GLOBAL ATMOSPHERIC WATER-BALANCE AND RUNOFF FROM LARGE RIVER BASINS

Atmospheric vapour flux convergence is introduced for the estimation of the water balance in a river basin. The global distribution of vapour flux convergence, -del(H) .($) over bar Q is estimated using the European Centre for Medium-Range Weather Forecasts global analysis data for the period 1980-1988. From the atmospheric water balance, the annual mean -del(H) .($) over bar Q can be interpreted as the precipitation minus evaporation. The estimated -del(H) .($) over bar Q is compared with the observed discharge data in the Chao Phraya river basin, Thailand. The mean annual values are not identical, but their seasonal change corresponds very well. The four year mean -del(H) .($) over bar 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.