Introduction

During the past several decades, anthropogenic impacts on climate have received tremendous attention in the climate-research community and in the public domain (Solomon et al. 2007). Human activities that influence the local and global climate can be divided into two main categories: changes in the atmospheric composition, including greenhouse gases and aerosols, and changes at the surface caused by urbanization, agriculture and irrigation, and deforestation (e.g., Pielke and Avissar 1990; Kanamaru and Kanamitsu 2008). Agricultural activities, including water withdrawal and irrigation, can significantly affect the energy and water exchanges between the land surface and the atmosphere by altering the flow regimes of both surface water and groundwater (Shah 2014; Zeng et al. 2016). These effects can enhance the local latent heat flux and thus affect the atmospheric circulation at local and global scales (Yu et al. 2014; Zou et al. 2014; Zeng et al. 2017). The exploitation and withdrawal of groundwater can lead to changes in the lateral flow and transport of groundwater from surrounding areas to local groundwater depressions, thus offsetting the loss of locally stored water (Xie et al. 2012; Fan 2015), and influencing latent heat flux and transpiration partitioning through groundwater redistribution (Maxwell and Condon 2016) that may affect the climate at local and larger scales (Maxwell et al. 2007; Maxwell and Kollet 2008). Anthropogenic nitrogen discharge from fertilizer applications, fossil fuel consumption, and crop production (Galloway et al. 2004), can be transferred and transported in soil and rivers, which may affect the development of biocenoses and ecosystem connections between land and oceans (Jickells 1998). It has also been shown that the discharge and transport of nitrogen can be regulated by human intervention (Maavara et al. 2015; Woli, Hoogenboom and Alva 2016; Van Cappellen and Maavara 2016; Liu et al. 2019). Denitrification in river and stream sediments is becoming increasingly recognized as an important source of N2O to the atmosphere (Beaulieu et al. 2011; Ivens et al. 2011). Activities that accompany urbanization, such as anthropogenic heat release (AHR) and urban water usage (UWU), along with the impact of urban spatial structures on the boundary layer, affect the local weather and climate (Sailor et al. 2004; Sailor 2011; Hendel et al. 2015; Hendel et al. 2016; Ketterer et al. 2017). The movement of freeze thaw fronts (FTFs), on the other hand, is an indicator of the climate status, which in turn affects the climate; this freeze and thaw process alters both the carbon-nitrogen cycle and the energy and water exchanges between the land surface and the atmosphere by affecting the soil’s hydrothermal characteristics, especially in frozen-soil regions (Zhang 2005; Iwata et al. 2010). These processes are closely impacted by the climate, and in turn affect the climate. Understanding these complex processes, and reasonably representing them in land-surface and global climate models, is very important for providing insights into weather and climate impacts of societally relevant quantities, such as water availability, environmental protection, and other ecosystem services (Wood et al. 2011; Tian et al. 2016; Xie et al. 2016; Bonan and Doney 2018).
Groundwater affects convection, advection, and precipitation by affecting the water-heat flux between the land and atmosphere (Chen and Hu 2004a; Haddeland et al. 2006; Zeng et al. 2016b). A number of studies in recent years have discussed incorporating groundwater lateral flow (GLF) into models (Xie et al. 2012; Fan et al. 2013; Maxwell and Condon 2016; Xie et al. 2018). Zeng et al. (2016) incorporated GLF into a LSM to investigate the effects of stream water conveyance over riparian banks on ecological and hydrological processes. The schemes were also modified to study GLF on global scales (Zeng et al. 2018). As GLF dynamics is often linked with human water use (Zeng et al. 2016; Xie et al. 2017) incorporated schemes describing GLF and human water use to investigate their effect on simulated land-surface processes. Zeng et al. (2017) also incorporated a human water-use (HWR) scheme to investigate the impact of groundwater exploitation on the global climate. Liu et al. (2020), on the other hand, incorporated schemes of anthropogenic heat release, and urban water use, as well as an urban-height scheme that considered height variations in the urban land model to study the effects of these processes on the urban climate. For nitrogen-related processes, Liu et al. (2019) included schemes of riverine dissolved inorganic nitrogen (DIN) transport and human activities, including nitrogen discharging and human water use, into a LSM coupled with a river transport model (RTM). In terms of the soil freeze and thaw processes, the Stefan method was recently incorporated in multilayered systems to simulate FTFs that were subsequently incorporated into a LSM for global simulations by Gao et al. (2016, 2018, 2019). Despite the abovementioned efforts; however, current LSMs do not synchronously describe all of the aforementioned processes, which makes it difficult to fully quantify the degree to which human activities affect the eco-hydrological system and global climate.
To better understand the responses of the Earth system to external forcing changes, and promote climate system model development, the Coupling Model Working Group of the World Climate Research Programme proposed the Coupled Model Intercomparison Project (CMIP). Among the participating models in the most recent phase of this project, CMIP6, CAS-FGOALS-g3 was developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences (LASG/IAP), which is considered to be a crucial modeling tool for climate science research (Li et al., 2020). CAS-FGOALS-g3 participates in CMIP6 by providing historical, Diagnostic, Evaluation and Characterization of Klima (DECK) simulations and other CMIP-Endorsed Model Intercomparison Projects (such as Scenarios, Paleo, Land, etc.), and it can serve as a platform as for the evaluation of climate response to anthropogenic forcing.
In this study, we implemented CAS-LSM, which described the aforementioned processes, into the climate system model CAS-FGOALS-g3 to study the impact of these processes on the global hydroclimate. This implementation of CAS-LSM expanded the range of studies with CAS-FGOALS-g3, which allowed us to further assess the impact of the interaction between anthropogenic activities and the eco-hydrologic system, and hence weather and climate.
The remainder of this paper is organized as follows. Section 2 describes the model development, and Section 3 provides the data and experiment design. Section 4 describes the validation of land surface climate simulations using the new CAS-FGOALS-g3 with a focus on variables such as surface air temperature, precipitation, soil water storage, evapotranspiration, river discharge, and the surface albedo, as well as an assessment of snow-albedo feedback. Section 5 describes the new features of CAS-FGOALS-g3 with the implementation of CAS-LSM, and Section 6 gives a discussion and summary.