Tetsuya Hiyama

A macro-scale hydrological analysis of the Lena River basin

A macro-scale hydrological analysis of the Lena River basin, located in eastern Siberia, Russia was carried out to understand both physical and biological roles in a cold region. A combined model which is composed of a SVAT model, runoff model and river routing model was proposed to explain snowmelt, evapotranspiration, thawing and freezing of permafrost, runoff formation and river flow. Two kinds of grid sizes were prepared for the combined model, in which a 1°×1° grid is used for the SVAT model and runoff model, and a 0·1°×0·1° grid for the river routing model. Both of them were averaged from GLOBE (NOAA, 1997). The river network was built up using digital elevation data of 0·1°×0·1° with the river discharge flow going to the largest difference in altitude of the surrounding eight grid points. Routine meteorological data at each 1°×1° grid over the basin were derived by spatial interpolation using observed data from forty meteorological stations. The analysed duration was set from October 1986 to September 1987. The results are as follows: The annual amount of estimated precipitation of the basin ranges from 100 mm to 900 mm. The area of over 400 mm is distributed mainly over the southeastern mountain area. The estimated evapotranspiration of the basin presents a clear grid distribution, the annual amount is larger than 200 mm in the most southern part and is less than 150 mm in the northern part. In spite of the fact that evapotranspiration in the southern part is large, the southern part of the basin becomes a sufficient region of runoff yield due to the large annual precipitation compared to the northern part. Copyright

A 6.5-year continuous record of sea surface salinity and seawater isotopic composition at Harbour of Ishigaki Island, southwest Japan

We produced continuous records of sea surface salinity and isotopic composition from 1998 to 2004 at Ishigaki Island, southwest Japan, and found clear seasonal variations in salinity and oxygen isotopic composition and increasing trends of them after 1999. These increasing trends could be principally due to the decreasing difference between local precipitation (P) and evaporation (E), as a result of the reduction of horizontal vapour transport from adjacent oceans. When samples collected in heavy rainfall events were excluded, the average Δδ18O/Δ salinity slope was obtained as 0.31, 0.35 in summer and 0.28 in winter. Estimated E/P ratios based on the isotopic box model are in good agreement with the ratios of independently estimated E to observed P. †Updated paper: originally presented on the IAEA International Symposium “Quality Assurance for Analytical Methods in Isotope Hydrology” (August 2004, Vienna).

Balance And Behavior Of Carbon Dioxide At An Urban Forest Inferred From The Isotopic And Meteorological Approaches

Diurnal variations in δ 14 C, δ 13 C and the concentration of atmospheric carbon dioxide in an urban forest were measured on 9 February 1999 to discriminate and quantify contributions from different CO 2 sources. The biogenic CO 2 concentration remained relatively constant throughout the day. However, anthropogenic CO 2 concentration fluctuated with the atmospheric CO 2 concentration, and seemed to be controlled by wind velocity and the amount of exhaust gases from fossil fuel burning. The vertical profiles of anthropogenic, biogenic, and total CO 2 showed a constant concentration within forest during daytime because of the large vertical CO 2 influx, strong winds, and neutral atmospheric condition. The biogenic contribution at night decreased from the forest floor upwards with a smooth gradient, while the anthropogenic contribution showed a direct mirror because of the location of respective CO 2 sources-the vertical gradient of wind velocity and the horizontal CO 2 supply.

How regional are the regional fluxes obtained from lower atmospheric boundary layer data

The contribution of the upwind source area to the surface fluxes determined from lower boundary layer profiles was studied. Use was made of the boundary layer data obtained from a 213-m meteorological tower and from a tethered sonde, and of surface fluxes from five stations deployed during the 3-week Tsukuba 92 intensive field experiment whose 16 by 16 km experimental area can be characterized by its generally flat topography and by its complex surface land usages. The profile-derived regional sensible heat fluxes were compared with the area-averaged fluxes from the flux data of the surface stations, evaluated for the upwind areas of different sizes from approximately 3 × 10−3 km2 to 70 km2. The best agreement was found for the comparison with the results for the upwind area of around 17 km2. This roughly corresponds to the height to fetch ratio of 200 when one considers the mean surface layer height of 40 m.

Variability of surface fluxes within a complex area observed during TABLE 92

Abstract A data set obtained in a 3 week intensive field experiment called Tsukuba Atmospheric Boundary Layer Experiment (TABLE) 92 allowed some investigation on the heterogeneity of the surface fluxes within a 16 km by 16 km experimental area at and around Tsukuba city, Japan. The area can be characterized by its generally flat topography and by its complex surface land usages. The land covers include crop fields, pine woods and forests, houses or buildings, grass fields, and paddy fields. Five flux stations were installed, in addition to two existing stations, to determine the energy and radiation balance components at each of the major surface types. The analysis of these data made in the present paper has revealed that the spatial variations of surface fluxes were caused by two independent factors. The first one is the non-uniform nature of the incoming solar energy Sd over the area, owing to the inhomogeneous presence of clouds, and the second factor is the variation of the surface cover. The former effect is important only when the sky is partly cloudy. The coefficient of variation for the hourly mean Sd, defined by the ratio of the standard deviation and the mean of Sd measured simultaneously at different stations, was of the order of V = 0.2, whereas the corresponding values of V for the net radiation, latent and sensible heat fluxes owing to the latter effect alone were around V = 0.3–1.0. Thus the variation of the surface cover had a somewhat larger effect on the inhomogeneity of the surface fluxes in the area.

Flux determination over a smooth surface under strongly unstable conditions

Careful micrometeorological measurements on an empty parking lot allowed determination of the surface fluxes of sensible heatH and of momentum by applying profile equations derived from Monin-Obukhov similarity theory with two sets of the stability correction function for momentum Ψm and sensible heat Ψh. These fluxes were compared with reference values ofH independently determined by means of an eddy correlation technique. In general, better agreement was found betweenH values derived from profiles with the stability functions of Brutsaert (1992) and referenceH values, than when the Businger-Dyer functions were used to deriveH. The disagreement in the latter comparison was especially serious under strongly unstable conditions, with the value ofy=−z/L (wherez is the height andL is the Obukhov length) larger than 10. A closer look at the procedure for calculatingH from the profiles revealed that the large differences between theH values derived with these two different versions of the stability correction functions were caused by the small differences of the Ψh values, and not by the larger differences of the Ψm values. This result stems from the strong sensitivity of the resultingH values on the choice of Ψh.

Land surface identification near Yakutsk in eastern Siberia using video images taken from a hedgehopping aircraft

Aircraft-based remote sensing was carried out over the Lena River region (approximately 100 km horizontal scale) near Yakutsk, in eastern Siberia, on nine days between 24 April and 19 June 2000. A home-use video camera was installed on the aircraft and aerial images of the land surface were recorded. By flying at very low altitudes (100 m and 150 m), we compensated for the low resolution of the camera. In all, 5515 scenes were sampled at 10-s intervals from the video, visually interpreted, and classified. The snow/ice and water cover conditions were also interpreted. The results showed that grassland covers the riverine lowland (RLL) of the Lena River, while the principal form of land cover over the terraces is larch forest (about 61%). There was a remarkable contrast between the left (LBT) and right (RBT) bank terraces of the Lena River, no-forest areas covered 28.8% of the RBT, whereas no-forest areas covered only 13.8% of the LBT. In addition, the LBT had a greater proportion of birch forest than the RBT (15.2% vs 2.5%). The video data over eight days showed that the snow thawed first in the RLL and last in the forested areas of the LBT and RBT. An increase in the area covered by water in the RLL in mid-May was probably due to flooding by the Lena River. Distinguishing insolated from cloud-shadowed scenes of the land surface revealed that the insolation rate was lower over the LBT and RBT than over the RLL. The land-cover database created in this analysis will play an important role as basic, reliable ground-truth information for studies using satellite images.

Evaluation of the Surface Water Distribution in North-Central Namibia Based on MODIS and AMSR Series

Semi-arid North-central Namibia has high potential for rice cultivation because large seasonal wetlands (oshana) form during the rainy season. Evaluating the distribution of surface water would reveal the area potentially suitable for rice cultivation. In this study, we detected the distribution of surface water with high spatial and temporal resolution by using two types of complementary satellite data: MODIS (MODerate-resolution Imaging Spectroradiometer) and AMSR-E (Advanced Microwave Scanning Radiometer–Earth Observing System), using AMSR2 after AMSR-E became unavailable. We combined the modified normalized-difference water index (MNDWI) from the MODIS data with the normalized-difference polarization index (NDPI) from the AMSR-E and AMSR2 data to determine the area of surface water. We developed a simple gap-filling method (“database unmixing”) with the two indices, thereby providing daily 500-m-resolution MNDWI maps of north-central Namibia regardless of whether the sky was clear. Moreover, through receiver-operator characteristics (ROC) analysis, we determined the threshold MNDWI (−0.316) for wetlands. Using ROC analysis, MNDWI had moderate performance (the area under the ROC curve was 0.747), and the recognition error for seasonal wetlands and dry land was 21.2%. The threshold MNDWI let us calculate probability of water presence (PWP) maps for the rainy season and the whole year. The PWP maps revealed the total area potentially suitable for rice cultivation: 1255 km2 (1.6% of the study area).

Determination of canopy emissivity: how reliable is it?

Abstract Two sets of simple equations to evaluate probable error in the determination of the canopy emissivity ϵ of the infrared region (8–14 μm) are presented. They can be used for prognostic as well as for diagnostic analysis of the error. In a prognostic mode, an application of the equations to a commonly used method to determine e developed by Fuchs and Tanner (Fuchs, M. and Tanner C.B., 1966. Infrared thermometry of vegetation. Agron. J., 58: 597–601), for various assumed conditions indicates that the error of the single emissivity determination of the order of 0.01 should be expected even if measurements are carefully carried out under favorable conditions. In a diagnostic mode, the equations are applied as an example to evaluate of the magnitude of error in the actual determination of emissivity of a pasture canopy with different LAI values.

A Comparison of Flux Variance and Surface Renewal Methods With Eddy Covariance

Sensible and latent heat fluxes are important parameters for understanding energy/mass interactions. At present, there are a number of methods available to estimate these fluxes, with varying degrees of difficulty and success. In the present study, we explore the use of the flux variance method and the surface renewal method to estimate the sensible and latent heat fluxes in two agricultural sites in China. The results of these two methods were compared to direct measurements using eddy covariance. The study sites, a wheat field in a semi-arid area and a rice paddy in a humid area provide an opportunity to examine two extreme conditions. In the arid conditions, both estimation methods provided similar sensible heat flux estimations but higher latent heat flux estimations with respect to the eddy covariance measurements. The sum of the estimated sensible and latent heat fluxes was in good agreement with measurements. In the rice paddy, sensible heat flux estimated by both methods showed similar results, whereas latent heat fluxes estimation were 1.2 times greater than eddy covariance measurements. The surface renewal method underestimated the Bowen ratio over the rice paddy, but overestimated it over the wheat field. In conclusion, both methods appear more appropriate for the estimation of sensible heat flux rather than for latent heat flux, in particular in humid environments.