How Do Gaining and Losing Streams React to the Combined Effects of Climate Change and Pumping in the Gharehsoo River Basin, Iran?

Projections of potential impacts of climate change and groundwater abstraction on gaining and losing streams, particularly in ephemeral river basins exhibiting sporadic and intricate flux exchanges, have remained largely unexplored. To fill this gap, we propose a promising modeling scheme based on the new fully integrated hydrological model SWAT‐MODFLOW‐NWT, calibrated and validated for 1978–2012, to quantify the intertwined surface‐groundwater interactions under a conjuncture of three climatic emission scenarios (RCP 2.6, 4.5 and 8.5) and two groundwater pumping variants: “pumping” (extending current groundwater utilization into the future) and “nonpumping” (assuming a complete cease of pumping in the future). By forcing the integrated model with future downscaled climate predictors of CanESM2 under the aforementioned RCPs for three time slices up to year 2100, projections of various water resources components for the Gharehsoo River Basin (GRB), in northwestern Iran were made. Results demonstrate that because of a general decrease of future precipitation, though with ups and downs across the total projection period, most of the surface and ‐subsurface budget quantities and fluxes are substantially affected. In particular, future groundwater discharge (baseflow) to the gaining streams will be more influenced by the “pumping” variant (increasing and decreasing for “nonpumping” and “pumping”, respectively) than the concentrated groundwater recharge from the losing streams (decreasing and increasing for “nonpumping” and “pumping”, respectively). Future water yield and groundwater storage will also diminish and, surprisingly, this cannot be alleviated by future “nonpumping”, indicating the groundwater overutilization is the compelling reason for the future water scarcity in the GRB, rather than climate change alone.

Plain Language Summary Understanding climate change impacts on surface‐groundwater interactions, accommodating nearly all of water resources components of a watershed, is of paramount importance in devising effective strategies to alleviate the adverse impacts of climate change. This is particularly important in semiarid regions where the surface water scarcity has markedly increased the dependency on groundwater resources which, in turn, has highly affected the interaction between these two. We have provided an innovative scheme to assess and project climate change impacts on water resources of a basin and, in particularly, on gaining and losing streams at different temporal and spatial scales. In addition, the net impacts of climate change impacts on the quantity of the water resources, compared with the groundwater overutilization, can be quantified using the proposed methodological approach. Our findings importantly highlight that the groundwater overutilization is the major reason, rather than climate change impacts, for extreme groundwater storage depletion in the Gharehsoo River Basin, in northwestern Iran. As a result, groundwater discharge to the river network will diminish tremendously under the changing climate projected for the region.