The Observed State of the Water Cycle in the Early Twenty-First Century

Abstract

This study quantifies mean annual and monthly fluxes of Earth's water cycle over continents and ocean basins during the first decade of the millennium. To the extent possible, the flux estimates are based on satellite measurements first and data-integrating models second. A careful accounting of uncertainty in the estimates is included. It is applied within a routine that enforces multiple water and energy budget constraints simultaneously in a variational framework in order to produce objectively determined optimized flux estimates. In the majority of cases, the observed annual surface and atmospheric water budgets over the continents and oceans close with much less than 10% residual. Observed residuals and optimized uncertainty estimates are considerably larger for monthly surface and atmospheric water budget closure, often nearing or exceeding 20% in North America, Eurasia, Australia and neighboring islands, and the Arctic and South Atlantic Oceans. The residuals in South America and Africa tend to be smaller, possibly because cold land processes are negligible. Fluxes were poorly observed over the Arctic Ocean, certain seas, Antarctica, and the Australasian and Indonesian islands, leading to reliance on atmospheric analysis estimates. Many of the satellite systems that contributed data have been or will soon be lost or replaced. Models that integrate ground-based and remote observations will be critical for ameliorating gaps and discontinuities in the data records caused by these transitions. Continued development of such models is essential for maximizing the value of the observations. Next-generation observing systems are the best hope for significantly improving global water budget accounting.

Global Precipitation: Means, Variations and Trends During the Satellite Era (1979–2014)

“…This 3% difference is within the error bars (Adler et al 2012) of GPCP, but is also of the same sign (and rough magnitude) of the adjustment needed for water cycle closure . Thus, for the same time period the new V2.3 numbers seem to compare very well with the Behrangi et al (2014) satellite estimates and the Rodell et al (2015) water balance values. Another merged data analysis product, the Climate Prediction Center Merged Analysis of Precipitation (CMAP) (Xie and Arkin 1997) has a very similar global ocean mean value (Behrangi et al 2014), but with a higher value in the tropics and a much lower value in high latitudes (Adler et al 2012;Behrangi et al 2014).…”

“…For example, Trenberth et al (2009) adjusted the GPCP value upward by 5% to achieve a global water balance. In another, more recent, example, Rodell et al (2015) examined the mean global water cycle for the first decade of the twenty-first century by using various satellite-based and conventional data sets, with reanalysis used to fill certain gaps. Initially, they used the magnitudes of the components as given by the means over the first decade of the twenty- first century.…”

“…On the other hand, Stephens et al (2012) argued that the GPCP value must be increased by roughly 15% to balance the energy budget. However, L 'Ecuyer et al (2015), in the companion energy budget analysis paper to the Rodell et al (2015) water cycle study, found an energy balance using the same GPCP values as Rodell et al These GPCP adjustments (up to 5% over ocean) fall within the estimated errors for GPCP (Adler et al 2012) and give some confidence that the GPCP global-scale magnitudes are close to the actual values, or at least fit comfortably with the estimates of the other water cycle components. The very small adjustment over land reflects the value of the gauge data (Schneider et al 2014) and the careful analyses thereof in GPCP, before its blending with the satellite data.…”

Understanding terrestrial water storage variations in northern latitudes across scales

“…Further, the model does not include any human-induced changes in water storages, which yet contribute to observed TWS variability in many regions Rodell et al, 2015). Other simplified or ignored hydrological processes include the coincident occurrence of rain and snowfall, liquid water capacity of snow, interception, freeze-thaw dynamics within the soil, capillary rise, and other surface-groundwater interactions, the effect of vegetation or other surface properties, and lateral flow from one grid cell to another.…”

Variations of global and continental water balance components as impacted by climate forcing uncertainty and human water use

“…Their values probably also included Antarctica, as they mention a land area of 144 000 km 2 , so a direct comparison is not straightforward. Based on the assessment of Rodell et al (2015) (see next paragraph), Antarctica's share in global P , AET, and Q is about 2.1, 0.2, and 4.9 %, respectively, and these percentages were added to the WaterGAP results. Surprisingly, global P of PGFv2.1 is 7.4 % lower than P of Hanasaki et al (2010).…”