Mitigation Wizard Methodology
How we estimate impacts and suggest mitigation opportunities
The Mitigation Wizard can both estimate the impacts of a project based on one or more footprint areas, and it can suggest mitigation for acreage impacts either from the tool's own estimates or from user-entered impact acreages for a range of species or habitats available in our Species Models. These models are described in detail below.
For estimating impacts, we use the full resolution data for each species model tied to a 30-meter "fishnet" grid, each covering 900 square meters or 0.22 acres. We have divided the entire Bay Area up into 30 meter squares and assigned an acreage amount to each square for each species under each model. When you draw, select, or upload a project area, we intersect that area with the fishnet and return the sum of acres for each species, as shown here:
We take a lower resolution approach for suggestion mitigation areas, to avoid false precision and allow for more filter attributes. With a set of 10 hectare (25 acre) hexagons, we attach total habitat acreage for species in each model, along with a range of other attributes. Many of those can be precalculated, such as land protection, land cover, habitat connectivity, and conservation priority (described below).
Other attributes of each hexagon are calculated dynamically as you use the tool. This is true of Habitat Coverage, Species Richness, and Habitat Rarity. Each of these is based on the suite of species you have selected for mitigation, so we can highlight those areas with the most or least habitat coverage, species richness, or habitat rarity for the specific species you have selected. You can use the info icons on each item to read more about how it is calculated.
The application relies on several species model approaches and we provide all as options to users to achieve the best outcomes from the tool.
We have models for two purposes:
- Protecting existing habitat
- Restoring or creating new habitat
And then within each purpose, we have models based on several different types of data:
- Suitable habitat within 2 mi of CNDDB occurrences (101 species) More constrained and will find less impact.
- Suitable habitat within 4 mi of CNDDB occurrences (103 species) Less constrained and will find more impact.
- Range defined by CWHR (14 species) Fewer species over larger range, so higher impacts per species.
The methodology below describes these options in more detail.
Habitat Preservation Models
For our habitat preservation models, we compiled existing spatial data on the locations of threatened and endangered species, the distribution of land cover or habitats, and the locations of important agricultural lands. The amount of habitat likely to require compensatory mitigation was identified through the following methodology:
- California Natural Diversity Database (CNDDB) points were selected that corresponded with species typically requiring mitigation when impacted by infrastructure projects. From the CNDDB dataset, those that were listed as "presumed Extant" were selected.
- The points were buffered by two and four miles.
- The California Wildlife Habitat Relationships model (CWHR), developed by California Department of Fish and Wildlife, includes relatively coarse range maps for terrestrial, vertebrate species. For this subset of the species identified in CNDDB, we collected range map shapefiles.
- The two CNDDB-based shapefiles as well as the CWHR range map shapefiles were then overlaid on the land cover dataset.
- Appropriate habitat types for each mitigation species were selected as follows:
- Vegetation types with a "High" rating in the California Wildlife Habitat Relationships model (CWHR) (California Department of Fish and Wildlife; http://www.dfg.ca.gov/biogeodata/cwhr/wildlife_habitats.asp) for terrestrial vertebrate species were selected.
- Calflora listed land cover types for plant species were selected (http://www.calflora.org/).
- Various online sources for invertebrate species were used to define appropriate habitat types.
- For plants, invertebrates, and unique types identified from the literature, the habitat requirements were cross-walked to CWHR types, so that their potential locations on the landscape could be identified using our reference maps.
- Species-specific habitat types were then selected from the buffer/land cover overlay for each species.
- Distribution data (NMFS) was used for salmonids.
Other non-species resources that require offsets if there are impacts are: riparian forest, wetlands, vernal pools, and oak woodlands. These were selected from the land cover dataset.
Habitat Restoration Models
Restoration models used the same approach as the conservation models just described. However, they included several additional land cover types where restoration activity for a species might occur: agricultural areas, barren land, and Eucalyptus groves.
For ecosystem rather than species mitigation, the following methods were used:
- Oak woodland: existing oak woodland polygons (blue oak, coastal oak, and valley oak) were selected from the land cover dataset. These were buffered by 0.5 miles and oak woodlands as well as agricultural areas, barren land, and Eucalyptus groves were identified as potential restoration areas.
- Wetlands: existing wetland polygons (freshwater emergent and saline emergent) were selected from the land cover dataset. These were buffered by 0.25 miles and wetlands as well as agricultural areas, barren land, and Eucalyptus groves were identified as potential restoration areas.
- Riparian forest: we selected the 'Stream Valleys' shapefile from the CLN 2.0 database. We then selected riparian forest (valley foothill and montane) agricultural areas, barren land, and Eucalyptus groves from the land cover dataset that intersected 'Steam Valleys' and identified these as potential restoration areas.
- Vernal pools: grassland (annual and perennial) land cover was selected as was agricultural areas, barren land, and Eucalyptus groves. Those polygons within the USFWS statewide vernal pool dataset were considered potential vernal pool restoration areas.
- Serpentine: all areas within serpentine types identified in the SSURGO database were identified as potential restoration areas.