In 1995, Thailand suffered from serious flood disaster. In Hydrology Division of the Royal Irrigation Department (RID), stress has been placed on flood problems again. In the present study, we studied relation between discharge of the Chao Phraya River at Nakhon Sawan and that from the upstream basin during flood period of 1994 to 1996. In and around low flat area near the Nakhon Sawan confluence point, habitual flooding and inundation occur particularly along the Yom and Nan Rivers, resulting in natural flood retarding effect. Flood storage volume in 1996 was calculated approximately 4.0 thousand million m3 for the study area, and actual capacity was estimated larger than that. Around a half of the upstream flood discharge originates from the Nan River. Also, we tried to make a simple and practical applied tank model which represents flood retarding effect. Main input data are sum of the three daily discharges on the Ping, Yom and Nan Rivers, and output is the Chao Phraya River daily discharge at Nakhon Sawan (RID C. 2 station). The obtained model, consisting of a river channel tank and three retarding area tanks, presented comparatively fine results. We discussed on probable effect induced by preparedness against flood in 1996. It was presented that degree of model adaptability to other floods may be variable, depending on selected floods for calibration.

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The functions that a dam reservoir increases the scanty runoff were evaluated at 11 catchments in Kanto area by comparing with the discharge-duration curves that were made of discharge from a forested catchment and dam. In this report, two indexes, the duration-stabilized-transform ratio, the achievement ratio, are introduced new. These indexes are used to discuss to what degree dam reservoir stabilizes discharge. In normal years, values of these indexes in 10 dams among 11 were small relatively, in other words these dam reservoirs didn't play a roll in increasing the scanty runoff. That means these dam reservoirs didn't contribute to stabilizing discharge. In the drought year, the duration-stabilized-transform ratio became larger in 7 catchments. And 5 dams among 7 had contributed to stabilizing discharge.

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The seasonal change of surface-soil layer processes in permafrost region was examined at several places in the central Tibetan Plateau from October 1991 to September 1993. Ground temperature and soil moisture conditions were much different among the observed places within an area of 10×10km2, though conditions of air temperature and precipitation were similar in the observed area. Soil moisture of surface layer decreased in winter season; this amount was estimated to be 5 to 36mm. Seasonal change of depth of melting layer caused change of void amount in melting layer, since the bottom of the melting layer behaved as impermeable interface in the permafrost. At the same time, the difference of depths of melting layer at several places caused difference of void amount and soil moisture in the melting layer. Furthermore, this difference of soil moisture caused difference of melting speed and thickness of active layer which is maximum depth of melting layer. Especially, difference of soil moisture at the initial stage of melting season among the observed sites expanded the differences of ground temperature and soil moisture conditions. The calculated thicknesses of active layer in 1992 and 1993 were 229 cm and 232 cm respectively; these values were similar in spite of smaller thawing index in 1993 than that in1992.

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The seasonal change and relation between evaporation and surface-soil layer processes in permafrost region of the central Tibetan Plateau were examined from June to September 1993 at 2 sites where conditions of soil moisture were different. The observation period was divided into two periods based on the evaporation ratio which was a ratio of evaporation from natural soil to that from the soil saturated with water. The former period was before 20 June, corresponding to the pre-monsoon season (dry period). The latter was after 20 June corresponding to the monsoon season (wet period). The evaporation was controlled by soil moisture in the dry period at both 2 sites which have different surface conditions, bare ground (Site A) and earth hummock covered with grass (Site B). The average daily evaporation in the dry period was 2.1 mm/d at the small soil moisture site (Site A), and was 2.6 mm/d at the large soil moisture site (Site B), while in the wet period at both sites were 3.3 mm/d. In this region where precipitation is much compared with other regions in the Tibetan Plateau, evaporation was different by surface-soil layer condition in the dry period, but evaporation became similar in the wet period. Total amounts of total evaporation in the whole periods did not so much depend on site conditions, since the total evaporation in the dry period was little.

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This study proposes a one-dimensional numerical model to estimate the heat transfer in permafrost regions by using the routine meteorological data. The model includes snowcover and active layer subprocesses, both of which are general characteristics of a permafrost region, and represents an interaction between atmosphere and land surface. A numerical simulation with this model is carried out over one year using the routinely recorded daily meteorological data obtained from four sites in Lena River basin, which is located in eastern Siberia, Russia. The result shows a good agreement with observed snow depth. Compared with daily air temperature and minimum land surface temperature data, the simulated temperatures of the land surface also represent understandable variations. In addition, the active layer depth in the four stations are determined. Because this model requires only the routine meteorological data as input data, it could be used not only to study past hydrological events and to predict the depth of snowcovers and active layers under the climate change in permafrost regions, but also to predict river discharge by connecting with a model of runoff process.

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As an engineering application of the stormwater runoff model which was presented by Fujimura and Ando(J. of Japan Society of Hydrology and Water Resources, 10(4)), we carried out a stormwater runoff analysis for three urban basins using actual and design rainfall on the assumption that infiltration facilities are set in these basins. In this study, the design infiltration intensity are given varying from 0mm/h to 40mm/h because the details of infiltration facilities setting for a basin, such as the numbers and kinds, are not considered so that this simulation will be utilized for a flood runoff control plan for a basin scale. To conclude, we evaluated that infiltration facilities are effective for the total runoff control rather than the peak runoff control, and also we showed the method how quantitatively estimate runoff control efficiency by infiltration facilities when these are set in a basin.

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Diurnal variations of convective activities in the southeast Asian monsoon region are examined using hourly data about 0.1 degree in spatial resolution from Geostationary Meteorological Satellite for one year. Large diurnal variations of convective activities exist over land, particularly in coastal area of the Indochina Peninsula. The variations are stronger than those over the Gulf of Thailand and the Andaman Sea. The large diurnal variations over land should be due to the strong surface heating during daytime and the interaction between land-sea circulations and the southwest monsoon. The characteristics of the diurnal variations during the rainy season are classified into two; one is May class (May, Sep. and Oct.) and the other is July class (Jun., Jul. and Aug.). The diurnal variations are strong in both classes over the Indochina Peninsula but they are stronger in July class than in May class over the Andaman Sea and North Vietnam. Seasonal changes of the diurnal variations of convetive activities is examined over five typical grids both over land and sea. Over land, the peak of the diurnal variations is found to be around on sunset, however, there are semi-diurnal cycles over ocean and the peaks are at 5 LST and 17 LST. It is also found that the seasonal changes of the diurnal variations of convective activities correspond to those of observed precipitation.

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A rice paddy field is one of the typical land cover in monsoon Asia, which affects both meteorological and hydrological aspects through water and heat budgets at land surface. In this study, we observed the rice canopy using a visible-near infrared video camera in order to vertify the ground truth for satellite data. Purposes of this study are given an information of the relationship among diurnal changes in spectral reflectance, vegetation status and land surface fluxes. The visible-near infrared video camera is available for measurement of the “spatial” spectral reflectance and diurnal changes in digital counts of video camera reflect these in canopy status.

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Irrigation schedules are discussed, focusing on crop rotation types recently introduced in the Shavour Areas located in the central parts of Khuzestan Province in Iran. The design is based on guidelines provided by the Ministry of Agriculture, Forestry and Fisheries of Japan. The farm pond irrigation system is applied in the fields of 300ha irrigated by existing diversion dams constructed in the Shavour River, and are designed under conditions of (1) four variations of amounts of river water resources, (2) economical volumes of the farm pond, (3) three types of crop rotation and (4) two cultivation types which show the monocultured and mixed cultured crop patterns per village. In the fields not irrigated from the Shavour River, the small size of irrigation system with pond and water harvesting apron is applied in the traditional cultivation fields of 300m2, and are designed under conditions of (1) harvesting water efficiency of 80% and application efficiency of 60% and (2) daily irrigation interval and designed irrigation intervals estimated by the soil moisture depletion method.

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