Abstract
Trend estimation of river discharge is an important but difficult task because discharge time series are nonlinear and nonstationary. Previous studies estimated the trend of discharge using a linear method, which is not applicable to nonstationary time series with a nonlinear trend. To overcome this problem, we used a recently developed wavelet-based method, ensemble empirical mode decomposition (EEMD), which can separate nonstationary variations from the long-term nonlinear trend. Applying EEMD to annual discharge data of the 925 world’s largest rivers from 1948–2004, we found that the global discharge decreased before 1978 and increased after 1978, which contrasts the nonsignificant trend as estimated by the linear method over the same period. Further analyses show that precipitation had a consistent and dominant influence on the interannual variation of discharge of all six continents and globally, but the influences of precipitation and surface air temperature on the trend of discharge varied regionally. We also found that the estimated trend using EEMD was very sensitive to the discharge data length. Our results demonstrated some useful applications of the EEMD method in studying regional or global discharge, and it should be adopted for studying all nonstationary hydrological time series.
Highlights
IntroductionThe amount of global river discharge is roughly balanced by land precipitation that originates from ocean evaporation
River discharge to the world oceans is an important component of the global hydrological cycle.In a steady state, the amount of global river discharge is roughly balanced by land precipitation that originates from ocean evaporation
This contrasts the result of no significant trend as estimated by the MK method that imposes a linear fit to a nonlinear trend
Summary
The amount of global river discharge is roughly balanced by land precipitation that originates from ocean evaporation. As a result of climate change, increasing atmospheric CO2 , land-use change, and so on, both the regional and the global water cycle deviated from the steady state [1,2,3,4]. Quantifying the trend of global river discharge is important for understanding how various external factors, climate change, land-use change, atmospheric CO2 , and water availability on land influenced the hydrological cycle. The estimated trends of global river discharge have not been consistent among different studies, ranging from a significant increase [5] to no significant trend [6,7]. The reason for this discrepancy can be explained by the differences in the number of gauging stations used, in the period of investigation, or in the method used to estimate trends [8]
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