The Hydrologic Response of Small Basins in Georgia

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THE HYDROLOGIC RESPONSE OF SMALL BASINS IN GEORGIA James F. Woodruffand J. D. Hewlett* The inventory of physical factors considered necessary to environmental planning generally includes topographic, pedologie, climatologie, biotíc, and hydrologie aspects. As part of the latter, various discharge and quality characteristics are invariably included. Although difficult to assess, it would also be desirable to be able to define and quantitatively express the manner in which a watershed responds to or "handles" the precipitation falling upon it. Specifically, how much and how fast does precipitation run off as storm flow? Such data, particularly from the small watershed of less than 200 square miles, would have importance in planning for the rational use of the environment for it is volumes of floodflow from these headward basins that have profound ramifications for downstream conditions. Inasmuch as geographers may be increasingly involved in such planning and have need for such data, this paper presents a means of measuring these characteristics and demonstrates its application by mapping it for the State of Georgia. An old method of describing the reaction of a watershed to its rainfall was the "runoff coefficient;" that is, runoff as a percentage of total rainfall. Lack of agreement on what was meant by runoff—total discharge, surface streamflow , stormflow, or surface runoff—negated the value of this coefficient and it was never widely adopted. Rather than reopen a question already too vigorously contested it is suggested that the concept of the runoff coefficient be retained, but that a new term and method of computing it be adopted. The term "quickflow" is proposed for that portion of the total runoff that is immediately attributable to the precipitation event. (1 ) The difficulty arises in identifying or separating this quickflow from the total flow. The hydrograph represents the volume of water at a given point and time in a stream channel. This volume is the sum of water contributed by channel interception, overland flow, interflow, and base flow. The stormflow or quickflow rise on the hydrograph would result from a combination of all of these types of flow except base flow. Base flow should only respond to ground water aquifers w ".iich are affected but slowly by a precipitation event. Separation of the various components from the hydrograph is arbitrary at best since the definitions are "discrete" in neither space nor time. Empirical studies of 15 small basins in the Piedmont involving 200 water-years of record, however, indicated that a line projected from the beginning of a stream rise on the hydrograph at a slope greater than .05 csm until it intersects the falling limb of the hydrograph realistically separated base flow from quickflow (Fig. 1 ). A Fortran IV program has been written to separate stormflow events from the hourly data. This program has been modified to *Dr. Woodruff is professor of geography at the University of Georgia, Athens. Dr. Hewlett is professor of forestry at the same school. The paper was accepted for publication in August 1970. Southeastern Geographer ce o -?------- The Event False Event Slope CO O IL (ß O ID C) (O ?- > < Hours q-i q0 q¿ Quick Flow (?) q ? Day Figure 1. Idealized hydrographs illustrating the separation of quickflow. The upper hydrograph shows the full separation method using hourly data from USGS records. The separation slope constant is 0.05 cubic feet per second per square mile per hour. The lower diagram illustrates the separation of quickflow from daily records. The separation slope is 1.2 cfsm/day. Vol. XI, No. 1, 1971 identify quickflow from the mean daily discharge data secured on tapes from the USGS' Automated Data Center in Washington, D. C. This modified program also accommodates those situations in which a storm commences the day before the separation constant (1.2 cfsm per day) is sufficiently steep to detect a rise in the hydrograph (Fig. 1). As written for computer programming the equation is: Quickflow = 0.03719 (inches ) A ? S (q.- q- 1.2.A) + (n + 1 ) (q - q i-1 » ° l ° -1 Where A is the area of the watershed in square miles, q is the average daily flow in cubic feet per second from USGS records, q is a value of q selected such that the next value q¿=i is greater than (q + 1.2A) and ? is the number of days that flow remains above the separation constant 1.2 csm/ day. The number 0.03719 is a conversion constant and the equation is valid only for positive values of (q¡ — q0 — 1.2-A). The second term (n + 1) (q0 — q_ ? ) ) adds the correction to accommodate the situation illustrated in Figure 1. (2) This method is valid only for basins from 2 to 200 square miles. The "full hydrograph" method is required on basins from a few acres to 2 square miles. (J ) It might be argued that the rate of discharge, in cubic feet per second per square mile, itself would be sufficient index of the behavior of small watersheds but it is not rate but volumes of water from these basins that contribute to the downstream flooding. In a similar vein, were merely the total volume of stormflow used, as separated on the hydrograph, there would be no indication of the relative "flashiness" of one basin or the "storage" capacities of another. It is for these reasons that it is suggested that the runoff coefficient idea be revived but made consistent and meaningful by the adoption of these terms and separation methods. The value thus established as stormflow if divided by the precipitation causing the stormflow is the hydrologie response. a ? TT ? ? ¦ p annual quickflowinn Annual Hydrologie Response = ------------3---------------- x luu annual precipitation As the name implies it is a value expressing the way the watershed responds to its precipitation. It also has the advantage of being able to convert precipitation data into volumes of discharge as...

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