The Water Budget in the Amazon River Basin during the FGGE Period

Abstract

The seasonal change of the water budget in the Amazon river basin during the FGGE period is investigated, using the global objective analyzed data set, precipitation data and river discharge data. Substituting these data into the atmospheric water balance equation and that of the basin, the evapotranspiration and relative value of basin storage are estimated and the relationships of water balance terms are examined.The annual water vapor flux convergence, calculated using the FGGE "main" III-b data set analyzed at the European Centre for Medium-Range Weather Forecasts, is estimated to be smaller than the annual runoff obtained by river discharge data. This is due to the characteristics of this global objective analyzed data set that it is made by the 4-dimensional data assimilation method which expresses the divergence and convergence of wind in the tropics as weaker than in the real state. The seasonal change pattern of precipitation shows good correspondence to that of water vapor flux convergence, which is multiplied by the factor 1.37 in order that its annual value meets the annual runoff. Evapotranspiration, estimated by the atmospheric water balance equation, remains almost constant within a year and the seasonal change of basin storage is very large. This result indicates that evapotranspiration in the entire Amazon river basin is not affected by the seasonal change of basin storage even in the dry season.The evapotranspiration ratio (evapotranspiration/precipitation) of the dry season is larger than that of the rainy season. It is concluded that the role of evapotranspiration on the water cycle in the Amazon river basin is relatively more important in the dry season than in the rainy season.

Spatial and temporal variabilities of rainfall in tropical South America as derived from Climate Prediction Center merged analysis of precipitation

“…In the Amazon basin, about half of the annual precipitation drains to the Atlantic, and the remaining half is recycled over the basin (e.g. Salati, 1987;Matsuyama, 1992b). In the light of this observation, the ratio obtained, 0.57 (1139/2014), agrees well with these previous studies.…”

The Seasonal Change of the Water Budget in the Congo River Basin

“…(2), basin storage is difficult to measure directly. In addition, it is difficult to estimate the actual evapotranspiration in the tropics on a monthly basis by using the climatological data and the method as mentioned in Matsuyama (1992). Only the atmospheric water balance method is available in this case, by which evapotranspiration can be estimated as precipitation minus vapor flux convergence on a monthly basis.…”

“…8. It is interesting that the difference between the maximum and minimum values of the entire basin storage is about half/twice as large as that in the Amazon/Mississippi river basin (Matsuyama, 1992;Roads et al, 1994). In the case of the entire Congo river basin, the relative value of basin storage at the end of December should be zero, but it is not, due to the underestimation of the annual vapor flux convergence.…”

“…Since the annual runoff values obtained by previous studies are more than 300mm/year (Tables 1 and 3) and considered to be more precise than the annual vapor flux convergence, it can be said that the vapor flux convergence is estimated to be a little smaller. This difference is considered to reflect the characteristics of the global objective analysis of the ECMWF which expresses the divergence of wind in the tropics weaker than the real state (e.g., Masuda, 1988;Oki et al, 1992;Matsuyama, 1992), although the treatment of vapor has been improved since 1985 (Hoskins et al, 1989). The seasonal changes of the atmospheric water budget and NDVI in the entire basin are shown in Fig.…”

“…The first method is to multiply by a factor each monthly vapor flux convergence as in Matsuyama (1992). In the case of the Congo river basin this factor is 1.31, which is the ratio of runoff (332 mm/year) to vapor flux convergence (254 mm/year).…”