The recognition of Variable Source Area (VSA) hydrology as an important hydrological process has implications on water quality risk assessment that are only recently becoming widely realized.
Consequences for Water Quality
The Variable Source Area (VSA) is a physical hydrology concept. The related water quality risk assessment concept is the "Hydrologically Sensitive Area" (HSA) which is used to determine a probability of pollutant transport risk. Together, these concepts have significant consequences for surface water quality and have direct impacts on many areas including:
By combining several existing ideas about how to describe VSAs, we are developing and assessing ways to identify and quantify HSAs. Places where HSAs coincide with potential pollutant loading areas are "Critical Management Zones" (CMZs) (Walter et al. 2000, Gburek et al. 2002); management strategies can be designed to avoid pollutant loading in these parts of a watershed.
Hydrologically Sensitive Areas (HSAs)
While VSAs are a physical hydrology concept, HSAs are a water quality risk assessment concept and are linked to a probability of pollutant transport risk. HSAs are defined as areas in a watershed especially prone to generating runoff that are therefore potentially susceptible to transporting contaminants to perennial surface water bodies. This section provides more detailed information about hydrologically sensitive areas (HSAs).
Importance of HSAs
Wherever VSA hydrology is a dominant process, there will be regions within a watershed that are more susceptible to producing runoff and delivering it to surface water bodies than other regions. These areas can be considered hydrologically sensitive areas (HSA). Recognizing the existence of HSAs allows watershed-scale water quality efforts to be focused on those areas where HSAs coincide with land uses that potentially contribute pollutants. This concept is schematically presented in Figure 1 (below) in which the intersection is referred to as the critical management zone (CMZ). The most obvious best management practice for this area would be to limit or prohibit potentially polluting activity from this region. A contrasting approach is finding methods and means of eliminating the hydrological sensitivity of the critical zones; the philosophy of manipulating the environment verses adapting to environmental constraints is fodder for another discussion. (Recent research suggests attempts to remove hydrological sensitivity through drainage practices may simply reroute pollutants from overland flow to subsurface flow with little reduction in concentration (Shalit and Steenhuis 1996, Gachter et. al. 1998, Geohring et al. 2001). In either case, the HSAs warrant primary attention when trying to preserve or improve water quality. A more complete discussion of HSAs and the water quality implications of VSA hydrology can be found in Walter et al. (2000), Walter et al. (2001), Zollweg et al. (1996), and Gburek et al. (2002).
HSA Environmental Planning Tool
An HSA environmental planning tool for Delaware County, NY is available online. This tool can be used by farmers, Cornell Cooperative Extension, and NRCS employees for determining which areas in a landscape would most likely be saturated (hydrologically sensitive areas). This knowledge can be used in environmental and nutrient planning purposes. An introduction is provided, followed by calculations and then links to the on-line GIS map. References regarding the development of this tool are availabe at our References and Links page.
Example of Use
One example of where potential management practices derived from a recognition of VSA hydrology is in direct conflict with currently mandated management practices is whether dairy operators in the New York City watersheds should spread manure on steep slopes or in flat areas. The current dogma is to avoid spreading on steep slopes and to maximize spreading in flat areas. Under Hortonian flow steep areas might arguably produce the most rapidly moving runoff and therefore the greatest potential for erosion and transport of manure. However, VSA hydrology is the dominant process in these watersheds (e.g., Brown et al. 1999, Walter et al. 2002) and in actuality the steep slopes infiltrate essentially all rainwater and drain very rapidly, resulting in almost no substantial runoff (Frankenberger et al. 1999, Mehta et al. 2002). Conversely, the flat areas, especially at the base of hill slopes, are especially prone to saturation and thus to runoff generation. Figure 2 below illustrates manure deposition in exactly the wrong place, from an HSA standpoint. A manure spreading policy that is more consistent with the recognized hydrology would promote spreading high in the watershed and minimize spreading on low, flat areas susceptible to runoff generation.
These critical management zones can be seen when field boundaries are spatially overlaid onto a "saturation days" map. Where these two parameters intersect, management is critical and application of potential pollutant should be minimized. Check out an illustration of monthly Soil moisture fluctuation in an animated GIS format online. In this example, field boundaries are outlined in black and a legend is shown outlining number of days in a month that each area in the watershed is saturated.
Ongoing research in the BEE department at Cornell is aimed at determining the location of these HSAs in agricultural areas and developing tools that can be used in environmental planning to minimize inputs of pollutants in these areas (Agnew et al, 2006).
VSA Water Quality Papers
Walter, M.T., M.F. Walter, E.S. Brooks, T.S. Steenhuis, J. Boll, K.R. Weiler. 2000. Hydrologically Sensitive Areas: Variable Source Area Hydrology Implications for Water Quality Risk Assessment. Journal of Soil and Water Conservation. 3:277-284.Full Text (.pdf format, 385 kb)