Basin‐Scale River Runoff Estimation From GRACE Gravity Satellites, Climate Models, and In Situ Observations: A Case Study in the Amazon Basin

Abstract

River runoff is estimated as a water budget residual using Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage time series, ERA5 reanalysis data, and precipitation observations for January 2003 through December 2015 for the Obidos upstream drainage basin and for the entire Amazon basin. Estimated runoff based on the water budget agrees remarkably well with in situ gauge observations at Obidos, especially at seasonal time scales, with nearly perfect phase agreement but slightly larger seasonal amplitude. The discrepancy in the seasonal amplitude may be attributed to underestimation of river gauge runoff during the wet season when water overflows the riverbanks. The ERA5 model appears to overestimate long-term mean evapotranspiration in the Amazon by~2 cm/month based on comparisons with precipitation and runoff observations. Using precipitation data based on satellites and gauge observations relative to gauge observations alone improved agreement between water budget runoff estimates and in situ runoff observations. Seasonal variations in ERA5-simulated runoff are about twice as large as those from in situ observations and show a large phase lag as well. Water budget-based runoff for the entire Amazon (~7,200 km 3 averaged over the 13-year period) is significantly larger than observed runoff (~5,700 km 3) at Obidos and notably larger than previous estimates for the entire Amazon. These differences may be partly related to submarine runoff from the Amazon basin that cannot be captured by surface gauges.

Runoff Variations in the Yangtze River Basin and Sub-basins based on GRACE, Hydrological models, and In-situ data

“…When the water budget closure equation is used to estimate runoff for the YRB, the equation is widely expressed (Chen et al, 2020;Chen et al, 2019) as:…”

“…Currently, there are many available global precipitation (P) data and evapotranspiration (ET) models (gauge-based, satellite-related, and reanalysis datasets) from different organizations (Sun et al, 2018). The mainstream land surface models (Chen et al, 2020;Jing et al, 2019;Li et al, 2018;Martin et al, 2020;Zhang et al, 2016) include the Global Land Data Assimilation System (GLDAS), Global Precipitation Climatology Project (GPCP), GLEAM (Global Land Evaporation Amsterdam Model), and ERA5 models. One reason for the popularization of these models is that these models merge various satellite-based estimates over both ocean and land with gauge measurements.…”

“…GRACE-derived TWSA is monthly average information, and time gaps between two released monthly TWSAs are not uniform. Currently, some researchers tried to fill the gaps using the dynamic interpolation method and redefined the epoch of each month as the exact middle of the month (Chen et al, 2020). However, the dynamic interpolation method contains strong constraint information (time series only include seasonal and trend variations).…”

“…Currently, most studies on basin-scale water budget closure (Chen et al, 2020;Li et al, 2018;Long et al, 2014;Lv et al, 2017;Syed et al, 2005) benefit from the Gravity Recovery and Climate Experiment (GRACE) mission. The GRACE-derived terrestrial water storage change (TWSC) represents residual water storage variation, including natural water supply and loss, and TWSC is regarded as a perfect fit for water budget studies (Rodell and Famiglietti, 1999).…”

Random Forest-Based Reconstruction and Application of the GRACE Terrestrial Water Storage Estimates for the Lancang-Mekong River Basin

“…Encouragingly, the water balance method describes the equilibrium of water revenue and expenditure, which provides an effective way to estimate discharge [29][30][31]. The terrestrial water storage change (TWSC) is negligible in the water balance at the annual level but is essential at the sub-annual level [32][33][34]. The water balance method allows us to better understand the water cycle, and discharge can be obtained by subtracting the evapotranspiration (ET) and the TWSC from precipitation [35,36].…”