Figure 1 presents the four strategies for characterizing potential water use.
Figure 1a displays the distribution of index values for the unweighted index.
Figure 1b presents the distribution of index values for the principal components derived index.
Figure 1c,d present the index values for the USGS and FAO weighted principal components indices respectively. Each of these figures displays the index of interest in terms of quintiles with breaks set at the 20th, 40th, 60th, and 80th percentiles. We present our results using quintiles for two reasons: quintiles contain approximately the same number of counties in each of the five groups and they simplify visualization and interpretation by using a standardized range of values for each version of the index. Lower quintiles represent lower water use while higher quintiles indicate higher water use.
4.1. Geographic Trends in Metrics
A visual comparison of the panels presented in
Figure 1 highlights differences in potential water use. The unweighted decile index (
Figure 1a) indicates five regions with high water use. The first region is in and adjacent to the San Joaquin River Valley in California. Counties of the Northern and Central Plains states where soybeans and corn are grown extensively also are delineated as having high water use. The industrial Rustbelt, consisting of counties located in Illinois, Indiana, Michigan and Ohio, is a third region identified as having high water use. A fourth region with high water use, the Mississippi River Valley, includes counties in the states of Mississippi, Louisiana and Arkansas which are some of the more intensive cotton producing locations in the country [
72]. The southeastern seaboard is a fifth region with high water use and includes counties in Georgia, South Carolina, North Carolina and Virginia, where the apparel and automobile industries are a strong presence [
73] as well as low-wage manufacturing and military defense industry [
74]. On the other hand, counties along the Northeastern seaboard and those located in Appalachia are classified as having lower water use. The same is true for most counties in Florida as well as counties surrounding Seattle and Portland in the Pacific Northwest.
The PCA-weighted version of the index presented in
Figure 1b illustrates several similarities and differences with the unweighted decile index. For example, this index also identifies higher water use in the North and Central Plains states but with greater concentration and intensity. The Rustbelt region and the Southeastern United States (with the exception of Appalachia) are also shown to be areas of higher water use according to the PCA-weighted index but, again, with varying intensities. Unlike the unweighted decile index, the West Coast states of Washington, Oregon, and California are shown to be areas having lower water use. An exception to this pattern is the Central Valley of California (which includes the San Joaquin River Valley). As well, the Northeast portion of the country, particularly New York, New Jersey and Pennsylvania, is shown to be an area with lower potential water use when compared against the unweighted decile index.
This picture of water use is quite different from the picture presented by the unweighted index. The reason for these differences is that the PCA index uses the proportion of cumulative variance explained by the individual components as weights and these vary across geographic space. For instance, our first component explains 17.3% of the cumulative variance and receives greater weight when calculating the index while our fifth component receives less weight because it explains only 9% of the cumulative variance. In other words, water-intensive crops such as corn and soy, which are the highest loading variables on our first component, contribute more to this index than the agricultural and energy generating establishments that define our fifth component.
Figure 1c,d present the geography of water use, as indicated by the USGS and FAO-Weighted indices respectively. Visually, these indices are quite similar to one another and they also share several similarities with the decile index. Like the decile index, the Central Valley farming region of California is highlighted as having high water use. The agriculturally intensive Plains states are also highlighted as having higher use for water. Pockets of higher use for water are noticeable in industry-intensive states including Illinois, Indiana, Michigan and Ohio. The Mississippi River Valley and the Southeastern seaboard are also pockets of higher water use. These results reflect a more nuanced weighting schema that accounts for distinct regional variation in agricultural, industrial, and residential characteristics across US counties. For this reason, it is not surprising that farming intensive regions (e.g., Central Valley of California or the Great Plains) show high potential water use considering that agricultural activities constitute a large share of overall water use. Likewise, industrial activity that remains in the traditional manufacturing belt continues to indicate potentially high use of water.
4.2. Metric Comparison
Given the similarities and differences highlighted, a series of contingency tables were tabulated (
Table 4 and
Table A1,
Table A2,
Table A3 and
Table A4 to quantify the degree to which results vary across the different indices of potential water use. The rows and columns of the tables may be interpreted as the number of counties that were classified in a particular quintile for one index but changed quintile values in another index version. The diagonals represent counties that were classified in the same quintile for both indices.
Table 4 compares the quintile assignments of the unweighted decile and PCA indices. This table shows some agreement in the lower decile. The unweighted decile index classifies 9% of observations in the first quintile, as does the PCA index. There is also some agreement in the assignment of counties to the fifth quintile; both indices classify 10% of their observations in this quintile. In quintiles 2–4 there is somewhat less agreement between the indices, as indicated by the off-diagonal values. To summarize similarities in quintile classification, the values along the diagonal are summed and added to 36%. This number means that 36% of all counties were assigned to the same quintiles by each index.
Table 5 summarizes the similarities and differences between the four indices computed in this study. The numbers in this table were computed by summing the diagonal elements of
Table 4 and
Table A1,
Table A2,
Table A3 and
Table A4. This table also presents the similarities and differences with total withdrawals data from the USGS 2010 estimates [
18]. To compute this comparison, water withdrawals were assigned to quintiles (
Table A4) and compared with the quintiles for the indices derived in this study.
Based on the information in this table, there are some similarities between the derived indices and the quintiles derived from USGS water withdrawal data. Of these indices, the weighted PCAs are most similar to one another; they classified 83% of all counties in the same quintile. Of these two indices, the FAO-weighted PCA has the most similarities with the other indices. It is 41% similar to the unweighted decile index and 45% similar to the unweighted PCA index. A comparison of the USGS water withdrawal data and the four indices indicates that they are dissimilar from one another; the indices classify counties into similar quintiles about 20% of the time. The unweighted decile index is the most similar of the derived indices to the USGS data with about 23% of counties classified similarly.
To expand on the analysis in
Table 5,
Figure 2,
Figure 3 and
Figure 4 depict the geographic similarities and differences between the four indices developed in this paper via difference maps. To construct these maps, the quintile of one index is subtracted from the quintile of the comparison index. For example,
Figure 2a compares the decile index to the principal components derived index. If the results of this difference are positive, the decile index has higher values than does the PCA index. These locations appear in green on the map. If the results of this difference are negative, meaning that the decile index produces lower index values than the PCA index, counties are displayed in brown. Gray counties in this figure are counties that were classified in the same quintile by each index.
Figure 2 summarizes the similarities and differences between the decile index and the other three indices. A comparison with the PCA index (
Figure 2a) reveals that the decile index presents a picture of higher potential use in the West and along the East coast. Alternatively, the PCA index indicates higher use in the Plains states of Iowa and Nebraska, as well as portions of Southeast states including Texas, Florida, Louisiana and Arkansas. These differences are derived from how the indices are constructed. The PCA index emphasizes agricultural and industrial variables while the decile-based index affords equal consideration to all variables including residential variables. As described in the contingency tables, there is some agreement between the two indices; counties in gray are evident in the Plains states, as well as counties in several states including Illinois, Indiana, and New York.
A comparison of the decile and USGS-weighted PCA index (
Figure 2b) conveys a somewhat different picture. The USGS weighted index categorizes the Western states as having higher potential use than does the decile index. This is particularly evident in Oregon, Washington, Montana, and Wyoming. The USGS weighted index also ascribes higher potential use to Plains states such as Nebraska and Iowa than does the decile index. The decile index, which ascribes equal weighting to all variables, indicates more potential for water use along the East Coast and in states such as Nebraska, Oklahoma, and Texas. A comparison of the FAO weighted index (
Figure 2c), which ascribes slightly more weight to industry than does the USGS weighting scheme, presents a similar picture of differences in potential use between the two indices. The decile index ascribes higher potential use in the Plains states and the Southeastern region of the country. These differences are largely attributable to several agricultural variables in the decile index that are capturing these regions even though their mix of crops is different.
Unlike the decile index, which produced somewhat mixed patterns of similarities and differences, a comparison of the PCA index and the other three indices in
Figure 3 produces more geographically pronounced trends.
Figure 3a contains a difference map of the PCA and decile indices for comparative purposes only since this difference map for the two indices was discussed above. In terms of how the PCA index compares to the USGS (
Figure 3b) and FAO weighted (
Figure 3c) indices, the similarities and differences are quite similar. Both the USGS and FAO indices, which are weighted more towards industry, present the West and Northeast as having higher potential water uses than the PCA index. Alternatively, the PCA index classifies counties in the Plains states, the Midwest, and portions of the Southeast as having higher potential water use.
A geographic comparison of the quintile classifications highlights that those based on the USGS withdrawal data present counties in the West as having higher potential water use than do the indices derived in this paper (
Figure 4). The same is true for counties in the Northeast and in Florida. These differences in classification are most stark in the comparison of the decile (
Figure 4a) and PCA-derived (
Figure 4b) indices to the quintiles based on withdrawal data. The differences between quintile classifications fade a bit once FAO and USGS weighting schemes are incorporated (
Figure 4c,d). More gray counties, indicating classification similarities between two indices, appear in the West and Northeast. All four indices characterize the agriculturally intensive Plains states as having higher potential use than do the withdrawal data. However, these findings are made cautiously since publications by the USGS indicate the types of data used in their analysis, but the precise weighting schemes are not presented [
18,
75].
4.3. Index Decomposition
One of the means of understanding variations in classification results between indices—drivers of index values and how these map to quintile classifications—is the decomposition of each index into individual components. This moves beyond industry decomposition into municipal, industrial, and agricultural uses, which is possible via information from Aquastat [
29], or, the decomposition of withdrawal data into different types of uses (i.e. irrigation, thermoelectric power, public supply). We use the unweighted decile index to demonstrate the value of this approach.
Figure 5a–d presents a decomposition analysis of the unweighted decile index and three of its constituent variables in decile form.
The unweighted decile index takes into account each of the 14 indicators of potential water use and inherently captures geographic variability in agricultural, industrial and municipal pressures. These patterns are shown in
Figure 5a where counties in the North and Central Plains, the Mississippi River Valley, the San Joaquin River Valley in California, the Rustbelt, and the Southeast are identified as areas with high potential water use. On the other hand, counties in Appalachia and the Northeast are depicted as regions with low water use.
Three variables are identified from the unweighted decile index for further examination and are used to help explain the distribution of index values in
Figure 5a. These variables are selected because they show the highest component loadings in our independent PCA models presented in
Section 3.3 and are displayed in
Figure 5b–d. For instance, corn production (
Figure 5b) is highly concentrated in counties located in the North and Central Plains states as well as Illinois, Indiana, Ohio, Michigan, Minnesota, and Wisconsin. The number of agricultural establishments portrayed in
Figure 5c shows counties with high potential water use located in the Southeast region as well as the Pacific Northwest. Single-family homes, depicted in
Figure 5d, represent heightened potential water use and counties positioned in higher deciles are located in/ around the North and Central Plains, the upper Midwest and Rustbelt, and several counties in the Rocky Mountain region. The contribution of various factors to potential water use across the United States is complicated, as demonstrated by the series of indices derived in this paper, but through a decomposition analysis, it is possible to begin to understand how, why, and where potential water use varies geographically.