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Structural Heterogeneity and Functional Homogeneity: Mapping Soils for Hydrology

Structural Heterogeneity and Functional Homogeneity: Mapping Soils for Hydrology. H. Edwin Winzeler Phillip R. Owens Zamir Libohova Nandita Basu Suresh Rao. Structural Heterogeneity. Structurally soils are complex, intricately. A New Approach to Soil Mapping.

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Structural Heterogeneity and Functional Homogeneity: Mapping Soils for Hydrology

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  1. Structural Heterogeneity and Functional Homogeneity: Mapping Soils for Hydrology H. Edwin Winzeler Phillip R. Owens Zamir Libohova Nandita Basu Suresh Rao

  2. Structural Heterogeneity • Structurally soils are complex, intricately

  3. A New Approach to Soil Mapping • Soil properties are of greater interest to hydrologists than soil series or taxonomic classification. • Complex soil maps showing soil series can be cumbersome to interpret and use. • Soil properties exist in a continuum on the landscape, not usually in discreet groupings with abrupt boundaries.

  4. Traditional Soil Map SSURGO Fuzzy Soil Map based on terrain attributes ~ Cartoon ~ Photograph

  5. Polygons • Discreet boundaries • Broken interconnectedness • Vague predictions (value ranges) • Incompatibility with raster-based models • Simplicity of representation, complexity of interpretation • Rasters • Fuzzy boundaries • High degree of interconnectedness • Specific predictions at specific geographic intervals • High compatibility with raster models • Complexity of representation, simplicity of interpretation

  6. Polygon vs. Raster Drummer Soil Value Range:11 - 15 Flanagan SoilValue Range:9 – 12

  7. ? Answer: Tie soil properties to terrain attributes and model using fuzzy logic and membership value distributions of terrain/soil relationships.

  8. Terrain Attributes: Terrain Attributes: TWI Holmgren Coefficient: 1 Holmgren Coefficient: 9 Holmgren Coefficient: 27 Holmgren Coefficient: 50

  9. Terrain Attributes: AACN Altitude Above the Channel Network

  10. Traditional Approach • Tacit knowledge gained from extensive field work • Development of soil landscape concepts • Block diagram model development • Application of model to landscapes through stereoscopic landscape evaluation

  11. Traditional Approach • Expert gains knowledge • Expert examines landscapes • Expert applies knowledge to new landscapes • Knowledge communicated via the soil map

  12. A New Approach • Digital elevation model used rather than stereoscopic aerial photography • Terrain attributes calculated • TWI = Topographical Wetness Index • TWI = ln (a/tanB) • a = area upstream contributing flow to a pixel • B = slope of the pixel or grid cell (raster) • Soil properties related to terrain attributes • Membership value frequency histograms developed relating soil properties to terrain attributes • Fuzzy logic used to apply soil properties to terrain attributes

  13. Relate Soil Mapping Units to Terrain Attributes AACN >3 TWI < 12 Floodplain Soils AACN < 3 TWI > 12 Outwash Plain Soils

  14. Elevation Model for Little Vermillion Watershed (DEM) Match Parent Material Zones to Elevation Data Meters Floodplain < 204 m Moraine > 213 m Outwash Plain > 204, < 213

  15. Develop Rules for Soil Mapping Decision Tree: Elevation < 204 m: Flood Plain and Alluvial Soils Apply floodplain decisions and choose representative soils High TWI(27), low AACN: Sawmill Low TWI(27), low AACN: Senachwine High AACN: Sabina Medium elevation: Outwash Plain Apply outwash plain decisions High TWI(27): Drummer Lower TWI(27): Flanagan Elevation > 213 m: Moraine Apply morain decisions High TWI(9): Drummer Lower TWI(9): Raub

  16. Membership Values, Little Vermillion Membership values are an assumed probability of the likelihood of the occurrence of a given soil at a given location based on how well the location approximates the landscape position ideal for that soil’s occurrence. Drummer Soil

  17. Membership Values Sawmill Soil

  18. Hydrological Characterization for Each Soil Field Capacity Permanent Wilting Point

  19. Output Maps: Hydrological Predictions Average of water deficit values for each soil weighted by its membership value at each geographical location (raster cell) Field Capacity Water Deficit(cm)

  20. Functional Homogeneity Water Deficit at Field Capacity About 50 mm~ 80% of surface Water Deficit at Field Capacity: Functional Homogeneity

  21. Discussion • There are no fixed rules about the relationship between terrain attributes and predicted soils. (e.g. TWI and Drummer relationship – specific values – will not necessarily hold in multiple watersheds) • Fuzzy membership maps are not proven to be better than traditional soil survey maps, but their utility is higher for some related disciplines. • Fuzzy membership maps can aid interpretation and prediction of specific soil properties • Soil mapping is STILL based on “expert” judgment, not fixed rules • Reliability of fuzzy membership maps should increase with increasing involvement of experts.

  22. Conclusions • We have the tools to map gradations of soil variability • Terrain attributes are useful for estimating soil properties • Structural heterogeneity of soils can be simplified for hydrological response predictions because of functional homogeneity of soil properties. • Homogeneity of soil properties is relative to the use to which the soil map is put

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