**Coupled Terrestrial Carbon and Water Dynamics in Terrestrial Ecosystems: Contributions of Remote Sensing**

Baozhang Chen

*Institute of Geographic Sciences and Nature Resources Research, Chinese Academy of Sciences, Beijing P.R. China* 

#### **1. Introduction**

The Earth climate is a complex, interactive system, determined by a number of complex connected physical, chemical and biological processes occurring in the atmosphere, land and ocean. The terrestrial biosphere plays many pivotal roles in the coupled Earth system providing both positive and negative feedbacks to climate change (Treut et al., 2007). Terrestrial vegetation via photosynthesis converts solar energy into carbon that would otherwise reside in the atmosphere as a greenhouse gas, thereby regulating climate. Vegetation also transfers water between belowground reservoirs and the atmosphere to maintain precipitation and surface water flows.The terrestrial carbon (C) cycle is closely linked to hydrological and nutrient controls on vegetation (Betts et al., 2000; Cox et al., 2000). Understanding the coupled terrestrial C and water cycle is required to gain a comprehensive understanding of the role that terrestrial ecosystems play in the global climate change. Much progress has been made in gaining insight of the coupling processes between C and water cycles across a range of time and spatial scales (Pielke Sr, 2001; Friedlingstein et al., 2003; Seneviratne et al., 2006; Betts et al, 2007a,b; Baldocchi, 2008). Since the early 1990s, there has been an increased interest in monitoring of the CO2, water vapor and energy exchange between the atmosphere and terrestrial ecosystems by a variety of methods, such as the eddy-covariance techniques (EC), satellite and other airborne remote sensing, CO2 concentration and isotope measurements. Meanwhile, there are various kinds of models have been developed to better understanding of these processes and for largescale C and water budgeting.

Remote sensing (RS) from satellite and airborne platforms, along with many other sources of land ground-based measurements (e.g., eddy covariance flux tower network, biometric plots, radar network, etc.) is playing and will continue to play a vital role in better understanding the coupled C and water cycle. Satellite RS allows the study of ecosystems from a completely new vantage point, facilitating a holistic perspective like viewing the Earth does for astronauts. Satellite-borne RS offers unique opportunities to parameterize land surface characteristics over large spatial extents at variable spatial and temporal resolutions. While there are challenges relating RS data recorded in radiance or backscatter

Coupled Terrestrial Carbon and Water Dynamics in

coupled cycles are still far from well understood.

interactions (Treut et al., 2007).

uncertainties and interannual variability.

**2.2 Terrestrial C cycling** 

**2. Scientific background and state of knowledge 2.1 Overview of terrestrial ecosystems and climate** 

Terrestrial Ecosystems: Contributions of Remote Sensing 225

The climate system is controlled by a number of complex coupled physical, chemical and biological processes (Figure 1). The terrestrial biosphere plays a crucial role in the climate system, providing both positive and negative feedbacks to climate change through biogeophysical and biogeochemical processes (Treut et al., 2007). Couplings between the climate system and biogeochemistry are mainly through tightly linked dynamics of C and water cycles. The importance of coupled C and water dynamics for the climate system has been increasingly recognized (Cox et al., 2000; Pielke Sr, 2001; Friedlingstein et al., 2003; Seneviratne et al., 2006; Betts et al, 2000, 2007a,b); however the mechanisms behind these

Fig. 1. Schematic view of the components of the climate system, their processes and

One of the crucial issues in the prognosis of future climate change is the global budget of atmospheric CO2. The growth rate of atmospheric CO2 is increasing rapidly. Three processes contribute to this rapid increase: fossil fuel emission, land use change (deforestation), and ocean and terrestrial uptake. As shown in Figure 2, terrestrial C budgets have large

Terrestrial ecosystems mediate a large part of CO2 flux between the Earth's surface and the atmosphere, with ~120 Pg C yr-1 taken up by photosynthesis and roughly the same amount

to variables of interest, and RS has poor temporal resolution compared to ground-based measurement devices, RS and spatial analytical techniques and distributed biogeochemical modeling embedded in Geographical Information Systems (GIS) have allowed us to better understand the coupled C and hydrological dynamics across a large range of temporal and spatial scales.

The large number of papers published since the 1980s on the terrestrial and C/water cycles have resulted in the publication of several major reviews from different perspectives. For example, Running *et al.* (2004) described a blueprint for more comprehensive coordination of the various flux measurement and modeling activities into a global terrestrial monitoring network by reviewing the literature published before the middle of 1990s. Baldocchi (2008) recently provided a comprehensive review of research results associated with a global network of C flux measurements systems. The topics discussed by this review include history of the network, errors and issues related with the EC method, and a synopsis of how these data are being used by ecosystem and climate modellers and the remote-sensing community (Baldocchi, 2008). Kalma *et al.* (2008) reviewed satellite-based algorithms for estimating evepotranspiration (ET) and land surface temperatures at local, regional and continental scales, with particular emphasis on studies published since the early 1990s; while Verstraeten *et al.* (2008) provided a comprehensive review of remote sensing methods for assessing ET and soil moisture content across different scales based on the literature published after 1990s. Marquis and Tans (2008) reviewed satellite-based instruments on CO2 concentration measurements.

In this chapter, I distil and synthesise the rapidly growing literature on C and water cycles using remote sensing in direct or indirect ways across local to global spatial scales and over a range of time scales. To give the reader a perspective of the growth of this literature, a search of Web of Science produced over 1500 papers with the key words 'ecosystem carbon, water cycles and remote sensing' published since 1990 which is indicative of the large amount of research recently being undertaken on these topics. In order to filter through this large body of literature, I concentrate on papers discussing on the coupling processes between C and water and I extract information from a database of published results that I have collated during the past decade (available on request). In terms of content, the report covers the state of knowledge, monitoring and modeling of the coupled terrestrial C and water cycles. My aim is to highlight the recent advances in this field, and propose areas of future research based on perceived current gaps in the literature.

This is a synthesis of state-of-the-art research on how RS has informed the study of coupled C and hydrology cycles. The review is divided into several inter-connected sections. First, I review the scientific background of the linkage between terrestrial ecosystems and climate, and revise the state of knowledge on terrestrial C cycling, coupling of the C and water cycles. Second, I discuss the ground-based and satellite-based monitoring methods and observation networks associated with measuring C and water fluxes, CO2 concentration and C isotopes. Third, I report on the recent advances in modeling approaches associated with the terrestrial biochemical and hydrological studies. Fourth, I discuss research gaps in C sinks/sources estimates and finally, I discuss the current research trends and the near-future directions in this field and propose an upscaling framework for landscape and regional C and water fluxes estimates.

to variables of interest, and RS has poor temporal resolution compared to ground-based measurement devices, RS and spatial analytical techniques and distributed biogeochemical modeling embedded in Geographical Information Systems (GIS) have allowed us to better understand the coupled C and hydrological dynamics across a large range of temporal and

The large number of papers published since the 1980s on the terrestrial and C/water cycles have resulted in the publication of several major reviews from different perspectives. For example, Running *et al.* (2004) described a blueprint for more comprehensive coordination of the various flux measurement and modeling activities into a global terrestrial monitoring network by reviewing the literature published before the middle of 1990s. Baldocchi (2008) recently provided a comprehensive review of research results associated with a global network of C flux measurements systems. The topics discussed by this review include history of the network, errors and issues related with the EC method, and a synopsis of how these data are being used by ecosystem and climate modellers and the remote-sensing community (Baldocchi, 2008). Kalma *et al.* (2008) reviewed satellite-based algorithms for estimating evepotranspiration (ET) and land surface temperatures at local, regional and continental scales, with particular emphasis on studies published since the early 1990s; while Verstraeten *et al.* (2008) provided a comprehensive review of remote sensing methods for assessing ET and soil moisture content across different scales based on the literature published after 1990s. Marquis and Tans (2008) reviewed satellite-based instruments on CO2

In this chapter, I distil and synthesise the rapidly growing literature on C and water cycles using remote sensing in direct or indirect ways across local to global spatial scales and over a range of time scales. To give the reader a perspective of the growth of this literature, a search of Web of Science produced over 1500 papers with the key words 'ecosystem carbon, water cycles and remote sensing' published since 1990 which is indicative of the large amount of research recently being undertaken on these topics. In order to filter through this large body of literature, I concentrate on papers discussing on the coupling processes between C and water and I extract information from a database of published results that I have collated during the past decade (available on request). In terms of content, the report covers the state of knowledge, monitoring and modeling of the coupled terrestrial C and water cycles. My aim is to highlight the recent advances in this field, and propose areas of

This is a synthesis of state-of-the-art research on how RS has informed the study of coupled C and hydrology cycles. The review is divided into several inter-connected sections. First, I review the scientific background of the linkage between terrestrial ecosystems and climate, and revise the state of knowledge on terrestrial C cycling, coupling of the C and water cycles. Second, I discuss the ground-based and satellite-based monitoring methods and observation networks associated with measuring C and water fluxes, CO2 concentration and C isotopes. Third, I report on the recent advances in modeling approaches associated with the terrestrial biochemical and hydrological studies. Fourth, I discuss research gaps in C sinks/sources estimates and finally, I discuss the current research trends and the near-future directions in this field and propose an upscaling framework for landscape and regional C

future research based on perceived current gaps in the literature.

spatial scales.

concentration measurements.

and water fluxes estimates.
