# Topography, Hydrology, & Soils

TerrAdapt:Cascadia uses a variety of datasets pertaining to the physical environment, including data on topography, hydrology, and soils. Though these geophysical features change over geological timespans, for our purposes they are considered to be static. 

The topographic and hydrological indices we use are all based on a digital elevation model (DEM) that represents the elevation at the surface of the earth. We use the [GLO-30](https://spacedata.copernicus.eu/documents/20123/121239/GEO1988-CopernicusDEM-SPE-002_ProductHandbook_I4.0.pdf) 30m resolution DEM developed by the European Space Agency and hosted in the Google Earth Engine (GEE) data repository (see entry in the catalog [here](https://developers.google.com/earth-engine/datasets/catalog/COPERNICUS_DEM_GLO30)).

From the DEM, we calculate various topographic indices in GEE, including:

* Slope
* Aspect
* Heat load index ([McCune and Keon, 2002](https://onlinelibrary.wiley.com/doi/10.1111/j.1654-1103.2002.tb02087.x))
* Topographic position index (Weiss, 2001)
* Height above nearest drainage ([Nobre et al., 2011](https://www.sciencedirect.com/science/article/abs/pii/S0022169411002599))
* Horizontal and vertical terrain curvature (Florinsky 2016)

In addition to the above topographic indices, we calculate the following additional indices using [SAGA GIS version 9.0](https://saga-gis.sourceforge.io/en/index.html): 

* Multiresolution valley bottom flatness
* Topographic wetness index
* Flow accumulation
* Surface area
* Texture
* Ruggedness
* Morphometric protection index

Soil data used by TerrAdapt:Cascadia comes from [SoilGrids.org](https://soilgrids.org/), which developed a global 250m resolution soils dataset (we use version 2.0). From this dataset, we use the following variables, all calculated at a soil depth of 0-30 cm. 

* % clay content
* % sand content
* % silt content
* bulk density
* pH
* Soil organic carbon
* Soil order


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