The Water Budget And Water Cycle Over The Yangtze River Basin And The Yellow River Basin

The Yangtze and Yellow River basins, the largest river basins in China, sustain the most densely populated region of China by providing water and energy resources. Being located in East Asia Monsoon area, the characteristics of the water budgets and water cycle over these two basins might be affected by different circulation systems in the low, middle, and high latitudes. Since limited efforts have been done on the water budgets and cycle over the two big closed river basins at a continental scale, especially from the aspect of the atmospheric branch, the climatology as well as the interannual and interdecadal variability of the water budgets and water cycle over this two closed basins are therefore studied in this paper. Firstly, the seasonal cycle, interannual variability, and linear trend of the water budgets over the two rivers are investigated from the observations. And the ability of NCEP/NCAR reanalysis (NRA) to describe the water budgets over the two basins are validated. Then the NRA is used to characterize the annual cycle of the vertical distribution of the water vapor flux through the boundaries of the river basins, and the contributions of the mean and transient eddy transport to the total moisture convergence for every month is studied. Furthermore, the structures of the large-scale moisture transport and circulation anomalies responsible for the interannual variability of the water vapor budget for each basin during summer monsoon month (May to September) are examined. And the roles of mean and transient eddy transport budgets in the interannual variations of the total moisture budget are compared. Finally, the mechanisms of the interdecadal variability of the water budget in the 'seasonal cycle progressing in early spring' for the two basins are investigated. The main conclusions are as follows: 1. The observation shows that the water budgets of the Yangtze River basin have larger values and more obvious seasonal and interannual variability, and lead one month in the seasonal cycle then those of the Yellow Rive basin. But when divided by the budgets themselves, the seasonal and interannual variations of the water budgets in the Yangtze River are less then those in the Yellow River. Evaporation is almost equal to runoff over the Yangtze River basin, but about four times as much as runoff over the Yellow River bain. For the Yangtze River, the precipitation in January, June, and July has increased significantly, the precipitation in May has decreased significantly during 1958 to 2000. And for the Yellow River basin, the precipitation in June has increased significantly while those in August, September, November, and annual mean has decreased significantly during the last 43-year period. During 1958-1990, a significant increasing can be found in runoff in June, but in August, November, and December runoff has decreased significantly over the Yellow River. 2. The artificial residual forcings in the NRA water budgets over the Yangtze and Yellow River basins greatly affect the budgets in total amount, seasonal cycle, interannual variability, and linear trend. The NRA overestimates most of the water budget components over the two River basins, except for underestimating precipitable water for both rivers and moisture convergence over the Yangtze River and having almost equal moisture convergence over the Yellow River with that of the observation. The water budgets in NRA can well characterize the seasonal and interannual variability, except for the wrong runoff seasonal variations over the Yellow River, and the bad annual mean evaporation interannual variations over the Yangtze River basin. Most of the significant linear trends in the water budgets in the observation can be caught in the NRA, but there are some exaggerative linear trends in it. 3. The Yangtze and Yellow River basins are always moisture sinks except in the winter months in climatology mean. The mean circulation brings plenty of moisture to the Yangtze River almost through out the year, but causes the moisture divergence over the Yellow River in most months except from June to September. The annual cycle of the mean circulation transport budgets is consistent with that of the total moisture convergence over both rivers. On the contrary, the transient eddy always causes moisture divergence for the Yangtze River through out the year, but takes more water vapor to the Yellow river except from July to August. The annual cycle of the transient eddy transport budgets is almost out of phase with the total moisture convergence over these two basins. 4. The temporal signatures of the moisture convergence of the two rivers show some significant cycles on different time-scales. The interannual variations of the moisture convergence due to the mean circulation transport are in good agreement with that of the total convergence over these two rivers, however the transient eddy dose not play any significant role on the interannual variability of the total moisture budgets. It is interesting to note that the vapor budgets due to the mean and transient eddy transport are negatively correlated with each other. 5. From May to September, the structures of the large-scale moisture transport and circulation anomalies responsible for the interannual variations of the monthly moisture budget are quit different. Accompanied with the strengthening and northward moving and then weakening and southward retreating of Asian Monsoon,the amount of the vapor fluxes at different boundaries vary largely. Therefore results in the key water vapor flux routeways and large-scale circulations, which are important to the interannual variations of the total moisture budget, are different in various months, especially over the Yangtze River basin. 6. Note that after the late 1970's, the weakening of the trough over East Asia becomes significantly slower, and the strengthening of the ridge over the west China becomes significantly faster at the middle troposphere during the early spring. At the same time, at the lower troposphere, the strengthening and northward extending of the west part of the subtropical high, and the weakening of the trough over the southwest part of the Aleutian low in the early spring becomes significantly slower. This significant decadal change of circulation causes a sharp decrease in the speed of the northward extending of the water vapor transport, and the speed of the increasing of the water vapor budgets over the Yangtze and Yellow River basins during the early spring after 1978.