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Atmospheric vapour flux convergence is introduced for the estimation of the water balance in a river basin. The global distribution of vapour flux convergence, -OH + Q is estimated using the European Centre for Medium-Range Weather Forecasfs global analysis data for the period 1980-1988. From the atmospheric water baknce, the annual mean -VH * Q can be interpreted as the precipitation minus evaporation. The estimated -VH -Q is compared with the observed discharge data in the Chao Phraya river basin, Thailand. The mean annual values are not identical, but their seasonal change corresponds very well. The four year mean -VH -Q is also compared with the climatological runoff of nearly 70 large rivers. The multi-annual mean runoff is calculated from the Global Runoff Data Centre data set and used for the comparison. There is generally a good correspondence between the atmospheric water balance estimates and the runoff observations on the ground, especially in the mid-and high latitudes of the northern hemisphere. However, there are significant differences in many instances. The results emphasize the importance of accurate routine observations in both the atmosphere and river runoff. The global water balance of the zonal mean is compared with prior estimates, and the estimated value from this study is found to be smaller than previous estimates. The annual water balance in each ocean and each continent are also compared with previous estimates. Generally, the global runoff estimation using the conventional hydrological water balance is larger than the result by the atmospheric water balance method. Annual freshwater transport is estimated by atmospheric water balance combined with geographical information. The results show that the same order of freshwater is supplied to the ocean from both the atmosphere and the surrounding continents through rivers. The rivers also carry approximately 10% of the global annual freshwater transport in meridional directions as zonal means.KEY WORDS Atmospheric water balance method Continental hydrology Large-scale evapotranspiration Total water storage Freshwater transport

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Abstract:The decrease of river runoff draining into Lake Balkhash in Central Asia was investigated using hydrological and meteorological data over a long-term period. The data from the difference integral curves of the annual runoff from 1911 to 1986 suggested that a low-flow period began in 1970 in the River Ili, and in 1973 in the east rivers, continuing until 1986. Compared with the runoff before 1969, the decrease of runoff in the upper reaches of the River Ili was less than those in the middle and lower reaches. The decrease in the upper reaches resulted from natural variability, whereas those of the middle and lower reaches were due to a combination of the effects of climate and human activity. There are two reasons for this. First, after 1970, precipitation did not decrease uniformly across the entire basin. Second, in 1970, the Kapchagay dam started operating. As a result, the annual inflow into Lake Balkhash from the River Ili after 1970 decreased to 77% of the pre-1969 mean. Similarly, after 1973, the inflow from the east rivers was 75% of the pre-1972 mean.The characteristics of the runoff in the growing season (April to September) and those in the non-growing season (October to March) were investigated for the upper, middle, and lower reaches of the River Ili. In all reaches, the runoff in the non-growing season remained unchanged both before and after 1969. However, from 1970 to 1986, decreases of river flow in the growing season were 2Ð0 to 2Ð5 times greater in the middle and lower reaches than in the upper reaches. This was primarily due to the impacts of human activity. In conclusion, it can be deduced that the decrease in runoff after 1970 in the middle and lower reaches was the result of human activity during the low-flow period.

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