River Sediment and Elemental Fluxes

Results from Mae Sa (Thailand)

Bedload

This paper reviews the current state of knowledge regarding bedload transport in SE Asian streams and presents the results from a case study on bedload transport in a mountain stream in northern Thailand.  Together, the review and new data help contextualize the paucity of work done in the region in the face of a rapid increase in development and reservoir building throughout SE Asia.  Data from both the reviewed studies and our case study indicate that bedload transport in many SE Asian streams (catchment areas < 100 km²) is often much higher than  is commonly assumed for tropical streams (i.e., about 10%). Estimated annual bedload yield was 18% of the annual sediment load in 74-km² Mae Sa Catchment, in northern Thailand. Bedload transport rates ranged from 0.001 to 1.1 kg s^-1; and measured total suspended solid (TSS) rates ranged from 0.01 to 39 kg s^-1, equivalent to TSS concentrations of 25 to 13,500 mg l^-1 (associated with flows ranging from 0.4 to 30 m³ s^-1). Event and annual loads of bedload and TSS were determined from rating curves based on automated measurements of discharge and turbidity (for TSS only). When taking uncertainty into account, the estimated range is 9-25% (equivalent to a yield of 81-279 Mg km^-2y^-1).  The corresponding TSS yield estimate ranged from 649-1037 Mg km^-2; and the total sediment load is an estimated 730-1316 Mg km^-2 y^-1.  The proportion of bedload was lower than that reported in some other Asia streams, probably due to the occurrence of extended periods with high TSS that dampened the bedload signal, which was sand-dominated during the low-energy events that were sampled.  Nevertheless, the bedload rate was generally higher than for most SE Asian locations, likely due to the occurrence of several road-related landslides the previous year.  Although we were not able to measure bedload transport for high energy flows (discharges > 4.5 m³ s^-1), we believe our upper estimates for bedload variables (25% of the total sediment load; and a yield of 279 Mg Km^-2 y^-1) provide reasonable upper bounds. Finally, the bulk of bedload transport is episodic in nature, with a higher proportion moved during high energy tropical storms that occur late in the monsoon rainy season, as well as in response to both natural and anthropogenic landscape disturbances.  The possibility that bedload proportion could exceed 20-40% for rivers and streams of various sizes reinforces the need for accurate estimates of all river sediment loads prior to building dams in the region. Past examples of reservoir closure following rapid infilling possibly stem from underestimating sediments loads, particularly the bedload component, and failing to factor in the very high sediment loads associated with large storm events.

Turbidity and Suspended Sediment

Annual total suspended solid (TSS) loads in the Mae Sa River in northern Thailand, determined with an automated, turbidity-based monitoring approach, were approximately 62,000, 33,000, and 14,000 Mg during the three years of observation.   These loads were equivalent to basin yields of 839 (603-1170), 445 (217-462), and 192 (108-222) Mg km^-2 for the 74.16-km² catchment during 2006, 2007, and 2008, respectively.  The yearly uncertainty ranges indicate our loads may be underestimated by 38-43% or overestimated by 28-33%. In determining the annual loads, discharge (Q) and turbidity (T) values were compared against 333 hand-sampled total suspended solid concentrations (TSS) measured during 18 runoff events and other flow conditions across the three-year period.  Annual rainfall varied from 1632 to 1934 mm; and catchment runoff coefficients (annual runoff/annual rainfall) ranged from 0.25 to 0.41. Measured TSS ranged from 8-15,900 mg l^-1; the low value was associated with dry-season base flow; the latter, a wet-season storm. Storm size and location played an important role in producing clockwise, anticlockwise, and complex hysteresis effects in the Q-TSS relationship.  Turbidity alone was a good estimator for turbidity ranges of roughly 10-2800 NTU (or concentrations approximately 25-4000 mg l^-1).  However, owing to hysteresis and high sediment concentrations that surpass the detection limits of the turbidity sensor during many annual storms, TSS was estimated best using a complex multiple regression equation based on high/low ranges of turbidity and Q as independent variables. Turbidity was not a good predictor of TSS fractions > 2000 µm.  Hysteresis in the monthly Q-TSS relationship was generally clockwise over the course of the monsoon season, but infrequent large dry-season storms disrupted the pattern in some years. The large decrease in annual loads during the study was believed to be related to depletion of fine sediment delivered to the stream by several landslides occurring the year prior to the study.  The study indicated the importance of monitoring Q and turbidity at fine resolutions (e.g., sub-hourly) to capture the TSS dynamics and to make accurate load estimations in this flashy headwater stream where hysteresis in the Q-TSS signature varied at several time scales.

Longchuan River (China)

Major Element Chemistry

Water samples were collected twice per month over a two-year period from the Longchuanjiang River (Yunnan Province, China) to understand monthly variations in major elements and solute fluxes as related to rock weathering and associated CO consumption rates. Solute concentrations were 5 times the median of 65 mg/l for global average. Total cationic exchange capacity (Tz+) ranged from 2.4 to 6.1 meq/l; and the mean (4.4 meq/l) was significantly higher than that of the global river waters. Calciumand bicarbonate dominated the annual ionic composition, accounting for more than 70% of the solute flux that exceeded 71×10^6 kg/yr. Lower concentrations of most measured elements during the monsoon high flow period could be explained by dilution effects from precipitation. Three major reservoirs contributed to the dissolved load: carbonates, silicates and anthropogenic inputs, i.e., some83% of the riverine cations fromcarbonates and 17% fromsilicates. The chemical weathering rate of 26.1 t/km2/yr, with respective carbonate and silicate weathering rates of 20.3 t/km2/yr (8.46 mm/kyr) and 5.75 t/km2/yr (2.13 mm/kyr), was comparable to the average for global rivers, but higher than that for the Changjiang River in China. The CO2 consumption rate was estimated to be 173.7×10/yr and 202.9×10^3 mol/km2/yr by silicate and carbonate weathering, respectively. The CO consumed by rock chemical weathering in the upper Changjiang River reduced the atmospheric CO2 level and constituted a significant part of the global carbon budget. Consequently the carbon sink potential of rock chemical weathering in the QinghaiPlateau deserves extra attention. Population density and anthropogenic activities, particularly agricultural practices, contributed remarkably to dissolved solutes and associated CO consumption worldwide, and anthropogenic inputs probably contributed some 10.4% to the dissolved solutes in the Longchuanjiang River.

Seasonal changes in elemental fluxes

Fluxes of dissolved and particulate nitrogen (N) and phosphorus (P) variables were measured monthly from September 2007 to March 2009 in the upper Longchuanjiang River (Yunnan Province, China) to determine annual loads and seasonal variability. Dissolved N (DN) and particle associated P (PAP) contributed 56% and 99% of the total nitrogen (TN) and total phosphorus (TP) yields of 549 and 608 kg/km2/yr. Fluxes of particulate N (PN), dissolved P (DP), PAP and TP exhibited great seasonality because they were highly correlated with water discharge. Areal export rates of NH

–N, PN, PAP and TP were higher than in the main channel and most tributaries of the Changjiang River. High particulate loads were contributed to erosion of phosphorus-rich soils during heavily rains in the wet season. Median measured concentrations of TN, NH –N and TP exceeded the maximum permissible limit for domestic and recreational use in China. High nitrogen and phosphorus concentrations draw attention to the potential for additional nutrient loading to foster the formation of algal blooms in locations where free-flowing river sections are changing into cascades of reservoirs. Importantly, the great seasonality in the data shows necessity of sufficient sampling for determining annual fluxes.

Organic Carbon Fluxes

To investigate the effects of anthropogenic activity, namely, land use change and reservoir construction, on particulate organic carbon (POC) transport, we collected monthly water samples during September 2007 to August 2009 from the Longchuanjiang River to understand seasonal variations in the concentrations of organic carbon species and their sources and the yield of organic and inorganic carbon from the catchment in the Upper Yangtze basin. The contents of riverine POC, total organic carbon and total suspended sediment (TSS) changed synchronously with water discharge, whereas the contents of dissolved organic carbon had a small variation. The POC concentration in the suspended sediment decreased non-linearly with increasing TSS concentration. Higher molar C/N ratio of particulate organic matter (average 77) revealed that POC was dominated by terrestrially derived organic matter in the high flows and urban wastewaters in the low flows. The TSS transported by this river was 2.7 10 t/yr in 2008. The specific fluxes of total organic carbon and dissolved inorganic carbon (DIC) were 5.6 and 6 t/km/yr, respectively, with more than 90% in the high flow period. A high carbon yield in the catchment of the upper Yangtze was due to human-induced land use alterations and urban wastes. Consistent with most rivers in the monsoon climate regions, the dissolved organic carbon–POC ratio of the export flux was low (0.41). Twenty-two percent (0.9 t/km2/yr) of POC out of 4 t/km2/yr was from autochthonous production and 78% (3.1 t/km2/yr) from allochthonous production. The annual sediment load and hence the organic carbon flux have been affected by environmental alterations of physical, chemical and hydrological conditions in the past 50 years,demonstrating the impacts of human disturbances on the global and local carbon cycling. Finally, we addressed that organic carbon flux should be reassessed using adequate samples (i.e. at least two times in low-flow month, four times in high-flow month and one time per day during the flood period), daily water discharge and sediment loads and appropriate estimate method.

Partial Pressures of CO₂

Rivers have been under sampled to investigate carbon degassing, especially in the tropical and subtropical regions. An unprecedented high-temporal-resolution (daily) sampling during July 2008–August 2009 was conducted in the Longchuan River of the upper Yangtze basin, a subtropical monsoon river in China, to reveal the daily-to-seasonal dynamics of the partial pressure of CO2 (pCO

degassing flux from the river using Henry’s constant and CO2. The pCO2 levels ranged from 230 to 8300 latm with an average of 1230 latm and obvious daily and seasonal variations. More than 92% samples were supersaturated with CO2 in contrast to the atmospheric equilibrium (380 latm). pCO values in the river water in the wet season were relatively low, except in the flooding event in November, due to a dilution effect by heavy rainfall. In contrast, the pCO levels in the dry season were much higher, due to lower pH resulted from anthropogenic activities. Net CO2 degassing and pCO were strongly correlated with dissolved nitrogen, but weakly with water temperature, dissolved inorganic carbon and water discharge, and uncorrelated with particulate nutrients and biogenic elements. The estimated water-to-air CO degassing flux in the Longchuan River was about 27 mol/m2) and CO2/yr, with the upper limit of 50 mol/m/yr. Our study also indicated that among the carbon remobilized from land to water, around 7% (2800 t C/yr) of the total carbon was emitted to the atmosphere, 42% (17,000 t C/yr) deposited in the riverreservoirssystem and 51% (21,000 t C/yr) exported further downstream. High spatial and temporal resolution of estimates of CO emission from the world large rivers is required due to extremely heterogeneous catchment characteristics and anthropogenic activities in space and time.

Support

Ziegler, AD (PI, NUS). Anthropogenic and Natural Controls on Stream Chemistry at Multiple Scales. 2012-2015. Ministry of Education (MOE) Academic Research Fund (AcRF, R-109-000-134-112), Singapore ($388,000): Thailand.

Ziegler AD (PI). 2009-2012. Landslide initiation mechanisms and fate of sediment in a headwater catchment in Thailand. NUS FASS Start-up grant (R-109-000-092-133) ($20,000): Thailand.

Ziegler AD (PI, NUS/UHM), RC Sidle (Kyoto), S Wood (BSU), XX Lu (NUS), A Snidvongs. 2009-2010. Sediment Dynamics in Tropical Streams Affected by Land-cover/Land-use & Climatic Change. Asia Pacific Network, grant no #ARCP2008-01CMY ($40,000): Thailand, Vietnam, China, India.

Lu Xi Xi (PI, Singapore), M Bird (Scotland), A Chen (Taiwan), D Higgitt (Singapore), M Kummu (Finland), R Robinson (UK), J Sarkkula (Lao PDR), AD Ziegler (NUS/UHM). 2008-2010. Sediment and carbon fluxes in large Asian rivers: Climate and human impacts. Ministry of Education (MOE) Academic Research Fund (AcRF), Singapore ($200,000): South and SE Asia.

Lu Xi Xi (PI, Singapore), M Bird (Scotland), A Chen (Taiwan), D Higgitt (Singapore), M Kummu (Finland), R Robinson (UK), J Sarkkula (Lao PDR), AD Ziegler (UHM). 2008-2010. Sediment and carbon fluxes in large Asian rivers: Climate and human impacts. National University of Singapore Bridge Funding, Singapore ($24,500): Red, Mekong, and Chao Phraya Rivers.

Ziegler AD (PI, UHM), RC Sidle (Kyoto), S Wood (BSU), XX Lu (NUS), A Snidvongs. 2007-2008. Sediment Dynamics in Tropical Streams Affected by Land-cover/Land-use & Climatic Change. Asia Pacific Network, grant no #ARCP2007-01CMY ($40,000): Thailand, Vietnam, China, India.

Ziegler AD (PI, UHM), Tantasarin C (PI, Thailand), XX Lu (Singapore), DL Higgitt (Singapore), S Benner (Boise State), TW Giambelluca. 2007-2008  Sediment and nutrient dynamics in a headwater tributary in SE Asia:  Building a foundation for investigating impacts of future anthropogenic change. South Asia Regional Committee for START, award no. 95/01/CW-005 ($15,000), northern Thailand.

Ziegler AD (PI) (UHM), R.C. Sidle (Kyoto), S. Wood (BSU), X.X. Lu (Singapore), A. Snidvongs (Bangkok). 2006-2007. Sediment Dynamics in Tropical Streams Affected by Land-cover/Land-use & Climatic Change – Thailand Phase. Asia Pacific Network, grant no #ARCP2006-06NMY ($40,000): Thailand.