| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Streams > What is a Stream? > Streamflow |
Brief Description: Streamflow varies with the volume of water, precipitation, surface temperature, and other climatic factors. For most streams (rivers), the highest water discharge is found close to the sea, but in arid regions discharge decreases naturally downstream. Land use in drainage basins also strongly affects streamflow. For a given area of 1 km2, the water discharge (specific run-off) may range from <0.1 l/s to >50 l/s. Major streamflow regimes include glacial (ice melt: regular high water period in early summer, with annual mean discharge q = 10-20 l/s/km2); nival (snow melt: late spring high water, with q = 3-15 l/s/km2); pluvial (high water in late autumn-winter, with q = 5-20 l/s/km2); dry tropical (high water in summer rainy season, with q = 0.5-10 l/s/km2); monsoon (q = 20-40 l/s/km2); equatorial (high water during two rainy periods, with q = 15-30 l/s/km2); and desert (non-perennial flow, with q <0.5 l/s/km2). Reversals in streamflow, in conjunction with indirect methods of paleoflood studies and paleohydrology, yield long-term indicators of changes in discharge that are valuable for responses to flooding, estimating long-term trends in water and sediment discharges, and for distinguishing possible long-term climate change. Significance: Streamflow directly reflects climatic variation. Stream systems play a key role in the regulation and maintenance of biodiversity. Changes in streams and streamflow are indicators of changes in basin dynamics and land use. One estimate puts the total annual losses to the economy from flooding of river and coastal plains worldwide at US$20,000 million. Human or Natural Cause: Natural variations in streamflow predominate, but they can be strongly modified by human actions. It is estimated that about 3/4 of the total water flow of the 139 largest river systems in North America, Europe and the former Soviet Union is significantly affected by dams and reservoirs, irrigation, and diversion for use outside the watershed. Environment Where Applicable: Fluvial systems - rivers, streams and channels Types of Monitoring Sites: Stream channels Spatial Scale: landscape (catchment) to mesoscale / regional to continental Method of Measurement: There are standard techniques for measuring streamflow. All measurements are based on the continuity equation, Q = AV, whereas streamflow is typically estimated from channel size using the power relation Q = aWb (where Q = discharge, A = areal cross-section, V = velocity, a = a coefficient, W = channel width, and b is an exponent). Where more quantitative data are not available, study of changes in biomass distribution (especially woody plants) can provide reliable qualitative measures of hydrologic and geomorphic events spanning the past several hundred years. Frequency of Measurement: continuous to periodic Limitations of Data and Monitoring: Streams in flood, and on deltas, alluvial plains and karst terrains, are difficult to gauge. The effectiveness of stream flow as an indicator depends strongly on a well- designed, systematic network of monitoring stations. Despite their importance for understanding climate change, assessments of temporal variations in runoff, evaporation and soil water storage have been neglected, in part because of a lack of monitoring efforts. Applications to Past and Future: Estimates of paleofloods and paleodischarge can sometimes be made through study of stream sediment deposits, channel morphology, and associated landforms. Possible Thresholds: NA |
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