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<br />of high and low water extremes of various duration provide measures of environmentaL stress <br /> <br />and disturbance during the year; the inter-annual variation (e.g., coefficient of variation) in the <br /> <br />magnitudes of these extremes provides another expression of contingency. <br /> <br />; ~ . ,. . , <br /> <br />IHA Group #3: Timing of Annual Extreme Water Conditions <br /> <br />- .1fl~ <br /> <br />Group #3 includes two attributes, one of which measures the Julian date of the I-day lIIlIlWlb" <br /> <br />minimum water condition,andcthe..other,which.measures- the Julian date of the I-day <br /> <br />maximum water condition (Fig. 5). The timing of the highest and lowest water condit,iOMft..i <br /> <br />within annual cycles provides another indicator of environmental disturbance or stress by . <br /> <br />describing the seasonal nature of these stresses; the inter-annual variation in timingof~~~. <br />. ,:/5:.f_'[.;'>:;:{,'.;:-', . <br />events reflects environmental contingency.,,~tf.kt1.:'. <br /> <br />~ . 'c-.J; <br /> <br />" -'", <br />:i~ <br /> <br /> <br />IHA Group #4: Frequency and Duration of IDgh and Low Pulses <br /> <br />o'f!tm~ <br /> <br />The four Group #4 attributes include two which measure the number of annual occurre~;;'''R' <br /> <br />during which themagnitude..oLthe"water.condition.exceeds an upper threshold or remains:~, ~. <br />,,_ ~.~q....l _ <br /> <br />below a lower threshold, respectively, and two which measure the mean duration of suc~Q.1fi.ai <br /> <br />". ,,-'_0' . <br /> <br />and low pulses (Fig. 6). These measures of frequency and duration of high- and 10w-~~t:ii.J~)..' <br />_ ..~ . . ':_:::\~f->;>:_::".-:". <br />conditions together portray the pulsing behavior of environmental variation within a y~, ~", . <br />. .""~- <br /> <br />provide measures. of the shape of these environmental pulses. Hydrologic pulses are d:ff~ <br /> <br />. ,->>, - ->~;!:,. <br /> <br />here as those periods within a year in which the daily mean water condition either rises above<. <br />. _' 'jiJa<;i.J <br /> <br />the 75th percentile (high pulse) or drops below the 25th percentile (low pulse) of all dailyCS;W( <br /> <br />mean values for the pre-impact time period. <br /> <br />11 <br />