**3. Results and discussions**

The model was built with DEM, soil classes, land use/cover, and slope types for the Genale watershed, which contained 25 sub-basins, 464 HRUs with a catchment area 54,942Km2 at the outlet.

#### **3.1 Model parameter sensitivity analysis**

Sensitivity analysis was performed to navigate the calibration action and pinpoint parameters that significantly affect the discharge and sediment flow. In a sensitivity analysis of the model, SCS curve number (CN2.mgt), an available water capacity of the soil layer (SOL\_AWC.sol), and saturated hydraulic conductivity (SOL\_K.sol) are the most sensitive parameters for runoff estimation. However, model efficiency is also influenced by the reliability of spatial and temporal data.

Sediment sensitivity analysis was carried out for three years warm-up period 1987 to 1989- and 16-years calibration period 1990 to 2005- and 8-years validation period 2006 to 2013. Based on the p-value and t-stat results obtained from sensitivity analysis, the ranks of parameters were finalized. The simulated sediment was sensitive to the amount of sediment re-entrained during channel sediment routing (SPCON.bsn), (SOL\_AWC.sol), CN2, etc., respectively.

A parameter with a larger absolute value of t-stat is more sensitive to flow. The p-value gives the relevance of the sensitivity. Thus, when the p-value is close to zero, then the sensitivity of the parameter is a priority (**Table 1**).

#### **3.2 Calibration/validation**

Calibrated parameters and the fitted values are final notes for the modeler from the calibration process used for the required objectives. Calibration of discharge and sediment flow was performed with several iterations of 500 simulations number; each was carried out for the calibration period of 1990–2005 monthly.

Validation is required to verify whether the calibrated parameters also work for other data of different years within the watershed. Validation time (2006–2013) results revealed a satisfactory performance, as statistical measures are in the acceptable range for discharge and sediment. **Table 2** shows the acceptable range for the model's performance in light of the calibration and validation process.

The results show satisfactory and well responded to calibration and validation process (**Table 3**).

The calibration was done from 1990 to 2005 & the validation period from 2006 to 2013, and the model performance shows satisfactory agreement between the observed and simulated flow (**Figure 7**). The calibrated/validated model also responded to the rainfall with the respective months.

As indicated, the simulated and observed sediment load agreed and showed a satisfactory performance during the calibration and validation action (**Figure 8**).

#### **3.3 Impact of climate change in the watershed**

#### *3.3.1 Climate change impacts on temperature and precipitation*

The climate change impact on hydrology was evaluated by driving the calibrated/validated SWAT model with the bias-corrected RCM-CORDEX weather corresponding to the present-day historical data and future emission scenarios. The analysis was executed on a monthly basis for streamflow and sediment yield.


*Evaluation of Climate Change-Induced Impact on Streamflow and Sediment Yield of Genale… DOI: http://dx.doi.org/10.5772/intechopen.98515*

#### **Table 1.**

*Fitted values and rank of parameters used in the SWAT model calibration/validation (1998–2012).*

The statistically downscaled Regional Climate Model (RCM) Bias-corrected Coordinated Regional Climate Downscaling Experiment (CORDEX), precipitation, min/mean/max temperature for Africa-Ethiopia, under RCP 4.5 and RCP 8.5. The average annual rainfall in the study climate stations during the baseline 24-years period (1990–2013) was 810 mm, and the maximum and minimum yearly rainfall accounts were 1,303 mm and 300 mm, respectively.

The monthly temperature of the catchment varies from 14.5°C to 24.6°C, with an average of 19.5°C. We predicted the long-term average precipitation with the historical data for two climate emission scenarios. As shown in **Figure 9**, significant changes occur in the dry season (December, January & February).


**Table 2.**

*SWAT statistical performance index acceptable range [25, 26].*


**Table 3.**

*Actual index value for SWAT output during calibration/validation process (1990–2013).*

#### **Figure 7.**

*Monthly calibration and validation of streamflow (1990–2013) for Genale River basin at Genale Halwen.*

#### **Figure 8.**

*Monthly observed and simulated sediment load plots for the calibration (1990–2005) and validation (2006–2013).*

*Evaluation of Climate Change-Induced Impact on Streamflow and Sediment Yield of Genale… DOI: http://dx.doi.org/10.5772/intechopen.98515*

**Figures 9** and **10** show the climate changes in the average monthly precipitation and the maximum and minimum air temperatures over the catchment between the historical and future periods (2022–2080) for the two emission scenarios. Generally, the climate change over the Genale basin will likely become warmer, especially in autumn and spring, considering the higher emission scenario (**Figure 10**).

#### **Figure 9.**

*Comparison of average observed monthly precipitation for baseline condition, RCP4.5, & RCP8.5 scenarios of four stations in the catchment.*

#### **Figure 10.**

*Comparison of mean temperatures for historical data, RCP4.5, & RCP8.5 scenarios of four stations in the catchment.*

Indistinct, the maximum temperature increase is somewhat higher than that of the minimum temperature in the region.

**Figure 10** shows an average of mean monthly changes in temperatures in the study watershed, and it is increasing under emission scenarios of RCP4.5 and RCP8.5.
