Evapotranspiration data obtained above various vegetated surfaces were analyzed. The amount of evapotranspiration E is depend on the meteorological parameters (radiation, air temperature, water vapor pressure, and wind speed), the ground surface parameters (vegetation type and leaf area index), and the precipitation amount. Since the potential evaporation Ep(Kondo and Xu, 1997) represents the meteorological parameters, the non-dimensional evapotranspiration E/Ep is investigated as functions of the precipitation amount and leaf area index. The amplitude of seasonal cycle of E/Ep is small for a typical Japanese basin composed of the evergreen and deciduous forests, but it is large for a deciduous forest and a grassland. The long-term mean of E/Ep for several months increases as the precipitation amount increases. The dependence of E/Ep on the leaf area index LAI is large for the forest, and it is small for the grassland. For the paddy field, on the other hand, E/Ep is independent of LAI. For the forest, the value of E/Ep is from 0.1 to 0.2 when LAI=0, and from 0.7 to 0.8 when LAI>5. For the grassland, the value is from 0.3 to 0.6 when LAI=0, and about 0.8 when LAI>2.

View full abstract

As proposed by the IHP-V projects (the fifth phase of the International Hydrological Programme), understanding of global hydrological and geochemical processes, and integrated water resources management in arid and semi-arid zones are important themes for the present and future researches on hydrology. In this paper, the global properties of annual evapotanspiration were discussed, based on the hydrological data which were collected in a catchment of a semi-arid zone, Central America and in several catchments of Japan. Aa a result, a global tendency of the relationship between the annual ratio of actural evapotranspiration to potential evaporation (Et/Ep) and annual precipitaion was found and it was concluded that the ratio for the semiarid catchment was very small, compared with those for catchments in Japan. The tendency suggested that annual evapotranspiration from catchments is greatly dependent on wetness and vegetation in catchments.

View full abstract

Observations of rainfall and runoff were made at the three experimental catchments in Malaysia and Indonesia to examine rainfall and runoff characteristics in the tropical forested headwater basin in the South-East Asia. The runoff from the catchment covered by a natural tropical rain forest has sometimes ceased whereas a continuous plentiful discharge was observed in the small stream from the catchment covered by a fast growing tree species after clear felling. Rainfall, evapotranspiration and bedrock were considered to be the three main factors for determining rainfall-runoff characteristics in the tropics. Recent progresses and problems of tropical forest hydrological experiments and further research needs in the 21st Century were discussed.

View full abstract

Historical and prospective direction of water management on floods was discussed and the functions of paddy areas were evaluated from the viewpoint of flood retention and mitigation in neighboring urban areas and of its application to watershed management. First, it was pointed out that detention capacity of paddies has to be assessed as inherent potential, and a method to evaluate its function in a macro-view was proposed. This result leads to the analysis of how to utilize the function of flood storage capacity of paddies on watershed management. Secondly, perspectives of flood control and floodplain management in the 21st century were found on records in history. That is, flood management of “to collect water and to drain it quickly” methods by dams and levees, was shifted to watershed management with some combination of flood and water use. This is just the same direction of flood management found as natural practices at Japanese rivers in the early stage of the Tokugawa period and/or as those at watersheds in Southeast Asia of today. Finally, it was shown that the direction of technology application depends on each hydrologic condition in the world by citing drainage management in the Netherlands, change of regional water resources by forest clearing and agricultural development in Australia, and effective combination of floods and irrigation in the lower Mekong River.

View full abstract

International organizations are supposed to be instrumental in abating or resolving conflicts among riparian countries in international water systems. International organizations may function to (i) mediate among basin countries to solve a conflict, (ii) elaborate a collaborative mechanism for riparian countries, (iii) let riparian countries develop a framework for collaboration, and (iv) formulate a global framework about use of water resources in an international water system. International organizations could be instrumental as a mediator on an emergency, such as on the verge of a “water war”. A collaborative mechanism like Mekong Committee may be useful as long as riparian countries are not hostile each other, while the same mechanism may not be instrumental to abate a serious conflict between basin countries. The General Assembly and the Security Council are mechanisms essentially for an emergency, as is the case with other sorts of wars (e.g. over land or oil), which may not be functional as a mediator for conflicts among riparians. The recently adopted United Nations Convention on the Law of the Non-Navigational Uses of International Watercourses has, as shown by latest cases, a potential to become a really useful code of conduct in international water systems, which may serve to abate conflicts among basin countries.

View full abstract

Glaciers in Asian high mountain regions are fed mostly during summer while ablation takes place simultaneously, which is in contrast with those in the other regions of the world, where accumulation and ablation occur in winter and summer respectively. Great abundance of supraglacial debris is another important nature of glaciers in Asia : total glacial area with surface debris is much larger than the area of debris free ablation zone of glaciers. Ablation processers below a supraglacial debris layer are, hence, of great importance. Very rapid rates of shrinkage identified for glaciers in Asian high mountan regions could be related to these two kinds of nature of those in the area. The recent shrinkage of glaciers contributes significantly to the amount of river discharge. In other words, the amount of water supply from mountains is currently maintained with the additional water through the decrease of glacier masses. A sharp decrease of available water could take place, therefore, in near future in association with disappearances of some glaciers. This would be bery important in arid and semi arid regions in Asia where relative importance of glacier melt water is comparatively great.

View full abstract

This paper describes recent advancement of hydrologic frequency analysis. Stressing the importance of hydrological and statistical consideration of data, it introduces modern frequency analysis methodologies such as goodness-of-fit criteria and resampling methods. Four goodness-of-fit criteria are used for quantitative evaluation of frequency analysis models. Resampling methods include the jackknife and the bootstarp that can correct the bias of parameter and quantile (T-year event) estimates and provide the accuracy (standard error or variability) of the estimates. Also discussed is flood frequency analysis based on partial duration series (or peaks-over-threshold series). In addition, incorporation of lower and upper bounds to frequency analysis models is proposed as a more advantageous future method in terms of scientific and practical rationality. Finally, the conclusions session provides comprehensive perspectives of hydrologic frequency analysis and river planning towards the 21st Century.

View full abstract

The author forecasted the future of irrigation water demand of Japan from 2001's WTO to 2010 by a scenario analysis. The results showed that paddy fields decreased and surplus irrigation water came out. Otherwise, recently, FAO, WB and WWI published food supply in comparison with population of 2020 to 2030. These estimations explain lack of food caused by increasing population. Therefore, even in Japan, food production should be increased with securing irrigation water. Analyzing balance of demand and supply of rice for the past 100 years in Japan, well balance condition was exceptional and bad balance was a normal state. Therefore, food and water resources policies with long view will be more excellent.

View full abstract

The characteristics of hydrological environment in Japan are discussed by comparing with these of foreign countries. The strategy of utilization for water resources in the 21st century was examined from the view point of paddy-based agriculture. The results are as follows. The large circulation mechanism of the atmosphere formed by the Himalayas guarantees the sustainability of the water resources and rice growing in Japan and Southeast Asia. The rice growing areas of Australia which belong to a semi-aridity region is suffering from salinization due to a large-scale irrigation system. The sustainability of the Australia agriculture is doubtful. The paddy-based agriculture in Japan and Thailand assume responsibility for food productions and flood control effects. Their utilization of the water resources are the most suitable for the climate. The 21st century of Japan is a decrease in population. Then, for the moment, the strategic utilization for water resources makes the supply of securer foods. We should conduct investigation in order to write out the prescription for the food crisis due to population explosion of the world.

View full abstract