Modeling surface water dynamics in the Amazon Basin using MOSART-Inundation v1.0: impacts of geomorphological parameters and river flow representation

Abstract

Abstract. In the Amazon Basin, floodplain inundation is a key component of surface water dynamics and plays an important role in water, energy and carbon cycles. The Model for Scale Adaptive River Transport (MOSART) was extended with a macroscale inundation scheme for representing floodplain inundation. The extended model, named MOSART-Inundation, was used to simulate surface hydrology of the entire Amazon Basin. Previous hydrologic modeling studies in the Amazon Basin identified and addressed a few challenges in simulating surface hydrology of this basin, including uncertainties of floodplain topography and channel geometry, and the representation of river flow in reaches with mild slopes. This study further addressed four aspects of these challenges. First, the spatial variability of vegetationcaused biases embedded in the HydroSHEDS digital elevation model (DEM) data was explicitly addressed. A vegetation height map of about 1 km resolution and a land cover dataset of about 90 m resolution were used in a DEM correction procedure that resulted in an average elevation reduction of 13.2 m for the entire basin and led to evident changes in the floodplain topography. Second, basin-wide empirical formulae for channel cross-sectional dimensions were refined for various subregions to improve the representation of spatial variability in channel geometry. Third, the channel Manning roughness coefficient was allowed to vary with the channel depth, as the effect of riverbed resistance on river flow generally declines with increasing river size. Lastly, backwater effects were accounted for to better represent river flow in mild-slope reaches. The model was evaluated against in situ streamflow records and remotely sensed Envisat altimetry data and Global Inundation Extent from Multi-Satellites (GIEMS) inundation data. In a sensitivity study, seven simulations were compared to evaluate the impacts of the five modeling aspects addressed in this study. The comparisons showed that representing floodplain inundation could significantly improve the simulated streamflow and river stages. Refining floodplain topography, channel geometry and Manning roughness coefficients, as well as accounting for backwater effects had notable impacts on the simulated surface water dynamics in the Amazon Basin. The understanding obtained in this study could be helpful in improving modeling of surface hydrology in basins with evident inundation, especially at regional to continental scales.

Toward continental hydrologic–hydrodynamic modeling in South America

“…River discharges at gauge stations with RC ranging between 0.3 and 0.6 5 are generally well represented in all spatial scales, while performance tends to be lower for RC < 0.3 and highly variable for rivers with lower drainage areas. basin (e.g., Obidos, NSE HD = 0.89) are comparable to other regional studies (e.g., Getirana et al, 2012;Paiva et al, 2013;Luo et al, 2017) while better performance is found over several of its tributaries (e.g., Purus, Madeira and Japura Rivers). …”

“…However, detailed information about channel geometry is usually not available for large-scale basins and a very common approach is to adopt classic hydraulic geometry relationships (HGs) (Leopold and Maddock, 1953) for specific sites according to drainage area or discharge (Decharme et 5 al., 2008;Yamazaki et al, 2011;Getirana et al, 2012;Paiva et al, 2013;Luo et al, 2017;Pontes et al, 2017). Here, the global database of Andreadis et al, (2013) was used to set initial values of bankfull widths and depths, which were derived from two-year return period flows using the Global Runoff Database Center (GRDC) data and universal HGs obtained from several rivers around the world.…”

“…On the other hand, even if discharges are well represented, there are uncertainties related to Manning values and also to river widths and 25 depths derived from HGs, which do not reflect singularities of cross sections such as narrowing or widening of rivers at both gauge and VS locations. Previous studies have demonstrated the large influence of channel geometry and roughness on both amplitude and timing of water levels, especially over the Amazon basin (e.g., Yamazaki et al, 2011;Paiva et al, 2013;Paris et al, 2016;Luo et al, 2017). Moreover, river bed profiles are subject to DEM errors that can hardly be reduced through simple profile-smoothing procedures.…”

Upper Blue Nile basin water budget from a multi-model perspective

“…Surface runoff and baseflow are converted into streamflow along the river network using a kinematic wave formulation, allowing the comparison against in-situ observations [more details can be found in Getirana et al, 2012]. In river routing schemes, channel geometry, floodplain topography, and roughness coefficient are an acknowledged source of uncertainty Yamazaki et al, 2011;Getirana et al, 2013;Luo et al, 2017). We implemented the HyMAP's global standard parameters except for river width in this study.…”

A substantial role of soil erosion in the land carbon sink and its future changes

“…In ELM, each land unit consists of multiple plant function types (PFTs) that share a soil column of the same topography and soil texture (Niu et al, 2005). Currently, water and biogeochemical exchanges between PFTs are not yet represented in ELM, and because ELM and the E3SM river model are one-way coupled (Li et al, 2013;Luo et al, 2017), the transport of sediment and C in rivers is also not included in our simulations. As such, for each land unit, we compared the total amount of rainfall-and runoff-driven erosion with the total amount of sediment transport capacity and defined sediment yield (i.e., the amount of detached soil particles that enter the rivers) as the smaller value between them.…”