Percent clay in surface soil horizons for the contiguous USA (VEMAP version)

The spatial resolution of this dataset is 50 km.

The Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) was a multi-institutional, international effort that addressed the response of biogeography and biogeochemistry to environmental variability in climate and other drivers in both space and time domains. The objectives of VEMAP were to study the intercomparison of biogeochemistry models and vegetation type distribution models (biogeography models) and determine their sensitivity to changing climate, elevated atmospheric carbon dioxide concentrations, and other sources of altered forcing.

Soil properties were based on a 10-km gridded EPA soil database developed by Kern (1994, 1995). Two soil coverages are provided in the Kern data set: one from the USDA Soil Conservation Service (SCS) national soil database (NATSGO) and the other from the United Nations Food and Agriculture Organization soil database (FAO 1974- 78).

Only the SCS NATSGO soils are included in the VEMAP set. Physical consistency in soils data was incorporated by representing a grid cell's soil by a set of dominant (modal) soil profiles, rather than by a simple average of soil properties. Because soil processes, such as soil organic matter turnover and water balance, are non-linearly related to soil texture and other soil parameters, simulations based on dominant soil profiles and their frequency distribution can account for soil dynamics that would be lost if averaged soil properties were used.

To spatially aggregate Kern data to the 0.5 degree grid, luster analysis was used to group the subgrid 10-km elements into up to 4 modal soil catagories (Kittel et al. 1995). In this statistical approach, cell soil properties are represented by the set of modal soils, rather than by an "average soil."