Purdum’s water vulnerability assessment model [12] was applied to the Thomas Brook watershed, located in the Annapolis Valley of Nova Scotia (Figure 1).

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 (Figure 2), with a total linear drainage length of about 6 km, all being fed by rainfall, ephemeral rivulets, and groundwater seepage. The average channel slope is 3.5%, being steeper in the upper watershed (>10%) and shallower in the lower reaches (0.5% - 1.3%) [13] . Soils are predominantly reddish brown sandy loam [17] (Figure 3). Land-use is varied [15] , being primarily (ca 57%) agriculture [14] (Figure 4) with large patches of forest in the upper watershed (Figure 5) and numerous dwellings in the mid- to lower watershed (Figure 6).

The Thomas Brook watershed has experienced degradation of both surface and groundwater quality due to agricultural and residential development [13] [14] . Because of this, the watershed was selected to be part of a long-term research program of Agriculture and Agri-Food Canada [18] . The Watershed Evaluation of Beneficial Management Practices (WEBs) program was set up to assess the environmental and economic impacts of different BMPs through hydrologic modes of sediment and nutrient export [15] [16] . As well, due to being labelled as one of the most attractive places in all of Canada in which to live, the predominantly agrarian Annapolis Valley is under threat from development, as witness to newspaper articles bearing titles such as “Farmers losing fight against urban sprawl”, “No farms means no food”, or “Giving away the farm”, for example.

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 [12] , from which the following shortened description is derived. Surface water vulnerability was assessed based on the serial integration and evaluation of three types of spatially-explicit geographic data: 1) Sources of Land Use-induced Change; 2) Intensity of Drainage; and 3) Sensitivity of Drainage Basins to Change. The model is based on the step-wise inclusion of information contained within three to four simple

rankings categorized based on values obtained from the literature and expressed (i.e. mapped) as dimensionless variables (Figure 7).

Each of the 5 sources of land-induced change produces its own output map (Figure 7). The three land use categories used in our present application of the Purdum model accurately condense the earlier, more detailed designations of 18 land use typologies used by Ahmad et al. [15] to run their engineering SWAT (Soil and Water Assessment Tool) model which simulated agricultural nonpoint source pollution in the Thomas Brook watershed. Most of the upper or top reaches of the watershed are covered with forests, with urbanization being largely centered along the central-eastern edge, and with much of the middle and lower or bottom of the watershed being agricultural. Ahmad et al. [15] show maps of soil typology and slopes for the Thomas Brook watershed. The rapid derivation of erosion hazard ratings for the present model adequately captures this spatial information determined from the labor-intensive engineering model. Locations of high erosion hazards were found to occur in regions of rock outcrop and steep riparian terrain, mostly in the central “neck” area of the watershed. Locations of low erosion hazard occurred in about half of the top reaches and most of the bottom reaches of the watershed, the rest of the watershed being categorized as medium erosion hazard. Derivation of stormwater runoff hazard ratings closely parallels land use. Very severe and severe categories were restricted to developed locations, and the very lowest and low categories occurred respectively in forested and in agricultural locations. Of more interest is that most of the riparian zones received a category of moderate runoff hazard. Most of the Thomas Brook watershed (from the central “neck” downstream) was designated as having a wetland loss hazard rating of moderate. Locations of high and severe rating were restricted to the riparian corridor in the bottom half of the watershed. In terms of the alteration of stream morphology, the riparian zones were predominantly categorized as low hazard. Only a few isolated locations, those with residential development or roads, were designated as medium hazard. No designations of high stream alteration hazard were found.

Together, all this information was integrated to produce the map of the Sum of Sources of Land Use-induced change shown in Figure 8. Most of the Thomas Brook watershed is categorized as having medium land use change. Locations of high land use change are predominantly restricted to the riparian zones within the central and bottom regions of the watershed, and locations of low land use change occur within the top reaches of the watershed.

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 Figure 9. Much of the top watershed was categorized with a low rating, whereas locations distant from the stream in the central and bottom of the watershed were categorized as moderate. Much of the linear length of the riparian zones were categorized as having a high intensity of change, and only a few locations in a contiguous band through the bottom of the watershed were designated with very high or severe ratings of intensity of change.

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. [15] . These included the ridge crest which spans the width of the watershed just above the “neck” region, as well as riparian zones in the central region

(Figure 10). Locations of the lowest sensitivity occurred in the bottom reaches of the watershed, where slopes were the lowest. The bulk of the watershed was categorized as medium drainage basin sensitivity.

The integration of this information with the Intensity of Change data produces the final map of the Vulnerability to Change shown in Figure 11. Locations in the topmost reaches of the Thomas Brook watershed are categorized as being of predominantly medium threat from land use-induced change to surface water vulnerability. Locations in the bottom of the watershed are categorized as being of largely low threat from land use-induced change to surface water vulnerability. And locations in much of the central region of the watershed are categorized as being of high threat from land use-induced change to surface water vulnerability.

From the present GIS analysis, using Purdum’s [12] spatial, exposure-based model, a suggestion can be made that in terms of water-sensitive planning, it would be wisest to direct future development in the Thomas Brook watershed to the bottom reaches where the predicted threats to stream vulnerability from land use-induced change will be the lowest. If this is not possible, the next most favorable region for development would be the top reaches where the predicted threats to stream vulnerability from land use-induced change will be of medium likelihood. Only as a last resort should the central rejoin of the watershed, where the predicted threats to stream vulnerability from land use-induced change will be the highest, be targeted for future development.

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) [3] .

The GIS model also suggests interesting ramifications for land use planning on a finer spatial scale. For as Purdum [12] stated, “though these sources [of land use-induced change] are mentioned frequently in the literature as contributors to stream and river degradation, their comparative importance is not clear.” The final map of the Sum of Sources of Land Use-induced change was found to most closely resemble the component map for stormwater runoff hazard, followed by those for wetland loss and for alteration of stream morphology hazards, and least so for those of nutrient loading sources and erosion hazard. For the Thomas Brook watershed, therefore, it appears that the legacy of past and the potential of future hazards for specific sites located within riparian zones will be a greater contributing factor to cumulative land use change than for other sites located upslope and distant from surface waters. In this regard, it is worth noting that the implications of such spatially-explicit results from the exposure-based assessments produced by landscape architects and land-use planners are different from those generated by effects-based, process models used by environmental engineers. This is not surprising given that motivations and methodologies can be dissimilar if not at outright cross-purposes between environmental researchers (environmental engineers) and natural resource managers (landscape architects and watershed land-use planners) [27] .

The primary objective of the modeling component of the Canadian WEBs program is to simulate the performance of agricultural BMPs on a watershed scale [18] . The SWAT model used by Ahmad et al. [15] requires GIS-based spatial information and temporal climate input variables to simulate levels and movements of water, sediment, and nutrients in relation to varying soils, land uses, and land management schemes. In the case of the Thomas Brook watershed, some of the data necessary for the SWAT model were obtained from a continuous monitoring program spanning years [13] . Ahmad et al. [15] calibrated and validated their model, finding it to perform satisfactorily in terms of simulating the export of sediment and nitrate. This led them to conclude that the SWAT model has the potential to be used as a decision support tool for watershed management, though no specific explanation is offered as to how this might be brought about. However, a follow-up paper [16] did use the same approach to simulate how different scenarios of tillage, crop rotation, and fertilizer application would affect crop yield, nitrate leaching, and sediment yield in the Thomas Brook watershed.

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. [15] depict a map showing the spatial distribution of average annual sediment yield measured from the sub-basins to Thomas Brook. The 6 sub-basins found to produce high sediment loads have steeper slopes or have more tillage practices and are situated in the central “neck” region and the top reaches of the watershed. It is important to note that there was a general concordance between these engineering-model results and the high erosion hazard rating category determined for these locations in the present study through use of the rapid GIS-landscape model. Seven of the 13 sub-basins found to produce low sediment loads have shallow slopes and are situated together in the bottom region of the watershed. Again, this matches the low erosion hazard rating category determined for these locations in the present study.

Ahmad et al. [15] also depict a map showing the spatial distribution of average annual nitrate export measured from the sub-basins to Thomas Brook. High nutrient losses occurred in 5 sub-basins, three of which were situated in the “neck” region, one in the upper reaches, and the other in the lower reaches of the watershed. There is less concordance between these engineering-model results and the nutrient loading sources mapped in the present study. This discrepancy may exist because the SWAT model determinations can reflect individual point-sources of pollution (farmsteads) that were situated directly above the monitoring inlets of their downstream sub-basins [15] .

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 [28] , treatment wetlands [29] , storage basins [30] , or alternative agricultural practices [31] . Again, our deliberate selection of Thomas Brook as the watershed in which to apply Purdum’s rapid assessment, exposure-based model enables comparisons to be made to findings derived previously from a multi-year study generating the detailed scientific, effects-based SWAT model. Ahmad et al.’s [15] SWAT model found that the drainage sub-basins of greatest contaminant (sediment and nutrient) loading were located across the “neck” region and within the central area of the top reaches of the watershed, and the sub-basins of lowest contaminant loading were found in most of the bottom reaches of the watershed. Although, using a finer spatial resolution based on precise sites rather than grouped sub-basins, the present GIS-Purdum model identified the very same locations as being meritorious for further study or BMP implementation due to their designation of being a high potential threat to water vulnerability. In other words, there is a close concordance between the two approaches in their identification of those locations in the watershed that are most in need of future research or immediate management.

Steinitz [32] argued that to be effective and efficient, land use planning should progress through a framework of inquisition and investigation by applying the appropriate models of representation, process, evaluation, change, impact, and decision. Process models such as SWAT, popular among environmental engineers, address the question about how landscapes operate, whereas evaluation (assessment) models such as Purdum’s, address the question of whether the current landscape is working well. The latter builds upon the former and segues into models for predicting the impacts of land-use changes, which in turn can be used for decision making in terms of avoiding or mitigating those changes [33] .

The present study found there to be a close concordance between the results of the process (SWAT) and evaluation (GIS) models. Ahmad et al. [15] may be overenthusiastic, and possibly unrealistic, in believing that a labor-intensive, and therefore costly model such as SWAT “has potential to be used as a decision support tool for agricultural watershed management”. This is because the scale upon which watershed degradation is occurring necessitates, above all else, rapidity in reaching management decisions [34] . The luxury of having four years for a doctoral student to reach a decision as to which portions of the watershed need “immediate” attention in order to ameliorate soil erosion and consequent degradation of receiving waters is simply not part of real-world praxis [3] . Effective and efficient land-use planning is based on “the notion that complex systems must be met by powerful simplifications that extract the essence of things” [35] . In this regard, we believe that the simple, spatially-explicit, exposure-based model developed by Purdum [12] and investigated in the present study, has the potential, like other GIS-based water-sensitive models [10] [33] [36] , to be useful in time-efficient and cost-effective land use planning. Using the present model offers promise as one stage in adopting a logical and operational framework of inquisition and investigation to help achieve the overall goal in watershed management of “being able to assess the vulnerability (the risk of potential harm from the impacts of land-use change) of the area under investigation” [3] .