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<br />Characterization of Hydrologic Regimes <br /> <br />llydrologic conditions can vary in four dimensions within an ecosystem (three spatial <br /> <br />dimensions and time), and this inherent variability necessitates a careful definition of the <br /> <br />spatial and temporal domains of hydrologic regimes. If the spatial domain is restricted to a <br /> <br />specific point within a hydrologic system (such as a measurement point in a river, a lake, or an <br /> <br />aquifer), then the hydrologic regime can be defined in terms of one temporal and one spatial <br /> <br />dimension -- changesin'waterconditions(e:g;; levelS', heads, rates) at a single location over' <br /> <br />" _ :"c>'.L~Ul <br />time; Such temporal changes in water conditions are eommonly portrayed as plots of water . <br /> <br />condition against time, orhydrographs (see Fig. IIor example). <br /> <br />.,.__.,',<",,:,d-(j; <br /> <br />":i~',:~~~ <br />Our goal is to characterize the temporal fluctuationS of hydrologic regimes in a <br />__ .., .'c. "';c;.,~'. <br />biologically releVant manner. Many differentattdbutes of hydrologic regimes can reUsedto <br />.:,C.OC')3} <br />characterize the "physical habitat templates" (sensu Southwood 1977, 1988 or Poff &'W'anl. <br /> <br />f::.>;..;.'.1 tm& <br />1990; Townsend & Hildrew 1994) or "environmental filters" (sensu Keddy 1992) that shape <br /> <br />;;) ',)~- <br />the biotic composition, structure, and function of aquatic, wetland, and riparian ecosystemS'. - <br /> <br />: ~'.rt- <br />However, all such attributes~are'based-upon -five, fundamental characteristics of hydrologic <br /> <br />. - ,~,~f'$iV <br />regimes: 1) the magnitude of the water condition, 2J the timing oIOccurrence of a specifiC: . <br />. .{,~ ii'!flU' .; <br />water condition, 3) the frequency of occUrrence of a specific water condition, 4) the duralmn: <br /> <br />of time over which a specific water Condition exists, and 5) the ~ateof change of the water . " <br /> <br />conditionovera'specified time interval. 'Statistical' characterization of a hydrologic regi~:-(If. <br />.;.c., I". it! <br />can be derived' from measurements of the central tendency (e.g., mean, median), extreme'. <br /> <br />. '. ..." ,.". ." ,,;;,..qmw <br />. conditions (e'.g.; maxima. minima),' and dispersion (e:g:, variance, coefficient of variation) of <br /> <br />'-~I-\fN)1q <br /> <br />8 <br /> <br />...'>,~ <br />