Hydrology is the study of inter-relationships and interactions between water and its environment in the hydrological cycle. As water moves within a watershed, it carries sediment, chemicals, heat and biota. (Dunne and Leopold, 5). The movement of water in the hydrologic cycle drives the watershed system and affects all aspects of watershed health.
Water is central to human existence and life in general. Unless trapped as groundwater within closed basins, water is constantly in motion. It may be detained in glacial ice, under ground, or in lakes or reservoirs; but eventually it flows and melts, seeps, or evaporates. This movement of water is continuous, but irregular in space and time. Because of this, even areas that are typically well supplied can experience droughts or floods at various times (Satterlund and Adams 1992).
Explore the Hydrology Health Scores to see a series of index values that show health trends in hydrologic systems of Minnesota.
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
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 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 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. https://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.
Measuring stream flow
The volume of water flowing in a stream generally cannot be continuously recorded. In order to calculate the volume of stream flow, a gaging station is established. The gage obtains a continuous record of water level in the stream.
A mathematical relationship must be established between the water level (stage) and stream flow volume (discharge) at the site. This relationship is called a rating curve. A field hydrologist will directly measure discharge at the gage at different water levels, particularly during flood flows. Using these measurements, a rating curve is developed. This table of related stage/discharge values can be used to estimate the amount of water discharged for any given stage reading.
Some major watersheds contain a continuous stream gaging station to quantify the flow regime other watersheds must extrapolate information from an adjacent watershed. (see map)
Rating curves must be revised if upstream land use changes significantly or if the stream cross-section changes at the gage site. Many gage sites are established where channel geometry is relatively stable such as a bridge or other stream crossing.
In addition to current stream flow, historic stream flow data are required to develop hydrologic time series analysis and if needed, water budgets. Stream flow records for gaged streams are available from the U.S. Geological Survey (USGS). If stream flow data have not been gathered over a sufficient period of record, several methods can be used to estimate hydrology (Bovee et al. 1998; Wurbs and Sisson 1999). Hydrologic simulation models (e.g., HEC-HMS, WMS) use watershed characteristics, precipitation, and runoff patterns to synthesize or extend a streamflow record.
Furthermore, streamflow data from gages in the same region can be used to synthesize runoff patterns for another watershed by establishing a statistical relationship. Accurate synthesis from one river system to another is only feasible if watershed characteristics such as soil, area, topography and precipitation patterns are similar.
Flow regime measurements
Scientists have described river flows in detail, identifying five (5) aspects:
- Magnitude – discharge value
- EXAMPLE: How large was the flood? What stage and discharge values were reached?
- Duration – length of time flows remain at the level of interest
- EXAMPLE: How long did the river stay above floodstage?
- Timing - seasonal and diurnal pattern of the event
- EXAMPLE: What time of year did the river flood? What time of day did flows peak?
- Frequency – number of events within a given timeframe
- EXAMPLE: How often did the river reach floodstage last year?
- Rate of change – rate at which water level of interest is obtained
- EXAMPLE: How quickly did the water level rise?
Discharge (also known as stream flow, flow or flow rate) is expressed as volume of water over a given time period. A variety of units are used to describe flow from near instantaneous terms such as cubic feet per second (cfs) to long time intervals such as acre-feet per year (afy). Cubic feet per second (cfs) is a measurement of stream flow rate that represents one cubic foot of water moving past a given point in one second; whereas acre feet per year (afy) is the volume of water necessary to cover one acre of surface area to a depth of one foot (43,560 cu ft) that moves past a given point in a year.
The exceedance value is the magnitude of discharge over a period of time. Using historic flow records, the Q value reflects the percentage of time stream flow has been found at that Q level. For example, Q50 is average flow; 50% of the time flows have been measured at or above that level and 50% of the time flows have been at or below that level. The Q90 value indicates that 90% of the time, stream flow has been greater than that value. In other words, the stream flow has only been that level or below 10% of the time. Q90 is considered protected low flow level in Minnesota and is used for suspending water appropriation permits.
Hydrologic records of flow regimes are critical for understanding and investigating stream components other than flow. A hydrologic record is needed to assess:
- Habitat changes
- Hydraulic functions
- Water quality factors
- Channel maintenance
- Riparian and valley forming processes
Limitations and constraints for using hydrologic data
There are limitations and constraints to consider when using hydrologic data. One constraint is the availability of adequate hydrologic data (e.g., streamflow and precipitation records). Data that is incomplete in space or time limits assessment of current conditions and prediction of potential change. Further, it should be understood that past records of precipitation or streamflows may not reflect current or future conditions and therefore may have limited applicability. Additionally, global climate change will introduce new uncertainty to extrapolation of past conditions. Changing patterns in precipitation, temperature, prevailing winds and storm events are examples of uncertainties that will need consideration in predicting future hydrologic data.