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.