Note: unless otherwise indicated, all numbers in this post are volumetric fluxes in thousand cubic kilometers per year. One cubic kilometer equals about 810,000 acre-feet. For reference, a full Lake Mead, the largest reservoir in the USA, holds about 30 cubic kilometers. All human use is approximately 9100 cubic kilometers per year - about 300 full Lake Meads.
A paper, The Observed State of the Water Cycle in the Early 21st Century, just accepted for publication in the Journal of Climate (2015) and authored by M. Rodell, H.K. Beaudoing, T.S. L’Ecuyer, W.S. Olson, J.S. Famiglietti, P.R. Houser, R. Adler, M.G.Bosilovich, C.A. Clayson, D. Chambers, E. Clark, E.J. Fetzer, X. Gao, G. Gu, K. Hilburn, G.J.Huffman, D.P. Lettenmaier, W.T. Liu, F.R. Robertson, C.A. Schlosser, J. Sheffield, and E.F. Wood, estimates mean annual and monthly global water fluxes for the first decade of the 21st century.
Here is a very good story that explains the paper.
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.
Here are the paper's estimates of global mean annual fluxes:
Groundwater discharge to oceans is rolled into the stream discharge because the former is so much lower and more difficult to quantify. The authors used 2.3.
So how do these new figures compare with older estimates?
Below is a graphic that is my favorite representation of the global water cycle for my undergraduate students. It's from the hydrology text by Hornberger et al. Note that the fluxes are easily comparable; units are relative to the 100 units of precipitation that fall on the land, so one unit = about 1200 cubic kilometers per year. That is 20% larger than the units used in the above graphic. To compare the estimates in the two graphics, multiply the numbers in the lower graphic by 1.2.
After making the 1.2 adjustment, the graphics' estimates are generally quite close. The worst agreements are those between the oceanic E and P estimates. Not surprising.
The paper includes estimates of mean annual continental fluxes (see below).
The paper provides quantitative estimates of global fluxes with uncertainties. It deals with water and energy in an integrated manner. One set of estimates, the blue figures in the top graphic, were obtained by forcing both water and energy budgets to 'close' (balance).
The information in the paper provides a solid, quantitative baseline for the future. The estimates will improve as our satellite data-collection techniques and modeling improve.
Enjoy!
"It's been real and it's been nice, but it hasn't been real nice." - Unknown
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