Development and Applications of the L-THIA Model
L-THIA was initially designed for planners and natural resource managers because they are familiar with land use change in a particular area, have perhaps the best access to land use information, and are often interested in environmental impacts. Whether it be past, present, or projected land use development scenarios, establishing land use areas and determining CNs as input variables to runoff estimation is a task well-suited to planners and resource managers. L-THIA and many other models determine runoff from precipitation data and a land use / soils index, the Curve Number (CN), developed from real-world data by the United States Department of Agriculture, Soil Conservation Service (USDA, 1986). The CN is used in an empirically based formula to determine how much of a given rainfall event becomes surface runoff. The relationship between rainfall, runoff and CN value is non-linear, meaning that small changes in land use or rainfall can produce large changes in runoff. Although used in everyday, simple stormwater management methods, the CN method is also often buried in complex models used in more sophisticated analyses. The use of the CN equation in L-THIA is a simple alternative to far more complicated hydrological models that require extensive data inputs which are often not available for most areas.
Early applications of L-THIA, such as Harbor (1994), McClintock et al. (1995), Ogden (1996) and Bhaduri et al. (1997) and were all conducted using L-THIA as a spreadsheet application. Like the TR-55 manual (USDA, 1986), the original spreadsheet L-THIA model calculated runoff generation only on a watershed or sub-basin level, using an area-weighted or "composite" CN. In this approach, CN values for large areas are averaged prior to calculating runoff. L-THIA has subsequently been developed as a GIS application (Ogden, 1996; Bhaduri et al., 1997; Grove, 1997; Harbor and Grove, 1997; Leitch, 1997; Grove et al., in press; Bhaduri, 1998; Leitch and Harbor, in review) which has several advantages, including simple implementation of a "distributed" approach to runoff calculation. In the distributed approach, runoff is calculated for all unique areas in the watershed, then summed, eliminating the need for averaging CNs which is required in the composite approach. Given the nonlinear relationship between runoff and CN, the averaging procedure required in composite analyses can lead to significant underestimation of runoff compared to a distributed approach (Grove, 1997; Grove et al., in press). In a GIS application, L-THIA can handle the computationally intensive task of distributing runoff calculations for numerous land use polygons over space.
Initial applications of L-THIA involved assessing the impact of land use change on groundwater recharge, and of suburbanization on runoff into a wetland in northeast Ohio (Harbor, 1994; McClintock et al, 1995). Ogden (1996) then applied the spreadsheet technique to town planning and coastal management in Barbados, and Bhaduri et al. (1997) used L-THIA to examine the hydrologic implications of future land use change for an urbanizing watershed in north-central Indiana, based on zoning maps. Leitch (1997) and Leitch & Harbor (in review) followed up on Ogden (1996) to provide a detailed assessment of how urbanization and agricultural transition in the Holetown watershed (Barbados) changed runoff inputs to the coastal zone, including comparisons with existing streamflow data. Grove et al (in press) performed an extensive sensitivity analysis on the L-THIA approach, examining how the results of analyses varied with distributed versus composite CNs, the spatial resolution of the input data, and differing climate data record lengths. Grove et al (accepted) then used a GIS version of L-THIA to examine the impact of historical land use change in a watershed in Indianapolis, using remote-sensing based land use maps, with a particular focus on spatial patterns of change within the watershed. Grove et al (accepted) also compared L-THIA predictions of runoff volume to river gauge data.
Recent applications of L-THIA include Minnerís (1998) analysis of variations in urban sprawl impacts for the major climate regions of the U.S., her assessment of the relative hydrologic impacts of conservation subdivision design versus traditional use patterns, and Minner et alís (in press) analysis of how L-THIA and other hydrologic techniques can be used in the apportionment of costs as part of a fee system to support maintenance costs for a drainage management programs. In addition, Bhaduri (1998) performed comparisons of L-THIA with other well-known hydrologic models, and developed a nonpoint source pollution routine for the GIS version of L-THIA that has been applied to an assessment of water quality impacts associated with past and planned land use change in a watershed in Indianapolis