Estimates of Freshwater Discharge from Continents: Latitudinal and Seasonal Variations

Abstract

Annual and monthly mean values of continental freshwater discharge into the oceans are estimated at 1Њ resolution using several methods. The most accurate estimate is based on streamflow data from the world's largest 921 rivers, supplemented with estimates of discharge from unmonitored areas based on the ratios of runoff and drainage area between the unmonitored and monitored regions. Simulations using a river transport model (RTM) forced by a runoff field were used to derive the river mouth outflow from the farthest downstream gauge records. Separate estimates are also made using RTM simulations forced by three different runoff fields: 1) based on observed streamflow and a water balance model, and from estimates of precipitation P minus evaporation E computed as residuals from the atmospheric moisture budget using atmospheric reanalyses from 2) the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) and 3) 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 unmonitored streamflow, in discharging runoff at correct locations, and in providing an annual cycle of continental discharge. The use of river mouth outflow increases the global continental discharge by ϳ19% compared with unadjusted streamflow from the farthest downstream stations. The river-based estimate of global continental discharge presented here is 37 288 Ϯ 662 km 3 yr Ϫ1 , which is ϳ7.6% of global P or 35% of terrestrial P. While this number is comparable to earlier estimates, its partitioning into individual oceans and its latitudinal distribution differ from earlier studies. The peak discharges into the Arctic, the Pacific, and global oceans occur in June, versus May for the Atlantic and August for the Indian Oceans. Snow accumulation and melt are shown to have large effects on the annual cycle of discharge into all 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-based 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.

Simulation of the Global Hydrological Cycle in the CCSM Community Atmosphere Model Version 3 (CAM3): Mean Features

“…Total discharge is 1.33 Sv. Discharge excluding Antarctica is about 1.25 Sv, which is about 6% higher than the estimate of Dai and Trenberth (2002). The river transport scheme does not account for loss of water due to human withdrawal or impoundment of water, seepage into groundwater, or evaporation from the river channel.…”

Evaluation of a near-global eddy-resolving ocean model

“…The SeaWiFS Kd-490 is a measure of the turbidity of the water column that quantifies the depth over which short-wave radiation penetrates the ocean. The model forcing includes climatological, seasonal river forcing estimated by Dai and Trenberth (2002) and Dai et al (2009). River forcing is applied as a water flux, with the injection of zero-salinity water and local SST distributed over the top 3 model layers at coastal grid points.…”

A 1/36° model of the Solomon Sea embedded into a global ocean model: On the setting up of an interactive open boundary nested model system

“…Therefore, the evolution of the model state feedbacks on the fluxes that drive it. Continental runoffs are provided as monthly means from the climatology by Dai and Trenberth 14 .…”