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Watershed Assessment Tool: Hydrology Concepts
Dimensions of Hydrology
To understand the continuous and complex interactions between water and its environment during the hydrologic cycle, watershed hydrology can be described and studied in four dimensions (Amoros 1987; Ward 1989).
- longitudinal (headwater to mouth),
- lateral (channel to floodplain),
- vertical (channel bed with groundwater),
- chronological (over time)
Rivers and their watersheds are shaped and characterized by movements of water through the longitudinal, lateral, and vertical dimensions, which transfer materials, energy, and organisms. Additionally, the time dimension (duration and rate of change) is critically important in managing stream systems because of the dynamic nature of the riverine components (Ward 1989).
Water withdrawal, bridge and dam building, and changing land use are examples of alterations that interrupt these processes. Due to the dynamic nature of the system, alterations will have impacts in all four dimensions.
The River Continuum Concept
|Figure redrawn from Vannote et al. 1980.|
The River Continuum Concept (RCC) emphasizes the longitudinal dimension of the stream ecosystem. This concept describes a progressive shift in the system from the headwaters to the mouth. A shift in physical gradients and energy inputs is accompanied by a shift in trophic organization and biological communities (From Vannote et al. 1980).
The RCC describes the entire river system as a continuously integrating series of physical gradients and associated biotic adjustments as the river flows from headwater to mouth.
Floodplain rivers experience seasonal variation in water levels which sustain riverine processes. This “flood pulse” emphasizes the importance of the lateral dimension to the stream ecosystem. Seasonal fluctuations in the lateral extent of a water body are essential for biological productivity and energy transfer in that system. The inundation and recession of water into and out of the floodplain is critical.
The timing of the flood pulse creates a pattern of inter- and intra-annual hydrologic variability. This timing needs to match the biological requirements (e.g. plant phenology, life histories of aquatic organisms) and environmental context (e.g. nutrient cycling, temperature regimes, sediment transfer and deposition) that exist within each particular river system. (Annear, 11).
The flood pulse concept applies the River Continuum Concept to the lateral dimension as a “batch process,” operating distinctly from upstream inputs and accounting for the existence, productivity, and interactions of major biota in river-floodplain systems (Junk et al. 1989). This same water level variability is essential for the health of the areas surrounding other hydrologic features such as lakes and wetlands.
Streams interact with groundwater in two basic ways: streams gain water from inflow of groundwater through the streambed; they lose water to groundwater by outflow through the streambed, or they do both-gaining in some reaches and losing in others (Winters et al. 1998). These processes are directly related to the five riverine components.
The main source of water for Minnesota watersheds is precipitation as rain or snow. The amount, timing, and kind of precipitation are key factors determining annual water yield from any specific watershed.
There are spatial and temporal influences on the availability of water that add to the complexity of the precipitation picture in Minnesota. For example, tropical maritime air moves into the State from the south and southeast. Therefore, southeastern Minnesota, averaging near 32 inches, receives more precipitation than northwestern Minnesota, which averages less than 19 inches.
Nearly two thirds of Minnesota’s annual precipitation falls during the growing season of May through September, a period during which Gulf of Mexico moisture is often available. Drought can occur in all areas of Minnesota, however it is more likely in western and northwestern areas more distant from Gulf of Mexico moisture. When Gulf moisture is abundant, repeated rain events can overwhelm surface water systems, raising lake levels and forcing streams out of their banks. Singular, intense rain events can lead to flash floods anywhere in the State.
Only eight percent of average annual precipitation falls in the winter (December through February) when the dry polar air masses prevail. Yet, large scale spring flooding can occur as a result of a combination of a deep late winter snow pack, frozen soil which prohibits infiltration, rapid snow melt due to an intrusion of warm air, and heavy early spring precipitation.
The presence of moist vs. dry air masses helps to determine the atmosphere’s ability to absorb water vapor evaporating from soil and open-water surfaces, or transpiring from leaf surfaces. Evaporation plus transpiration is called “evapotranspiration”.
Minnesota is located on the boundary between the semi-humid eastern U.S., and the semi-arid west. Semi-humid climates are areas where average annual precipitation exceeds average annual evapotranspiration, leading to a net surplus of water. In semi-arid areas, evapotranspiration exceeds precipitation on average, creating a water deficit. In Minnesota, the boundary between the climate regimes cuts the State roughly into east-west halves. http://www.dnr.state.mn.us/climate/water_availability.html
Runoff occurs when the rate of rainfall on a surface exceeds the rate at which water can infiltrate the ground, and any depression storage has already been filled. Not all parts of a watershed are equal when it comes to generating runoff.
- Some portions of the land are more tightly linked to the stream system than others.
- Some areas away from the perennial channel may be more consistent runoff generators than other areas nearby the channel.
- If a storm is short, only a small part of the watershed is likely to generate runoff, either as storm flow or base flow.
- The entire watershed may generate storm flow if the storm lasts long enough and antecedent conditions have been wet.
Variable source concept
Variable source areas show seasonal variation in runoff generation. Solid line: source of perennial stream flow; hatched line: source of late winter, spring and early summer intermittent flows; dotted lines: source of ephemeral flow during wet seasons. The entire watershed may generate runoff for a few days during a long storm in a wet season or during snowmelt.
- Most of the time, only a portion of the watershed actively generates runoff in response to precipitation or snowmelt.
- The portion generating runoff changes. It will grow during a storm or snowmelt and shrink after the end.
- The pattern of runoff generation varies from storm to storm, partly because each storm is different. In winter, precipitation may fall as snow, which is retained in storage until it melts. But even if all storms were uniform rainstorms, variation would exist due to differences in the amount and distribution of moisture in the watershed.
- Despite all the variation, the runoff response of a watershed varies in a recognizable pattern with season
Water yield is the runoff from the drainage basin, including ground-water outflow that appears in the stream plus ground-water outflow that bypasses the gaging station and leaves the basin underground. Water yield is the precipitation minus the evapotranspiration. (USGS Langbein)
In addition to precipitation, there are other factors that determine annual water yield:
- Type of vegetation and how much
- Land slope
- Soil type (or surface type)
- Soil condition (e.g., degree of compaction, permeability)
- Amount of storage (surface and groundwater)
- Land use activities in the watershed
The mechanisms by which these factors affect water yield can be subtle or straightforward. For example:
- Steep hills drain quickly.
- Level land pocked with bogs, wetlands, or lakes retains and meters water more slowly to streams.
- Bare bedrock and impermeable clay drain more quickly than sand.
- Porous soils and fractured bedrock allow runoff to enter groundwater, further metering discharge to a nearby stream.