The utility of a spatially-explicit, exposure-based model was examined for its suitability as a tool for rapidly assessing surface water vulnerability in watershed planning. This simple GIS-model uses three types of easily obtainable spatial information: (1) sources of land use-induced change; (2) intensity of watershed drainage; and (3) sensitivity of drainage basins to change. This model was applied to the Thomas Brook watershed in Nova Scotia, Canada, which has been the site of previous studies, conducted over multiple years, using detailed, effects-based, hydrologic models. Doing so allowed us the opportunity to compare the two approaches. Results showed a good concordance in the derived mapped outputs between the two models. Given the rapid ease and inexpensive cost of using the GIS, exposure-based model, we believe it to offer great promise in terms of prioritizing locations for further study or for intervention of best management practices, as well as for planning where to best direct future water-sensitive development through build-out analyses.
Deterministic models have been used to simulate complex processes in the movement of water, sediment and contaminants in the scientific study of watersheds [
The accelerating pace of the environmental degradation of watersheds necessitates developing more rapid approaches for identifying those locations most in need of applying best management practices (BMPs) to protect ecosystem services [
Of paramount importance for water-sensitive land use planning is the need to be able to rapidly prioritize the suitability of different locations in order to regulate land use development in the most environmentally benign way possible [
One useful (and simple to use) GIS-based model for predicting the aquatic impacts of site development is that of Purdum [
Purdum [
Purdum’s water vulnerability assessment model [
This small, 784 ha watershed is part of the larger 360 km2 Cornwallis River Catchment draining into the Bay of Fundy. The stream network is relatively simple with several upland streams merging into a single channel about a third of the way down the watershed (
The Thomas Brook watershed has experienced degradation of both surface and groundwater quality due to agricultural and residential development [
The model is based on an existing database comprised of 12 parameters available from easily obtainable government and academic sources: from the Government of Nova Scotia’s GeoNOVA portal: topography (orientation, derived from DEM), topography (slope, derived from DEM), water table (seasonal high), water (wetland type), open water (predominant type, i.e. lake, river, stream, etc.), woodlands (presence, absence, type), vegetation (predominant type, i.e. forest, grass, etc.), transportation (type, i.e. gravel, bridge, class of highway), urban land (dwellings, from zoning of land), and agriculture (existing agricultural land uses); from Natural Resources Canada: soil erodibility index (derived from detailed soil survey); and from Dalhousie University’s GISciences Centre: water (watershed boundary for Thomas Brook catchment area).
Detailed explanations behind the rationale for including model variables, as well as the step-by-step developmental process, are described in [
rankings categorized based on values obtained from the literature and expressed (i.e. mapped) as dimensionless variables (
The first step of model development is to determine the Sum of Sources of Land Use-induced Change. This was determined by integrating database overlays from 5 spatially-assessed variables: nutrient loading, erosion and sedimentation, stormwater runoff, adjacent wetland loss, and alteration of stream morphology (
The potential for nutrient (both phosphorus and nitrogen) loading from each location is dependent on land use and vegetative cover. Nutrient loading weights were assigned in relation to agriculture (highest), urban (medium), and forests (lowest) [
The potential for erosion and sedimentation is based on the spatial assessment of three determinants: soil typology combined with slope; distance from surface water; and land use. The relative erosion hazard is estimated by the nature of the soils and the slope of the land. Each location was given a value in relation to high, moderate, or low soil erodibility in relation to categorization by the USDA Soil Conservation Service [
The stormwater runoff hazard assessments (i.e. coefficients of runoff) were ranked as very low, low, moderate, severe, and very severe based on the land uses of urban, cultivated, residential, forest, road/bridge as in [
Because wetlands operate as hydrological sponges and purifying kidneys on the landscape [
Because streams in urbanizing environments are in states of dynamic disequilibrium [
Each of these 5 individual sources of land use-induced change generate their own mapped output (
Identification of the locations more susceptible to change is estimated by the drainage intensity, which is dependent on the area of the drainage basin, land use and cover typology, and the movement of rainfall through the drainage network [
The assessment of the Intensity of Change is determined from a matrix combining each location’s drainage intensity with its previously derived Sum of the Sources of Land Use-Induced Change. These values are rated and shown on the resulting map as very low, low, moderate, high, very high, and severe potential energy (
The shape of the land will influence the movement of runoff and the consequent transport of contaminants [
for collection zones in which runoff is concentrated, and high for conveyance zones where runoff can directly enter surface waters.
The final assessments of the Vulnerability to Change of surface waters is determined from a matrix combining each location’s sensitivity of drainage basin zones with its previously derived Intensity of Change (
Each of the 5 sources of land-induced change produces its own output map (
Together, all this information was integrated to produce the map of the Sum of Sources of Land Use-induced change shown in
With the exception of a few locations in the central “neck” region and a linear
strip through the bottom region of the watershed, both of which were categorized as having medium or high drainage intensity ratings, the rest of the Thomas Brook watershed was designated as low drainage intensity. Combining this information with the Sum of Sources of Land Use-induced change produces the map of the Intensity of Change shown in
Sensitivity of drainage basin zones depends on topography. For the Thomas Brook watershed, locations of highest sensitivity occurred in the region of greatest elevation change, as shown in the DEM-determined slope map in Ahmad et al. [
(
The integration of this information with the Intensity of Change data produces the final map of the Vulnerability to Change shown in
From the present GIS analysis, using Purdum’s [
It is important to remember that water-sensitive planning assessments of surface water quality vulnerability (in addition to its contributing factors of wetland loss and presence and extent of forested buffers), such as measured and used in the present GIS model, are but one part of a framework of comprehensive watershed development planning in both professional and pedagogical undertakings. Other variables to consider include wildlife-sensitive planning (endangered species, biodiversity, fragmentation, connectivity), site amenities (agricultural potential, visual quality, historic/cultural resources), and site construction and maintenance (energy and microclimate, projected construction costs, wastewater treatment) [
The GIS model also suggests interesting ramifications for land use planning on a finer spatial scale. For as Purdum [
The primary objective of the modeling component of the Canadian WEBs program is to simulate the performance of agricultural BMPs on a watershed scale [
The spatial scale of the SWAT model used in the Thomas Brook watershed was based on lumping data from similar hydrologic response units (characterized by their land use, soil type, and slope) in each of 28 delineated sub-basins. Although this is not as spatially-explicit in terms of site-specificity as the present GIS model, it is still useful for enabling comparisons in water protection assessments made between the patterns of the two mapped outputs.
Ahmad et al. [
Ahmad et al. [
A benefit of the present spatially-explicit, exposure-based GIS-model is that it identifies those locations with the greatest potential threat to surface water vulnerability. In so doing, these specific locations can become the focus of later, more-detailed research regarding the conceptual modeling of installing any of a number of BMPs of known utility, such as forested buffer strips [
Steinitz [
The present study found there to be a close concordance between the results of the process (SWAT) and evaluation (GIS) models. Ahmad et al. [
France, R.L. and Pardy, G. (2018) Spatially-Explicit, Exposure-Based Assessment of Surface Water Vulnerability from Land Use Threats for Time-Efficient and Cost-Effective Watershed Development Planning. Journal of Geoscience and Environment Protection, 6, 35-55. https://doi.org/10.4236/gep.2018.66003