2. Study Area
3.1. Base-Flow SEPARATION
3.1.1. Graphical Method (Constant Slope Method)
- Identify the start of direct runoff;
- Estimate the duration of the direct runoff period N after the peak of the storm by the empirical relationship (N = A0.2) proposed by Linsley et al. , where N is the number of the days after the peak and A is the area of the watershed in square miles (Mi2) above the gauge station;
- Draw a line connecting the start of direct runoff to the end (inflection point after N days). This is the base-flow hydrograph (Figure 3).
3.1.2. Automated Web GIS-Based Hydrograph Analysis Tool (WHAT)
- Eckhardt method: Eckhardt  proposed in Equation (3) two parameters in the digital filter base-flow, the base-flow filter parameter α (0.98) and BFImax (base-flow index (BFI); the maximum value of long-term ratio of base-flow to total stream-flow). To reduce the subjective influence of using BFImax, Eckhardt  proposed that BFImax in perennial and ephemeral streams with porous aquifers be 0.8 and 0.5, respectively, and 0.25 with a hard rock aquifer. In this study, a BFImax value of 0.8 was used.
- Local minimum method: The local minimum method begins by constructing a sequence of “local minima”. The procedure depends on determining the lowest discharge value in one half of the interval minus one day (0.5(2N − 1) days) before and after the day being considered and then connects the adjacent local minimum by an interpolated line .
- Obtain monthly base-flow from the base-flow-record estimation;
- Obtain long-term mean monthly base-flow. In our case, this was done for the period 1969 to 1981;
- Perform data processing by sorting and accumulating the long-term mean monthly base-flow. In this way, a new series of long-term mean monthly accumulated base-flow values is obtained;
- Choose the most stable (near-linear) segment and obtain the slope of the stable base-flow;
- Use linear interpolation in the remaining months; finally, the mean annual base-flow is obtained.
3.2. Displacement Recession Curve Method
3.3. Water Table Fluctuation Method
4. Results and Discussion
|Method||Base-Flow Records (m3/y)||Base-Flow Indices (BFI)||Stable Base-flow records (m3/y)||Stable Base-Flow Indices (BFI)|
|Graphical method||4.9 × 108||0.68||3.8 × 108||0.52|
|Single parameter method||5.51 × 108||0.75||4.2 × 108||0.56|
|Eckhardt method||5.1 × 108||0.69||3.9 × 108||0.52|
|Local minimum method||5.48 × 108||0.75||4.2 × 108||0.56|
- Recharge estimated by the hydrograph analysis method represents an integrated long-term recharge over a large area in different mechanisms (diffuse recharge and focused recharge through river bed leakage). However, the main source of recharge calculated by water table (WT) fluctuation is diffuse recharge, and it is less affected by indirect recharge, especially in areas far away from the main river course;
- Base-flow may be overestimated by bank storage, which is considered as a short-term storage discharge, in addition to snow melt in the spring season;
- The probability of leakage from the confined system becomes higher downstream, because the difference in hydrostatic pressure increases with the decreasing of the unconfined aquifer thickness;
- Recharge obtained by the WT fluctuation method does not represent the entire watershed, because neither monitoring wells nor pumping test wells cover the entire watershed area.
Conflicts of Interest
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