Baseflow contribution to nitrate-nitrogen export from a large, agricultural watershed, USA
Introduction
Export of nitrate-nitrogen (nitrate) from the midwestern region of the United States is receiving increasing attention due to concerns regarding excessive nutrient enrichment and eutrophication in streams (Dodds and Welch, 2000, US Environmental Protection Agency (USEPA), 2000) and development of hypoxic conditions in the Gulf of Mexico (Rabalais et al., 1996, Goolsby et al., 1999). Nitrate export from the State of Iowa, located in the middle of the US corn belt region, has been identified as a major contributor to Mississippi River pollutant loads. Average annual export of nitrate from surface water in Iowa was estimated to range from approximately 204,000 to 222,000 Mg, or about 25% of the nitrate that the Mississippi river delivers to the Gulf of Mexico, despite Iowa occupying less than 5% of its drainage basin (Schilling and Libra, 2000). Nitrate export from the Raccoon River watershed in west-central Iowa is among the highest in the interior US. Average annual nitrate yield from the Raccoon River watershed was estimated to be 26.1 kg/ha/year, which ranked as the highest loss of nitrate out of 42 Mississippi River subwatersheds evaluated for a Gulf of Mexico hypoxia report (Goolsby et al., 1999).
The major nonpoint source of nitrate in Iowa is agriculture, primarily the widespread use of nitrogen fertilizers, application of livestock manure, legume fixation and mineralization of soil nitrogen (Hallberg, 1987, Goolsby et al., 1999, Burkart and James, 1999). In particular, nitrogen fertilizer use is a major nitrate source. Use of nitrogen fertilizers has increased substantially since the early 1960s, increasing from less than 200,000 tons/year in early 1960s to nearly 1,000,000 tons/year in the 1990s (IAS, 2001). In the Raccoon River drainage basin from 1979 to 1990, the average nitrate load in streamflow was estimated to 25% of the nitrogen fertilizer applied in the watershed (Lucey and Goolsby, 1993).
Nitrate is primarily delivered to Iowa streams through groundwater discharge as baseflow and tile drainage (Hallberg, 1987). Schilling (2002) reported that nitrate export occurred primarily with baseflow in two central Iowa watersheds with baseflow transport greatest during the late summer and fall. Transport of nitrate has also been shown to vary markedly with season (Alberts et al., 1978, Owens et al., 1991, Pionke et al., 1999, Jaynes et al., 1999) and vary due to geologic controls on groundwater discharge (Schnabel et al., 1993) and land use differences (Owens et al., 1991, Gburek and Folmer, 1999, Schilling and Wolter, 2001, Schilling, 2002). Inputs from storm events further accentuate intermittent loading of nonpoint source pollutants (Carpenter et al., 1998, Pionke et al., 1999).
Mitigating the effects of nitrate export from a large agricultural watershed such as the Raccoon River requires an understanding of the primary path of pollutant delivery to the stream and its spatial and temporal variation patterns. While baseflow is hypothesized to be the predominant delivery mechanism for nitrate loading to the Raccoon River, quantification of the magnitude, variability, and timing of the baseflow component of nitrate loading is needed before proper best management practices can be put in place to reduce or intercept the nitrate before it is discharged to streams. Reconciling the timing of fertilizer applications or establishment of deep-rooted riparian buffers with periods of greater baseflow would be a step towards providing control strategies that reduce nitrate losses within the field before downstream transport occurs.
The objectives of this study were to: (1) quantify fundamental hydrological processes of groundwater recharge and discharge (evapotranspiration and baseflow), and (2) evaluate annual and seasonal patterns of nitrate losses in streamflow and baseflow from the Raccoon River over a continuous 28-year period (1972–2000). To accomplish our objectives, we used daily streamflow measurements and nitrate concentration data collected as part of one of the longest running monitoring programs in the Midwest. Our study demonstrates the importance of maintaining a long-term approach to environmental monitoring in order to overcome short-term climate variability and provide accurate assessment of hydrologic and chemical transport relationships.
Section snippets
Watershed description and sources of data
The Raccoon River in west-central Iowa drains a watershed of 16,861 km2 above the City of Van Meter in west-central Iowa (Fig. 1). The North, Middle and South Raccoon rivers form major tributary branches to the Raccoon River. The North and Middle Raccoon Rivers flow through the recently glaciated Des Moines Lobe landform region of Iowa, a region dominated by low relief and poor surface drainage (Prior, 1991). The South Raccoon river drains an older pre-Illinoian glacial landscape with higher
Nitrate load estimation
Nitrate export from a watershed may be affected by stream discharge, baseflow, cropping patterns, land use among others. Various methods for estimating of chemical loads have been developed during the last two decades. Some of these methods were evaluated recently by Guo et al. (2002) to assess the uncertainty of nitrate load computations. The USGS program ESTIMATOR (Cohn et al., 1989, Cohn et al., 1992, Gilroy et al., 1990) was used to estimate daily loads of nitrate at the Van Meter gauging
Estimation of ET and groundwater recharge
From 1972 to 2000, variable precipitation falling in the Raccoon River watershed produced more than a 20-fold range of annual discharge (Fig. 3; Table 1). Annual precipitation varied from 513 mm in 2000 to 1208 mm in 1993 and averaged 870 mm (Fig. 3a), whereas discharge varied from 27 mm in 1977 to 573 mm in 1993 and averaged 223 mm (Fig. 3b). Lower annual discharge was typically associated with the second year of below normal precipitation. Baseflow trends followed total discharge patterns (
Summary
The long-term record of streamflow and nitrate concentration data from the Raccoon River provided an opportunity to assess nitrate losses in a highly agricultural region at a timescale rarely afforded by other watershed monitoring projects. Nitrate losses averaged 26.1 kg/ha over the 28-year period, with baseflow contributing approximately two-thirds of the nitrate load (17.0 kg/ha). The average total annual nitrate export from the Raccoon River was 23,100 Mg, which is more than 10% of Iowa's
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