Watershed description

I use the available data for two typical Midwestern watersheds, both located in Iowa: the Boone River Watershed (BRW) and the Raccoon River Watershed (RRW). The National River and Streams Assessment 2008–2009 includes Iowa in the Temperate Plains Ecoregion.18 The survey finds high levels of nitrogen in 58% of the rivers, and medium levels of nitrogen in 13% of the rivers. At the same time, 31% (24%) of the rivers have high (medium) levels of

phosphorus.

The Boone River Watershed

The BRW is located in the north central part of Iowa. The watershed covers more than 537,000 acres (2,370km2_{) in six counties (Hamilton, Hancock, Humboldt, Kossuth, Wright, and Webster)}

as shown in Figure 3-1. The watershed area is crop intensive, with the surface being intensively tile drained; consequently, the wetlands area has been reduced significantly. Moreover, the Boone River Watershed agricultural area has been found to be responsible for some of the highest nitrogen loadings among Iowa’s watersheds (Libra et. al 2004).

18 _{Other states included in the same ecoregion are eastern North and South Dakota, western}

Minnesota, portions of Missouri, Kansas, Nebraska, western Ohio, central Indiana, Illionois, and southeastern Wisconsin.

Figure 3-1. Boone River Watershed.

Land use in the watershed is dominated by agriculture: cropland represents 89.7% of total area, retired land represents 5.6% of total area, forestry represents 2.6% of total area, and urban areas and water surfaces account for the remaining 2.1% of total area. Most of the land is a flat, characterized by soils with low slopes (i.e., 73% of the areas have a slope less than 0.01 inches). The corn suitability rating (CSR) is another characteristic that defines potential yield.19 It is an index that ranges between 0 and 100, where high values are associated with high quality soils. A soil with a high corn suitability index value is less likely to have high rates of fertilization, and at the same time is less likely to be considered for land retirement as a solution for reducing nitrate

19 _{CSR is a ranking that rates different kinds of soils for their potential row crop productivity. It}

loadings. In the BRW more than 50% of the soil has been rated with corn suitability index values ranging from 50 to 79, and 40% of the soil has corn suitability rating values higher than 80.

The required data for our modeling system (i.e., SWAT 2009) was collected at Common Land Unit (CLU) level.20 More than 16,300 unit levels have been identified in the BRW. As an HRU is the unit required by SWAT model, the common land unit levels were regrouped into roughly 2,968 HRUs. Data related to crop rotation, land uses, fertilizer management, tillage, and conservation practices were provided by a field-level survey conducted by Kiepe (2005). Figure 3-1 shows the subbasin boundaries together with the location of the weather stations that

provided the historical weather data to calibrate the model. The approach for simulating the water quality impact of conservation practices as well as weather, soils, and management characteristics are described in detail in Gassman (2008).

The Raccoon River Watershed (RRW)

The Raccoon River Watershed (RRW), as shown in Figure 3-2, is one of the largest watersheds in the state of Iowa. It covers an area over 9,400km2 in west-central Iowa, being the Des Moines River major tributary. The RRW flows approximately 300 km from its origin in Buena Vista County to the confluence with the Des Moines River in the city of Des Moines.

The landscape in the south part of the watershed is characterized by higher slopes with many hills and a well-developed drainage system, while the landscape in the northern part is

20 _{“A Common Land Unit (CLU) is the smallest unit of land that has a permanent, contiguous}

boundary, a common land cover and land management, a common owner and a common producer in agricultural land associated with USDA farm programs. CLU boundaries are delineated from relatively permanent features such as fence lines, roads, and/or waterways”( http://www.fsa.usda.gov).

characterized by lower slopes and poor surface drainage system (Schilling et al. 2008). With more than 73% of the planted area being use for corn and soybeans, the land use is dominated by

Figure 3-2. Raccoon River Watershed.

agricultural row production. Other land uses include grassland (16.3%), woodland (4.4%), and urban (4.0%) (Gassman and Jha, 2011). The applied fertilizer for corn is one of the main sources of nitrogen and phosphorus in the RRW. There are a significant number of cattle feedlots (135) and confinement operations (424), but there is little impact from cattle grazing on pasture (Schilling et al. 2008). Additionally, 77 waste facilities operate under the National Pollution Discharge Elimination System permit, which contributes a small amount of nitrate. During recent decades, nonpoint sources have been identified as the main contributing source to the high levels of nitrogen, phosphorus, and sediment (Jha et al. 2010; Schilling et al. 2008).

As stated earlier in SWAT, a watershed is divided into multiple subbasins or

calibration and for the results calibration (Schilling et al. 2008; Jha et al. 2010). In this framework, the RRW is divided into 112 subbasins and 3,640 HRUs, with 1,569 being agricultural HRUs.

Figure 3-2 shows the subwatershed boundaries, the watershed stream network, and the location of climate stations used for establishing baseline stream flows and model testing, and impaired stream segments requiring the establishment of Total Maximum Daily Loads (as described in Schilling et al. 2008, and Jha et al. 2010).

Table 3-1. Watersheds: Summary Information

Watershed Baseline N (kg) Baseline P (kg) Subbasins Fields21 Area (km2) Boone 4,725,826 218,828 30 2,968 2,370

Raccoon (RRW) 18,604,642 632,406 112 1,569 9,400

Table 3-1 summarizes the baseline nitrogen (N) and phosphorus (P) emissions as well as some of the characteristics for the two watersheds. The baseline values for both nitrogen and phosphorus represent the annual mean values computed using the available historical data for 1995–2001, and 1994–2000 for RRW with the first two years being dropped out for both watersheds.