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Hydrologic Soil Group


The primary source of soils data was the Gridded Soil Survey Geographic Database (gSSURGO), (SoilSurveyStaff, 2018). The gridded soils database contains 10-meter rasterized coverage of surface soils derived from National Cooperate Soil Survey (NCSS) maps. These maps are generally drawn at 1:24000 scale. NCSS designates soils by a “map-unit name,” which can be joined with other attribute data. Map units in the study area were joined with the soils component table, containing hydrologic-soil group designations. NCSS classifies hydrologic soil groups according to estimates of runoff potential. Soils are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D) as defined below:

  • Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.
  • Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission.
  • Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission.
  • Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission.

If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. In certain locations, data were augmented with the SSURGO Value added tables (SoilSurveyStaff, 2016) using the Potential wetland soil landscapes field.

In areas where gSSURGO data were not available, we used the Global Hydrologic Soil Groups (HYSOGs250m) for Curve Number-Based Runoff Modeling developed by Oak Ridge National Laboratory. This dataset contains world-wide hydrologic soils groups derived at a 250 meter resolution from machine learning predictions. Hydrologic soil groups were given the same designation as the SSURGO data above.

To account for wetlands and saturated soils not included in the above datasets, we used the USGS GAP/LANDFIRE National Terrestrial Ecosystems data set, which includes nationwide vegetation and land cover data.

Layer Access in Earth Engine​

The javascript commands below can be used to access this layer within the Google Earth Engine Code Editor. A Google Earth Engine account is required.

// Import the layer data dictionary
var data = require('users/stormwaterheatmap/apps:data/public')

// To view data dictionary, print to the console:
print('Data:', data)

//Get this layer from the layer data dictionary:
var layer_name = data.rasters["Soils"]


Individual objects contain all the info used in the stormwater heatmap. To add it to the map, add the layer object.

var display_image = layer_name.layer


To get the raw image data for analysis, access the eeObject key.

var raw_image = layer_name.layer.eeObject



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