Vol.:(0123456789) 1 3 Environmental Earth Sciences (2023) 82:341 https://doi.org/10.1007/s12665-023-10988-y ORIGINAL ARTICLE Application of SWAT and MODFLOW models for characterization of surface–groundwater interaction in the Modjo River catchment, central Ethiopia Belay Molla Sisay 1,2  · Dessie Nedaw 1  · Behailu Birhanu 1  · Amanuel Godie Gigar 1,3 Received: 5 September 2022 / Accepted: 13 May 2023 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023 Abstract Assessing the relationship between Surface Water (SW) and Groundwater (GW) is an important aspect of hydrology, as it aids in the conjunctive and efective management of water resources. This study characterized the surface–groundwater interac- tion in the Modjo River catchment, central Ethiopia using the Soil and Water Assessment Tool (SWAT) and MODFLOW models. The SWAT model adequately simulated the hydrology of the study area for the calibration (2007–2011, R 2 = 0.83) and validation (2012–2015, R 2 = 0.7) periods. The semi-distributed average GW recharge from the SWAT model simula- tion with other hydrogeologic parameters and boundary conditions was utilized to construct the GW fow model using the MODFLOW–NWT version. The integrated modeling result showed that about 43Mm 3 of water leaks to the groundwater system annually, whereas the fux from GW to rivers is about 165Mm 3 /yr. Most reaches of the Modjo River in the northern and central areas of the catchment are gaining, with fuxes reaching up to 4940 m 3 /day. The downstream part of the catch- ment is characterized by a losing-disconnected river, with a lower fux of up to 675m 3 /day. In general, most reaches of the Modjo River are interconnected with the GW system and the GW infow to streams is higher. Based on these fndings, future exploitation and management of both SW and GW resources should consider the nature of their interconnection for sustain- able utilization of the water resources in the catchment. Keywords Surface water–groundwater interaction · Groundwater modeling · SWAT  · Modjo River catchment · MODFLOW Introduction Surface and groundwater have been perceived and managed traditionally as separate resources. This is mainly because the chemical, biological and physical properties of surface and groundwater are usually diferent (Kalbus et al. 2006; Fleckenstein et al. 2010). However, surface water (which includes rivers, lakes, reservoirs, wetlands, etc.) and ground- water are not separate entities and interact continuously with each other in the hydrologic cycle under diferent climatic and physiographic conditions (Winter et al. 1998; Sophocle- ous 2002; Khan and Khan 2019). The study of conjunctive management of surface and groundwater has been given more attention in recent times, because it has been increas- ingly understood that reliable water resource management can only be achieved by accurate determination of the inter- actions between the whole hydrologic cycle, which includes sub-surface, surface, and atmospheric processes (Sophocle- ous 2002; Khan and Khan 2019; Woessner 2020). However, the management of GW and SW resources as a single resource and the determination of the interplay between them is usually overlooked in Ethiopia and only little eforts have been made to understand the interaction between the two systems (Birhanu et al. 2021). There are few regional studies dealing with the characterization of sur- face and groundwater interaction in the Awash River basin (Ayenew et al. 2008; Kebede et al. 2021; Yifru et al. 2022). However, there are no previous works in the Modjo River catchment dealing with surface–groundwater interaction. It * Belay Molla Sisay belaymolla2009@gmail.com 1 School of Earth Sciences, Addis Ababa University, Addis Ababa, Ethiopia 2 Department of Geology, Debre Berhan University, Debre Berhan, Ethiopia 3 Department of Geology, Wollo University, Dessie, Ethiopia