A Procedure for Separation of Baseflow

1. A Procedure for Separation of Baseflow MESUT CIMEN KEMAL SAPLIOGLU Suleyman Demirel University, Facultyof Engineering -Architecture, Department of Civil Engineering, Isparta, TURKEY

2. ABSTRACT In modeling of streamflow, there are many difficulties changing respect to annuals and even seasons such as rainfalls occurring after from starting of recession, snow melts, interflow, properties of the aquifer recharging the stream, evapotranspiration, geological and morphological structure of the catchment. However, identification of response of the baseflow (BF) affecting stream hydrograph is important for evaluation of the water resources. In this study, the procedures, which are used at separation of BF, are considered, and a procedure for this purpose is suggested, andthe procedures are compared each other.

3. INTRODUCTION AND PURPOSE Analyses on the streamflow (SF) are carried a great importance due to the increasing of demanding water. The recession works on the SF, which are provided limit values on subjects such as water supply, irrigation, hydropower and water quality, are especially necessary in determination of low SF characteristics. While a BF hydrograph is dependent upon hydrological and geomorphologic conditions, a shape ofBF recession continues after the rise of the stream hydrograph may be characterized by:(1) BF recession continues after the rise of the stream hydrograph; (2) BF will peak after the stream hydrograph peak due to the storage–routing effect of the subsurface stores; (3) BF recession will likely follow an exponential decay function; (4) BF hydrograph will rejoin the stream hydrograph as direct runoff ceases.

4. PREVIOUS WORKSVarious studious have been made on prediction of BF which is occurred a great amounts of SF during the dry periods, especially. The studies taking into consideration the rising and recession of BF are the empiric or theoretic studies with the simple assumptions.These;

5. (a) Smoothed Minimum TechniqueIn this procedure, the span between the two ends of the hydrograph is joined by a straight line.(b) Half-Linear Minimum/Maximum TechniqueBF rate taking into consideration the rate of contribution and ability of transmissivity of the aquifer to thestream is found as either the min. value of BF which is had the curve of previous recession or the max. value of BF which is had the curve of that recession continuing towards back. The state which is the min.or max. of BF is connected by the other end. (c) Efficient Subsurface Flow TechniqueSeepage from rainfall is taken into consideration during the recession periods. Thus, a SF hydrograph is separated to three zones which are influenced of the surface, subsurface and groundwater flows. The reverse recession curve is plotted from the point where is efficiently BF to the point (inflection point) where is not efficiently surface flow. Later, the previous recession curve is extended to below of SF, and these points are joined by a straight line

6. (d) Wittenberg TechniqueIn this procedure, the reverse computed BF recession curve is intersected the rising limb of the SF hydrograph, and then, a transition point is adapted as the peak of BF. Values of the rising limb of the BF hydrograph are found as the computed recession for one time–step forward for each given SF value.(e) Separation of Baseflow on Streamflow Hydrographs with Multiple PeaksThe reverse computed BF recession curve and the recession curve of SF hydrograph occurred from previous rainfall are joined. This point and the initial end of the rising limb are connected with a straight line

7. PROPOSED TECHNIQUEThis procedure may be applied for separation of BF on SF with both the single and multiple peaks.In the single peak case, it is assumed that the lag time between the SF and BF peaks was one month (this study considers monthly data of streamflow). The reverse computed BF recession curve is extended, and the peak value of BF is obtained. After this stage, two processes are made. First the peak point of BF is intersected with the initial point of rising limb of the SF hydrograph by a straight line. Later, a curve is plotted from the initial point of rising limb to the forward that it is calculated to the peak time of SF hydrograph with the well-knownrecession formula Q=Q0exp(t) (here, Q and Q0are SF rates during times t and zero;  is the recession coefficient). Finally, average of the straight line and curved line values is taken, and thus the error occurred from the BF values calculated during the rising limb is decreased.

8. Streamflow Baseflow Streamflow Baseflow Time In multiple peak case, extending the reverse computed BF recession curve to the peak of SF hydrograph, the final peak value of BF is found. A horizontal line from this point to backward is plotted to the initial point of rising limb of the final SF hydrograph, and also the final peak point of BF is joined with initial point of first rising limb of the SF hydrograph by a straight line. Thus, average of these lines are taken, and the recharge of BFby rainfalls is taken into consideration during this period. Later, the reverse computed BF recession curve is extend to the previous peak of SF hydrograph. The same processes are made for the peak number of SF hydrograph again and again. Discharge

9. APPLICATION In this study, it is aimed the comparison of the BF separation procedures in literature and the suggested procedure. For this purpose, it is taken into consideration the monthly discharges of the Beskonak streamgauge station between 1976-1990 years on the Koprucay river, Turkey. The observed discharge hydrographs and the recession curves calculated by the available and suggested procedures. Furthermore, Table 1 is given to be seen the differences between the procedures. The proportions of the calculated BF volumes to the measured SF volumes are obtained. As shown from the average values, the proposed procedure value is found between the Wittenberg and ‘Separation of baseflow on the streamflow hydrographs with multiple peaks’ procedures. The other procedures are also obtained as quite difference from the suggested procedure.

10. Discharge Time (month) Figure 11. Separation of baseflow with the Wittenberg (1999) technique - - - Streamflow Baseflow Discharge Time (month) Figure 12. Separation of baseflow on the streamflow hydrographs with multiple peaksprocedure Discharge - - - Streamflow Baseflow Time (month) Figure 13. Separation of baseflow with the proposed procedure

12. CONCLUSIONS A new procedure for the baseflow separation from total streamflow hydrograph is suggested in this paper, and it is compared the available procedures in literature. The proposed procedure gave quite close results to the results of the Wittenberg (1999) and ‘Separation at baseflow on the streamflow hydrographs with multiple peaks’ procedures. However, this procedure may be developed for thedaily discharges on the various stations.