Southeast Asia

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 , 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.


The carbon "balance" of the Mekong River system, as the composite of the sources, processing, and transport of dissolved and particulate carbon from land to the sea, constitutes an important part of the overall carbon balance of the Mekong, and integrates across hydrology, sediment transport, and landuse. This is a particularly relevant topic, because recent evidence is showing that the carbon cycle through rivers is much more important in the global carbon cycle than previously thought. Establishing this balance was a principal charge to the UW group from IKMP, to build on the work that the UW team was doing through its principal funding (NASA, NSF). The components of the river carbon balance are summarized below.

   Few studies have investigated seasonal and interannual variations in the pCO2 and organic carbon 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. The question then is, where does this pCO2 come from, and does the source vary seasonally? Alin et al (in prep) argued that the outgassed CO2 from the Mekong River is youngest during the dry season, when inputs from terrestrial environments are at a minimum and in situ productivity may yield the peak availability of young organic matter. In contrast, during the wet season, when partial pressures of CO2 (pCO2) peak, inputs of dissolved and particulate carbon from upland environments and the concomitant reduction of in situ primary production due to increased suspended sediment concentrations result in a relatively aged source for the outgassed CO2. Lockwood et al. (in prep) carried the analysis further, establishing annual carbon budgets for the lower Mekong, and the sequence of metabolic processes producing those budgets.
   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. (2012) examined the changes in the elemental and lignin compositions of POM, showing pronounced seasonal differences. Ellis et al. (2013) 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 (review) 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.
The next big question is, what drives the metabolism of the Tonle Sap, relative to the Mekong itself, ultimately producing the trophic basis for the fisheries? Using continuous monitoring of turbidity, dissolved oxygen, conductivity, temperature, and fluorescence, Irvine et al (2011) showed claear patterns in the seasonal heterogeneity of water mixing between the mainstem, open Tonle Sap, and fringing forests, over an annual cycle. Using a state-space oxygen mass balance model and continuous dissolved oxygen measurements, Holtgieve et al (in press) provided the first direct estimates of gross primary productivity (GPP) and ecosystem respiration (R). They then estimated that the fisheries harvest is equivalent of 7-69% of total annual NPP, which is substantially larger than global average for marine and freshwater systems.

  The University of Washington team and regional colleagues in Thailand, Cambodia, and Laos, conducted a series of field campaigns, to examine the carbon and nutrient dynamics of the Mekong basin (funded by NASA and the US National Science Foundation). The following are abstracts from the respective papers.

Alin, S.R. M.Fátima 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. Geophysical Research 116, G01009, doi:10.1029/2010JG001398, 2011 Outgassing of carbon dioxide (CO2) from rivers and streams to the atmosphere is amajor 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.  

Irvine, K.N., 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 (DO), 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, DO, 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 organic-rich particulate layer near the bottom of the lake. Grab samples for total suspended solids (TSS) were collected at the Preak Leap (PL) site (Mekong River) in 2009 and 2010. An excellent relationship was established between daily mean turbidity and TSS concentration for the PL site, with r2 = 0.95. Autoregressive, integrated, moving average models adequately forecast water level and water quality data one month ahead.  

Ellis, E.E., R.G. Keil, A.I. Ingalls, and J.E. Richey. 2012. Seasonal variability in the sources of particulate organic matter of the Mekong River as discerned by elemental and lignin analyses. J. Geophys. Res., doi:10.1029/2011JG001816. 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.

Holtgrieve, G.W, M.E. Arias, K.N. Irvine, D. Lamberts, E.J. Ward, M. Kummu, J. Koponen, J. Sarkkula,and J.E.Richey. (2013). Patterns of ecosystem metabolism in the Tonle Sap Lake, Cambodia with links to capture fisheries. PLoS ONE 2013 Vol: 8(8):. DOI: 10.1371/journal.pone.0071395 The Tonle Sap Lake in Cambodia is a dynamic flood-pulse ecosystem that annually increases its surface area by over 600% with monsoon-driven rains. This flooding is thought to structure many of the critical ecological processes, including aquatic primary and secondary productivity. The lake also has a large fishery that supports the livelihoods of nearly 2 million people. Management of fisheries is hindered by a lack of scientific knowledge, in particular ecological data relevant to fish production. We used a state-space oxygen mass balance model and continuous dissolved oxygen measurements from four locations to provide the first estimates of gross primary productivity (GPP) and ecosystem respiration (ER) for the Tonle Sap. GPP averaged 4.1 ± 2.3 g O2 m-3 d-1 with minimal differences among sites. There was a negative correlation between monthly GPP and lake level (r=0.45) and positive correlation with turbidity (r=0.65). ER averaged 24.9 ± 20.0 g O2 m-3 d-1 but had >6-fold variation among sites and minimal seasonal change. Repeated hypoxia was observed at most sampling sites along with persistent net heterotrophy (GPP<ER), indicating significant bacterial metabolism of organic matter that is likely incorporated into the larger food web. Using our measurements of GPP, we calibrated a hydrodynamic-productivity model and predicted aquatic net primary production (aNPP) of 2.0 ± 0.2 g C m-2 d-1 (2.4 ± 0.2 million tonnes C y-1). Considering a range of plausible values for the total fisheries catch, we estimate that fisheries harvest an equivalent of 7-69% of total aNPP, which is substantially larger than global average for marine and freshwater systems. This is likely due to relatively efficient carbon transfer through the food web and support of fish production from terrestrial NPP. These analyses are an important step in quantifying the resource pathways that support fisheries and people in this important ecosystem.  

Martin, E.E., A.E. Ingalls, J.E. Richey, R.G. Keil, L.T. Carlson, G.M. Santos, S.R. Alin, and E.R. M. Druffel. (2013). Age of riverine carbon suggests rapid export of terrestrial primary production in tropics. Geophysical Res. Letters 40, 1-5, doi:10.1002/2013GL057450, 2013 The balance between the storage of vascular plant carbon in soils, oxidation to carbon dioxide, and export via rivers affects calculations of the strength of terrestrial ecosystems as carbon sinks. The magnitude and timescale of the riverine export pathway are not well constrained. Here we use radiocarbon dating of lignin phenols to show that plant-derived carbon carried by the suspended sediment of the Mekong River is very young, having been produced within the last 18 years. Further, this plant-derived carbon remains consistently young across seasons, even though the bulk carbon varies from modern to over 3,000 radiocarbon years old. Agreement between the age of riverine lignin phenols and vascular-plant derived carbon in upland soils suggests riverine lignin reflects the residence time of terrestrial carbon, and that primary-production derivatives are exported rapidly. These results are relevant for modeling predictions of the influence of the terrestrial biosphere on atmospheric carbon dioxide levels.  

Alin, S.R., J.E. Richey, G.W. Holtgrieve, E.E. Ellis, and D. Lockwood. (in advanced prep, to be submitted 2014). Seasonal dynamics of the sources and isotopic composition of carbon dioxide outgassed from large tropical river systems: the Mekong and Chao Phraya rivers of Southeast Asia River systems link critical domains within the global carbon cycle, carrying globally significant carbon fluxes from terrestrial environments to the global oceans, as well as providing a conduit for the return of carbon gases (carbon dioxide [CO2], methane [CH4]) produced by respiration in soil and aquatic environments to the atmosphere. The seasonal wet-dry alternation typical of tropical river systems results in oscillation of hydrological conditions at these environmental interfaces, which may produce "hot moments" when rates of biogeochemical reaction or throughput are transiently elevated compared to background rates. In an effort to gauge the seasonal variability in the sources of carbon dioxide outgassed by tropical river systems like the Amazon, we examined changes in the concentration and stable and radiogenic carbon isotope composition of dissolved inorganic carbon (DIC) throughout the annual hydrological cycle in two major lowland river systems in Southeast Asia, the Mekong and Chao Phraya rivers. Here we argue that the outgassed CO2 from the Mekong River is youngest during the dry season, when inputs from terrestrial environments are at a minimum and in situ productivity may yield the peak availability of young organic matter. In contrast, during the wet season, when partial pressures of CO2 (pCO2) peak, inputs of dissolved and particulate carbon from upland environments and the concomitant reduction of in situ primary production due to increased suspended sediment concentrations result in a relatively aged source for the outgassed CO2. Although regional geology is characterized by a greater influence of carbonate weathering in the Mekong than in the Amazon Basin, and land use is more extensive in Southeast Asia, we suggest that this pattern holds as a general model for reconsidering the net annual source of CO2 outgassed by rivers in lowland tropical river systems.  

Lockwood, D., J. Richey, P. Quay, M. Sampson, and M. Ung. (in advanced prep. To be submitted 2014). CO2 outgassing flux and ecosystem metabolism over an annual cycle in the Lower Mekong River. Outgassing of carbon dioxide from inland waters including rivers is a significant source of carbon to the atmosphere on the order of 1 Pg C yr-1, but this value is not well constrained. We determined the magnitude of CO2 outgassing flux and the factors controlling it in the world's eight largest river by discharge, the Mekong River. We surveyed inorganic chemical parameters over an annual cycle on the mainstem near Phnom Penh, Cambodia, in the Lower Mekong Basin, including pCO2, alkalinity, pH, δ13C-DIC and concentrations of nutrients and fine suspended sediment. We also determined river metabolism (ratio of respiration to photosynthesis, R:P) by measuring the concentration and δ18O of dissolved oxygen (O2). Throughout the year, the partial pressure of CO2 (pCO2) was supersaturated with respect to the atmosphere and R:P was >1, indicating net heterotrophy. Both pCO2 and R:P were highly correlated with the hydrograph. Mean annual CO2 outgassing flux at this site was 214 ± 86 mmol C m-2 d-1 (error estimates reflect 1 σ confidence intervals); it was lowest in the dry season (50 ± 20 mmol C m-2 d-1) and 6x greater in the flood season (321 ± 129 mmol C m-2 d-1). Scaling this up to the Lower Mekong mainstem, we estimate an annual CO2 outgassing flux of 5.7 Tg C year-1 from the China border to the beginning of the Mekong Delta, or 1.5x the DIC load of the Lower Mekong River. Through analysis of DIC and O2 mass and isotopic budgets, we find that this outgassing is explained by in situ respiration of C3 plant material. In contrast, in the Tonle Sap River, a major tributary of the Mekong that seasonally reverses flow direction, in situ net heterotrophy explains only 5% of CO2 outgassing flux in the flood season during downstream flow from Cambodia's great lake, the Tonle Sap, with the rest presumably driven by CO2 advected from the soil and floodplain.  

Ellis, E.E., A.E. Ingalls, L.T. Truxal, R.G. Keil, and J.E. Richey. (in advanced prep, to be submitted 2014). 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 Rivers export between 0.38 - 0.53 Pg of OC each year. Although much of this flux is mineralized within seawater and surficial sediments, terrigenous carbon burial is on the same order of magnitude as that derived from marine inputs in coastal 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. Because the organic composition of material carried by rivers can vary significantly between seasons seasonal variability of the BIT index in rivers should be well quantified. However, to our knowledge this data is entirely lacking. Accordingly, 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. To aid in the interpretation of the variability, other measures of terrestrial carbon, such as lignin-derived phenols, N:C ratios, and δ13C were concurrently measured for these same sediments. Based on the results of this study, branched GDGTs can no longer be considered to be produced strictly in soils. 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. Considering that the BIT index of sediment was not correlated with any measure of terrestrially-derived carbon, such as lignin phenols, N:C ratios, or δ13C, we hypothesize that the relative ratios of branched GDGTs and GDGT IV produced within the river are responsible for this variation.


    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.

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.

Kummu, M., S. Tes, S. Yin, J. Sarkkula, J. Koponen, J. Józsa, J. Richey, and P. Adamson. 2013. Water balance model for a complex lake-floodplain system: Case Tonle Sap Lake. Hydrological Processes

The Tonle Sap Lake of Cambodia is the largest freshwater body of Southeast Asia, forming an important part of the Mekong River system. The lake has an extremely productive ecosystem and operates as a natural floodwater reservoir for the lower Mekong Basin, offering flood protection and assuring the dry season flow to the Mekong Delta. In light of the accelerating pace of water resources development within the Mekong Basin and the anticipation of potentially significant hydrological impacts, it is critical to understand the overall hydrologic regime of Tonle Sap Lake. 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 was modelled with the EIA 3D hydrodynamic

model. We found that majority (53.5%) of the water originates from the Mekong mainstream, but the lake's tributaries also play an important role contributing 34% of the annual flow, while 12.5% is derived from precipitation. The water level in the lake is mainly controlled by the water level in the Mekong mainstream. The Tonle Sap system is hence very vulnerable, from a water quantity point of view, to possible changes in the Mekong mainstream and thus, development activities in the whole Mekong basin. From a biogeochemical point of view, the possible changes in the lake's own catchment are equally important, together with the changes in the whole Mekong Basin. Based on our findings, we recommend of continuing the monitoring programmes in lake's tributaries and urgently starting of groundwater measurement campaign within the floodplain, and including the groundwater modelling to be part of the hydrodynamic models applied for the lake. 

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.

     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: . 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.