INTRODUCTION 



A hydrograph is a curve depicting a 

 stream's rate of flow throughout a given pe- 

 riod of time. If the records are complete, the 

 curve is continuous; this curve reveals to the 

 trained observer the innermost secrets of a 

 watershed's behavior — nothing is concealed. 

 Every change in the curve reflects the water- 

 shed's reaction to a multitude of internal and 

 external forces. A steep rise in the curve could 

 reflect the passage of a high intensity storm; a 

 gradual descent might reflect the watershed's 

 high water storage capacity. Every change in 

 the hydrograph can be readily interpreted in 

 the light of forces acting on the watershed. A 

 study of all changes in the hydrograph is a 

 key to our understanding of the functioning of 

 a watershed. 



Streamflow is one of the end products of 

 precipitation that falls on a watershed. Some 

 of this precipitation falls directly into the 

 stream channel and brings about an immedi- 

 ate rise in the rate of streamflow. Similarly, 

 another portion of the precipitation quickly 

 enters well defined channels; it also contri- 

 butes to the rapid rise in streamflow rate. 

 Some of the rain enters the soil mantle, and 

 thence, by circuitous paths, finally enters the 

 stream channel. Another portion, pulled by 

 the force of gravity, finds its way to the water 

 table (the surface of the groundwater), and 

 eventually flows into the stream channel. The 

 shape of the hydrograph is determined by the 

 amount and rate at which each of these com- 

 ponents enters the stream channel. This shape 

 is a unique expression of a watershed's re- 

 action to its environment. 



Although the ultimate purpose of hydro- 



graph analysis is an understanding of a water- 

 shed's behavior, this can be achieved only if 

 .we have a thorough understanding of the flow 

 patterns that together make up the hydro - 

 graph. For this reason, hydrologists have de- 

 vised many methods to separate the com- 

 ponents, despite the belief that "it is prac- 

 tically impossible to measure the volumes of 

 water following each path ... 



The first and most important step in 

 hydrograph analysis is to partition the hydro- 

 graph into its components: 

 Total flow = Base flow + Interflow + Rapid flow 



Base flow is the contribution from ground- 

 water — water that moves under differences in 

 head in a saturated zone. Interflow is water 

 that percolates through the soil mantle and, 

 after some delay, enters the stream. Rapid 

 flow (also called surface flow) is water that 

 enters the stream channel with a minimum 

 delay; it includes channel interception. The 

 sum of interflow and rapid flow are desig- 

 nated as stormflow. 



The techniques now employed in hydro- 

 graph analysis are highly subjective. Moreover, 

 the analyst starts his analysis at the end of the 

 storm hydrograph and proceeds backwards in 

 time towards the beginning of the storm 

 hydrograph.^ Linsley and others discuss thor- 

 oughly the procedures and arbitrary rules that 

 are used in hydrograph separation. 



This paper presents a computer program 

 designed to analyze automatically hydrologic 



^ Ray K. Linsley, Jr., Max A. Kohler, and Joseph 

 L. H. Paulhus. P. 387, in: Applied Hydrology, 689 p. 

 New York: McGraw-Hill Co. 1949. 



^Ibid, Ch. 15. 



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