New Strategies for America's Watersheds (1999) / Chapter Skim
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2 Spatial and Temporal Scales for Watersheds
2 Spatial and Temporal Scales for Watersheds
Pages 37-55
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From page 37... ...
Effective watershed science and policy require understanding the effects that variations in scale and scope have on watershed management efforts. This chapter explores the implications of scale, scope, and structure by (1)
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1o2 103 104 105 1o6 107 1o8 109 101° 1011 1012 1013 1014 1 1 A_ . Upper Rock Creek .> 1 1 Mississippi River : : ~///////////////////,~: : USGS Cataloging Units : am: USGS Accounting Units , ', ,~//~////////////i/~ House lot Towns USGS Regions .
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, the ocean, or another larger river. Drainage basins have perimeters defined by relatively high portions of the topography that form drainage divides.
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Data sets for very large river basins can be constructed in additive fashion by cumulating the data for smaller climatic regions. Although economic and social data are not intentionally collected and reported according to watershed boundaries, nearly coincidental economic and social regions can be defined at least for the large water regions of the United States.
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For smaller watersheds, census data collected on divisions smaller than counties are also available, with data aggregated according to census tracts and census blocks that have resolution to a scale equal to about four city blocks. Watershed approaches are therefore geographical perspectives that can integrate of physical, chemical, some biological, economic, and social data.
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As assemblages of hillslopes and channels, watershed sizes have specific functions. The smallest drainage basins, ranging from a few square meters to perhaps a square kilometer, often resemble simple hillslopes in their behavior, and they are source areas for water, sediment, and chemicals.
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Human attempts to control water flow begin to reach their effective limit in watersheds and rivers of this size; large dams, channel engineering, and floodplain protective works such as levees can influence the processes and results of most events short of large "50-year" or "100year" extremes, but larger and rarer events may be beyond direct human control. Large watersheds those with areas greater than 10,000 square kilometers (3,860 square miles)
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As a result, change in order rarely occurs in the downstream direction after the trunk stream achieves an order of six or seven. The Mississippi River in its lower reaches is probably tenth order.
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the typically used Strahler method, and (b) the more useful but less commonly applied Shreve method that shows increases in stream order more closely associated with hydrologic reality.
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Watershed management may be successful for certain purposes at the small scale (for example, erosion control) , and transmit changes to the larger system (for example, through changes in sediment yield)
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Physical factors change their roles in watershed processes depending on the temporal scale of analysis. For example, if an analyst considers watershed changes on a time scale of decades to centuries, the drainage network morphology (numbers and arrangements of channel segments)
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. Daily weather : Seasonality FIGURE 2.4 Temporal scales for watersheds.
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Status of Variables During Designated Time Spans Greater Drainage Basin Variables Than a Millennium Decade to Centuries Year to Decade Time Controlling Not Relevant Not Relevant Initial Relief Controlling Not Relevant Not Relevant Geology Controlling Controlling Controlling Climate Controlling Controlling Controlling Vegetation Responding Controlling Controlling Local Relief Responding Controlling Controlling Hydrology (runoff and sediment Responding Controlling Controlling yield per unit area) Drainage network morphology Responding Responding Controlling Hillslope morphology Responding Responding Controlling Hydrology (channel discharge of Responding Responding Responding water and sediment)
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Water Resources Council, published maps for each state showing the locations of the boundaries, as well as a national map. A few years later during the Second National Water Assessment, the Council reclassified the 222 planning regions into 106 assessment subregions, which have hydrologic boundaries that can be approximated by county boundaries.
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These 2,150 units, each with an average area of about 1,750 square kilometers (700 square miles) , combine to form planning subregions and accounting units (Figure 2.7~.
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Water Resources Council have defined watershed boundaries within the United States, providing a nested series of 2,150 small hydrologic units within 222 subregions, all subsumed by 21 water resource regions or large river basins. The boundaries of these units
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, energy, nutrients, or contaminants are not necessarily directly additive as small watersheds combine into successively larger watershed scales, because smaller systems have internal storage. Management efforts must also consider the temporal scale of the various watershed processes they seek to respond to.
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1996. Tennessee Valley Authority's Clean Water Initiative: Building Partnerships for Watershed Improvement.
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