Baseflow Separation

3.6.5 Baseflow Separation

A surface stream hydrograph is the final quantitative expression of various processes that transform precipitation into streamflow. Separation of the surface stream hydrograph is

a common technique of estimating the individual components that participate in the flow formation. Theoretically, they are divided into flow formed by direct precipitation over the surface stream, surface (overland) runoff collected by the stream, near-surface flow of the newly infiltrated water (also called underflow), and groundwater inflow. However, it is practically impossible to accurately separate all these components of streamflow generated in a real physical drainage area. In practice, the problem of component sep- aration is therefore reduced to an estimation of the baseflow, formed by groundwater, and surface runoff, which is the integration of all the other components. In natural long- term conditions and in the absence of artificial groundwater withdrawal, the rate of groundwater recharge in a drainage basin of a permanent gaining stream is equal to the rate of groundwater discharge. Assuming that all groundwater discharges into the surface stream drainage network, either directly or via springs, it follows that the stream baseflow equals the groundwater recharge in the drainage basin. Although some pro- fessionals view the hydrograph separation method as a “convenient fiction” because of its subjectivity and lack of rigorous theoretical basis, it does provide useful information in the absence of detailed (and expensive) data on many surface water runoff processes

GroundwaterRecharge

and drainage basin characteristics that contribute to streamflow generation. In any case, the method should be applied with care and regarded only as an approximate estimate of the actual groundwater recharge. In addition, geologic and hydrogeologic character- istics of the basin should be well understood before attempting to apply the method. The following examples illustrate some situations where baseflow alone should not be used to estimate groundwater recharge (Kresic, 2007):

1. Surface streamflows through a karst terrain where topographic and groundwater divides are not the same. The groundwater recharge based on baseflow may be grossly over- or underestimated depending on the circumstances.

2. The stream is not permanent, or some river segments are losing water (either always or seasonally); locations and timing of the flow measurements are not adequate to assess such conditions.

3. There is abundant riparian vegetation in the stream floodplain, which extracts a significant portion of groundwater via ET.

4. There is discharge from deeper aquifers, which have remote recharge areas in other drainage basins.

5. A dam regulates the flow in the stream.

A simple hydrograph generated by an isolated precipitation event and the principle of baseflow separation is shown in Figure 3.28. In reality, unless the surface stream is intermittent, the recorded hydrograph has a more complex shape, which reflects the in- fluence of antecedent precipitation. Actual hydrographs are formed by the superposition of single hydrographs corresponding to separate precipitation events.

The first method of hydrograph component separation shown in Fig. 3.28 (line ABC) is commonly applied to surface streams with significant groundwater inflow. Assuming that point C represents the end of all surface runoff, and the beginning of flow generated solely by groundwater discharge, the late near-straight line section of the hydrograph is extrapolated backward until it intersects the ordinate of the maximum discharge (point

N (days)

Discharge

A Baseflow

Time

F IGURE 3.28 Single hydrograph formed by isolated rainfall event showing two common methods of baseflow separation.

280 ChapterThree

B). Point A, representing the beginning of surface runoff after rainfall, and point B are then connected with the straight line. The area under the line ABC is the baseflow or the groundwater component of the surface streamflow.

The second graphical method of baseflow separation is used for surface streams in low-permeable terrain without significant groundwater flow. It is conditional since point

D (the hydrograph falling time) is found by the following empirical formula (Linsley et al., 1975):

(3.35) where A is the drainage area in square kilometers. In general, this method gives short

N = 0.8A 0.2 (days)

falling times: for an area of 100 km 2 , N is 2 days; for 10,000 km 2 , N is 5 days. Thus, the method should be applied cautiously after analyzing a sufficient number of single hydrographs and establishing an adequate area-time relationship.

As illustrated in Fig. 3.29, graphical methods of baseflow separation may not be ap- plicable at all in some cases. A stream with alluvial sediments having significant bank storage capacity may, during floods or high river stages, lose water to the subsurface so that no baseflow is occurring (Fig. 3.29a). Or, a stream may continuously receive base- flow from a regional aquifer that has a different primary recharge area than the shallow aquifer and maintain a higher head than the stream stage (Fig. 3.29b). Although one could use the same approach and “separate” either of the two hydrographs, it would

Time F IGURE 3.29 Stream hydrograph showing flow components after major rise due to rainfall when (a)

Time

the stream stage is higher than the water table and (b) the stream stage is higher than the water table in the shallow aquifer, but lower than the hydraulic head in the deeper aquifer that is discharging into the stream. (1) Initial stream stage before rainfall and (2) stream stage during peak flow.

GroundwaterRecharge

not be possible to make any conclusions as to the groundwater component of the surface streamflow without additional field investigations. One such field method is hydrochem- ical separation of the streamflow hydrograph using dissolved inorganic constituents or environmental tracers. It is often more accurate than simple graphoanalytical techniques because surface water and groundwater almost always have significantly different chem- ical signatures.

Risser et al. (2005b) present a detailed application and comparison of two automated methods of hydrograph separation for estimating groundwater recharge based on data from 197 streamflow gauging stations in Pennsylvania. The two computer programs— PART and RORA (Rutledge, 1993, 1998, 2000)—developed by the USGS are in public domain and available for free download from the USGS Web site. The PART computer program uses a hydrograph separation technique to estimate baseflow from the stream- flow record. The RORA computer program uses the recession-curve displacement tech- nique of Rorabaugh (1964) to estimate groundwater recharge from each storm period. The RORA program is not a hydrograph-separation method; rather, recharge is deter- mined from displacement of the streamflow-recession curve according to the theory of groundwater drainage.

The PART program computes baseflow from the streamflow hydrograph by first identifying days of negligible surface runoff and assigning baseflow equal to streamflow on those days; the program then interpolates between those days. PART locates periods of negligible surface runoff after a storm by identifying the days meeting a requirement of antecedent-recession length and rate of recession. It uses linear interpolation between the log values of baseflow to connect across periods that do not meet those tests. A detailed description of the algorithm used by PART is provided in Rutledge (1998, pp. 33–38).

Rorabaugh’s method used by RORA is a one-dimensional analytical model of ground- water discharge to a fully penetrating stream in an idealized, homogenous aquifer with uniform spatial recharge. Because of the simplifying assumptions inherent in the equa- tions, Halford and Mayer (2000) caution that RORA may not provide reasonable estimates of recharge for some watersheds. In fact, in some extreme cases, RORA may estimate recharge rates that are higher than the precipitation rates. Rutledge (2000) suggests that estimates of mean monthly recharge from RORA are probably less reliable than estimates for longer periods and recommends that results from RORA should not be used at time scales smaller than seasonal (3 months), because results differ most greatly from manual application of the recession-curve displacement method at small time scales. It should

be noted that neither RORA nor PART computer programs can account for situations shown in Fig. 3.29 or other possible complex relationships between surface streams and groundwater.

Spring Flow Hydrograph Although the processes that generate hydrographs of springs and surface streams are quite different, there is much that is analogous between them, and the hydrograph ter- minology is the same. Increase in spring flow after rainfall events is a direct indicator of the actual aquifer recharge. Knowing the exact area where this direct recharge from rainfall takes place, and the representative (average) amount of rainfall, enables rela- tively accurate determination of the aquifer recharge based on the measured increase of spring discharge rate. However, it is often difficult to accurately determine a spring drainage area, especially in karst. In addition, the spring hydrograph reflects the response to rainfall of all porosity types in the entire volume of the aquifer and the response to

282 ChapterThree

all water inputs into the aquifer; these inputs may include percolation of sheet surface runoff from less permeable areas beyond the aquifer extent and direct percolation from surface streams.

The impact of newly infiltrated water on spring discharge varies with respect to predominant type of porosity and position of the hydraulic head in the aquifer. In any case, the first reaction of karst or fractured aquifers to recharge in form of a rapid initial increase in discharge rate is in many cases the consequence of pressure propagation through karst conduits and large fractures, and not necessarily the outflow of newly infiltrated water (see also Fig. 2.23). The new water arrives at the spring with certain delay, and its contribution is just a fraction of the overall flow rate. The contribution (percentage) of the newly infiltrated water discharging at the spring can be determined using hydrochemical separation methods (Section 2.9.3) and environmental tracers.