Determining Seasonal Variability in the Source and Age of Carbon transported by the Mekong River

  Southeast Asia provides interesting contrasts to the Amazon, with different geologies, weather patterns, and especially human pressure. The extraordinary pace of development and population growth   in the region has placed dramatically increasing pressure on river basins and their downstream coastal ecosystems. The impact on river systems occurs through erosion of the land surface, changes in the nature of the sediment and its associated organic matter, and nutrient content from agricultural and urban sources. Changes in hydrology are immediate consequences of dam construction and large-scale water diversion for irrigation. Longer-term changes in regional weather patterns and climate will result in altered flow regimes and thus impact downstream ecosystems including the coastal zone. Coastal ecosystem production relies very strongly on material inputs from the land. Deterioration of water quality, due to natural causes such as salt and acidity, and anthropogenic causes such as domestic, agriculture and industry, is problematic in most if not all countries in this region. These changes have major consequences for economic opportunities and hence are risks for investments.

     To extend and test our understanding of basin-scale dynamics derived from the Amazon in tropical basins where human impacts were greater, we initiated the SEA-BASINS project, as a joint endeavor of the UW (School of Oceanography and Department of Civil Engineering) and SEA START (Chulalongkorn University, Bangkok, responsible for networking multiple institutions for training and information throughout Southeast Asia , http://www.start.or.th). SEA/BASINS began with a series of information exchange and technical training workshops, starting in July of 1998. The First Partners Workshop, in Chiang Rai, November 1998, involved 40 scientists and engineers from across Southeast Asia, including universities, NGOs, government agencies, and UNESCO. Between workshops, development work continues at the UW and SEA START. Initial funding was  provided by START, the U.S. National Science Foundation, the NASA Earth Observing System program, Asia -Pacific Network (APN), START, UNOPS/GEF program, the World Resources Institute (WRI), and the Association Liaison Office (ALO) For University Cooperation in Development of AID. With a grant from NSF, we began more in-depth sampling and model development, to examine the magnitude and dynamics of CO₂ outgassing, as a test on our Amazon observations.

      The project we are currently finishing up is Connectivity of the Landscape of Southeast Asia with the South China Sea: Scaling of Hydrologic and Biogeochemical Processes (NASA IDS). The focus of this work is to examine regional-scale landscape dynamics in river basins of Southeast Asia, relative to their connectivity to the sea. By focusing on how transient forcing of the atmosphere combines with land-use change at multiple space and scales time- to impact the land surface and mobilize water and carbon to the sea, we are examining the critical and poorly understood interfaces between the atmosphere, land surface and sea function. Project elements include:

 Project Strategy Overview

      The objective of the current iresearch is to examine regional-scale landscape dynamics in river basins in Southeast Asia, relative to their connectivity to the South China Sea, with an emphasis on the Mekong River.  The geographic and geopolitical provenance of this project covers a diverse set of environments, being subject to rapid changes. Over the past year, it has become clear  that the Mekong River may be subject to rapid development of hydropower, where not only the upper "Chinese Cascade" of dams is in place, but that an additional 12 mainstem dams are under discussion, along with a projected 97 (!) on tributaries in Laos.  The cumulative effect of these dams would fundamentally alter the overall hydrologic and biogeochemical regime of the Mekong, (as well as fisheries and livelihoods) with consequences for the South China Sea. This project is in the position of both fundamental science challenges to understand what this might mean, as well as in a position of responsibility for not only analyzing the potential outcomes, but for making the results more broadly known.

    Our focus is on "high resolution but regional" work A basic premise is that the understanding of regional scale processes requires the higher resolution now possible with satellite data, process-based models, and field measurements, with the convergence of data and models from multiples sources. The work can be significantly extended, with the inclusion of information from specialized local and regional institutions. But such information is typically not readily accessible. A key regional player is the Mekong River Commission. Through a formal Letter of Agreement in place with the MRC, we are jointly developing a "Virtual Mekong Basin," to provide a portal to the world what the current and projected status is of the basin, under different conditions. This agreement opens the door further, to important data resources, as well as providing a portal for the use of NASA-information to broader communities.

      Our strategy has been to establish the overall climatology and hydrology of the broader region, then dialing down through to the biogeochemistry and ecosystem productivity of the Mekong and the Tonle Sap Lake, culminating in the hydrologic and chemical export to the sea. All of this has taken considerable time to assemble, with the not-unexpected hiccups along the way (especially for such a challenging geopolitical region). In the following sections we summarize thework on biogeochemistry.

Climatology/Hydrology

    Climatological forcing of the land surface (model) requires developing datasets that can be used to drive the models; preferably, with some understanding of the dynamics that produced those forcings. The question arises, how accurate at what resolutions are the data used to represent the forcings? Actual coupling can be passive (the provision of the necessary datasets) or active. To meet these requirements, we developed climate forcing data to drive the hydrology models. We collaborated with Dr. Ruby Leung, PNNL, to produce a data set from the Weather Research and Forecasting model (WRF), and then to use those results as part of a study to look at variations in precipitation across the region (Leung et al in prep). This collaboration also led to a data archive of model results at the UW, to be used in this and related studies, and it promoted the development of the direct coupling between WRF and VIC. Sonessa et al (in prep) used the WRF results in comparison to NCEP/NCAR reanalyses and ERA-Interim reanalysis to evaluate the water balance terms of the seven primary rivers of Southeast Asia. Beyenne et al (in prep) used the Mekong VIC forcings and climate projections from IPCC scenarios to examine the interaction of climate change and dams scenarios on the overall flow patterns of the Mekong, and changes in percent regulation.

Leung , R., et al. (in preparation). Diurnal and seasonal variations in precipitation and atmospheric heating in Asia based on observations and model simulations. In preparation. To be submitted to J. of Climatology. 

    Due to the influence of the large-scale circulation and its interactions with the complex terrain, precipitation over Asia varies at a wide range of temporal and spatial scales. The Weather Research and Forecasting model has been used to simulate the regional hydrological cycle in Asia at 18 km grid resolution using boundary conditions from a global analysis for 1997- 2008. Interior nudging was used to provide stronger constraints from the global reanalysis on the simulated large-scale circulation to better simulate seasonal and interannual variability associated with large-scale conditions. The diurnal precipitation variability in Asia has been analyzed using a combination of observations and the downscaled simulation to elucidate the dynamical and thermodynamical influence of orography and subsequent effects on the hydroclimate of the region. The TRMM precipitation data show distinct early morning rainfall timing at the foothills of the plateau and late afternoon/early evening timing on top of the plateau. Analysis of the simulation shows important influence of plateau-plain circulation and nocturnal drainage/upslope flow on the development of the boundary layer and diurnal rainfall. Comparison of the seasonal and interannual variability of rainfall among different remote sensing and ground based measurements and model simulation shows large differences and highlight the need to better constrain precipitation variability to yield improved estimates of the surface water budgets in the topographically diverse region.

WRF and VIC Data Transfer and Coupling

     Results from the WRF modeling were transferred from PNNL to the UW.  The domain of focus covered the project region, 9.09 - 33.9285N and 91.1573 - 108.843 E (roughly 9 - 34 N and 91 - 109 E), for the period 01/1998 to 12/2006. Results were also catalogued for a broader domain, for future analysis.  Data includes: P, Tmax, Tmin, Wind Speed (vertical and horizontal components) data from 01/1998 to 12/2008.  The data is of 18km resolution. In preparation for more seamless operation, and laying the groundwork for future applications, we coupled VIC with WRF through the CCSM4 coupler CPL7. In CCSM4, the communication process is separated from the component integration process. All communication processes are performed by Cpl7 and the components run by themselves. Coding work therefore was mainly focused on replacing CLM with VIC. VIC was extracted as it runs in an existing MM5-VIC coupling system for interaction with the flux coupler (because VIC in MM5 is in image mode, i.e., runs at all space for a given time step, as contrasted with point mode, which runs all time steps at a given grid node before proceeding to the next grid node). Based on the VIC version with MM5, we developed our own interface subroutine, which was suitable for the CCSM4 environment to connect coupler and VIC. The WRF-VIC coupling work is essentially completed, with work under other projects on-going to improve the performance.      

Sonessa, M.Y., J.E.  Richey, and D.P. Lettenmaier. (in prep). Evaluation of Water Balance Terms of the SEA with a Land Surface Model and ERA Interim Reanalysis

      The South East Asia (SEA) region includes seven major basins (Mekong, Irrawaddy, Salween, Chao Phraya, Hong (Red ) River, Sittang, Song Ma). Added to the impact of climate and land use changes, there are increasing number of dams being constructed or planned for the near future, especially on the main stream of Mekong and its tributaries. We evaluate the spatial and temporal variability of the water balance terms across SEA using the Variable Infiltration Capacity (VIC) land surface hydrology model, forced by gridded precipitation and temperature, from the ERA-Interim reanalysis and the WRF model.

      The first set of meteorological forcing data for the VIC model, were derived as precipitation (monthly P from the University of Delaware (UDel), adjusted for gauge under catch and for orographic effects, and then to daily from NCEP/NCAR reanalysis), and temperature (monthly CRU, then NCEP/NCAR to daily), The second set of meteorological forcing used as VIC input is based on simulation produced by the Weather Research and Forecasting (WRF) model, applied at 18 km horizontal resolution over Asia to resolve the complex terrain and its influence on meteorological conditions. Soil properties were obtained from the FAO Soil Program. The land use data for the model was obtained from MODIS 2003 (MOD12Q1 Land Cover Product - MODIS/Terra Land Cover 96 Day L3 Global 1 km ISIN Grid - IGBP land cover classification) tiles were acquired through the NASA ECHO website: http://www.echo.nasa.gov/index.html . The ERA interim archive constitutes basically the same land surface variables produced by VIC including surface fluxes of both water and energy, as well as atmospheric moisture flux and storage at multiple levels, which can be vertically integrated to produce a gridded atmospheric water balance. The ERA Interim data cover the period from January 1989 onwards.

      The P from all the three sources, VIC-NCEP, VIC-WRF and ERA-INT, showed similar seasonal pattern as well as comparable magnitudes. P peaked during the northern hemisphere summer season (JJA) contributing about half of mean annual P; while winter (DJF) is the lowest season contributing less than 6 % of mean annual P. As in the case of P, JJA contributes the highest percentage of mean annual ET for all the sources. Even though ET from all the sources showed similar seasonal pattern, the ERA-INT ET is the least variable seasonally with only 11% difference between the peak season (JJA) and the low season (DJF). For the other two sources, VIC-NCEP and VIC-WRF, the difference between the peak and low season in terms of contribution to annual mean ET is 35 %.RF, next to P, is the flux term where all the three sources matched each other both in terms of seasonal pattern as well as flux magnitude. The summer season (JJA) contributes the lion's share of the mean annual RF for all the sources, VIC-NCEP and ERA-INT (52%) and VIC-WRF (44%), while winter season (DJF) contributes the least, VIC-NCEP and ERA-INT (4%) and VIC-WRF (7%). VIC balances the surface water budget by construct; thus, non-closure term for VIC-NCEP and VIC-WRF is less than 1% of annual mean P. However, the non-closure term for ERA-INT is 7.1% of the annual mean P for the SEA region overall. For the seven basins individually, ERA-INT non-closure term ranges from 1.2% for Red basin to 9.8% for Mekong basin. The P and RF have similar spatial pattern with high values in the northwest SEA (Irrawaddy basin) basin and low values near the north and south ends of the SEA. ERA INT P and ET showed less spatial variability relative to those from the other sources. ET increases from north to south annually as well as when divided into four seasons. 

Beyene, T., D.P. Lettenmaier, and J.E Richey. (in prep).  Mekong river reservoir simulation for Current and Future Climate

     Climate change coupled with economic and social development specifically in recent development of construction and operation of dams in the Mekong River basin raises and poses a greater contention and pressure on the limited water resources of the region. In addition to the Chinese cascade of dams, multiple dams are now being proposed for the lower Mekong, as far as Cambodia. The net consequences are subject to extensive debate, but little quantitative analysis. This study investigates the projections of climate change induced hydrologic and water resources implications during the twenty-first century simulated by 20 coupled atmosphere-ocean general circulation models based on the Special Report on Emissions Scenarios A1B and B1. To reduce model bias and uncertainty, ensembles mean (EM) is used for multi-model projections. Although it is difficult to reproduce the present Mekong river discharge in any single model, the EM results produce more accurate representation of the basins river flow. The paper is mainly focused on the state of the art of simulating and optimizing the reservoirs in the upper  and lower Mekong both for retrospective (observed climate) and future climate (IPCC 2007 ) emissions to quantify and asses the combined effect of climate change and reservoir development mainly for hydropower production. The overall approach is to use the VIC macroscale land surface hydrologic model, forced with a regional gridded data set of precipitation, temperature, and wind data. A reservoir simulation and optimization model would be forced using simulated fluxes by VIC. The reservoir model is mainly optimized for maximum hydro power generation.

      Our multi-model median warming by 2070-2099 relative to 1978-2000 was 2.12 and 2.34 ^o C for A1B and B1 emissions scenario respectively. For the same periods, the models project median precipitation increases of ~ 0.4 and 0.41 m (25 and 27%) with corresponding median runoff changes of  0.25 and 0.27 m (50 and 57%), an increase of ~250,000 mcm, with largest runoff increases resulting mainly from an increase in wet season (May to October) precipitation in all catchments, for A1B and B1 respectively. We perform quantitative analysis of the signatures of the degree of regulation of six upper Mekong reservoirs and 7 lower Mekong reservoirs to downstream riparian countries in term of shifts in seasonal streamflow characteristics. A sharp decrease in wet-season streamflow and an increase in dry-season streamflow were predicted to occur following completion and operation of six dams in the Chinese headwaters and seven lower Mekong dams. Historical degree of regulation of 3% at Gongg and 16% at Jingh in the upper Mekong and  15 % in Pakbeng and 20 % in Latsua are predicted using simultaneously simulating and optimizing  all the dams for aggregated regulation effect in the rivers streamflow.


Biogeochemistry

      Rivers transport dissolved inorganic carbon (DIC) and organic carbon (OC) from land to the ocean, but they also chemically transform this carbon during its transit through in situ respiration and photosynthesis, and exchange CO₂ with the atmosphere through gas exchange. Most rivers are supersaturated with CO₂, and as such are a net source of carbon to the atmosphere on the order of 1 Pg C/year.  However, this value is poorly constrained and thought to be an underestimate. This CO₂ supersaturation in rivers is driven by two processes, advection of CO₂ produced by respiration in soil of their watersheds and/or hyporheic zones and the prevalence of net heterotrophic metabolism in rivers. Tropical rivers carry two-thirds of the global riverine organic carbon load, and many of them experience strong seasonal flooding, bringing large amounts of allochthonous carbon into the river. This high organic carbon load combined with high respiration rates in tropical climates creates the potential for high CO₂ outgassing rates in tropical rivers. 

      Few studies have investigated seasonal and interannual variations in the pCO₂ of large tropical rivers. We investigated the cumulative sequences of river metabolism organic carbon composition, and outgassing in the Mekong, as a large, tropical, carbonate-rich river influenced by a significant flood-drought cycle. Alin et al (2011) examined the physical controls on outgassing, with the all-important evaluation of the gas transfer velocity. An important parameter linking sediment transport to organic carbon is the changes in the composition of particulate organic matter (POM) over the course of the hydrograph. Ellis et al. (in review) examined the changes in the elemental and lignin compositions of POM, showing pronounced seasonal differences. Ellis et al. (in prep, near final) carried the compositional analysis further, to establish a baseline for terrestrial versus marine derived OM in coastal sediments, using a new proxy, the branched/isoprenoid tetraether (BIT) index. Finally, Ellis et al (draft) assessed the time scales over which terrestrial organic matter is exported from the Mekong watershed via fluvial processes, by looking at the ∆¹⁴C of the lignin phenols. Lockwood et al. (about to be submitted) established annual carbon budgets for the lower amazon, and the sequence of metabolic processes producing those budgets.

Alin, S. R., M. F. F. L. Rasera, C. I. Salimon, J. E. Richey, G. W. Holtgrieve, A. V. Krusche, and A. Snidvongs (2011). Physical controls on carbon dioxide transfer velocity and flux in low‐gradient river systems and implications for regional carbon budgets, J. Geophys. Res., 116, G01009, doi:10.1029/2010JG001398

      Outgassing of carbon dioxide (CO₂) from rivers and streams to the atmosphere is a major loss term in the coupled terrestrial‐aquatic carbon cycle of major low‐gradient river systems (the term "river system" encompasses the rivers and streams of all sizes that compose the drainage network in a river basin). However, the magnitude and controls on this important carbon flux are not well quantified. We measured carbon dioxide flux rates (FCO₂), gas transfer velocity (k), and partial pressures (pCO₂) in rivers and streams of the Amazon and Mekong river systems in South America and Southeast Asia, respectively. FCO₂ and k values were significantly higher in small rivers and streams (channels <100 m wide) than in large rivers (channels >100 m wide). Small rivers and streams also had substantially higher variability in k values than large rivers. Observed FCO₂ and k values suggest that previous estimates of basinwide CO₂ evasion from tropical rivers and wetlands have been conservative and are likely to be revised upward substantially in the future. Data from the present study combined with data compiled from the literature collectively suggest that the physical control of gas exchange velocities and fluxes in low gradient river systems makes a transition from the dominance of wind control at the largest spatial scales (in estuaries and river mainstems) toward increasing importance of water current velocity and depth at progressively smaller channel dimensions upstream. These results highlight the importance of incorporating scale‐appropriate k values into basinwide models of whole ecosystem carbon balance.


Ellis, E.E., R.G. Keil, A.I. Ingalls, and J.E.Richey. (in review). Seasonal variability in the sources of  particulate organic matter of the Mekong River as discerned by elemental and lignin analyses.  J. Geophys. Res. Biogeosciences (2011JG001816).

      The Mekong River ranks within the top ten rivers of the world in terms of water discharge and sediment load to the ocean, yet its organic matter (OM) composition remains unstudied. This river is experiencing anthropogenically-forced changes due to land use and impoundment, and these changes are expected to intensify in the future. Accordingly, we monitored bulk OM composition and vascular-plant signatures (using lignin phenols) of Mekong River particulate OM (POM) over a one-year period. Autochthonous production comprises a greater proportion of POM during the dry season than in the rainy season, as demonstrated by higher percent organic carbon values (7.9 ± 2.4 vs. 2.2 ± 0.4%), lower yields of lignin normalized to carbon (0.40 ± 0.05 vs. 1.1 ± 0.3 mg (100 mg OC)-1, and an increase in N:C ratios towards phytoplankton values during the dry season (from 0.06 to 0.11). Changes in the lignin-phenol composition of POM suggest that gymnosperms contribute more toward OM composition during the dry season, with angiosperms dominating in the wet season. This is supported by calculations of the lignin phenol vegetation index of riverine OM, which is statistically different among seasons (dry: 29.4 ± 15.6 vs. wet: 74.6 ± 15.6). These changes likely reflect seasonal differences in the proportion of flow that is coming from the Upper and Lower Basin, corresponding to compositional differences between the vegetation of these regions. Therefore, this work provides a baseline understanding of OM variability that can be used to assess how future change will affect this river.

Ellis, E.E., A.I. Ingalls,  L.T. Truxal, R.G. Keil, and J.E.Richey (in prep. Draft in near-final revision). Sources and Temporal Variability of Branched and Isoprenoid Tetraether Lipids Exported by a Large Tropical River

      The organic carbon (OC) discharged by rivers is a key component of the global carbon cycle due to the potential for this material to be permanently buried in marine sediments. Accurate accounting of the proportion of terrestrial verses marine-derived organic matter preserved in coastal sediments is of considerable interest due to the paradox that less terrestrial carbon is preserved in ocean sediments than that which is predicted from riverine fluxes.  Recently, a new proxy has been developed to trace the proportion of terrestrially verses marine-derived organic matter in coastal sediments, known as the branched/isoprenoid tetraether (BIT) index. Although the BIT index has since been widely applied to assess the proportion autochthonous verses allochthonous carbon buried in marine sediments, the variability, and especially the seasonal variability, of the BIT index in the suspended sediment carried by rivers remains largely uninvestigated.  The objective of this study was to determine the variability of the BIT index of suspended sediments carried by the Mekong River, in Cambodia, just upstream of the delta, over the course of one year. Results showed that branched GDGTs can no longer be considered to be produced strictly in soils.  In this study, between 48 to 73% of the branched GDGTs associated with suspended and bed sediment of the Mekong River were intact, indicating that the cells from which these biomarkers are derived are viable.  Further, the percentage of intact branched GDGTs found in river bed and lake bed sediments, and suspended sediments generally exceed that found in soils.  Therefore, the significant in situ production of branched, in addition to isoprenoid, tetraether lipids suggests that aquatic environments substantially modify the original BIT index imparted on sediment in upland environments.

Ellis, E.E., J.E.Richey, R.G. Keil, A.I. Ingalls, G. dos Santos, and E. Druffel. (in prep, first draft outlined). Seasonal Variability of the ∆¹⁴C of Organic Matter Carried by the Mekong River:  Insights from Radiocarbon Analysis of Lignin Phenols.

      The source and age of dissolved and particulate organic matter that is transported by the Mekong River provides dynamical information that bulk concentrations alone do not. We are assessing the time scales over which terrestrial organic matter is exported from the Mekong watershed via fluvial processes, and determining how seasonality affects the type of organic carbon being exported.  Lignin-derived phenols are a biomarker for vascular plants, as they are diagnostic of different plant material types and diagenetic condition, they are abundant, and they are fluvially transported. With a recently-developed technique, it is possible to radiocarbon date individual lignin phenols. Measurements were made from February 2009-February 2010, and analyzed for δ¹³C (organic matter source) and Δ¹⁴C (age), in the lower Mekong, near Phnom  Penh. Fine suspended sediments (FSS) and POC track each other and the hydrograph. DOC is enriched in ∆¹⁴C relative to POC, but depleted relative to lignin. Lignin results showed that plant-derived material that is characterizable as lignin is always modern..   The  ∆¹⁴C of POC is predictable and highly variable, with values ranging between -327.7 to +25.9‰, with the lowest values during low water and the highest at high water.  Such variability is consistent with the conclusion that organic carbon inputs are controlled by mountainous sources during high water and lowland and autochthonous sources during low water.

Lockwood, D., J.E. Richey, P.D. Quay, M. Sampson, and M. Ung. (in prep, to be submitted November 2011). CO₂ outgassing flux and ecosystem metabolism over an annual cycle in the Lower Mekong River  

     The main objectives of this paper are to (1) To estimate the CO₂ degassing rate in the Lower Mekong River over an annual cycle. (2) To measure river metabolism over an annual cycle using the stable isotopic tracer δ¹⁸O, which has been used to estimate the metabolic state of river systems on integrated space and time scales. (3) To determine the processes controlling the CO₂ degassing rate and observed seasonal cycles of biogeochemical variables using a combination of dissolved inorganic carbon (DIC) and O₂ mass and isotope budgets. Measurements were made at a downstream site, by Phnom Penh, tracking the hydrographic seasons. Many of the biogeochemical indices we measured at the Phnom Penh site varied seasonally and in concert with the hydrograph.  DIC and alkalinity were both negatively correlated with discharge, however, the flux of DIC and alkalinity werepositively correlated with discharge, suggesting that weathering is enhanced by precipitation during the flood season.  pH was not significantly correlated with discharge, but notably, pH was highly correlated with the calcium carbonate (calcite) saturation index, suggesting that the geology of the basin strongly influences pH, particularly the carbonate source. However, for this site to be supersaturated throughout the entire year there must be a pervasive input of CO₂ from soil or in situ respiration that keeps the water supersaturated. The seasonal trend in pCO₂ opposes the alkalinity and DIC trends, peaking in the flood season and lowest in the dry season. O₂ was undersaturated throughout our study, and O₂ and CO₂ were also inversely correlated. Together with the pCO₂ supersaturation throughout the year, these observations suggest that respiration (or advection of soil CO₂) causes the pCO₂ supersaturation.  Using the dual isotopes of O₂ and O₂ saturation measurements, calculated R:P suggested that the Phnom Penh site was net heterotrophic during the entire year, and extremely heterotrophic during the flood season. Examining δ¹³C-DIC versus δ¹⁸O provides a view of which process controls river dissolved gas concentration at a certain time-photosynthesis, respiration or gas exchange. Calculations of the  δ¹³C of CO₂ suggests that the CO₂ outgassing from the Mekong River is very close to the average signature expected for terrestrial plant material, and is slightly heavier than the δ¹³C of FPOM.  The observation that δ¹³C of outgassing CO₂ is -26‰ during the flood season suggests minimal influence of CO₂ from the respiration of freshwater plankton; this supports our observation of a high R:P ratio and the possibility that P approaches zero at this time of year. CO₂ outgassing fluxes are lower but on the same order of magnitude as flux measurements from the Amazon River basin. We calculated an annual flux of 5.4 Tg C. Although it is small compared to the outgassing contributed by the Amazon River (estimated at 0.5 Pg C/yr), this value is a significant on a global scale and is comparable to the DIC load of the Mekong River, which is 3.9 Tg C/yr. This estimate is most likely an underestimate of CO₂ outgassing in the Lower Mekong Basin because we are not including outgassing from the Mekong delta or the Tonle Sap Great Lake in this calculation.

The Tonle Sap

        The largest freshwater body in South East Asia, the Tonle Sap Lake (TSL) is a large fluvial lake and a unique ‘pulsing'ecosystem with a large floodplain, abundant biodiversity, and high seasonal sediment and nutrient fluxes from the Mekong River.  It is located in central Cambodia and features an unusual bi-directional hydraulic connection with the Mekong River. During the dry season, the TSL covers an area of about 2500 - 3500 km² and drains into the Mekong via the Tonle Sap River (TSR). In the active southeast monsoon phase, the flow of the Tonle Sap River reverses as the Mekong swells with flood water, delivering more than 51,000 million m³ to the TSL. As a result, the lake expands more than five-fold flooding the surrounding alluvial plain and covering an area of about 14,500 - 16,000 km².   The Mekong-Tonle Sap is also the most productive freshwater fishery in the world with 50 million people dependent on fisheries for nutrition, income, and cultural identity. Large-scale development in the upstream part of the Mekong may threaten the natural flow regime and reduce the flood pulse amplitude, impacting on the Tonle Sap ecosystem as well. Despite its importance, there is little basic ecological information about the TSL ecosystem and many scientific questions about the fishery remain unknown.

      Irvine et al (2011) established the overall chemical continuity between the Mekong River and the Tonle Sap. Kummu et al (in revision) developed a water balance model for the TSL, identifying the different water sources and outputs from the lake. Data from regional observations and from the3D-EIA model were used. An interesting aspect is that during the time of peak outflow from the TSL, when the Mekong flow is low, much of the water that transits downstream to the Delta and on to the ocean is actually from the TSL, with a totally different chemistry than the Mekong itself. (Hence any calculation of Mekong discharge to the South China Sea must include this aspect). The overall distributions of sediments in the TSL, and the consequences for ecosystem production, are very poorly known. Kirschke et al (in prep, near final) examined the spatial patterns of turbidity in different seasons, using spatial pattern metrics of MODIS 250 m data. They then related the remotely-sensed patterns to predictions from the 3D EIA model of suspended sediments and phytoplankton production (the same model used by Kummu et al). Finally, Holtgrieve et al. (in prep) evaluated long-term autonomous sensor data and dynamic Bayesian mass balance models to estimate rates of gross primary productivity (GPP) and ecosystem respiration (ER), with comparisons to the 3D-EIA model.

K.N. Irvine, J.E. Richey, G.W. Holtgrieve, J. Sarkkula, and M. Sampson. 2011. Spatial and temporal variability of turbidity, dissolved oxygen, conductivity, temperature, and fluorescence in the lower mekong river-tonle sap system identified using continuous monitoring. International Journal of River Basin Management. DOI: 10.1080/15715124.2011.621430

     Continuous monitoring of turbidity, dissolved oxygen, conductivity, temperature, and fluorescence was done at five locations on the Tonle Sap Lake and the Mekong-Bassac Rivers near Phnom Penh, Cambodia, between 2004 and 2010 using autonomous datasondes. Seasonal, daily, and spatial trends were clearly identified in the data and were related to the annual monsoon rainy season-dry season cycle, system metabolism, system hydraulics, and in some cases, localized phenomena such as waste discharges. The datasondes were particularly useful to track the oxygenation of anoxic black water areas in the flooded forest fringe of the Tonle Sap that occurred during the rainy season freshwater pulse. A strongly developed vertical variation of turbidity, dissolved oxygen, and conductivity in the flooded forest fringe may be related to a combination of factors, including dissolved material release from bed sediment and a floating organics-rich particulate layer near the bottom of the lake. Grab samples for total suspended solids were collected at the Preak Leap site (Mekong River) in 2009 and 2010. An excellent relationship was established between daily mean turbidity and total suspended solids concentration for the Preak Leap site, with r² = 0.95. ARIMA models adequately forecast water level and water quality data one month ahead.

Kummu, M., S. Tes, S. Yin, J. Sarkkula, J. Koponen, J. Józsa, J. Richey, and P. Adamson. (in revision). Water balance model for a complex lake-floodplain system: Case Tonle Sap Lake. Hydrological Processes

     We present here a detailed water balance model, based on  observed data of discharges from the lake's tributaries, discharge between Mekong and the lake through the Tonle Sap River, precipitation, and evaporation. The overland flow between the Mekong and lake is modelled with EIA 3D hydrodynamic model. Over the eight-year simulation period, the model replicated the observed data rather well. There were six sub periods when the water balance model performed less well, particularly for the lower water levels occurring towards the end of the dry season during late April and early May. Breaking the overall water balance into its components shows the relative magnitude of each term. On average, 53.5% of Tonle Sap Lake volume originates from the Mekong mainstream, either via the Tonle Sap River (50%) or overland flow (3%). The system's own tributaries contribute 34%, and the balance of 12.5% is sourced from precipitation. The annual inflow during the eight-year study period ranged from 51 km3 during the dry conditions of 1998 to 109 km3 during the 2000 high flood episode. The estimated eight-year average inflow was 83.1 km3. Around 88.5% of the total annual outflow from the lake is discharged into the Mekong mainstream via the Tonle Sap River (84%) while the overland drainage back to the Mekong constitutes a fraction of 3%. The open water evaporative losses are assessed at 13%. The mean annual outflow during the study period was 81.9 km³, with annual volumes varying between 46 km³ (1998) and 114 km³ (2000). Flow alterations in the mainstream would have direct impacts on the Tonle Sap water levels and hydrology as well. Recent research has shown that the relatively small rises in the dry season lake water level would permanently inundate disproportionately large areas of floodplain, rendering it inaccessible to floodplain vegetation and eroding the productivity basis of the ecosystem by reducing the inundated area, and duration and amplitude of flooding. The lake extension would thus cause permanent submersion; in essence destruction, of considerable areas of the gallery forest stripe surrounding the lake in the floodplain. 

Kirschke, S., J. E. Richey, M. P. Logsdon, and M. Kummu. (in prep).Variability of the spatial structure of turbidity in the Tonle Sap Lake , Cambodia. (Hydrology and Earth System Sciences).

     The flood pulse of the Mekong River carries dissolved and suspended solids to the lake and its floodplain, thus directly influencing the nutrient status of the flooded ecosystem.  But surprisingly few sediment data exist to assess system dynamics; precise sediment accumulation rates in the lake are unknown and estimates vary considerably. In Part 1 of this paper, we examine the seasonal variability in the spatial structure of turbidity. We use MODIS (Moderate Resolution Imaging Spectrometer) 250m level-1B satellite data for the TSL and an in situ data set of lake surface turbidity and total suspended sediments (TSS) to generate a turbidity index/product for the lake and analyze the variability in the spatial structure of turbidity between low, rising, high and falling water periods. The objectives are to (1) determine the correlation/relationship between the MODIS 250m band 1 and band 2 spectral band ratio and the in situ observational measurements of turbidity (NTU) and TSS; (2) calibrate the MODIS 250m derived turbidity index using the above mentioned relationship (spectral characteristics - NTU/TSS); (3) construct a data set of calibrated turbidity images for the TSL based on hydrographic stage data (low, rising, high, falling water stages); (4) identify spatial structure of turbidity in the data set by characterizing turbidity fronts on the turbidity (lake) surface; and (5) test the hypothesis that a relationship between the variability of turbidity and the spatial extent/stage height/time of year of the lake exists (on different scales) and can be analyzed using MODIS 250m satellite data. Geospatial metrics were used in the analysis, based on the Topographic Position Index (TPI). A series of metrics describing the TPI includes rugosity, number of patches, the perimeter-area fractal dimension, the largest patch index, and the splitting index illustrate seasonal differences in lake structure. In Part 2 of this paper, we compare the resulting spatial patterns to results from a model of TSS and phytoplankton distributions in the lake, as an independent and process-based evaluation of the observed patterns. Comparison to the EIA-3D model results, done with a reduced set of the spatial parameters, shows consistencies in patterns between the model and remotely sensed products, allowing a partitioning of net turbidity into suspended sediments and plankton.

Holtgrieve, G.W.,  K.N. Irvine, J.E. Richey, M.  Kummu, and D.Lockwood, (in prep).  Ecosystem metabolism and support of freshwater capture fisheries in the Tonle Sap Lake, Cambodia.

      Numerous recent studies have highlighted the important role of freshwaters in the global carbon cycle (Cole et al. 1994, Richey et al. 2002). Mechanisms which increase the connectivity of inland waters to their surrounding watershed, for example, though annual flooding of riparian forests, are thought to highly influence internal ecosystem functioning of lakes and rivers through the supply of terrestrially derived organic matter (Cole et al. 2007). However, direct empirical evidence of geophysical processes controlling ecosystem processes is generally sparse. This project evaluated long-term autonomous sensor data and dynamic Bayesian mass balance models (Holtgrieve et al. 2010) to estimate rates of gross primary productivity (GPP) and ecosystem respiration (ER) in the Tonle Sap Lake over a full hydrologic cycle. There was a strong seasonal cycle in both the rates of GPP and ER that were correlated with lake water level.  High-water was associated with decreased GPP, elevated ER, and increasingly heterotrophic ecosystem dominated by respiratory process. This information is crucial for understanding the role of the Mekong-Tonle Sap Ecosystem in regional carbon sequestration. The results of our analyses are being integrated with previous modeling work to estimate the overall reliance of the fishery on in situ primary production.  How hydrology influences the energetic basis of these fisheries through fixation and respiration of organic matter is a particularly pressing question since current proposals for hydroelectric dam projects may threaten the long-term sustainability of the fishery by potentially removing the annual flood dynamics.