Project Overview

Full Title
Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies - 2009 Updates

Location
 Statewide

Cooperating Agencies
Public Water Supply Section
Source Water Assessment and Protection Program

Project Chief
Silvia Terziotti
seterzio@usgs.gov

Period of Project
1/2009 - 6/2009

Team Members
Kirsten Tighe
Ramona Traynor

USGS IN YOUR STATE

USGS Water Science Centers are located in each state.

Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies - 2009 Updates

Changes in Ratings from 2000 to 2009

Updates to the source factors that are used to generate the layers for watershed characteristics and unsaturated zone ratings in North Carolina did create slight shifts in the overall ratings.

Unsaturated Zone Ratings

Figure 1 shows the overall change in the unsaturated zone rating from 2000 to 2009. These changes are due to either improvements in the resolution of the source data, or to actual changes in the land cover or land use since 2000.

Figure 1 -- Changes in unsaturated zone rating due to updates in the GIS layers.

For the unsaturated zone, all of the factor layers were updated. The shift from 1992 to 2001 land cover and land use layers increased the amount of impervious surface (more urbanization) for land cover causing a slightly lower trend in permeability to the unsaturated zone. Conversely, the increase in urbanization and road networks slightly increased risk from the land use layer. The slope factor should not have seen a major change within the State, but the improvement in data capture from 2000 to 2009 has changed dramatically. Slopes that were not captured with the original source of data in 2000 can now be easily picked up with the 2009 elevation data. This resulted in more areas with defined or higher slopes throughout the State. For the overall unsaturated zone rating, this would mean that there were fewer areas that were flat and more susceptible to permeability to ground water. The created a slightly lower mean for the slope classes overall. The vertical series hydraulic conductance included new detailed soil permeability data for 21 counties that was not available in 2000. The county data is of a much higher resolution providing roughly 1000 values of permeability with the new soils for 1 value of permeability from the lower-resolution data. There was no obvious trend that this would cause in the overall unsaturated zone ratings, but it did raise the mean from 3.91 to 2.48 overall (though presumably, no change should have occurred outside the 21 counties). Overall, the mean value for the unsaturated zone rating did not change significantly. The changes in mean for each factor are shown in table 1.

At site specific locations, changes from the updated land cover, slopes and soils data can be seen. More refined delineations of urbanized areas, as well as better defined slopes are visible in the mapped values of the unsaturated zone rating. This may change individual well ratings, especially if they lie in urbanizing or low slope areas (figure 2).

Figure 2 -- Comparison of site specific changes between 2000 and 2009 unsaturated zone ratings.

Watershed Ratings

Figure 3 shows the areal extent of changes to the watershed characteristics rating.

Figure 3 -- Changes in watershed ratings due to updates in the GIS layers.

For the watershed rating, all five of the factor layers were updated. For this rating, the shift from 1992 to 2001 land cover and land use layers to more impervious surface (more urbanization) increases runoff, so would increase the ratings for land cover and land use. The slope factor, as discussed for the unsaturated zone, identifies more high-slope areas, resulting in more areas with defined or higher slopes throughout the State. For the overall watershed vulnerability rating, this would create more areas for runoff, increasing the mean slope rating since 2000. Annual average precipitation was changed to reflect more current years, included 2 major drought periods in the 1990's and 2000's. The years for the 2000 rating included 1961 to 1990. The new factor is the average from 1971 to 2007. The slightly lower average rainfall during this period, should lower the mean rating since less rainfall causes less runoff. The final factor is the ground-water contribution. This applies the unsaturated zone rating to buffers around each stream in North Carolina, with areas outside of the buffered zone given a value of 0 for the rating. This factor should not change much, since the average value of unsaturated zone rating did not change much overall. The overall mean for the watershed characteristics rating changed from 31.62 to 31.13. The changes in mean for each factor are shown in table 2.

For watersheds, changes from the updated land cover, slopes and precipitation data may change individual watershed ratings, especially if they lie in urbanizing or high slope areas (figure 4).

Figure 4 -- Comparison of site specific changes in the watershed characteristics rating between 2000 and 2009 for one public supply watershed.