The SCIMAP QGIS plugin allows you to run the SCIMAP-Sediment and SCIMAP Flood tools and calculate the Network Index of hydrological within QGIS. The input datasets for the DEM, land cover map, and rainfall map need to be aligned, clipped to the same extent, and at the same spatial resolution.
Install the plug-in
From the plug-in menu, search for SCIMAP and install 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, down to 0.4 m, and coarser data, up to 5m. The approach can be applied to data such as the TanDEM 12m dataset, but the representation of hydrological connectivity will be degraded. In the UK, this data can come from the EA or NRW LiDAR datasets. The Scottish data is being developed by SEPA.
Land Cover: The land cover maps need to represent the key different land covers within the catchment. For a UK catchment, we normally use arable, improved grassland, rough grass, woodland, moorland and urban. For your catchment, you need to define the key classes. If there are local differences in soil type that affect erosion, they can be encoded 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. The can cover weights can use the default parameter set or be calculated 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 CHIRIPS.
The tool will give three outputs:
- Erosion potential
- Connectivity
- In-stream risk concentration.
If stream power is selected, then the erosion potential is calculated as the land cover weight multiplied by the stream power. Otherwise, the erosion risk is the land cover weights. The connectivity is calculated with the network index, with values of one representing the 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. This can be calculated 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 the suitability for implementing flood mitigation solutions that slow and store flood water. 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 area:
- Digital Elevation Model (DEM)
- Stream Initiation threshold (m2).
- WhieboxTools 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.