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Quantitative Soil Descriptions for Ecoregions of the United States

^a Western Ecology Division, National Health and Environmental Effects Research Laboratory (NHEERL), USEPA
^b USDA Natural Resources Conservation Service, Watershed Science Institute
^c Indus Corporation, 200 SW 35th Street, Corvallis, OR 97333

View larger version (201K): [in a new window]   Fig. 1. Soil texture (USDA5) and rock contents of State Soil Geographic database (STATSGO) soil map units of the conterminous United States and the boundaries of 84 Level III Omernik ecoregions. The yellow color represents coarse (cr) soils with colors darkening through moderately coarse (mocr), medium coarse (mecr), moderately fine (mofn), and fine (fn) soils in the reddest shade. A higher percentage of rocks adds coarseness to a soil and is depicted as decreasing color intensity within the texture class.

View larger version (27K): [in a new window]   Fig. 2. Comparison of U.S. and Ecoregion 39 soil characteristics. (a) The baseline represents each mean soil characteristic for the USA and the bars above and below represent the amount of difference for Ecoregion 39. For example, the cation exchange capacities (cath, catl), percent clay, and liquid limits (liqh, liql) are greater than the national averages whereas soil permeability (perh, perl) and percent sand are less. (b) The bars indicate error differences between the U.S. model and the standard errors of prediction for the Ozark Highlands. Values for percent clay and percent rocks are above the U.S. average in this ecoregion, and perl is below average. Rsc, compares mean characteristics to the national average; Rer, error of prediction relative to the U.S. models.

View larger version (35K): [in a new window]   Fig. 3. Analysis of Ecoregion 39. (a) The relationships between map unit areas (solid lines, Eq. [3]) and inclusion areas (dashed lines, Eq. [2]) for Ecoregion 39. The vertical axis represents dimensionless areas, . From left to right, the circular markers locate map unit areas 1m, sm, and tm on the solid lines, and 1i, si, and ti on the dashed lines. The 1 denotes the largest single contributor of area, s is the "switch point" from inside to outside the ecoregion, t is the total number of map units, and the subscript i refers to the inclusion zone. The vertical difference between the solid and dashed lines represents the size of the exclusion zone for the ecoregion. At 1, the exclusion zone (1m - 1i) is minimal whereas the inclusion area, 1i, is the largest for a single map unit. (b) The spatial relationships of intersecting map units of Ecoregion 39 demonstrate the "switch point" between the inclusion and exclusion zones. Map unit areas < s are dark gray, parts of which are outside the ecoregion boundary. Although the light gray areas extend far beyond the ecoregion boundary, only those within the ecoregion boundary and the dark gray areas outside the boundary determine the boundary agreement and the neighbor–host ranks (Rmu and Rnh).

View larger version (26K): [in a new window]   Fig. 4. A summary of the standard errors of predicting soil characteristics of the State Soil Geographic database (STATSGO) map units of the United States and three groups of Level III ecoregions. The groups were based on the relationships among the five soil ranks compared with the U.S. values. Group A has the highest predictability (lowest total error) and Group C has the lowest predictability (highest errors). Group A = Ecoregions 8, 32, 37, 54, 24, 34, 65, 70, 26, 52, 35, 40, 47, 73, 43, 46, 6, 72, 67, 55, 7, 30, 42, 71, 22, 74, 39, and 61. Group B = Ecoregions 17, 79, 59, 69, 62, 82, 3, 48, 60, 81, 31, 36, 53, 20, 56, 51, 33, 23, 21, 9, 45, 10, 29, 66, 78, 68, 19, and 28. Group C = Ecoregions 49, 76, 77, 75, 84, 50, 12, 63, 58, 2, 1, 15, 14, 18, 5, 27, 80, 11, 64, 41, 38, 13, 16, 57, 44, 83, 4, and 25. Soil characteristics and standard errors of ecoregions in italic type are listed in Table 3.