環(huán)境工程外文翻譯——美國大型農(nóng)業(yè)流域基流對硝酸鹽流失的貢獻(xiàn)研究（英文）

1、Baseflow contribution to nitrate-nitrogen export from a large, agricultural watershed, USAKeith Schillinga,*, You-Kuan ZhangbaIowa Geological Survey, 109 Trowbridge Hall, Iowa City, IA 52242-1319, USA bDepartment of Geos

2、cience/IIHR Hydroscience and Engineering, University of Iowa, Iowa City, IA 52242, USAReceived 10 November 2003; revised 12 March 2004; accepted 19 March 2004AbstractNitrate-nitrogen export from the Raccoon River watersh

3、ed in west-central Iowa is among the highest in the United State and contributes to impairment of downstream water quality. We examined a rare long-term record of streamflow and nitrate concentration data (1972–2000) to

4、evaluate annual and seasonal patterns of nitrate losses in streamflow and baseflow from the Raccoon River. Combining hydrograph separation with a load estimation program, we estimated that baseflow contributes approximat

5、ely two-thirds (17.3 kg/ha) of the mean annual nitrate export (26.1 kg/ha). Baseflow transport was greatest in spring and late fall when baseflow contributed more than 80% of the total export. Herein we propose a ‘basefl

6、ow enrichment ratio’ (BER) to describe the relation of baseflow water with baseflow nitrate loads. The long-term ratio of 1.23 for the Raccoon River suggests preferential leaching of nitrate to baseflow. Seasonal pattern

7、s of the BER identified the strong link between the baseflow nitrate loads and seasonal crop nitrogen requirements. Study results demonstrate the utility of assessing the baseflow contribution to nitrate loads to identif

8、y appropriate control strategies for reducing baseflow delivery of nitrate. q 2004 Elsevier B.V. All rights reserved.Keywords: Nitrate-nitrogen; Stream discharge; Baseflow; Chemical loads; Hydrograph; Agricultural hydrol

9、ogy1. IntroductionExport of nitrate-nitrogen (nitrate) from the mid- western region of the United States is receiving increasing attention due to concerns regarding excessive nutrient enrichment and eutrophication in str

10、eams (Dodds and Welch, 2000; 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 inthe middle of the US c

11、orn 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 M

12、g, 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 water

13、shed 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 420

14、022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.03.010Journal of Hydrology 295 (2004) 305–316www.elsevier.com/locate/jhydrol* Corresponding author. Fax: þ1-319-335-

15、2754. E-mail address: kschilling@igsb.uiowa.edu (K. Schilling).separated into baseflow and stormflow components using an automated hydrograph separation program (Sloto and Crouse, 1996). A local-minimum method was utiliz

16、ed, which essentially connects the lowest points on the hydrograph and provides estimates of daily baseflow discharge between local minimums by linear interpolation (Sloto and Crouse, 1996). Daily runoff discharge was de

17、termined at each stream gauge site by subtracting daily baseflow discharge from daily streamflow discharge. For purposes of this study, runoff was defined as contributions from overland flow and subsurface stormflow (or

18、interflow) (Freeze and Cherry, 1979). Water quality data for the Raccoon River were obtained from the Des Moines River Water Quality Network operated by Iowa State University’s Department of Civil, Construction and Envir

19、on- mental Engineering and supported through the Rock Island District of the Army Corps of Engineers (Lutz and Esser, 2002). A total of 981 grab samples, corresponding to a weekly or bimonthly frequency, were collected f

20、rom the Raccoon River from 1972 to 2000 and analyzed by the Analytical Services Laboratory at Iowa StateUniversity. All concentrations are reported as nitrogen with an analytical detection limit of 0.01 mg/l. Nitrate com

21、prises approximately 81% of the total nitrogen exported from the Raccoon River (Goolsby et al., 1999). Nitrate data for the 1972–2000 period were tested for temporal trends using non-parametric seasonal kendall tau stati

22、stics with WQSTAT Plus version 1.56. No trends were significant at p , 0:05: Lack of temporal trends in nitrate concentrations and streamflow variables for the 1972–2000 period indicated that the entire data set could be

23、 grouped together for analyses. Nitrate concentrations in surface water samples exhibited a wide range of values, ranging from 17.0 mg/l in 1982 and 1990 to several occasions less than 0.05 mg/l in 1977 (Fig. 2). Of the

24、981 samples collected for nitrate analysis over the 28-year period, 252, or 26% of the total, were greater than the maximum contaminant level of 10 mg/l for US drinking water supplies (Fig. 2). The mean of all nitrate an

25、alyses was 6.8 mg/l. More information related to nitrate concentration patterns in the Raccoon River can be found in Schilling and Lutz (2004).Fig. 1. Location of Raccoon River watershed. Precipitation stations located i