Water Quality Analysis

Water quantity and quality issues are also important factors in choosing between various planning scenarios for the CAMP region.  Total runoff and nitrogen loading are two of the most salient and for this work were estimated within the Long-Term Hydrologic Impact Assessment (LTHIA) model.  This section discusses the methodology used for this analysis, and the results thus obtained.

L-THIA is a GIS-based export coefficient model developed at Purdue University with support of US Environmental Protection Agency. L-THIA calculates mean surface runoff and NPS pollutant loading for a given region and a period using daily precipitation series, a land use map and a hydrological soil group map (figure 1). Selected for this study are total nitrogen (TN) and total annual runoff.

LTHIA uses a basic “curve number” algorithm for calculating runoff rates.  As used by LEAM, every cell in the simulation area is assigned a soil type and land use classification.  These numbers are then combined into a so-called curve number (CN) which is meant to capture the basic hydrological and pollutant-loading properties of the cell in question. 

In the graphic below, the gray box marked “link” represents the application of the LTHIA model which gives runoff and pollutant-loading numbers for each land-use and soil type combination.  The model was run with land use maps simulated by LEAM from 2005 to 2040 with five-year intervals and 1961-1990 daily precipitation series. LEAMwq was run for five development simulations: reference, agriculture protection districts, 40-acre minimum zoning, compact contiguous growth (CCG), and conservation districts.

The final LTHIA results can be represented in a look-up table with one entry for each  land-use, and soil type combination.  Once this table is generated, it is linked back the results to the input maps to give an approximate spatialization of runoff and pollutant contributions.  In the McHenry County LEAMwq analysis, only total runoff and nitrogen loading were assessed. 

Sample Analysis (McHenry County's Watershed 601)

At 2023 runoff in the Green Infrastructure scenario increases at a much higher rate than before and in comparison to all except the Groundwater Protection scenario.  Results of these two scenarios indicate that development occurs in contiguous islands throughout the county, which would create greater amounts of contiguous impervious surfaces within the developed areas.  Alternatively, farmland in these scenarios is still available for development, creating a greater rate of decrease in nitrogen loading. 

Results of the Land Evaluation scenario indicate that protecting farmland via the criteria used in the simulation would be effective in leveling out runoff volume in this watershed. the converse is true for nitrogen loading.  The decrease in nitrogen loading resulting from the Groundwater Protection scenario is likely a result of farmland being converted in place of groundwater protection areas.  It is important to note that the scale of nitrogen loading changes is very small; the largest change being only 30 lbs over 25 years.