# Integrated Hydrological Analysis of Little Akaki Watershed Using SWAT-MODFLOW, Ethiopia
## 3 Results and Discussions
### Groundwater Modeling
Little variations in most sensitive parameters could result in a large change in model simulation outputs. Parameter sensitivity analysis (Figure 12) shows that the hydraulic conductivity value is highly sensitive in hydraulic conductivity zone HK_5, which is located below the Gefersa reservoir. The hydraulic conductivity value near the center of the watershed, below the Gefersa Lake, could be due to the presence of significantly different geological features, like faults, in those areas [11]. In this study, hydraulic conductivity is the only parameter obtained through the calibration process. The MODFLOW numerical model response to the deviation from the calibrated hydraulic conductivity during the simulation, while keeping other parameters constant was used as a tool to evaluate the sensitivity of the model to that particular parameter. To check the sensitivity of the model to the parameters, continuous simulations were made by changing the calibrated values of hydraulic conductivity both in increases and decreases by 10%, 25%, and 50%. After the continuous run of six times, the corresponding root means squared error (RMSE) for head changes (%) in observation wells from the calibrated value were plotted in Figure 13. It was noted that the model was less sensitive to a small change in parameter value and more sensitive to the big changes of the parameters from the calibrated values. Figure 12: Spatial parameter sensitivity from Little Akaki watershed. (e) Groundwater Balance and Scenario Analysis  
One of the fundamental ways to quantitatively estimate the flow of groundwater within an aquifer system is through a calculation of the water budget. The basic water budget equation is that the difference between inflows and outflows equals the change in storage of the groundwater system. The water balance is established based on the modeling water budget tool in MODFLOW. The water balance of the baseline scenario (Table 4) shows that inflow and outflow of the total Source/Sink are equal. The baseline groundwater pumping recorded considered was the static pumping rate obtained from 17 pumping wells (Figures 14 and 15). Because of the increasing groundwater withdraws as a result of the increasing population and development activities in the area, five pumping scenarios were adopted to test the response of the aquifer system under variable groundwater abstraction rates. In addition to the pumping scenarios, increasing and reducing groundwater recharge scenarios are considered because of the recharge in the area is significantly dependent on land use type (reported in the recharge estimation section). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Scenarios**} & **Water Balance** & **Inflow** & **Outflow** & **Difference** \\  & **Components** & **(m3/Day)** & **(m3/Day)** & **(m3/Day)** \\ \hline \multirow{6}{*}{**Baseline Scenario**} & Constant Head & 120,604 & 20,462 & \multirow{6}{*}{} \\  & Wells & - & 20,993 & \\ \cline{1-1}  & Drains & - & 19,665 & \\ \cline{1-1}  & River Leakage & 622,224 & 748,583 & \\ \cline{1-1}  & Head Dep Boundary & 3617 & 2211 & \\ \cline{1-1}  & Recharge & 65,448 & - & \\ \cline{1-1}  & Total Source/Sink & 811,893 & 811,913 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Baseline Scenario of calibrated model. Figure 13: Parameter sensitivity analysis result. The considered pumping scenarios are the current pumping scenario (Scenario 1) and the increasing of the current abstraction by 10% (Scenario 2), 25% (Scenario 3), 50% (Scenario 4), 100% (doubling the current pumping, scenario 5), and 200% (increasing fourfold, Scenario 6) from the existing pumping wells (Table 5). Figures 14 and 15 summarizes the results of the groundwater response to the pumping scenarios. Simulation results of stream base flow, water table elevations (drawdown), and subsurface outflows during the pumping scenario (Figure 14). Figure 14: Groundwater head at observation wells in response to different pumping scenarios.