment, and nutrient regimes. It is useful to consider a suite of different indices to represent all of the key elements of a loading regime when developing regional comprehensive management efforts. The US Geological Survey (USGS) has developed and provided the statistical package EflowStats in R computer language that can be used to calculate 171 "ecologically relevant" hydrologic indices. The indices were determined to be ecologically relevant from previous works, as noted by Olden and Poff (2003) and others [13,              . While the hydrologic indices that are included in EflowStats were originally intended for defining a flow regime, the same statistics can be used to quantify the magnitude, duration, frequency, timing, and rate of change of sediment and nutrient loading. The resulting loading indices may also be deemed ecologically relevant, as previous studies showed that the magnitude, duration, frequency, timing, and rate of change in sediment and nutrient delivery are important factors of ecological health      . EflowStats is a reimplementation of the Hydrologic Index Tool (HIT)  . Unlike the original HIT, EflowStats has been redesigned to use hydrologic timeseries data and is not restricted to data formats used by the USGS National Water Information System. Thus, timeseries sediment and nutrient daily loading data can be used as input data to generate 171 loading statistics. However, many of the resulting values of those indices show similar information (i.e., redundancy) and they are linearly correlated, which can cause multicollinearity issues. Such problems with redundancy and multicollinearity can invalidate statistical assumptions, resulting in the redundancy of the statistical significance of a particular component of a loading regime. In which case, principal components analysis (PCA) can be used to reduce redundancy and to succinctly define key elements of a loading regime [11, 13, 24] . When defining a loading regime, it is important to quantify the difference between a loading regime prior to anthropic alteration (i.e., pre-development conditions) and current conditions to assess the extent of current alterations. However, the historical data required to define pre-development conditions are usually not available (e.g., hydrologic conditions circa 1800 in the central US). In which case, watershed-scale hydrologic modeling tools can be used to simulate timeseries flow, sediment and nutrient data. Such information can be used to develop a priori recommendations (i.e., loading targets) necessary to inform policy makers and managers what it might take to reach pre-development water quality conditions. The Soil and Water Assessment Tool (SWAT) is a continuous-time physically-based semi-distributed watershed-scale hydrologic model that can be used to simulate long term influences of climate, topography, soils, land use, and management operations on water, sediment, and chemical yields, without large investments of resources (e.g., time, money and labor)  . Soil series information can be used to estimate the inputs of pre-development land cover with native vegetation. For example, Ecological Site Descriptions (ESD) (https://esis.sc.egov.usda.gov/) and Official Soils Series Descriptions (OSD) (https://soilseries.sc.egov.usda.gov/) that were developed by United States Hydrology 2018, 5, 29 3 of 19 Department of Agriculture National Resource Conservation Service (USDA-NRCS) show native vegetation that is dependent on climate and soils properties. While the SWAT model can be used to simulate the daily timeseries flow, sediment, and nutrient data needed to quantify loading regimes, there is a need to assess SWAT simulated pre-development conditions of sediment and nutrient regimes that are important for defining a pre-development loading regime. The primary objective of the current work was to assess the difference in loading regimes between SWAT simulated pre-development conditions and observed developed conditions to inform policy makers and managers of what it might take to reach pre-development conditions. Sub-objectives were to quantify: (i) a sub-set of statistically significant indices to represent each key element of a pre-development loading regime; (ii) the extent of alterations to sediment and nutrient loading regimes; and, (iii) loading targets based on pre-development loading estimates using four years of observed suspended sediment, total inorganic nitrogen, and total phosphorus data collected at five nested gauging sites in a representative mixed-land-use watershed of the central US. Materials and Methods Study Site Hinkson Creek Watershed (HCW) is a mixed-land-use (i.e., 32% forested, 39% agricultural, 26% urban, and 3% wetland/water) catchment, which was located in the Lower Missouri River Basin (Figure 1) . The catchment has a drainage area of approximately 228 km 2 and elevations that range from 274 m above mean sea level (AMSL) in the headwaters to 177 m AMSL at the outlet. The headwaters of HCW are primarily rural agricultural lands and forested areas; and, the lower reaches are mostly urbanized (Table 1) . Approximately 60% of the city of Columbia, Missouri (population 113,225) is located in the lower elevations  .