The Hydrologic Response of Small Basins in Georgia

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 stormflow in figures consistent with those used by the climatologists and familiar to the general public. Hydrologie response is a function of the various physical properties of the basin including basin area. It is also partly a function of mean precipitation and precipitation anomalies. Basin area affects the response in that quickflow volumes tend to become attenuated downstream and, therefore, appear reduced on the hydrograph of larger basins. Also contributing to smaller quickflow volumes per square mile on the larger basins is the fact that the larger basins are seldom entirely covered by the intense storm. Regression analysis, made on 178 years of data from 55 watersheds, gave the following formula for correcting response for these factors of area and deviation from Southeastern Geographer mean precipitation: Rp = R + .00027(A) - .00241 (P-P) Where Rp is actual response for any year, Rp is mean corrected response, A is area and P is precipitation for the same year as R„. (3) The area correction factor is apparently valid for watersheds up to 200 square miles. Data from watersheds smaller than 20 square miles probably needs no correction in that it would yield a negligible value of less than % of 1 percent. To test the value of hydrologie response as a geographic parameter, a map of this environmental factor was drawn for the State ofGeorgia (Fig. 2 ). This map is based on 30 stations having at least 15 years of discharge records and 55 stream gauges whose length of record was only 3 years or less. Response values from the latter were corrected for both area and departure from average precipitation. Certainly a large part of the problem of constructing useful maps of hydrologie response is the paucity and poor distribution of necessary data. This mal-distribution of stations may tend to produce greater complexity of pattern in the northern part of the state where T.V.A. activity has provided a closely spaced network of stations and a broader, more generalized pattern of response in the southern portion. Although simplicity of response pattern is consistent with the greater regional homogeneity of the Coastal Plain, a larger number of stations might enhance the detail, but the general pattern would not be altered significantly. The distinct overall pattern, in any event, indicates that hydrologie response is not a random meaningless parameter but is one which consistently reflects the complex of physical factors controlling the rapidity of discharge from drainage basins. Although there have been some studies of individual basins which indicate relationships between discharge volumes and physical attributes of the basin, no statistically valid correlations have been established for a relatively large, widely scattered sample of basins. In this regard, the distribution of response as indicated by Figure 2 has relevance beyond its value as a source of general information useful in environmental planning. That is, this map may give some insight, inductive though it may be, into the physical factors controlling the manner in which a basin sheds its water. In essence, it may be indicating broad patterns of variation in "hydrologie depth" generally reflect lithology. Some interesting aspects of this distribution seems to support this contention. Patterns of variation in hydrologie response at the largest scale trend northeast-southwest generally paralleling the strike of lithology (Figs. 2 and 3). The Coastal Plain is a province of less than average response undoubtedly reflecting the storage capacities of the sedimentary beds, and the trend of isolines is consistent with the outcropping structures. In the Coastal Plain an increased network of data might further emphasize this parallelism and might actually indicate the manner in which individual stratigraphie members affect the "hydrologie depth" of watersheds occurring Vol. XI, No. 1. 1971 on them. For example, the immediate coastal area, about coincident with Quaternary deposits with values in excess of 10, tends to have a higher response than the Middle and Inner Coastal Plain. This higher response may possibly reflect basins which are hydrologically shallow because they have high water tables; although low in relief, they are almost immediately affected by rainfall. In effect, they are somewhat analogous to a lake whose entire surface would be one of "channel interception" and, therefore, least retentive and most responsive to precipitation. Antithetical is the exceptionally low response of the Inner Coastal Plain Sand Hills region. Here average annual stormflow is as low as two percent of the average annual precipitation. Undoubtedly these highly permeable PERCENT OF PRECIPITATION 12-16 Com Serv Lot).. Univ. of Go. Figure 2. Hydrologie response for the state of Georgia based on the mean annual responsemean annual quickflow divided by mean annual precipitation for 85 stations. Southeastern Geographer sands and hydrologically deep soils tend to suppress the fluctuations of discharge associated with precipitation events. Whether a correct comparison or not, this situation is similar to the even discharge of some of the streams flowing across the vesicular lavas of the Columbia Plateau. Response values on the Piedmont are generally higher than those on the Coastal Plain. They are sharply differentiated by the sudden rise in response along the Fall Line; values increase from 2 in the Sand Hills to 10 immediately above the Fall Line. This overall higher hydrologie response of the Piedmont is undoubtedly associated, at least indirectly, with the relatively low "infiltration rates" of the residual clay soils. This apparently strong relationship of hydrologie response and lithology is enhanced by the values of specific basins and the occurrence of lithologie properties. For instance, a number of basins with values of about 11 are found in the wedge of Precambrian schists and phyllonites in the northeast part of the state; and basins on Triassic slates in both Georgia and North Carolina tend to have higher response values. One of the most obvious features of the pattern is the marked increase in values in the northwest corner of the state. Values ranging from about 12 to 16 percent of the total rainfall seem to be characteristic of the sediments and metasediments of the Ridge and Valley sections. Even higher responses are apparently related to the greater relief and stratigraphically younger Figure 3 Vol. XI, No. 1, 1971 sediments of the Plateau. Here the average response is about 14 although some basins may have values as high as 18. Initial studies of surrounding areas indicate that this northwest corner of the state is only a small portion of a continuous band of higher response stretching from central Alabama to Pennsylvania. (2) Particularly rapid and high response are apparently related more to the Plateau than to the Ridge and Valley and probably to the presence of solution features. In the preceding discussion, emphasis has been on the lithology rather than some other physical factor which might conceivably control stormflow. INCHES 68 70 ISOHYETS Coft Serv Lob. Univ of Go. Figure 4. Average annual stormflow discharge from small basins expressed as inches of water. Southeastern Geographer This assumption that lithology through its affect upon hydrologie depth is the principal control of response may seem unusual. It is based, however, upon the fact that response does not tend to vary with such marked changes in relief as those occurring along the margins of the Blue Ridge but does vary along lithologie contacts. This inductive conclusion is supported by the results of an empirical study made of one hundred small basins scattered across the Eastern U. S. (2) In this study, correlation analysis indicates no significant control upon response by a wide variety of basin parameters including various measurements of relief and slope. Inasmuch as the response is expressed as a percentage of the total precipitation , it does not give in volumes the amount of water being released from the small watersheds as stormflow. This value can be readily determined , however, and expressed in units with which the public is familiar rather than rates of flow. Figure 4 indicates the variations in average annual stormflow for the State of Georgia. It should be noted that these values range from a low of less than 1 inch of storm runoff in the Sand Hills to a maximum of about 8 inches in the northwest corner. Although maximum average annual precipitation of about 75 inches is occurring in the extreme northeast, the area responds with only about 6 inches of average annual quickflow. Parenthetically, experience with very small watersheds above 4,000 feet elevation indicates that responses may be, under certain conditions, much higher for certain local conditions. These details would be indicated if response were mapped at a sufficiently fine scale although not apparent on this map (Fig. 4). It is hoped that these maps demonstrate that hydrologie response, as defined and determined here, is a useful and concise parameter with wide application for those engaged in environmental planning. It is also suggested that such maps would be valuable not only to planning activities but should also give insight into the mechanics of how watersheds retain or discharge their precipitation. Response data would help in identifying the important factors contributing to that process of "basin storage" which is at present only intuitively known and if accumulated over a period of years would be invaluable in determining man's real effect upon basin discharge. The research upon which this paper is based was supported by funds provided under the Water Resources Research Act of 1964 administered by OWRR, USDI, through the Water Resources Center at the Georgia Institute of Technology. (1)Hewlett, J. D., and Hibbert, A. R., "Factors Affecting the Response of Small Watersheds to Precipitation in Humid Areas," International Symposium on Forest Hydrology, Ftergamon, Oxford, 1967, pp. 275-290. (2)Woodruff, J. F., and Hewlett, J. D., "Determining and Mapping the Average Hydrologic Response of the Eastern United States," Final Report, OWRR Project A-013-Ga., Georgia Water Resources Center, Georgia Institute of Technology, Atlanta, 1969. (3)Hewlett, J. D., "A Hydrologie Response Map for the State of Georgia," Water Resources Bulletin, Vol. 3, No. 3, 1967, pp. 4-20. ...