| | A watershed, or drainage basin, is the land that drains runoff to a particular point in a surface water body. Watersheds for AESA Stream Survey were selected through a process that took into account:
- agricultural intensity; and
- runoff likelihood.
Agricultural intensity
Agricultural data obtained from 1996 Census reports were statistically analyzed to determine the main agricultural activities in Alberta; these activities were found to be grain/oilseed, cow-calf, and intensive livestock operations.
To determine the agricultural intensity in a watershed, 1996 Census data were examined for fertilizer and chemical expenses (indicators of crop production), and manure production (an indicator of livestock production). These indicators were used to rank agricultural intensity into three categories: high, medium, and low. Alberta watersheds were then classified into one of the three intensity categories.
Runoff risk potential
Runoff refers to the portion of rain, snowmelt, or irrigation water that flows over the land. Landforms, soil characteristics, and precipitation strongly influence the potential for runoff to occur. Various landforms in Alberta were categorized according to how likely, and how rapidly, they would deliver runoff to nearby surface water. Soils were also categorized as having either high, moderate, or low runoff potential based on soil texture (clay, loam, silt), and depth of the topsoil layer. Total annual rainfall was classified as high, moderate, or low for each watershed area. Evaluations of these three characteristics determined the runoff potential in all Alberta watersheds.
Selection process
Watersheds greater than 1500 km2 (370 640 acres) were dropped from the selection process, as larger watersheds tend to have non-agricultural influences, like cities. In addition, watersheds needed to have an active stream gauging station for interpreting water quality data. A list of 221 watersheds from throughout the province, which met both the size and gauging station criteria, was used for the next step of the selection process.
Evaluation of agricultural intensities and runoff potential narrowed the list to 65 watersheds. Of the 65 watersheds, 35 were identified by expert opinion as suitable for monitoring. Field surveys narrowed the selection to the final list of 23 watersheds. If funding availability increases, more watersheds may be added to the program.
These 23 watersheds (Figure 1) represent regional and provincial characteristics in agricultural production and runoff processes, and may be used to represent similar watersheds from across Alberta.
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