Katsuro Ogawa

The Ganges and Brahmaputra rivers in Bangladesh: basin denudation and sedimentation

Every year the Ganges and Brahmaputra rivers in Bangladesh transport 316 and 721 million tonnes of sediment, respectively. These high loads of suspended sediment reflect the very high rate of denudation in their drainage basins. The average mechanical denudation rate for the Ganges and Brahmaputra basins together is 365 mm 103 yr−1. However, the rate is higher in the Brahmaputra Basin than that in the Ganges Basin. Several factors, including mean trunk channel gradient, relief ratio, runoff, basin lithology and recurring earthquakes are responsible for these high denudation rates. Of the total suspended sediment load (i.e. 1037 million tonnes) transported by these rivers, only 525 million tonnes (c. 51% of the total load) are delivered to the coastal area of Bangladesh and the remaining 512 million tonnes are deposited within the lower basin, offsetting the subsidence. Of the deposited load, about 289 million tonnes (about 28% of the total load) are deposited on the floodplains of these rivers. The remaining 223 million tonnes (about 21% of the total load) are deposited within the river channels, resulting in aggradation of the channel bed at an average rate of about 3·9 cm yr−1. Although the Brahmaputra transports a higher sediment load than the Ganges, the channel bed aggradation rate is much higher for the Ganges. This study also documents a wide range of interannual, seasonal and daily variation in suspended sediment transport and water discharge. Interannual variation in sediment deposition within the basin is also suggested. Copyright

Distribution of suspended sediment in the coastal sea off the Ganges–Brahmaputra River mouth: observation from TM data

Abstract Remote sensing technique was applied to estimate suspended sediment concentration (SSC) and to understand transportation, distribution and deposition of suspended sediment in the estuary and throughout the coastal sea, off the Ganges–Brahmaputra River mouth. During low river discharge period, zone of turbidity maximum is inferred in the estuary near the shore. SSC map shows that maximum SSC reaches 1050 mg/l in this period. Magnitude of SSC is mainly owing to resuspension of the bottom surface sediments induced by tidal currents flowing over shallow water depths. The influence of depth on resuspension is farther revealed from the distribution and magnitude of SSC along the head of Swatch of No Ground (SNG) submarine canyon. During high river discharge period, huge river outflow pushed the salt wedge and flashes away the suspended sediments in the coastal sea off the river mouth. Zone of turbidity maximum is inferred in the coastal water approximately within 5–10 m depth of water, where the maximum SSC reaches 1700 mg/l. In this period, huge fluvial input of the suspended sediments including the resuspended bottom sediments and the particles remaining in suspension for longer period of time since their initial entry control mainly the magnitude of SSC. In the estuary near the shore, seasonal variation in the magnitude of SSC is not evident. In the coastal sea (>5 m water depth), seasonal influence in the magnitude of SSC could be concluded from the discrepancy between SSC values of two different seasons. Transportation and deposition of suspended sediments also experiences seasonal variations. At present, suspended sediments are being accumulated on the shallow shelf (between 5 and 10 m water depth) in low discharge period and on the mid-shelf (between 10 and 75 m water depth) during high discharge period. An empirical (exponential) relationship was found between gradual settle down of suspended sediments in the coastal sea and its lateral distance from the turbidity maximum.

Comparison of global net primary production trends obtained from satellite-based normalized difference vegetation index and carbon cycle model

The global terrestrial net primary production (NPP) trend was estimated from two independent methods, satellite observation data and a carbon cycle model, and the results were compared for validation. The satellite-based NPP trend was estimated from the incoming surface solar radiation data set and a National Oceanic and Atmospheric Administration/ Advanced Very High Resolution Radiometer data set that was corrected by normalized difference vegetation index in areas of desert and dense vegetation. The increase in NPP from the Goddard Institute for Space Studies solar radiation data set and from the LaRC solar radiation data set over 10 years in the 1980s was estimated to be 1.8 and 4.4%, respectively. The NPP trend based on a carbon cycle model was estimated from a simple carbon cycle model that was established for the period 1850–1990 with biospheric and oceanic carbon cycle history constraints. The historical trend obtained from the model correlates well with the time variation of not only the observed atmospheric CO2 but also the biospheric and oceanic carbon cycle history. Terrestrial NPP shows an increasing trend beginning in 1930 and is estimated to increase at a rate of 1.1% over the 10-year period in the 1980s. Although all these methods show a recent increase in NPP, satellite-based estimation using the LaRC data set shows a larger trend than the others. A comparison of he trends estimated by these methods indicates that it is necessary to improve the accuracy of incoming surface radiation data, CO2 emission history from changes in land-use change and model structure.

A monthly stream flow model for estimating the potential changes of river runoff on the projected global warming

Hydrological processes in high altitude mountainous regions differ from those in more temperate regions, primarily due to such influences as cold temperatures, large and rapid change in surface energy balance during snowmelt, a long period at low-temperature environmental condition and the existence of permafrost. A physically based, semi-distributed water balance model to quantitatively simulate the hydrological processes and stream flow, as well as to estimate the potential consequences of projected global warming on stream flow for such high altitude mountainous regions was constructed. Distributed meteorological data from the interpolation of the point measurements by means of a digital elevation model (DEM) of the basin, such as air temperature, precipitation, snowfall ratio, wind speed, etc., have been used as model input. Several other hydrological parameters, such as soil moisture content and evapotranspiration, which are essential in simulation of river runoff in a water balance state, were estimated by the combination of Landsat TM and a DEM with the utilization of the distributed meteorological data. The model uses only a few crucial parameters for calibration, and the model structure is based upon estimating the stream flow components. Simulated results of spatially distributed soil moisture content, evapotranspiration and monthly discharge yield reasonable agreement, both spatially and temporally, to the field observations or the estimated results by the other approaches. This physically based model has the potential to project stream flow under the possible climate scenarios. Copyright

Observation and estimation of daily actual evapotranspiration and evaporation on a glacierized watershed at the headwater of the Urumqi River, Tianshan, China

In order to develop an effective approach to estimate evaporation and evapotranspiration (ET) in the glacierized watershed for hydrological process studies, the meteorological and energy budget data obtained in 1986 from a small watershed at the headwater of Urumqi River Basin, Tianshan, China were utilized. The field observation of ET and surface soil water content was briefly described. The performance of several existing approaches in estimating the daily actual ET and evaporation for the two kinds of predominant ground surface in the basin (i.e. the alpine tundra and glacier surface) was examined. The estimated daily ET from the alpine tundra for the summer season by Mortons equation correlates fairly well with the lysimeter-observed data, and the proposed wetness index by means of the principle of Mortons complementary relationship, provided reliable information of the surface soil wetness condition for the same period. For the other seasons, Kojimas equation was adopted for estimation of daily evaporation on both alpine tundra and the glacier surface when most parts of the watershed were snow covered. The daily evaporation on the glacier surface for the melting season was ultimately estimated by Ohnos equation for the non-melting season. The monthly mean evaporation estimated for the glacier surface was close to the field observation data obtained in 1987–1988. The annual evaporation obtained on both sites by this approach was in good agreement with that estimated with the water balance method for the same sites by Zhang et al. (1992). Copyright

A simple global carbon and energy coupled cycle model for global warming simulation: sensitivity to the light saturation effect

A simple Earth system model, the Four-Spheres Cycle of Energy and Mass (4-SCEM) model, has been developed to simulate global warming due to anthropogenic CO2 emission. The model consists of the Atmosphere—Earth Heat Cycle (AEHC) model, the Four Spheres Carbon Cycle (4-SCC) model, and their feedback processes. The AEHC model is a one-dimensional radiative convective model, which includes the greenhouse effect of CO2 and H2O, and one cloud layer. The 4-SCC model is a box-type carbon cycle model, which includes biospheric CO2 fertilization, vegetation area variation, the vegetation light saturation effect and the HILDA oceanic carbon cycle model. The feedback processes between carbon cycle and climate considered in the model are temperature dependencies of water vapor content, soil decomposition and ocean surface chemistry. The future status of the global carbon cycle and climate was simulated up to the year 2100 based on the “business as usual” (IS92a) emission scenario, followed by a linear decline in emissions to zero in the year 2200. The atmospheric CO2 concentration reaches 645 ppmv in 2100 and a peak of 760 ppmv approximately in the year 2170, and becomes a steady state with 600 ppmv. The projected CO2 concentration was lower than those of the past carbon cycle studies, because we included the light saturation effect of vegetation. The sensitivity analysis showed that uncertainties derived from the light saturation effect of vegetation and land use CO2 emissions were the primary cause of uncertainties in projecting future CO2 concentrations. The climate feedback effects showed rather small sensitivities compared with the impacts of those two effects. Satellite-based net primary production trends analyses can somewhat decrease the uncertainty in quantifying CO2 emissions due to land use changes. On the other hand, as the estimated parameter in vegetation light saturation was poorly constrained, we have to quantify and constrain the effect more accurately.

Evaluation of the Effect of Pre-processing of the Remotely Sensed Data on the Actual Evapotranspiration, Surface Soil Moisture Mapping by an Approach Using Landsat, DEM and Meteorological Data

Abstract A practical approach that combines Landsat TM, DEM and in situ conventional meteorological data to estimate the actual evapotranspiration (ET), and to predict the surface soil moisture content (SSMC) in a heterogeneous terrain was proposed and applied to the Urumqi River Basin, Tianshan, China. The successful application of the proposed approach to this meso‐scale watershed was found closely related to the procedures involved in the preprocessing of the remotely sensed data. The detailed investigation suggested that the atmospheric correction procedures is very sensitive to the resulted land cover classification, NDVI and surface albedo retrieved, which would ultimately affect the correct estimation of actual ET and accurate prediction of SSMC. This paper starts from the detailed description of the procedures in the preprocessing of the Landsat TM, DEM data and their combinations for the proposed ET, SSMC mapping approach. The different procedures were selected in the pre‐processing of the remotely sensed data to examine to what degree they will affect the final performance of the proposed approach on actual ET, SSMC estimations over the heterogeneous terrain on the Urumqi River Basin, Tianshan, China.

Reflectances in the Ganges and Brahmaputra rivers and in the adjacent coastal sea

The distribution patterns of reflectance were analysed from Landsat TM band 3, along the course of the Ganges and Brahmaputra rivers and their adjacent coastal sea. It was observed that during the high discharge period, reflectance values in the Ganges river are higher than that in the Brahmaputra. But in the low discharge period, it is vice versa. Reflectances varied significantly throughout the river courses but increased distinctly at their confluence zones. In the coastal sea, reflectance values decreased markedly with a narrow zone, which corresponded to the zone of spectral contrast as seen from band 3 image. Influence of river discharge in reflectance values is not distinct in the coastal sea.

Studies on Surface Temperature using Remote Sensing Technique in the NW Part of Bangladesh

Abstract A simple and straightforward technique based on remote sensing was applied for the estimation of surface emissivity and surface temperature at different dates in the northwestern part of Bangladesh for the summer and autumn period (March through October). Surface temperature was estimated from Pathfinder AVHRR Land (PAL) daily data. For this estimation different split window algorithms were employed. Estimated results show a discrepancy ranging from 0.04°C to 4.0°C. Surface emissivity was calculated based on fractionation of vegetation cover and NDVI at pixel level calculated from TM data. TM data was corrected for atmospheric effect. PAL data set was provided after atmospheric and geometric correction. Both air temperature and estimated surface temperature shows a decreasing trend from south to farther north. However, no distinct trend could be recognized in the temporal variation in surface temperature and in measured air temperature. A general trend of negative relationship could be concluded between NDVI and surface temperature from this study.

Cross-Well Tomography Experiment In a Highly Fractured Area

Sumzary A preliminary cross-well tomography experiment was carried out in the Tanna area, where the Tanna fault is well defined through extensive geological and geophysical investigations. The purpose of the test was to compare some commercially available borehole energy sources for subsequent full scale cross-well tomography survey and to figure out the subsurface geology by means of the preliminary tomographic data. Two wells were drilled on each side of the fault. The geology around two well showed a large difference indicating a considerable amount of lateral displacement along the fault. Two borehole energy sources, Bolt airgun and Prakla BASD, were applied and it was concluded that both energy sources could provide seismic waves at least UP to 400 Hz. Also it was found that the energy of the 40 cubic inch 2400 PSI airgun was almost equivalent to that of 8 seismic detonators undrer the given condition in the Tanna test field. A preliminary reconstructed velocity profile with limited number of raypaths well coincided with sonic and density data at the source and receiver wells. The effect of deviation of the well to tomographic image reconstruction was also investigated.