The SCIMAP QGIS plugin lets you run the SCIMAP-Sediment and SCIMAP Flood tools and calculate the Network Index of hydrological connectivity in QGIS. Align the input datasets for the DEM, land cover map, and rainfall map, then clip them to the same extent and spatial resolution.
Install the plug-in
From the plug-in menu, search for SCIMAP and install it from there. The plugin depends on Whitebox Tools for the DEM processing. You need to install a local copy of Whitebox Tools: The plugin can make better use of the multiple processors on modern computers with the Numba package. This is a recommended but optional installation. Detailed instructions for installing both Whitebox Tools and Numba are here.
Calculate the SCIMAP Sediment Map
To calculate SCIMAP-Sediment, you will need a DEM, a land cover map and a rainfall pattern.
DEM: The DEM needs to be high quality, and SCIMAP works best with LiDAR data at a ground resolution of 1m. The approach will still work with finer data, down to 0.4 m, and coarser data, up to 5m. The approach can be applied to data such as the TanDEM-X 12 m dataset, but the representation of hydrological connectivity will be degraded. In the UK, this data can come from the EA or NRW LiDAR datasets. SEPA is developing the Scottish data.
Land Cover: The land cover maps need to represent the key land cover types within the catchment. For a UK catchment, we normally use arable, improved grassland, rough grass, woodland, moorland and urban. For your catchment, define the key classes. If local soil differences affect erosion, you can encode them here, such as ‘grassland on sand’ and ‘grassland on clay’. In the UK, this data can come from the CEH land cover maps or from Living England. You can use the default parameter set for land cover weights or calculate them from a spatial pattern of observed in-stream values using the SCIMAP-Fitted approach.
[table of default weights]
Rainfall: The rainfall map should reflect the average rainfall pattern across the catchment. In the UK, this data can come from the HadGrid dataset; globally, it can come from datasets such as CHIRPS.
The tool will give three outputs:
- Erosion potential
- Connectivity
- In-stream risk concentration.
If stream power is selected, erosion potential is calculated as the land cover weight multiplied by stream power. Otherwise, the erosion risk is the land cover weights. Connectivity is calculated using the network index, with values of one representing areas most likely to connect to the river channels. The in-stream risk concentration represents the balance between the accumulation of the risk load across the catchment and the potential for higher flows to dilute it. Areas with greater risk accumulation than dilution potential indicate where sediment is likely to be coming from and where mitigation actions are more likely to be successful.
Calculate the SCIMAP-Flood Map
To calculate a SCIMAP-Flood map, you will need a set of pre-processed data files, which are described in detail here. These are
- Network index of hydrological connectivity. You can calculate this with the Network Index in this plugin; see below.
- A set of rainfall maps, typically six to 12.
- A set of overland flow travel maps to the impact points. These impact points may be key settlements, critical infrastructure or key links in the transport connectivity.
The output is a map of suitability for implementing flood mitigation solutions that slow and store floodwater. These actions include nature-based solutions, such as leaky debris dams, ponds and soil structure improvements, or engineered solutions, such as dams or water storage reservoirs.
Calculate the Network Index of hydrological connectivity
To calculate the Network Index of hydrological connectivity, select the Network Index option and put your DEM into the DEM field. The options are:
- Digital Elevation Model (DEM)
- Stream Initiation threshold (m2).
- WhiteboxTools executable
The output is the map of the relative hydrological connectivity, where a value of zero represents the area in the catchment least likely to connect the river channels, and a value of one represents the areas most likely to connect. The values are relative within the catchment; the same value between different catchments, calculated separately, does not represent the same level of connectivity.