The sentences: "In this report, loss through volatilization was considered in determining the nitrogen content of manure from each livestock group. Subsequent deposition of volatilized nitrogen is included in the measurements of inputs from the atmosphere, described in the next section of this report," should be replaced by: "Estimates of nitrogen input from manure in this report do not account for loss through volatilization. Thus, these estimates represent the total nitrogen content in manure as excreted by each livestock group." We apologize for the inconvenience of this errata sheet. A .pdf of the errata sheet is available (10 KB).

Summary Regional ground-water recharge estimates for Minnesota were compared to estimates made on the basis of four local-and basin-scale methods. Three local-scale methods (unsaturated-zone water balance, water-table fluctuations (WTF) using three approaches, and age dating of ground water) yielded point estimates of recharge that represent spatial scales from about 1 to about 1000 m 2 . A fourth method (RORA, a basin-scale analysis of streamflow records using a recession-curve-displacement technique) yielded recharge estimates at a scale of 10-1000s of km 2 . The RORA basin-scale recharge estimates were regionalized to estimate recharge for the entire State of Minnesota on the basis of a regional regression recharge (RRR) model that also incorporated soil and climate data. Recharge rates estimated by the RRR model compared favorably to the local and basin-scale recharge estimates. RRR estimates at study locations were about 41% less on average than the unsaturated-zone water-balance estimates, ranged from 44% greater to 12% less than estimates that were based on the three WTF approaches, were about 4% less than the age dating of ground-water estimates, and were about 5% greater than the RORA estimates. Of the methods used in this study, the WTF method is the simplest and easiest to apply. Recharge estimates made on the basis of the UZWB method were inconsistent with the results from the other methods. Recharge estimates using the RRR model could be a good source of input for regional ground-water flow models; RRR model results currently are being applied for this purpose in USGS studies elsewhere. ª

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific information that helps to enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on the Nation's water resources is critical to ensuring long-term availability of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish and wildlife. Population growth and increasing demands for water make the availability of that water, now measured in terms of quantity and quality, even more essential to the long-term sustainability of our communities and ecosystems. The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, regional, State, and local information needs and decisions related to water-quality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the condition of our Nation's streams and groundwater? How are conditions changing over time? How do natural features and human activities affect the quality of streams and groundwater, and where are those effects most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide science-based insights for current and emerging water issues and priorities. From 1991-2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation's river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html). In the second decade of the Program (2001-2012), a major focus is on regional assessments of waterquality conditions and trends. These regional assessments are based on major river basins and principal aquifers, which encompass larger regions of the country than the Study Units. Regional assessments extend the findings in the Study Units by filling critical gaps in characterizing the quality of surface water and groundwater, and by determining status and trends at sites that have been consistently monitored for more than a decade. In addition, the regional assessments continue to build an understanding of how natural features and human activities affect water quality. Many of the regional assessments employ modeling and other scientific tools, developed on the basis of data collected at individual sites, to help extend knowledge of water quality to unmonitored, yet comparable areas within the regions. The models thereby enhance the value of our existing data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various resource-management scenarios and in predicting how our actions, such as reducing or managing nonpoint and point sources of contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect water conditions within a region. Other activities pla...

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A mass-balance budget of N cycling was developed for an intensive agricultural area in west-central Minnesota to better understand NO 3 -contamination of ground water in the Otter Tail outwash aquifer. Fertilizer, biological fixation, atmospheric deposition, and animal feed were the N sources, and crop harvests, animal product exports, volatilization from fertilizer and manure, and denitrification were the N sinks in the model. Excess N, calculated as the difference between the sources and sinks, was assumed to leach to ground water as NO 3 -. The budget was developed using ground water data collected throughout the 212-km 2 study area. Denitrification was estimated by adjusting its value so the predicted and measured concentrations of NO 3 -in ground water agreed. Although biological fixation was the largest single N source, most was removed when crops were harvested, indicating that inorganic fertilizer was the primary source of N reaching the water table. It was estimated that denitrification removed almost half of the excess NO 3 -that leached below the root zone. Even after accounting for denitrification losses, however, it was concluded that the ground water system was receiving approximately three times as much N as would be expected under background conditions. The N cycle of agro-ecosystems is by definition dominated by agricultural sources and sinks, the magnitude of which greatly exceeds natural cycles that evolved over many millennia. Agricultural inputs of N in the USA have increased 20-fold in the past 50 yr, and the most dramatic increases have taken place in the past 30 yr (Puckett, 1995). Vitousek (1994) has estimated that of the total N used by humans throughout history up to 1992, approximately half was applied from 1982 to 1992. Consequently, many agro-ecosystems and their associated ground water and surface water systems may still be receiving increasing N amounts.One of the least appreciated components of the alteration of the natural cycle is the dramatic increase in N inputs caused by increased cultivation of N fixing crops such as alfalfa (Medicago sativa L.), soybean [Glycine max (L.) Merr.], and other legumes. These crops have a much greater capacity to fix atmospheric N than natural vegetation. For example, alfalfa can fix approximately 218 kg N ha -1 yr -1 , whereas deciduous forests can fix only about 12 kg N ha -1 yr -1 (Keeney, 1979;Jordan and Weller, 1996). In some agro-ecosystems, legume fixation may even be the largest source of N. For example, Keeney (1979) estimated that for the state of Wisconsin, 2 N fixation by legumes exceeded inputs from commercial fertilizer by 2.5 times. Peterson and Russelle (1991) estimated that for the eight states of the U.S. Corn Belt, alfalfa alone contributed 1 × 10 9 kg of N compared with 4 × 10 9 kg from commercial fertilizer. It is important to consider this N source because it may contribute NO 3 -to ground water and surface waters either directly through N added to the soil or indirectly as the result of mineralization of plant re...