LEAMwq assesses the potential impacts of land use changes on surface water runoff and NPS pollutant loading, using LEAM output and the Long-Term Hydrologic Impact Assessment (L-THIA) model.
For this analysis, LEAM land use change results from each scenario were use as inputs to the Long-Term Hydrologic Impact Assessment (L-THIA) model to estimate potential changes in runoff and nitrogen for each watershed.
For display purposes, the region is divided in to eight HUC 10 watershed areas as shown in the map to the left (for more info on Hydrologic Unit Codes (HUCs) Click here .
Results of the LEAMwq analysis can be viewed by watershed through the corresponding number links below.
Background and Methodology
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.
The idea is shown schematically below: