Dr. Yijian Zeng is Assistant Professor at the Faculty of Geo-Information Science and Earth Observation (ITC), the Department of Water Resources (WRS) of the University of Twente in Enschede, the Netherlands.
He serves as a panel member for GLASS (Global Land/Atmosphere System Study Panel) of GEWEX (The Global Energy and Water Exchanges) Project. GEWEX is part of the World Climate Research Programme (WCRP), and is dedicated to understanding Earth’s water cycle and energy fluxes at, and below the surface and in the atmosphere. The GLASS panel focuses on model development and evaluation, concentrating on the new generation of land surface models.
Dr. Zeng also serves as an Executive Board Member of ISMC (International Soil Modeling Consortium). The ISMC aims to integrate and advance soil systems modeling, data gathering, and observational capabilities, addressing key global issues and stimulate the development of transdisciplinary and translational research activities.
He is currently the co-lead of GEWEX-SoilWat Initiative, a joint project between the GEWEX and ISMC, which brings together two research communities to improve the representation of soil and subsurface processes in climate models, identifying the most pressing challenges and topics related to this effort.
Dr. Zeng received his PhD (Cum Laude) in vadose zone hydrology in 2012 from the University of Twente, Netherlands, in cooperation with the China University of Geosciences (Beijing) and Cold and Arid Regions Environmental and Engineering Research Institute (CAREERI, now as NIEER, North Institute of Eco-Environment and Resources), Chinese Academy of Sciences (CAS).
His PhD focused on “Coupled dynamics in soil : understanding the transport mechanism of liquid water, water vapor, dry air and heat by field experiments and numerical simulation”. Dr. Zeng’s PhD work has been awarded:
Dr. Yijian Zeng specializes at the interface of soil-water-plant-energy interactions and Earth observations, focusing on:
- Physically-based modeling of soil and subsurface processes;
- Understanding water, energy and carbon fluxes at leaf, plant and ecosystem levels;
- Coupled surface-groundwater interactions;
- Soil-water-plant-energy interactions in cold regions
- Forward observation simulator for microwave observations and Data Assimilation;
- Climate Data Records (CDRs) for climate services;
- Multi-scale and -sensor Earth Observations (EO) of the ecohydrological dynamics
(1) Physically-based modelling of soil and subsurface processes
This research topic focuses on understanding the coupling mechanism between liquid water, water vapor, dry air and heat transport in the soil, using field observations and numerical simulations. This study starts from a sand bunker experiment, illustrating the sub-diurnal pattern of thermal/isothermal soil moisture flux profiles [Zeng et al. 2009a Environ. Geol]. With the knowledge of moisture fluxes, it was able to identify the effective precipitation over the Badain Jaran desert. The effective precipitation is a certain portion of infiltrated water that will be remained in the soil, without being evaporated back to atmosphere, and eventually contribute to the water balance of the desert lakes over Badain Jaran desert [Zeng et al. 2009b HESS].
With the above studies, it was demonstrated that a single-phase transport mechanism cannot explain the model-observation mismatch of the vapor fluxes in the arid and semi-arid areas. To overcome this discrepancy, a two-phase heat and mass transport model (STEMMUS – Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil) was developed [Zeng and Su 2013b WRR]. It shows that the STEMMUS outperforms the traditional theory in estimating surface evaporation over arid and semi-arid regions [Zeng et al. 2011a WRR; Zeng et al. 2011b JGR; Zeng and Su 2013a WRR; Yu et al. 2016 HESS; Yu et al. 2018 JGR; Yu et al. 2020 HESS].
Figure 1. Diagram of STEMMUS Physics (http://blogs.itc.nl/stemmus/)
(2) Understanding water, energy and carbon fluxes at leaf, plant and ecosystem levels
There are currently two state-of-the-art approaches for monitoring and understanding water, energy and carbon fluxes of soil-plant-atmosphere continuum (SPAC): (1) remote sensing of the plant-water relation proxy, solar-induced chlorophyll fluorescence (SIF), and (2) terrestrial biosphere models incorporating plant xylem hydraulics along the SPAC. The SIF remote sensing can acquire explicit information about photosynthetic light responses and steady-state behaviors in vegetation to evaluate photosynthesis and water-stress effects, across a range of biological, spatial and temporal scales. The plant-hydraulics-based SPAC model links mechanistically tissue-level stress to ecosystem-level water and carbon fluxes, via a resistor-based manner with the tissue-level hydraulic traits (of roots, stems and leaves) and stomatal optimality theory (i.e., photosynthetic gain vs. hydraulic risk).
Although the abovementioned two approaches, individually, have been substantially advanced in the last several decades, the hand-shaking between the two has only been acknowledged important lately. This research topic will integrate the SIF remote sensing with the plant-hydraulics-based SPAC model to advance our mechanism understanding of the complex soil-water-plant-energy interaction, via coupling STEMMUS model with SCOPE (Soil-Canopy Observation of Photosynthesis and Energy) model.
Figure 2 The coupling scheme of STEMMUS-SCOPE (Wang et al. 2020 GMDD)
(3) Coupled surface-groundwater interactions
Land-surface processes (e.g. water, energy, and carbon fluxes) are moderated by water table elevations, via its influences on soil moisture availability and surface thermal states (especially over cold regions with seasonal frozen ground and permafrost, and with shallow groundwater table). For example, over Tibetan Plateau (TP), the rise in mean annual ground surface temperature has triggered permafrost degradation and associated profound hydrological and eco-environmental changes. Some studies report the intensification of the water cycle over the TP, with increased precipitation and surface runoff. However, in other studies, the time-series of river discharge throughout the source area of Yellow River over TP until 2010 suggested a steady decline of mean average discharge.
Many of these studies rely on statistical analysis between climate variables and a limited number of field observations. A process-based assessment of ongoing permafrost-degradation-induced surface-groundwater interactions remains challenging. STEMMUS is currently being coupled with a groundwater model (via a PhD project) considering freezing and thawing to investigate the surface-groundwater interactions over TP. Furthermore, to serve this research aim, we are currently undertaking the geophysical survey over the Maqu catchment (500 km2) located at Eastern TP.
Figure 3. 3D Hydro-geophysical characterization of the Maqu catchment located at the Eastern Tibetan Plateau (Li et al, 2020 ESSDD)
(4) Soil-water-plant-energy interaction in cold regions
With the backdrop of climate warming, Tibetan Plateau is experiencing the permafrost thawing with the increased active layer thickness, and the reduced maximum frost depth of seasonal frozen ground. The consequence of such changes, in organic-rich soils over Tibetan Plateau, is the enhanced greenhouse gas release (e.g. CO2, CH4) and the accelerated decomposition of soil organic matter (permafrost-carbon-feedback, PCF).
Recent reviews emphasize the need to address our incomplete understanding of the coupling mechanism between hydrological process and permafrost dynamics, and the associated carbon response through the atmosphere or hydrosphere. To this purpose, STEMMUS is currently being coupled with an ecohydrological model (TeC model, by Simone Fatichi). Together with the integrated surface-groundwater interactions, we will be able to improve our understanding of soil-water-energy-plant interactions in a cold region like Tibetan Plateau.
Figure 4 The soil-water-energy-plant interaction in cold regions (Yu, et al. 2020 TCD)
(5) Forward observation simulator & Data assimilation
When we want to estimate states and fluxes of the Earth system (e.g., water, energy, and carbon cycles), at any arbitrary past, present, and future time, we always encountered two complementary, but both incomplete and inaccurate, sources of information: the observations and the model. Data Assimilation (DA) provides the tool to tackle the problem by extracting synergies between model and observations, and by exploiting their respective information content. Here the observation is referred as Earth Observations, including satellite-, airborne-, drone-, and in-situ-based measurements. To optimize the synergy between the model and observations, it is necessary to transform physical-based process modelling into observables, which needs a forward observation simulator.
Coupling the STEMMUS (& its coupled models) with CMEM (Community Microwave Emission Model) and TorVergata model (an emission-scattering model) can form such a forward observation simulator for microwave observations (i.e., forward simulation of brightness temperature - TB). The developed forward observation simulator will be integrated into a consistent DA framework to assimilate EO data for sustainable water resources management. This will facilitate the development of an open source ‘Earth System Science Model’ to facilitate EO data integration for water management. Such DA framework can be applied generally to assimilate other observations, if with a dedicated observation simulator (e.g., cosmic ray neutron counts) (Mwangi et al., JGR, 2020; Zhao et al., JAMES, 2020).
Figure 5 The diagram illustrating the development of a forward observation simulator by coupling physically-based soil process model with an emission-scattering radiative transfer model.
(6) Climate Data Records (CDRs) for climate services
Climate services are becoming the backbone to translate climate knowledge, data & information into climate-informed decision-making at all levels, from public administrations to business operators. It is essential to assess the technical and scientific quality of the provided climate data and information products, including their value to users, to establish the relation of trust between providers of climate data and information and various downstream users. The climate data and information products (i.e., from satellite, in-situ and reanalysis) shall be fully traceable, adequately documented and uncertainty quantified and can provide sufficient guidance for users to address their specific needs and feedbacks. To achieve such aims, the quality assurance (QA) framework was developed to deliver timely assessments of the quality and usability of Essential Climate Variable (ECV) products. Such a QA framework will support a traceable climate service, in terms of understanding how the uncertainty propagates into the resulting benefit (utility) for the users of the climate service, other than rigorously evaluating the technical and scientific quality of ECV products that represent the upstream of climate services.