Estimates of Freshwater Discharge from Continents: Latitudinal and Seasonal Variations

TLDR

The study estimates annual and monthly continental freshwater discharge into oceans at 1° resolution using multiple methods. The authors combine streamflow data from 921 rivers with runoff–drainage ratios for unmonitored areas and run river transport model simulations forced by observed streamflow, water‑balance runoff, and P–E residuals from NCEP–NCAR and ECMWF reanalyses to derive river mouth outflows. The river‑based estimate of global continental discharge is 37,288 ± 662 km³ yr⁻¹, about 7.6 % of global precipitation, and shows a 19 % increase over unadjusted streamflow, with distinct oceanic peaks in June (Arctic, Pacific, global), May (Atlantic), and August (Indian), a strong snow‑melt influence except in the Indian, Mediterranean, and Black Seas, and ECMWF‑based runoff aligning with river estimates while NCEP–NCAR shows a negative bias.

Abstract

Annual and monthly mean values of continental freshwater discharge into the oceans are estimated at 1° 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 ∼19% compared with unadjusted streamflow from the farthest downstream stations. The river-based estimate of global continental discharge presented here is 37 288 ± 662 km3 yr−1, which is ∼7.6% 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.