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INTRODUCTION
Mountain regions are a major source of surface water and groundwater recharge in the world (Viviroli et al. 2007; Dettinger 2014). Watersheds in mountainous regions provide between 40 and 80% of the water resources available to lowland settlements (Messerli et al. 2004), therefore water resources recharge is the focus of environmental benefits in headwater catchments (Křeček et al. 2017; Ostad-Ali-Askari & Shayannejad 2021). The water balance in mountainous regions is defined by the interactions between the climate, cryospheric, and hydrological systems (De Jong et al. 2005; DeWalle & Rango 2008). The semi-humid Taihang Mountain region in the North of China is the most important groundwater recharge area of the North China Plain (Sakakibara et al. 2017). Besides, Taihang Mountain plays important roles in ecological service, water conservation and climate regulation for surrounding regions (Hu et al. 2019), especially for Xiongan New Area, ‘a strategy crucial for a millennium to come’ by the Chinese government (Song et al. 2018). One noticeable characteristic of Xiongan is that it is located in the region of slow flooding and stagnation of the Daqing River, which originates from the eastern Taihang mountain and its current flood recurrence period is only one in ten years, and it has suffered many flood disasters (Xia & Zhang 2017). Taihang Mountain is located in a region with large relief caused by limestone uplift (Wang & Li 2008) and is greatly affected by the Asian monsoon during the rainy season (Yamanaka et al. 2004; Liu et al. 2010). These factors generate topographical and meteorological heterogeneities (Sakakibara et al. 2017). Due to physical, chemical, biological weathering and other geological processes, the geologic formation of Taihang Mountain is comprised mainly of featured rock-masses of granitic gneiss origin. Strata with extensive weathered rock underlying thin surface soil layers are typical geologic features in this critical headwater mountain region (Cao et al. 2005). The mechanism of the water cycle and the characteristics of the water conversion in the Taihang Mountain is not yet clear (Song et al. 2007).
Previous studies have attempted to estimate the effective saturated permeability coefficient of an unsaturated soil using a constant head bore hole test (Sasidharan et al. 2019). For example, analytical solutions have been developed to determine the hydraulic properties of...