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)
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 Minner’s (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 al’s (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.