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Competency Area 6: Watershed hydrology AEM

PO 57. Understand the major inputs and outputs of water in a watershed.

The following terms are thus described in the context of watersheds.

  • Precipitation – often varies across a watershed, and the spatial variation tends to be even greater in watersheds with significant topographic relief because the cooler air at higher elevations has a lower saturation vapor pressure and the humidity condenses easily. The amount of precipitation also varies more with convective type storms as compared to broadly occurring frontal storms.
  • Storms – are classified as frontal, convective, and orographic.
    • Frontal storms occur as warm or light air masses meet cold or heavy air masses. This is the dominant type of precipitation that occurs in the Northeast, generally producing the low intensity, long duration rainy days.
    • Convective storms occur as a result of air expanding when heated by solar energy, and the lighter air moistened with evapotranspiration rises, cools, and condenses. In the Northeast, this produces the summer thunderstorms, often delivering high intensity but short duration rains.
    • Orographic storms occur as wind forces warm, moist air masses to rise over hills and mountains, so although this occurs in conjunction with frontal and convective storm types in high relief watersheds, this type of storm process better explains why watersheds in the western side of the Adirondacks (the Tug Hill Plateau) gets more precipitation than the eastern side (Hudson Valley and Lake Champlain).Some important considerations of watersheds in regards to storm direction are the watershed size, shape, gradient, and aspect. Storms moving slowly over large, elongated, steep gradient watersheds in the opposite direction of the aspect are likely to create flashy increases in stream flow and depth, often leading to flooding conditions in the lower end of the watershed.
  • Infiltration and percolation – will vary substantially across a watershed. Generally, infiltration and percolation tend to be higher in soils that are higher in elevation than the other soils in the same association in the watershed, or higher in soils that straddle convex slopes. However, there are numerous exceptions to this because of the variation in soils, soil layering and geologic material.
  • Storage (Depression, Detention, Channel, Groundwater, Retention) – refers to the locations in the watershed where surface runoff or excess drainage water is held, at least temporarily.
    • Depression storage is the water stored in field surface depressions and therefore not contributing to surface runoff.
    • Detention storage refers to the water in excess of the depression storage which is temporarily stored somewhere in the watershed while enroute to streams. Wetlands are a good example of detention storage.
    • Channel storage is the water temporarily stored in channels while enroute to an outlet or it also refers to the drainage water that can be stored above the start pumping level in ditches and floodways without flooding adjacent land.
    • Groundwater is stored until such time that the water table starts to rise above the stream.
    • Retention is the precipitation on an area that does not escape as runoff.