Re: Ground And Surface Water Hydrology Mays Pdf Download
Christal Rasband
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Ground water exchange affects the ecology of surface water by sustaining stream base flow and moderating water-level fluctuations of ground water-fed lakes. It also provides stable-temperature habitats and supplies nutrients and inorganic ions. Ground water input of nutrients can even determine the trophic status of lakes and the distribution of macrophytes. In streams the mixing of ground water and surface water in shallow channel and bankside sediments creates a unique environment called the hyporheic zone, an important component of the lotic ecosystem. Localized areas of high ground water discharge in streams provide thermal refugia for fish. Ground water also provides moisture to riparian vegetation, which in turn supplies organic matter to streams and enhances bank resistance to erosion. As hydrologists and ecologists interact to understand the impact of ground water on aquatic ecology, a new research field called "ecohydrology" is emerging.
The hydrologic cycle describes the continuous movement ofwater above, on, and below the surface of the Earth. Thewater on the Earth's surface--surface water--occurs asstreams, lakes, and wetlands, as well as bays and oceans.Surface water also includes the solid forms of water-- snowand ice. The water below the surface of the Earth primarilyis ground water, but it also includes soil water.
The hydrologic cycle commonly is portrayed by a verysimplified diagram that shows only major transfers of waterbetween continents and oceans, as in Figure 1. However, forunderstanding hydrologic processes and managing waterresources, the hydrologic cycle needs to be viewed at a widerange of scales and as having a great deal of variability intime and space. Precipitation, which is the source ofvirtually all freshwater in the hydrologic cycle, fallsnearly everywhere, but its distribution is highly variable.Similarly, evaporation and transpiration return water to theatmosphere nearly everywhere, but evaporation andtranspiration rates vary considerably according to climaticconditions. As a result, much of the precipitation neverreaches the oceans as surface and subsurface runoff beforethe water is returned to the atmosphere. The relativemagnitudes of the individual components of the hydrologiccycle, such as evapotranspiration, may differ significantlyeven at small scales, as between an agricultural field and anearby woodland.
To present the concepts and many facets of theinteraction of ground water and surface water in a unifiedway, a conceptual landscape is used (Figure 2). Theconceptual landscape shows in a very general and simplifiedway the interaction of ground water with all types ofsurface water, such as streams, lakes, and wetlands, in manydifferent terrains from the mountains to the oceans. Theintent of Figure 2 is to emphasize that ground water andsurface water interact at many places throughout thelandscape.
Movement of water in the atmosphere and on the landsurface is relatively easy to visualize, but the movement ofground water is not. Concepts related to ground water andthe movement of ground water are introduced in Box A. Asillustrated in Figure 3, ground water moves along flow pathsof varying lengths from areas of recharge to areas ofdischarge. The generalized flow paths in Figure 3 start atthe water table, continue through the ground-water system,and terminate at the stream or at the pumped well. Thesource of water to the water table (ground-water recharge)is infiltration of precipitation through the unsaturatedzone. In the uppermost, unconfined aquifer, flow paths nearthe stream can be tens to hundreds of feet in length andhave corresponding traveltimes of days to a few years. Thelongest and deepest flow paths in Figure 3 may be thousandsof feet to tens of miles in length, and traveltimes mayrange from decades to millennia. In general, shallow groundwater is more susceptible to contamination from humansources and activities because of its close proximity to theland surface. Therefore, shallow, local patterns ofground-water flow near surface water are emphasized in thisCircular.
Small-scale geologic features in beds of surface-waterbodies affect seepage patterns at scales too small to beshown in Figure 3. For example, the size, shape, andorientation of the sediment grains in surface-water bedsaffect seepage patterns. If a surface-water bed consists ofone sediment type, such as sand, inflow seepage is greatestat the shoreline, and it decreases in a nonlinear patternaway from the shoreline (Figure 4). Geologic units havingdifferent permeabilities also affect seepage distribution insurface-water beds. For example, a highly permeable sandlayer within a surface-water bed consisting largely of siltwill transmit water preferentially into the surface water asa spring (Figure 5).
Changing meteorological conditions also strongly affectseepage patterns in surface-water beds, especially near theshoreline. The water table commonly intersects land surfaceat the shoreline, resulting in no unsaturated zone at thispoint. Infiltrating precipitation passes rapidly through athin unsaturated zone adjacent to the shoreline, whichcauses water-table mounds to form quickly adjacent to thesurface water (Figure 6). This process, termed focusedrecharge, can result in increased ground-water inflow tosurface-water bodies, or it can cause inflow tosurface-water bodies that normally have seepage to groundwater. Each precipitation event has the potential to causethis highly transient flow condition near shorelines as wellas at depressions in uplands (Figure 6).
Transpiration by nearshore plants has the opposite effectof focused recharge. Again, because the water table is nearland surface at edges of surface-water bodies, plant rootscan penetrate into the saturated zone, allowing the plantsto transpire water directly from the ground-water system(Figure 7). Transpiration of ground water commonly resultsin a drawdown of the water table much like the effect of apumped well. This highly variable daily and seasonaltranspiration of ground water may significantly reduceground-water discharge to a surface-water body or even causemovement of surface water into the subsurface. In manyplaces it is possible to measure diurnal changes in thedirection of flow during seasons of active plant growth;that is, ground water moves into the surface water duringthe night, and surface water moves into shallow ground waterduring the day.
These periodic changes in the direction of flow also takeplace on longer time scales: focused recharge fromprecipitation predominates during wet periods and drawdownby transpiration predominates during dry periods. As aresult, the two processes, together with the geologiccontrols on seepage distribution, can cause flow conditionsat the edges of surface-water bodies to be extremelyvariable. These "edge effects" probably affectsmall surface-water bodies more than large surface-waterbodies because the ratio of edge length to total volume isgreater for small water bodies than it is for large ones.
Streams interact with ground water in all types oflandscapes (see Box B). The interaction takes place in threebasic ways: streams gain water from inflow of ground waterthrough the streambed (gaining stream, Figure 8A), they losewater to ground water by outflow through the streambed(losing stream, Figure 9A), or they do both, gaining in somereaches and losing in other reaches. For ground water todischarge into a stream channel, the altitude of the watertable in the vicinity of the stream must be higher than thealtitude of the stream-water surface. Conversely, forsurface water to seep to ground water, the altitude of thewater table in the vicinity of the stream must be lower thanthe altitude of the stream-water surface. Contours ofwater-table elevation indicate gaining streams by pointingin an upstream direction (Figure 8B), and they indicatelosing streams by pointing in a downstream direction (Figure9B) in the immediate vicinity of the stream.
Losing streams can be connected to the ground-watersystem by a continuous saturated zone (Figure 9A) or can bedisconnected from the ground-water system by an unsaturatedzone. Where the stream is disconnected from the ground-watersystem by an unsaturated zone, the water table may have adiscernible mound below the stream (Figure 10) if the rateof recharge through the streambed and unsaturated zone isgreater than the rate of lateral ground-water flow away fromthe water-table mound. An important feature of streams thatare disconnected from ground water is that pumping ofshallow ground water near the stream does not affect theflow of the stream near the pumped wells.
In some environments, streamflow gain or loss canpersist; that is, a stream might always gain water fromground water, or it might always lose water to ground water.However, in other environments, flow direction can vary agreat deal along a stream; some reaches receive groundwater, and other reaches lose water to ground water.Furthermore, flow direction can change in very shorttimeframes as a result of individual storms causing focusedrecharge near the streambank, temporary flood peaks movingdown the channel, or transpiration of ground water bystreamside vegetation.
A type of interaction between ground water and streamsthat takes place in nearly all streams at one time oranother is a rapid rise in stream stage that causes water tomove from the stream into the streambanks. This process,termed bank storage (Figures 11 and 12B), usually is causedby storm precipitation, rapid snowmelt, or release of waterfrom a reservoir upstream. As long as the rise in stage doesnot overtop the streambanks, most of the volume of streamwater that enters the streambanks returns to the streamwithin a few days or weeks. The loss of stream water to bankstorage and return of this water to the stream in a periodof days or weeks tends to reduce flood peaks and latersupplement stream flows. If the rise in stream stage issufficient to overtop the banks and flood large areas of theland surface, widespread recharge to the water table cantake place throughout the flooded area (Figure 12C). In thiscase, the time it takes for the recharged floodwater toreturn to the stream by ground-water flow may be weeks,months, or years because the lengths of the ground-waterflow paths are much longer than those resulting from localbank storage. Depending on the frequency, magnitude, andintensity of storms and on the related magnitude ofincreases in stream stage, some streams and adjacent shallowaquifers may be in a continuous readjustment frominteractions related to bank storage and overbank flooding.
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