Modelling interactions between groundwater and surface water at catchment-scale influenced by groundwater abstractions and climate change
With intensifying water crisis, environmental and ecological degradation, as well as ongoing climate change worldwide, integrated water resources management, which considers surface water (SW) and groundwater (GW), is becoming increasingly important. As integrated surface–subsurface hydrological models are capable of simulating water processes in an integrated and holistic fashion, provide spatially and temporally detailed description of the catchment-scale hydrological cycle, enable scenario analysis, and may be coupled with other models (e.g. solute transport model), they are essential and useful tools in integrated water resources management. The SWAT-MODFLOW is such a surface-subsurface model.
Excessive groundwater abstractions can decrease the groundwater table, and thereby affect the aquifer-connected surface water bodies, which may deteriorate the quality of aquatic ecosystems. At the same time, climate change affects inland water ecosystems not only by increasing the water temperatures but also by influencing hydrological processes (e.g. evapotranspiration) and thereby alter the flow regime. The overall objective of my Ph.D. project was to further develop and apply a newly integrated surface-subsurface model SWAT-MODFLOW in order to improve the understanding of SW-GW interactions, and to assess the impacts of groundwater abstractions and climate change on the hydrological regime and on stream biota.
In the first part of my study (presented in manuscript 1), we further developed the SWAT-MODFLOW complex based on the previous publically available version (v.2) to enable the application of a Drain Package and an auto-irrigation routine. To better understand how groundwater pumping wells may influence streamflow patterns, we applied both the semi-distributed SWAT model and the further developed the integrated surface–subsurface hydrological, SWAT-MODFLOW model to a Danish, lowland, groundwater-dominated catchment - the Uggerby River Catchment (357 km2). Both models were calibrated and validated, and an approach based on PEST (Model-Independent Parameter Estimation and Uncertainty Analysis) was developed and utilized to enable simultaneous calibration of SWAT and MODFLOW parameters. The performance of the models when simulating streamflow and the simulated streamflow signals when running four groundwater abstraction scenarios through the two models were analyzed and compared. Both models demonstrated generally good performance of the temporal pattern of streamflow, albeit SWAT-MODFLOW performed somewhat better. In general, the simulated signals of SWAT-MODFLOW appeared more plausible than those of SWAT, and the SWAT-MODFLOW decrease in streamflow was much closer to the actual volume abstracted. The impact of drinking water abstraction on streamflow depletion simulated by SWAT was unrealistically low, and the streamflow increase caused by irrigation abstraction was exaggerated compared with SWAT-MODFLOW.
To quantitatively assess the effects of groundwater abstractions and climate change on the hydrological regime and on stream biota, we combined the SWAT-MODFLOW model with novel nationwide-scale flow-biota empirical models for three key biological taxonomic identities (fish, macroinvertebrates, and macrophytes). We applied the integrated approach to the Uggerby River Catchment and assessed to what extent the flow regime and key biota in stream segments of different sizes may be altered by groundwater abstractions and climate change. In the second part of my study (presented in manuscript 2), we therefore analyzed and assessed the impacts of the present level of groundwater abstractions and a scenario with extreme groundwater abstraction for three subbasin outlets representing stream segments of different sizes. The current groundwater abstraction level had only minor impacts on the flow regime and stream biotic indices at the three subbasin outlets. The simulated extreme abstractions, however, led to significant impacts on the smallest stream but had comparatively minor effects on the larger streams. The fish index responded most negatively to the groundwater abstractions, followed by the macrophyte index, decreasing, respectively, by 23.5% and 11.2% in the small stream in the extreme groundwater abstraction scenario. No apparent impact was found on the macroinvertebrate index s in any of the three subbasin outlets.
In the third part of my study (presented in manuscript 3), we analyzed and assessed the effects of predicted climate change towards the end of this century in two climate change scenarios of different greenhouse gas emission levels (RCP2.6 and RCP8.5) for all subbasin outlets classified into streams of three size classes, and we compared the results with the reference period (1996-2005). The overall streamflow and groundwater discharge in the catchment decreased slightly in the RCP2.6 scenario, while it increased in the RCP8.5 scenario. The differently sized streams underwent different alterations in flow regime and also demonstrated different biotic responses to climate change as represented by the fish and macrophyte indices. Large and some small streams suffered most from climate change, as the fish and macrophyte quality indices decreased up to 14.4% and 11.2%, respectively, whereas these indices increased by up to 14.4% and 6.0% respectively, in medium and some small streams. The climate change effects were larger in the RCP8.5 scenario than in the RCP2.6 scenario, as expected.
In conclusion, the further developed SWAT-MODFLOW model calibrated by PEST provided a better hydrological simulation performance and much more realistic signals relative to the semi-distributed SWAT model when assessing the impacts of groundwater abstractions for either irrigation or drinking water on streamflow; hence, it has great potential to be a useful tool in water resources management in groundwater-dominated catchments. The novel approach of combining SWAT-MODFLOW and flow-regime biota models is a useful tool to quantitatively assess the effects of groundwater abstractions on stream biota and thereby support water planning and regulations related to groundwater abstractions. To the best of my knowledge, the third part of my study is the first to quantitatively assess the impacts of streamflow alterations induced by climate change on stream biota beyond specific species, which would assist in water planning and regulations in the response to the challenges posed by climate change.
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