# Estimating Groundwater Recharge in the Semiarid Al-Khazir Gomal Basin, North Iraq

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. Study Area

^{2}and is located in the northern part of Iraq at the border of the high and low folded zones (Figure 1). Shaded relief has been used to highlight the topographic features of the basin. The surface elevation ranges from 216 m at the lowest point, close to the watershed outlet, to 2165 m at the highest point, in the mountainous area in the far northern parts. The basin is crossed by the Al-Khazir River and its tributary, the Gomel River, which meet near Bardarash Mountain. The annual stream-flow of the perennial Al-Khazir River measured at Manguba station near the outlet of the basin is 7.4 × 10

^{8}m

^{3}/y [24].

^{2}. The aquifer in this region consists of thick beds of conglomerate and sandstone alternated with thin layers of claystone and siltstone. This aquifer is considered the main and most important aquifer with regard to the quality and quantity of groundwater. The third region is specified by a triangular outlet area that has the same hydrogeological conditions as the second region, but is separated from it by the Bardarash Mountains. Most human activities are concentrated in the second region, which occupies 38% of the basin and includes many towns and villages. Rain-fed agriculture is one of the most important activities of the peoples in this region. Domestic use is the main use of groundwater in the basin, in addition to the irrigation of small home farms. Industrial activities are limited to one drinking-water bottling plant and quarries of sand and gravel along Al-Khazir River.

**Figure 1.**Map of Al-Khazir Gomel basin illustrating the basin boundary, the three regions’ boundaries, the stream-flow gauge and pumping and observation wells.

## 3. Methodology

**Figure 2.**Flowchart showing the methodology of the recharge estimation by base-flow separation methods, the displacement recession curve method and the water table (WT) fluctuation method.

#### 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 = A
^{0.2}) proposed by Linsley et al. [26], where N is the number of the days after the peak and A is the area of the watershed in square miles (Mi^{2}) 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)

_{t}is the filtered surface runoff (quick response) at the t time step (one day); Q is the original stream-flow and α the base-flow filter parameter (0.925). The value of 0.925 was determined by Arnold et al. [12] to give realistic results compared to manual separation techniques.

_{t}− R

_{t}

- Eckhardt method: Eckhardt [14] proposed in Equation (3) two parameters in the digital filter base-flow, the base-flow filter parameter α (0.98) and BFI
_{max}(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 BFI_{max}, Eckhardt [14] proposed that BFI_{max}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 BFI_{max}value of 0.8 was used.$${B}_{t}=\frac{\left(1-BF{I}_{\text{max}}\right)\times \text{\alpha}\times \text{}{B}_{t-1}+\left(1-\text{\alpha}\right)\text{}\times \text{}BF{I}_{\text{max}}\text{}\times \text{}{Q}_{t}}{1-\text{\alpha}\times \text{}BF{I}_{\text{max}}}$$

_{t}is the filtered base-flow at the t time step, B

_{t}

_{−1}is the filtered base-flow at the t − 1 time step, BFI

_{max}is the maximum value of long-term ratio of base flow to total stream-flow, α is the filter parameter and Q

_{t}is the total stream-flow at the t time step.

- 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 [13].

- (1)
- Obtain monthly base-flow from the base-flow-record estimation;
- (2)
- Obtain long-term mean monthly base-flow. In our case, this was done for the period 1969 to 1981;
- (3)
- 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;
- (4)
- Choose the most stable (near-linear) segment and obtain the slope of the stable base-flow;
- (5)
- Use linear interpolation in the remaining months; finally, the mean annual base-flow is obtained.

_{b}) to the total stream runoff (Q), is calculated for base-flow records and stable base-flow.

^{bf}is base-flow; ET

^{gw}is ET from the groundwater table; ΔS

^{gw}is change in groundwater storage.

^{bf}, can be neglected, and the net recharge can be estimated as:

^{bf}

#### 3.2. Displacement Recession Curve Method

_{c}= 0.2144K

_{1}is the groundwater discharge at a critical time after the peak of the previous recession curve, Q

_{2}is the groundwater discharge at the critical time after the peak on the current recession curve and K is the recession index.

**Figure 6.**Recession segments and master recession curve (MRC) for the period 1969–1981 determined by the RECESS program.

#### 3.3. Water Table Fluctuation Method

_{y}= specific yield; Δh = change in water table elevation and Δt = the time period.

## 4. Results and Discussion

_{max}) in the digital filter base-flow, where the BFI

_{max}parameter takes the physical properties of the basin into account.

Method | Base-Flow Records (m^{3}/y) | Base-Flow Indices (BFI) | Stable Base-flow records (m^{3}/y) | Stable Base-Flow Indices (BFI) |
---|---|---|---|---|

Graphical method | 4.9 × 10^{8} | 0.68 | 3.8 × 10^{8} | 0.52 |

Single parameter method | 5.51 × 10^{8} | 0.75 | 4.2 × 10^{8} | 0.56 |

Eckhardt method | 5.1 × 10^{8} | 0.69 | 3.9 × 10^{8} | 0.52 |

Local minimum method | 5.48 × 10^{8} | 0.75 | 4.2 × 10^{8} | 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.

**Figure 9.**Estimated recharge using the base-flow separation methods (graphical method, single parameter method, Eckhardt method, local minimum method), WT fluctuation method and displacement recession curve method.

_{3}, and the average total dissolved solid (TDS) and Cl were 406 and 3.5 mg/L, respectively). This indicates renewable groundwater, the short residence time of the water near the surface and a high infiltration capacity. In addition, the analysis of stable isotopes (deuterium and O

^{18}) of groundwater showed a match and a very slight shift from the local meteoric water line, which means that the recharged water is not affected by high evaporation. Moreover, rainfall happens in the cold months when evapotranspiration is minimal, and thus, more water will be available to contribute to groundwater recharge.

## 5. Conclusions

^{8}m

^{3}/y) to ensure a sustainable use of groundwater and to save the river environment.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**MDPI and ACS Style**

Jassas, H.; Merkel, B.
Estimating Groundwater Recharge in the Semiarid Al-Khazir Gomal Basin, North Iraq. *Water* **2014**, *6*, 2467-2481.
https://doi.org/10.3390/w6082467

**AMA Style**

Jassas H, Merkel B.
Estimating Groundwater Recharge in the Semiarid Al-Khazir Gomal Basin, North Iraq. *Water*. 2014; 6(8):2467-2481.
https://doi.org/10.3390/w6082467

**Chicago/Turabian Style**

Jassas, Hussein, and Broder Merkel.
2014. "Estimating Groundwater Recharge in the Semiarid Al-Khazir Gomal Basin, North Iraq" *Water* 6, no. 8: 2467-2481.
https://doi.org/10.3390/w6082467