Impact analyses add value to McHenry County LEAM simulation results by revealing the potential impacts that future growth may have on County assets
LEAM adopts a hybrid dynamic spatial modeling approach that combines regional drivers of land-use change along with drivers that operate in 30m x 30m cells across the landscape. At each time step in a LEAM simulation, the probability of land-use change in a cell is computed based on the combined probability associated with a number of factors. Then, the regional demand for new land uses are assigned to cells based on these probabilities. To capture the randomness of some land-use change, LEAM also produces land-use changes in a few cells in which the probability of change may not be high.
The “stress” natural, agricultural, and cultural resources face from future development can be measured from the likelihood of development occurring within their boundaries, based on the probability values projected in LEAM. Using this approach, we can differentiate between impacts of different land-use patterns on resource areas. One land-use pattern might have less of an impact on wetlands when compared to another, but may also have more of an impact on prime farmland and threatened and endangered species. In comparing different future land-use scenarios, tradeoffs may have to be made based on the impacts on different resources.
The Long-term Hydrologic Impact Assessment (L-THIA) is used to illustrate the effects of land use change on hydrology. LEAM combines land use change results with L-THIA to show how the different scenarios, and the policy interventions that define them, are likely to affect annual runoff volume and nitrogen loading in streams at the HUC 8 watershed level.
As residential and commercial growth occurs so does the demand on regional water resources. The methodology employed for this analysis combines LEAM land use change results for all scenarios with water demand data to project the likely trends in water demand that each watershed can expect over time. Tables provide cumulative data and graphs illustrate the annual changes expected for each watershed in the Chicago Region.
LEAM used r.hydro.CASC2D GRASS GIS hydrology model to illustrate the effects of land use change on hydrology during a 100-year, 24-hour storm event at the HUC 10 watershed level. Time-series maps illustrate surface water depth at snap shots of every six hours. Tables provide cumulative data and graphs demonstrate discharge changes at the outlet. Animations are expanded versions of time-series maps and display hourly changes in water quantity during 24 hours of storm event and 24 hours after the event.
The Green Infrastructure scenario takes a tiered approach to protecting critical natural areas throughout project region. The scenario utilizes maps based on the LEAM's green infrastructure process based on Maryland Green Infrastructure Assessment (http://www.dnr.state.md.us/greenways/gi/gi.html) as a reduced probability layer.
The concept behind this analysis is to project future development patterns resulting from protecting natural areas. The Green Infrastructure areas represent a broad swath community-based approach. The analysis used identifies areas within the area that are crucial to the ecological health and biological diversity of the project region; they simply represent those areas at different scales. An analysis reveals that they perform a synergistic function in terms of resource protection.