Scientific Investigations Report 2006–5137
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5137
A GUI was created to simplify the alteration and display of the flow and transport model described above. The GUI was created using ESRI’s royalty-free software MapObjects LT. The GUI provides a GIS interface for viewing and editing the nitrogen loading conditions, running the flow and transport model, and viewing the results of the modeling. The flow and transport model along with the GUI are collectively called the SRNSS. The components of the SRNSS and other associated files are available in a zip file that accompanies this report. One of the files, readme.txt, provides installation instructions. The appendix of this report provides detailed instructions for using the SNRSS software.
At startup, the SRNSS loads a base distribution of nitrogen loading and nitrate concentration distribution that represents the conditions in approximately the year 2000. The base-case run represents the end of a 90-year historical period simulation of the increase in the area’s farming and the introduction and increase of dairy farms. The increases in agriculture in the area are estimated from the U.S. Department of Agriculture’s Census of Agriculture.
Users can then choose from predefined zones to increase or decrease the nitrogen loading for those zones by a multiplication factor of 0.1 to 5.0 (fig. 8). We delineated these predefined zones to maintain political, hydrological, and predominant land-use boundaries while preventing any one zone from becoming so small that the SRNSS could not simulate that zone effectively. For the same reason, the SRNSS does not allow users to create loading zones. After users edit the nitrogen loading zones, the SRNSS software prompts the users to run MODFLOW-MOC3D with the new nitrogen loading conditions. When MODFLOW-MOC3D completes its model run, the results are displayed in the simulator, allowing users to view the results in 5-year increments up to 30 years.
Altering the nitrogen loading factors simulates land‑use changes for a zone. Nitrogen loads represent an entire zone that can contain multiple land-use types. Therefore, a change in nitrogen loading will represent a change from one predominant land use to another for the entire zone. The loading factor selected depends on the type of land-use change being modeled. For instance, a land-use change from rangeland to agriculture results in an increase in nitrogen loading that would be modeled by a loading factor between 1 and 5, depending on the amount and extent of the land-use change. Conversely, a land-use change from agriculture to residential results in a reduction of nitrogen loading and requires a loading factor between 0.1 and 0.9. Other typical land-use change scenarios would be to convert agricultural land to rangeland, resulting in a decrease of loading (or a loading factor between 0 and 1), or to increase the dairy cattle in an area, modeled with a loading factor between 1 and 5, depending on the magnitude of the increase in dairy cattle. Choosing appropriate loading factors for simulating land-use change is left to the user, however, some guidance is provided below.
The nitrogen loading values used by the flow and transport model are based on fertilizer, cattle manure (from dairy and beef operations), septic systems, atmospheric deposition (precipitation), and legume crops (alfalfa and beans), with the largest inputs coming from fertilizer and dairy cattle. The nitrogen input data also account for crop uptake of nitrogen, listed as crop losses. Nitrogen inputs are county-level estimates except those for dairy cattle and atmospheric deposition. Table 1 lists the nitrogen input and loss values for each county and land-use type as estimated in Skinner and Donato (2003), and describes the loading values for the 30-year run used to create the base case in the SRNSS flow and transport model. Table 2 lists the percentage of area that each nitrogen input type (except precipitation, which is 100 percent for each zone) occupies within each zone. Nitrogen input for dairy cattle was applied by determining spatial densities of dairy cattle from known dairy locations. Dairy cattle densities ranged from 1 to 450 animals per square mile with an increase of 33 lbs of nitrogen per acre per year for an increase of 50 per square mile (fig. 9). A more complete explanation of the nitrogen-loading layer is provided in Skinner and Donato (2003).
An example change in nitrogen loading input follows in which the predominant land-use changes from rangeland to dairy agriculture. The high nitrogen input values in southern Gooding and western Jerome Counties are primarily due to dairy cattle inputs (figs. 3 and 9). To change the nitrogen-loading estimate for other parts of Gooding County to mimic the introduction of similar dairy cattle facilities would increase nitrogen loading by a multiplication factor of 2 to greater than 4, depending on the original nitrogen input value. For example, the zone in northern Gooding County, primarily rangeland, would require a loading factor of 4 to resemble the nitrogen loading in southern Gooding County, primarily from dairy agriculture, (less than 55 lbs/acre/yr to greater than 220 lbs/acre/yr).
In simulating changes in nitrogen inputs, be aware that the nitrogen inputs discussed above are general values representing large areas. Small spatial changes in land use cannot be simulated effectively with the SRNSS. However, large-area land-use changes occurring over long periods are ideal scenarios to simulate with the SRNSS.
U.S.
Department of the Interior | U.S. Geological
Survey
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