Abstract:

Hydrological modeling is an important tool for estimating hydrological responses, not only for current conditions but also for future scenarios by optimizing hydrological parameters. In cases where direct measurements are difficult to obtain, modeling can be used to fill in the gaps and provide vital information for water resources planning and management. However, it is important to note that hydrological models need to be calibrated and validated to ensure that the parameter values are optimized for the specific watershed being studied.

The study shows that the baseline period of performance measure values is RMSE = 12, NSE = 0.76, PBIAS = 10.5% and R² = 0.78, and RMSE = 3.65, NSE = 0.85, PBIAS = 9.9%, and R² = 0.85 indicate that the model has performed well in simulating the streamflow in both the calibration and validation periods, respectively. . In the future (2015–2100) the average Tmax 1.59, 1.93, and 2.48 °C, and Tmin will rise by 1.83, 2.33, and 2.85 °C under SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively.  The calibrated parameters of the XAJ model were critical to assess the future hydrological responses of the Dirima watershed under the three SSP scenarios. The precipitation and streamflow values are consistently lower than those of ET and temperature across all SSP scenarios. This indicates that future water availability is likely to be under pressure and calls for appropriate strategic measures to create a more resilient water resource system.

Materials and methods

The studied Dirima river watershed is a subwatershed located in the upper Lake Tana basin, covering an area of 162.2 km2 and an altitudinal range of 1786–2746 masl. Geographically, the watershed is located between 12°25̍’06″–12°39′36″ N latitude and from 37°13’06″–37°30′36″ E longitudes at the outlet point (bridge) where the discharge observation station is located and is shown in Fig. 1. It has a tropical savanna climate, with an average annual minimum and maximum temperature of 16.7 °C and 27.2 °C, respectively, and a long-year average annual precipitation of 1,254.50 mm shown in (Fig. A). The annual flow of the river is 53.7 m 3/s and evapotranspiration is 1280 mm. The rainfall distribution is quite uneven throughout the year in that precipitation in summer accounts for more than 74%. The Dirima River is the main tributary of Lake Tana, with the longest main flow path of 48.53 km.

Xinanjiang (XAJ) conceptual hydrological model
The XAJ model is a conceptual hydrological model that predicts discharge at a basin outlet by simulating runoff, generation, and concentration within a catchment (Hao et al. 2015; Ren-Jun 1992; Zhijia et al. 2013). Its major characteristic is the formation of runoff during storage replenishment (saturation excess overland flow), which implies that runoff does not occur until the SM content of the unsaturated zone exceeds the field capacity, after which runoff equals the rainfall surplus without a further loss (Hao et al. 2015; Weimin and Qian 2012; Zhijia et al. 2013; Zhu et al. 2017).

The Xinanjiang (XAJ) model have been used to estimate the potential hydrological responses under current and future scenarios. In this study, an attempt was made to optimize the parameters for the Dirima watershed with a total area of 162 km² using DEoptim algorithms. In this study, the calibrated parameters were used to simulate the watersheds’ hydrological response for baseline (1996–2009) and three climate change scenarios (SSP2-4.5, SSP3-7.0, and SSP5-8.5) from a CMIP6 multimodel ensemble (2015–2100).

Results and discussion

The study shows that the baseline period of performance measure values is RMSE = 12, NSE = 0.76, PBIAS = 10.5% and R² = 0.78, and RMSE = 3.65, NSE = 0.85, PBIAS = 9.9%, and R² = 0.85 indicate that the model has performed well in simulating the streamflow in both the calibration and validation periods, respectively. Lower RMSE, higher NSE, and lower PBIAS indicate better model performance and suggest that the model has performed well in all these aspects. In the future (2015–2100) the average Tmax 1.59 , 1.93  and 2.48 °C, and Tmin will raise by 1.83 , 2.33  and 2.85 °C under SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively. Additionally, the mean annual future hydrological responses in the upper soil layer, including evaporation, soil moisture, and runoff will be 1.72, 1.71, and 1.79 mm, 0.63, 0.62, 0.64 mm, and 1.07, 0.97, and 1.18 mm under SSP2-4.5, SSP3-7.0, and SSP5-8.5. The future projections show an increase in average maximum and minimum temperatures under different SSP scenarios.

Future hydroclimate responses:
Climate change can have a significant impact on hydrological responses, and future climate models suggest that these responses may vary depending on location and global temperature increase. One of the potential effects of climate change on hydrological responses is an increase in evapotranspiration rates, which can lead to drier conditions in some regions. This could be particularly problematic in areas that are already experiencing water scarcity. Additionally, changes in precipitation patterns and intensity may lead to more frequent and severe flooding events in some regions. 

Conclusion

The calibrated parameters of the XAJ model were critical to assess the future hydrological responses of the Dirima watershed under the three SSP scenarios. The precipitation and streamflow values are consistently lower than those of ET and temperature across all SSP scenarios. This indicates that future water availability is likely to be under pressure and calls for appropriate strategic measures to create a more resilient water resource system. Streamflow reductions can occur due to changes in precapitation patterns, such as less frequent or intense rainfall events, and increased evapotranspiration from plants due to warmer temperatures. These changes can lead to decreased water availability in rivers, lakes, and groundwater systems, which can affect water supply for human consumption, agriculture, and other uses. Increased air temperatures can also exacerbate these effects, as warmer air can hold more water vapor, leading to more evapotranspiration and drier conditions. The relationship between higher air temperatures and increased evapotranspiration is particularly important. Warmer air can hold more water vapour, which accelerates evapotranspiration rates. As a result, the overall moisture content in the soil and vegetation decreases, contributing to drier conditions and potentially increasing the risk of droughts.