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<br />001499 <br /> <br />15 <br /> <br />base flow during the periods January 1 to February 10 and June 22 to July <br />l2. Similar base-flow periods can be found in other years. A smooth curve <br />averaging the variations in discharge during one of these periods provides <br />a sample of the general trend of the base-flow recessions during that time <br />of the year. The average of a number of these recession curves for dif- <br />ferent times of the year provides templates for estimating the time of term- <br />ination of direct runoff and for estimating base flow when the actual base <br />flow is unknown. Recession curves have also been drawn for all major <br />rises as shown in figure 4 to define the total flow expected if there had been <br />no further precipitation. The curves become progressively steeper from <br />winter to summer due to the depletion of flows by evapotranspiration. <br /> <br />The recession curves drawn in figure 4 obviously do not represent the <br />base flow for more than short segments of their length. At the beginning <br />of the recession, both direct runoff and base flow are present, and at the <br />end of the recession another rise has occurred with an accretion to base <br />flow. Therefore, it is necessary to determine for each recession when the <br />direct runoff ceases and when the accretion to base flow starts. <br /> <br />Various investigators have considered the possibility of two extreme <br />interpretations for base flow during a stream rise. The one extreme would <br />be that the increasing stage in the stream would cause the base flow to be- <br />come zero, or even negative if some of the stream discharge passed into <br />temporary bank storage and reversed the direction of ground-water move- <br />ment. This reversed movement would continue until the stage in the stream <br />dropped sufficiently to allow base flow to start. Then the base flow would <br />increase to some peak and follow a recession until the next flood flow oc- <br />curs. The other extreme interpretation would be for the base flow to start <br />to rise at about the same time as the stream, peak at about the same time <br />and then follow a recession until the next flood flow. In other words the <br />base flow would follow the same pattern as the total flow. The actual base <br />flow is probably somewhere between these two interpretations and for this <br />report is rather arbitrarily assumed to follow the previous recession un- <br />til the time of the peak discharge. Each recession curve drawn in figure 4 <br />has been terminated at the time of the following peak, as shown in figure 5. <br />For example, the recession following the peak of May 8 is ended at the peak <br />of May 24. <br /> <br />The time of cessation of direct surface runoff is the most uncertain of <br />all the steps in base-flow separation. Two approaches to this problem <br />have been used: (1) A study of the segments of reliably defined base flow <br />discloses a consistent shape for various seasons and conditions of flow, <br />