Groundwater is a strategic resource in all climatic regions of Ethiopia, contributing about 80% of the domestic supply of urban and rural populations. However, little research has been available compared with extensive geographical coverage and increasing population growth rates. Hence, the present study aimed to review published groundwater research of Ethiopian aquifers to realize potential research challenges and suggest future research directions. We focused on groundwater potential, recharge process, and qualities. The total potential groundwater of the country ranges from 2.5 to 47 billion cubic meters. The study depicted that the mean annual recharge estimate varies from 24.9 mm to 457 mm at catchments scales. However, the overall country was about 39.1 mm. The study found a need for a detailed investigation of different factors susceptible to groundwater pollution, as some of the evaluations indicated exceeding acceptable standards. This study observed that the main challenge was the lack of data and convergence research trends. Henceforth, future research in different climate regions should focus on multifaceted technical and stakeholder settings. This study gives the insight to integrate palatable research findings with the national policy and decision-making process to enhance the sustainability of groundwater resources significantly.

Recognizing how human activities affect hydrological systems is vital for the sustainable preservation and effective management of water resources in the watershed. Hence, this paper focuses on the hydrological response to land use and land cover (LULC) change scenarios in the Anyang watershed, South Korea. We obtained LULC data maps for the years 2000, 2013, and 2022 from the local government, revealing significant changes over the years. Agricultural lands experienced a 6.2% increase from 2000 to 2022, and pastureland expanded by 8.67% over two decades. The SWAT model was utilized to assess the impact of LULC on the hydrological components of the study watershed. Model calibration and validation for each LULC change were carried out using the SWAT-CUP program, considering the recorded streamflow information of the region. An excellent agreement was reached between the simulated and measured streamflow in both the calibration and validation stages under various LULC conditions. The Nash–Sutcliffe model efficiency (NSE), the objective function, demonstrated values of 0.9, 0.89, and 0.89 during the calibration for 2000, 2013, and 2022, respectively, in the LULC scenario, while for the validation, we obtained values of 0.82, 0.78, and 0.80 for 2000, 2013, and 2022, respectively. Our findings indicate that the surface runoff rise contributed much to the water yield increase over the two decades compared to the other components in terms of the water yield, while the contribution of evapotranspiration (ET) to the watershed hydrological cycle declined by 1.66% from 2000 to 2022. The southeastern sub-basin part showed a high groundwater recharge distribution due to agricultural land, rice area, and forest area changes.

Recharge is a crucial section of water balance for both surface and subsurface models in water resource assessment. However, quantifying its spatiotemporal distribution at a regional scale poses a significant challenge. Empirical and numerical modeling are the most commonly used methods at the watershed scales. However, integrated models inherently contain a vast number of unknowns and uncertainties, which can limit their accuracy and reliability. In this work, we have proposed integrated SWAT-MODFLOW and Transient Water Table Fluctuation Method (TWTFM) to evaluate the spatiotemporal distribution of groundwater recharge in Anyang watershed, South Korea. Since TWTFM also uses SWAT model percolation output data, calibration was performed for individual models and a coupled model. The coupled model was calibrated using daily streamflow and hydraulic head. The SWAT-MODFLOW model performed well during the simulation of streamflow compared to the SWAT model. The study output showed that the study watershed had significant groundwater recharge variations during the simulated period. A significant amount of recharge happens in the wet season. It contributes a significant amount of the average annual precipitation of the region. The direct flow components (surface and lateral) showed significant contributions when the water balance components were evaluated in the region. TWTFM showed a glimpse to estimate recharge, which requires representative monitoring wells in the study region. Comprehensively, the SWAT-MODFLOW model estimated groundwater recharge with reasonable accuracy in the region.

Recharge is a crucial section of water balance for both surface and subsurface models in water resource assessment. However, quantifying its spatiotemporal distribution at a regional scale poses a significant challenge. Empirical and numerical modeling are the most commonly used methods at the watershed scales. However, integrated model inherently contain a vast number of unknowns and uncertainties, which can limit their accuracy and reliability. In this work, we have proposed integrated SWAT-MODFLOW and Transient Water Table Fluctuation Method (TWTFM) to evaluate the spatiotemporal distribution of groundwater recharge in Anyang wa-tershed, South Korea. Since TWTFM also uses SWAT model percolation output data, calibration was performed for individual models and coupled model. The coupled model was calibrated using daily streamflow and hydraulic head. SWAT-MODFLOW model performed well during the simulation of streamflow compared SWAT model. The study output showed that the study watershed had significant groundwater recharge variations during the simulated period. A sig-nificant amount of recharge happens in the wet season. It contributes about 34% of the average annual precipitation of the region. The direct flow components (surface and lateral) showed sig-nificant contributions when the water balance components were evaluated in the region. TWTFM showed a glimpse to estimate recharge, which requires representative monitoring wells in the study region. Comprehensively, the SWAT-MODFLOW model estimated groundwater recharge with reasonable accuracy in the region.

Multi-purpose dams in a river basin frequently result in variations in downstream flow. Precisely assessing the reservoir operation effects can improve management strategies and alleviate extreme hydrological events. This study assesses the impact of reservoir operation scenarios on the downstream flow in the Seomjin River basin in South Korea. Four reservoir scenarios were developed utilizing observed daily inflow and outflow data from the reservoirs. A semi-disturbed hydrological model, SWAT (Soil and Water Assessment Tool), was employed to simulate the flow for each reservoir operation scenario in the downstream section of the study basin. Model execution was evaluated by comparing the simulated and measured streamflows using performance metrics, including R², NSE, and PIBAS, which displayed very good compatibility. The sensitivity of calibration parameters varied across different reservoir operation scenarios. The results of this study indicate that the operation scenarios for the Seomjin and Juam reservoirs led to a maximum downstream flow reduction of 32%. Additionally, the monsoon season exhibited a lower percentage reduction in flow compared to the dry season, which was influenced by the frequency of rainfall in the region. Annual assessment indicated that streamflow reduction varies between 1.35% and 32.9% across all reservoir operation scenarios. Reservoir operations have demonstrated their effect on the alteration of downstream flow in the Seomjin River basin. This study demonstrates that the operation of the Seomjin reservoir has a more significant impact on downstream flow than that of the Juam reservoir in the study region. This study analyzed a substantial basin with various reservoir operation scenarios to assess the influence of flow on the downstream section, yielding important insights for efficient water resource management.