> For the complete documentation index, see [llms.txt](https://terradapt.gitbook.io/terradapt-cascadia-documentation/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://terradapt.gitbook.io/terradapt-cascadia-documentation/data-inputs/topography-hydrology-and-soils.md). # 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 --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://terradapt.gitbook.io/terradapt-cascadia-documentation/data-inputs/topography-hydrology-and-soils.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.