**1. Introduction**

Distinguishing the contributors of different streamflow components is vital to the effective management of catchment water resources. Baseflow is the contribution of delayed pathways to stream discharge that maintains streamflow during drought periods, characterized by low precipitation, the dominance of groundwater discharge and/or snow

**Citation:** Zhang, J.; Zhao, P.; Zhang, Y.; Cheng, L.; Song, J.; Fu, G.; Wang, Y.; Liu, Q.; Lyu, S.; Qi, S.; et al. Long-Term Baseflow Responses to Projected Climate Change in the Weihe River Basin, Loess Plateau, China. *Remote Sens.* **2022**, *14*, 5097. https://doi.org/10.3390/rs14205097

Academic Editors: Luis Garrote and Alban Kuriqi

Received: 16 September 2022 Accepted: 8 October 2022 Published: 12 October 2022

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meltwater from upstream regions [1–3]. Baseflow influences the water quality/supply and the health of the catchment ecosystem in regional development [4]. It has a profound influence on the hydrologic cycle in prolonged dry periods [5–7]. It is essential for the provision of water resources and water security that can be influenced by climate conditions [8–10]. Therefore, estimating projected baseflow drought is critical to escalating our understanding of hydrological processes in the changing climate.

Climate variability is the primary factor influencing the terrestrial hydrologic cycle (e.g., baseflow) at regional and global scales [11–14]. For example, baseflow has a close link with the redistribution of precipitation due to infiltration providing a vital contribution to groundwater flow [15], which is characterized by a close interaction between groundwater and surface water. Trancoso, et al. [2] showed that reduced precipitation diminished baseflow and precipitation and positively affected baseflow in eastern Australia. However, the land-surface air temperature has increased over the past three decades and led to an energized/accelerated hydrological cycle by influencing precipitation amounts [16,17] and by capturing longwave radiation [13]. Li, et al. [18] used an analytical approach that integrated water balance and the Budyko hypothesis (evaporative index (ET/P) and aridity index (PET/P) were used to describe the long-term water and energy balance [19,20]) to separate the contributions of climate and anthropogenic effects on streamflow. Li, et al. [21] investigated the response of baseflow to climate variability in a large forested catchment and found that the contribution of climate variability to annual baseflow were greater than the impacts from forest disturbance. Trancoso, et al. [2] predicted a decreasing baseflow trend under certain climate changes (e.g., decreasing precipitation and increasing evapotranspiration related to CO2–vegetation feedbacks). Ficklin, et al. [16] assessed the impacts of climate change on baseflow and stormflow and found that baseflow had consistent trends with stormflow across the northeastern and southwestern United States. Additionally, Singh, et al. [4] quantified the response of baseflow levels to climate variability cycles (e.g., the Pacific Decadal Oscillation) in the Flint River.

Hydrological models are often used to estimate the effects of climatic factors on water yield. Climate projections have predicted that the frequency and intensity of extreme events (e.g., droughts and floods) will increase under future climate conditions [22]. However, the direct consequences of baseflow responses to future climate change are poorly understood. Therefore, assessing baseflow responses under climate change is imperative to facilitate the understanding of groundwater-related hydrological processes and provides scientific guidelines for water adaptation measures [23] in water-limited regions to face future droughts.

Generally, this approach uses alternative emission scenarios to investigate hydrological responses to climate change [24,25]. For instance, Yang, et al. [26] used 16 climate models from CMIP5 (the fifth phase of the Coupled Model Intercomparison Project) to assess the responses of hydrologic drought/aridity to climate change. They demonstrated that climate models did not capture vegetation water use under elevated CO2 conditions. Semi-distributed rainfall-runoff models based on SWAT (Soil & Water Assessment Tool, https://swat.tamu.edu/, accessed on 9 July 2016) have been widely used to evaluate streamflow variations in complex catchments [27]. Zhang, et al. [28] compared the performances of two distributed hydrological models (e.g., SWAT and the Distributed Hydrology Soil Vegetation Model) in separating the impacts of climate change and LUCC (land-use cover change) on catchment hydrology. Lauffenburger, et al. [29] evaluated the effects of agricultural irrigation and future climate change on groundwater recharge in the northern High Plains aquifer, USA, and found a significant bidirectional shift, leading to a reduction in future groundwater recharge. While those efforts improved our understanding of climate-variability effects on hydrological processes, baseflow responses to future climate change are poorly understood for semi-arid catchments in loess deposition regions, in which baseflow provides a significant water source for ecological restoration and environmental protection.

The Weihe River Basin (WRB) is a representative catchment on the Loess Plateau. It is one of the most important water sources for the environment and regional society of northwest China. In this study, to attenuate the uncertainties of baseflow estimation (e.g., signal and magnitude [30]), historical daily streamflow data and future streamflow data projected by two climate scenarios from three presentative GCMs were used to assess temporal variations in baseflow and the dynamics of baseflow characteristics under future climate changes in the WRB. The specific objectives were (1) projecting baseflow under two scenarios (RCP4.5 representing a lower emissions scenario, and RCP8.5 representing a higher emission scenario) from three GCMs (CSIRO-Mk3-6-0, MIROC5, and FGOALSg2); (2) assessing baseflow responses under future climate conditions; and (3) highlighting the role of baseflow in drought events at the catchment scale. Thus, this study provides drought assessment for water-resource managers to face the future changing climate.
