Estimates of Freshwater Discharge from Continents: Latitudinal and
Seasonal Variations
Aiguo Dai and Kevin E. Trenberth
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
email: trenbert@cgd.ucar.edu
voice: (303) 497 1318
fax: (303) 497 1333
Annual and monthly mean values of continental freshwater discharge into the
individual and global oceans are estimated at 1o resolution using several
methods. The most accurate estimate is based upon stream-flow data from the
world's largest 921 rivers, which are supplemented with estimates of discharge
from un-monitored regoins by means of a river transport model (RTM) forced by a
runoff field. The RTM simulation is also used to derive the river mouth outflow
from thr faerthest downstream gauge records. Separate estimates are also made
using RTM simulations forced by three different runoff fields, the first of
which is based on observed stream-flow and a water balance model, and the others
are from estimates of precipitation P minus evaporation E computed as residuals
from the atmospheric moisture budget using atmospheric reanalyses from the
National Centers for Environmental Prediction/National Center for Atmospheric
Research (NCEP/NCAR) and the European Centre for Medium Range Weather Forecasts
(ECMWF). Compared with previous estimates, improvements are made in extending
observed discharge downstream to the river mouth, in accounting for the
un-monitored stream-flow, in discharging runoff at correct loations, and in
providing a annual cycle of continental discharge. The use of river mouth
outflow increases the global continental discharge by ~19% compared with that
based on un-adjusted stream-flow from the farthest downstream stations. Out
river-based estimate of global continental discharge is 37288±662 km
3 yr-1 (~7% of global P). While this number is comparable
to earlier estimates, its partitioning into individual ocean basins and its
latitudinal distribution differ substantialy from earlier studies. The total
discharge into the Arctic, the Pacific, and global oceans peaks in June whereas
it peaks in May for the Atlantic and in August for the Indian Ocean. Snow
accumulation and melt have large effects on the annual cycle of discharge into
all the ocean basins except for the Indian Ocean and the Mediterranean and Black
Seas. The discharge and its latitudinal distribution implied by the
observation-based runoff and the ECMWF reanalysis-bases P-E agree well with the
river-based estimates, whereas the discharge implied by the NCEP/NCAR
reanalysis-based P-E has a negative bias. This result suggests that the P-E data
from the reanalyses may be used to study the interannual to decadal variations
in continental discharge.
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Hongjun Zhang:
zhangho@ucar.edu