Evaluations of global land-to-oceans fresh water discharge and evapotranspiration

We estimate global fresh water discharge from land to oceans (R) 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 (dSo/dt,dSl/dt) from GRACE as well as the space-based measurements of oceanic and land precipitation (Po,Pl) and oceanic evaporation (Eo). R can be estimated through the water balance equation over oceans, R=Eo-Po-dSo/dt, and then ET is estimated based on water mass balance over land, ET=Pl-R-dSl/dt. Monthly time series of R show peaks in July and December, and those of ET shows peaks in March and August. In general, our estimates of R and ET are correlated with Pl well indicating qualitatively that our estimates capture temporal patterns of R and ET reasonably. We compare our estimation for R and ET to 20th century simulations from the WCRP CMIP3 multi-model archive that are assessed in the IPCC 4th Assessment Report. R and ET from AOGCMs mainly show annual cycles, but those estimated in this study exhibit additional semi-annual changes. The estimates of R and ET examined in this study are potentially useful to constraint current AOGCMs models.