Abstract
Summary We estimate global fresh water discharge from land-to-oceans ( Q ) and evapotranspiration ( ET ) on monthly time scales using a number of complimentary hydrologic data sets. This estimate is possible due to the new capability of measuring oceanic and land water mass changes from GRACE as well as the space-based measurements of oceanic and land precipitation ( P l ) and oceanic evaporation. Monthly time series of Q show peaks in July and January, and those of ET show peaks in March, May and August. Our estimates of Q and ET are correlated with P l indicating qualitatively that our estimates capture temporal patterns of Q and ET reasonably well. Comparison of our Q with two other previous estimates based on the Global Runoff Data Centre (GRDC) river gauges network shows that our maximum peak in Q occurs about a month later than previous estimates. In addition, we compare our estimation of Q and ET to 20th century simulations from the WCRP CMIP3 multi-model archive assessed in the IPCC 4th Assessment Report. Runoff ( R ) and ET from AOGCMs tend to only exhibit the annual cycle, but the Q estimated in this study exhibits additional semi-annual variations that exists in P l as well. In addition, R from the models shows a maximum peak 2 months earlier than the estimated Q , which is due partly to the river discharge time lag that most AOGCMs do not take into account. These results indicate that current AOGCMs exhibit basic shortcomings in simulating Q and ET accurately. The new method developed here can be a useful constraint on these models and can be useful to close budget of global water balance.
Highlights
Evaluation of terrestrial water balance components, such as water storage changes, precipitation (Pl), evapotranspiration (ET) and fresh water discharge to oceans (Q), has been an important issue in terms of the global water and energy cycles, sediment transport and renewable water resources (e.g., Trenberth et al, 2007)
We evaluate global Q and ET on monthly time scales using a new approach without the influences from uncertainties in the estimations of atmospheric moisture flux from reanalysis or the in situ measurement limitations in Q
We estimate global Q and ET in monthly time scales from September 2002 to November 2006 using satellite-borne remote sensing and water balance equations between land and oceans
Summary
Evaluation of terrestrial water balance components, such as water storage changes (ddStl), precipitation (Pl), evapotranspiration (ET) and fresh water discharge to oceans (Q), has been an important issue in terms of the global water and energy cycles, sediment transport and renewable water resources (e.g., Trenberth et al, 2007). There have been many studies to estimate the terrestrial water balance components (e.g., Schlosser and Houser, 2007). Terrestrial water storage changes have been measured with the Gravity Recovery and Climate Experiment (GRACE) launched March 2002 (Tapley et al, 2004). Limitations of direct measurements for Q and ET hinder their accurate global monitoring.
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