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Competency Area 6: Watershed hydrology AEM

PO 66. Understand and apply the concepts of hydrologically sensitive areas and critical management zones at the field, farm and watershed levels. Be able to give examples.

Hydrologically sensitive areas (HSA's) are areas in the landscape where there is a high hydrologic risk for water movement off-site. Thus, HSA's have a high potential for transporting pollutants off-site.

Examples of HSA's include:

  • Impervious surfaces (barnyards, bunker pads).
  • Relatively impermeable (crusted or frozen soil, soil with shallow hardpans or fragipans) and erosion prone (low permeability, sloping) soils.
  • Saturated soils (poorly drained soils, variable source saturating areas, seeps).
  • Flood plains (especially those that flood frequently).
  • Flowing surface water areas (perennial and intermittent streams, concentrated flow paths).
  • Groundwater recharge areas (highly permeable soils, sinkholes, shallow soil over Karst – a special type of landscape that is formed by the dissolution of soluble rocks, including limestone and dolomite).

The first two examples of HSA's occur as a result of rainfall and processes of infiltration-excess. For nutrient management planning and CAFO permitting purposes, various NRCS standards are available to address these areas, and RUSLE is used to address the erosion prone soil areas. The latter four examples are induced by saturation-excess hydrologic processes, and these areas are addressed with the NRCS nutrient management standard, in conjunction with the P and N Leaching Index risk tools.

Critical management zones are where the application or loading of potential pollutants, and their availability, overlap with the HSA, resulting in the mixing and transport of the pollutants. A common example of a critical management area is when manure is applied in a low lying area of the field that saturates quickly during a rain event, producing runoff that transports the manure off-site.