Global River Routing Model Research Articles

Evaluating a reservoir parametrization in the vector-based global routing model mizuRoute (v2.0.1) for Earth system model coupling

Abstract. Human-controlled reservoirs have a large influence on the global water cycle. While global hydrological models use generic parameterizations to model dam operations, the representation of reservoir regulation is still lacking in many Earth system models. Here we implement and evaluate a widely used reservoir parametrization in the global river-routing model mizuRoute, which operates on a vector-based river network resolving individual lakes and reservoirs and is currently being coupled to an Earth system model. We develop an approach to determine the downstream area over which to aggregate irrigation water demand per reservoir. The implementation of managed reservoirs is evaluated by comparing them to simulations ignoring inland waters and simulations with reservoirs represented as natural lakes using (i) local simulations for 26 individual reservoirs driven by observed inflows and (ii) global-domain simulations driven by runoff from the Community Land Model. The local simulations show the clear added value of the reservoir parametrization, especially for simulating storage for large reservoirs with a multi-year storage capacity. In the global-domain application, the implementation of reservoirs shows an improvement in outflow and storage compared to the no-reservoir simulation, but a similar performance is found compared to the natural lake parametrization. The limited impact of reservoirs on skill statistics could be attributed to biases in simulated river discharge, mainly originating from biases in simulated runoff from the Community Land Model. Finally, the comparison of modelled monthly streamflow indices against observations highlights that including dam operations improves the streamflow simulation compared to ignoring lakes and reservoirs. This study overall underlines the need to further develop and test runoff simulations and water management parameterizations in order to improve the representation of anthropogenic interference of the terrestrial water cycle in Earth system models.

The effect of surge on riverine flood hazard and impact in deltas globally

Current global riverine flood risk studies assume a constant mean sea level boundary. In reality high sea levels can propagate up a river, impede high river discharge, thus leading to elevated water levels. Riverine flood risk in deltas may therefore be underestimated. This paper presents the first global scale assessment of the joint influence of riverine and coastal drivers of flooding in deltas. We show that if storm surge is ignored, flood depths are significantly underestimated for 9.3% of the expected annual population exposed to riverine flooding. The assessment is based on extreme water levels at 3433 river mouth locations as modeled by a state-of-the-art global river routing model, forced with a multi-model runoff ensemble and bounded by dynamic sea level conditions derived from a global tide and surge reanalysis. We first classified the drivers of riverine flooding at each location into four classes: surge-dominant, discharge-dominant, compound-dominant or insignificant. We then developed a model experiment to quantify the effect of surge on flood hazard and impacts. Drivers of riverine flooding are compound-dominant at 19.7% of the locations analyzed, discharge-dominant at 69.2%, and surge-dominant at 7.8%. Compared to locations with either surge- or discharge-dominant flood drivers, locations with compound-dominant flood drivers generally have larger surge extremes and are located in basins with faster discharge response and/or flat topography. Globally, surge exacerbates 1-in-10 years flood levels at 64.0% of the locations analyzed, with a mean increase of 11 cm. While this increase is generally larger at locations with compound- or surge-dominant flood drivers, flood levels also increase at locations with discharge-dominant flood drivers. This study underlines the importance of including dynamic downstream sea level boundaries in (global) riverine flood risk studies.

Coupling a global hydrodynamic algorithm and a regional hydrological model for large-scale flood inundation simulations

Abstract To bridge the gap between 1D and 2D hydraulic models for regional scale assessment and global river routing models, we coupled the CaMa-Flood (Catchment-based Macro-scale Floodplain) model and the regional hydrological model SWIM (Soil and Water Integrated Model) as a tool for large-scale flood risk assessments. As a proof-of-concept study, we tested the coupled models in a meso-scale catchment in Germany. The Mulde River has a catchment area of ca. 6,171 km2 and is a sub-catchment of the Elbe River. The modified CaMa-Flood model routes the sub-basin-based daily runoff generated by SWIM along the river network and estimates the river discharge as well as flood inundation areas. The results show that the CaMa-Flood hydrodynamic algorithm can reproduce the daily discharges from 1991 to 2003 well. It outperforms the Muskingum flow routing method (the default routing method in the SWIM) for the 2002 extreme flood event. The simulated flood inundation area in August 2002 is comparable with the observations along the main river. However, problems may occur in upstream areas. The results presented here show the potential of the coupled models for flood risk assessments along large rivers.

Compound simulation of fluvial floods and storm surges in a global coupled river‐coast flood model: Model development and its application to 2007 Cyclone Sidr in Bangladesh

AbstractWater‐related disasters, such as fluvial floods and cyclonic storm surges, are a major concern in the world's mega‐delta regions. Furthermore, the simultaneous occurrence of extreme discharges from rivers and storm surges could exacerbate flood risk, compared to when they occur separately. Hence, it is of great importance to assess the compound risks of fluvial and coastal floods at a large scale, including mega‐deltas. However, most studies on compound fluvial and coastal flooding have been limited to relatively small scales, and global‐scale or large‐scale studies have not yet addressed both of them. The objectives of this study are twofold: to develop a global coupled river‐coast flood model; and to conduct a simulation of compound fluvial flooding and storm surges in Asian mega‐delta regions. A state‐of‐the‐art global river routing model was modified to represent the influence of dynamic sea surface levels on river discharges and water levels. We conducted the experiments by coupling a river model with a global tide and surge reanalysis data set. Results show that water levels in deltas and estuaries are greatly affected by the interaction between river discharge, ocean tides and storm surges. The effects of storm surges on fluvial flooding are further examined from a regional perspective, focusing on the case of Cyclone Sidr in the Ganges‐Brahmaputra‐Meghna Delta in 2007. Modeled results demonstrate that a >3 m storm surge propagated more than 200 km inland along rivers. We show that the performance of global river routing models can be improved by including sea level dynamics.

The critical role of the routing scheme in simulating peak river discharge in global hydrological models

Global hydrological models (GHMs) have been applied to assess global flood hazards, but their capacity to capture the timing and amplitude of peak river discharge—which is crucial in flood simulations—has traditionally not been the focus of examination. Here we evaluate to what degree the choice of river routing scheme affects simulations of peak discharge and may help to provide better agreement with observations. To this end we use runoff and discharge simulations of nine GHMs forced by observational climate data (1971–2010) within the ISIMIP2a project. The runoff simulations were used as input for the global river routing model CaMa-Flood. The simulated daily discharge was compared to the discharge generated by each GHM using its native river routing scheme. For each GHM both versions of simulated discharge were compared to monthly and daily discharge observations from 1701 GRDC stations as a benchmark. CaMa-Flood routing shows a general reduction of peak river discharge and a delay of about two to three weeks in its occurrence, likely induced by the buffering capacity of floodplain reservoirs. For a majority of river basins, discharge produced by CaMa-Flood resulted in a better agreement with observations. In particular, maximum daily discharge was adjusted, with a multi-model averaged reduction in bias over about 2/3 of the analysed basin area. The increase in agreement was obtained in both managed and near-natural basins. Overall, this study demonstrates the importance of routing scheme choice in peak discharge simulation, where CaMa-Flood routing accounts for floodplain storage and backwater effects that are not represented in most GHMs. Our study provides important hints that an explicit parameterisation of these processes may be essential in future impact studies.

Modeling complex flow dynamics of fluvial floods exacerbated by sea level rise in the Ganges–Brahmaputra–Meghna Delta

Global warming is likely to exacerbate future fluvial floods in the world’s mega-delta regions due to both changing climate and rising sea levels. However, the effects of sea level rise (SLR) on fluvial floods in such regions have not been taken into account in current global assessments of future flood risk, due to the difficulties in modeling channel bifurcation and the backwater effect. We used a state-of-the-art global river routing model to demonstrate how these complexities contribute to future flood hazard associated with changing climate and SLR in the world’s largest mega-delta region, the Ganges-Brahmaputra-Meghna Delta. The model demonstrated that flood water in the main channels flows into tributaries through bifurcation channels, which resulted in an increase in inundation depth in deltaic regions. We found that there were large areas that experienced an increase in inundation depth and period not directly from the SLR itself but from the backwater effect of SLR, and the effect propagated upstream to locations far from the river mouth. Projections under future climate scenarios as well as SLR indicated that exposure to fluvial floods will increase in the last part of the 21st century, and both SLR and channel bifurcation make meaningful contributions.

Discharge of major global rivers in the late 21st century climate projected with the high horizontal resolution MRI‐AGCMs

AbstractThis study used an atmospheric general circulation model (AGCM) with a 20‐km grid spacing and a global river routing model with 0.5° grid resolution to project climatological mean river discharges in the late 21st century under a changing climate. We also performed an ensemble simulation with the same AGCM except with a resolution of 60 km and forced with different lower boundary conditions to quantify the level of confidence in the projections. By the late 21st century, projected annual mean river discharges increased in the high latitudes and in India and the southeastern United States but decreased in broad regions of Europe, western Asia, the western United States, Central America, and the southern half of the Amazon River basin. Projected future changes in monthly river discharges include unique seasonal details not present in the annual discharge projections: in the Amazon River, for example, river discharges were projected to increase in the high‐water season but decrease in the low‐water season. These seasonal changes suggest that monthly river discharge projections should be useful for assessing the human impacts and risks in future planning of water resources and flood control. We devised a way to measure the consistency between the various projections made by our 60‐km ensemble simulations and our standard 20‐km high‐resolution model. Wherever the projected changes were consistent, the future changes also tended to be statistically significant in our high‐resolution model. These results encourage a future projection of discharge for a specific river basin in the world. Copyright © 2013 John Wiley & Sons, Ltd.

First impact assessment of hydrological cycle in the Tana River Basin, Kenya, under a changing climate in the late 21st Century

This study projects the hydrological cycle in the Tana River Basin, Kenya, under a changing climate due to anthropogenic greenhouse gas emissions in the late 21st century, using a 20-km mesh atmospheric global climate model (AGCM) and a 0.5°–mesh global river-routing model. In addition, 60-km mesh AGCM ensemble experiments forced with four different projected sea-surface temperatures were performed to quantify the uncertainty in the climate projections. All four climatological annual mean hydroclimate variables: precipitation, evaporation, total runoff, and soil water storage: are projected to increase in the future climate. With the exception of total runoff and river discharge in the headwater basin, these climatological annual mean increases are generally robust. The four climatological monthly mean hydroclimate variables at Garissa are projected to exhibit different seasonal changes. The long rains season is projected to have almost the same peak precipitation amount in April with an earlier onset, while the short rains season is projected to have significant precipitation increases. River discharges are projected to increase significantly in November to March with high consistent change in sign and in June. These results should guide the awareness programs of the changing hydrological cycles in the Tana River Basin and assist in formulating mitigation and adaptation strategies to meet these challenges.

Floodplain dynamics included in river model

Surface water flow is important to the climate system and to water resources management. Floodplain inundation dynamics is a key part of surface water flow, but most current river routing models do not include realistic representation of floodplain dynamics. A new global river routing model created by Yamazaki et al. does include these dynamics. The researchers tested the model by comparing results of their model simulation with in situ and satellite observations and found that the model improved simulation of river discharge in most of the world's major river basins. (Water Resources Research, doi:10.1029/2010WR009726, 2011)

A physically based description of floodplain inundation dynamics in a global river routing model

Current global river routing models do not represent floodplain inundation dynamics realistically because the storage and movement of surface waters are regulated by small‐scale topography rather than the commonly used spatial resolution of global models. In this study, we propose a new global river routing model, CaMa‐Flood, which explicitly parameterizes the subgrid‐scale topography of a floodplain, thus describing floodplain inundation dynamics. The relationship between water storage, water level, and flooded area in the model is decided on the basis of the subgrid‐scale topographic parameters based on 1 km resolution digital elevation model. Horizontal water transport is calculated with a diffusive wave equation, which realizes the backwater effect in flat river basins. A set of global‐scale river flow simulations demonstrated an improved predictability of daily‐scale river discharge in many major world rivers by incorporating the floodplain inundation dynamics. Detailed analysis of the simulated results for the Amazon River suggested that introduction of the diffusive wave equation is essential for simulating water surface elevation realistically. The simulated spatiotemporal variation of the flooded area in the Amazon basin showed a good correlation with satellite observations, especially when the backwater effect was considered. The improved predictability for daily river discharge, water surface elevation, and inundated areas by the proposed model will promote climate system studies and water resource assessments.

Using moisture conservation to evaluate oceanic surface freshwater fluxes in climate models

We apply a diagnostic based on moisture conservation in the atmosphere, integrated over planetary-scale ocean basins and drainage areas to evaluate freshwater fluxes over the ocean surface to three generations of the Hadley Centre climate model (HadCM3, HadGEM1 and HadGEM2-AO). The coherent inclusion of runoff by the diagnostic enables model surface freshwater fluxes to be compared directly with observational estimates of precipitation, evaporation and river discharge. We also introduce a normalised metric, based on model-observation RMS differences, to assess the representation of the fluxes by the model. This methodology could be a powerful tool for evaluating model performance during future model development and model intercomparison exercises. Using this diagnostic, and defining the drainage areas from the global river routing model TRIP, we obtain large-scale surface oceanic fluxes from ERA40 and NCAR-NCEP reanalysis data, which we compare with analogous budgets computed from a set of individual observational estimates of evaporation, precipitation and river discharge. The sum of errors in the Hadley Centre climate model in all ocean basins suggests a steady improvement over the three generations of the model. However, an analysis of sources and sinks of water vapour shows common errors in the models, like an excess of evaporation in the tropical-subtropical Atlantic, and a surplus of water vapour export from tropical-subtropical areas to the mid-latitude regions, making the oceanic surface fluxes too fresh at mid latitudes. Errors in the models are consistent with an excessively strong hydrological cycle.

First Projection of Climatological Mean River Discharges in the Magdalena River Basin, Colombia, in a Changing Climate during the 21st Century

This study projects the river discharge in the Magdalena River basin, Colombia, considering projected climate conditions for the 21st century, by using a 20-km-mesh atmospheric global climate model and a 0.5°-mesh global river routing model under a greenhouse gas emission scenario. The climatological annual mean river discharges along the main stream of the Magdalena River do not change significantly, however precipitation, evaporation, and total runoff into the river show statistically significantly changes over most of the Magdalena River basin. By the end of the 21st century, the projected climatological monthly mean river discharge at Puerto Berrio decreases statistically significantly in April, October, and November (P < 0.05), compared to current values, whereas it shows a distinct increase for June through August, thereby reducing the present bimodality of its seasonal variation. Minimum climatological monthly mean river discharge in February could be lower at the end of the 21st century than in the current condition. These results should help increase the awareness of the changing river discharge in the Magdalena River basin, and prepare adaptation strategies to face these challenges.

Deriving a global river network map and its sub-grid topographic characteristics from a fine-resolution flow direction map

Abstract. This paper proposes an improved method for converting a fine-resolution flow direction map into a coarse-resolution river network map for use in global river routing models. The proposed method attempts to preserve the river network structure of an original fine-resolution map in the upscaling procedure, as this has not been achieved with previous upscaling methods. We describe an improved method in which a downstream cell can be flexibly located on any cell in the river network map. The improved method preserves the river network structure of the original flow direction map and allows automated construction of river network maps at any resolution. Automated construction of a river network map is helpful for attaching sub-grid topographic information, such as realistic river meanderings and drainage boundaries, onto the upscaled river network map. The advantages of the proposed method are expected to enhance the ability of global river routing models by providing ways to more precisely represent surface water storage and movement.

A reservoir operation scheme for global river routing models

A global river discharge simulation was conducted that accounted for 452 individually operated reservoirs by locating them on the digital global river network map of TRIP, a global river routing model. An operating rule was determined for each reservoir using a newly-developed algorithm that used currently available global data such as reservoir storage capacity, intended purposes, simulated inflow, and water demand in the lower reaches. This algorithm reduced the root mean square error of reservoir release and river discharge simulations from that of earlier global river discharge simulations that neglected reservoir operations or substituted an algorithm for natural-lake outflow. The 2-year global simulation results indicate that reservoir operations substantially altered monthly discharge for individual basins (i.e., by more than 20%). Averaged over the continental scale, the maximum change in monthly river discharge varied from 0% to 34% and the changes in reservoir storage were small as a proportion of the total storage capacity.

A GLOBAL INTEGRATED WATER RESOURCES MODEL BASED ON A BUCKET TYPE LAND SURFACE MODEL

A global integrated water resources model was developed. The model consists of four sub models, a bucket type land surface model, a global river routing model, a process based agricultural model and a reservoir operation model. The model was designed to work with near surface meteorological data and currently available global grid-based digital GIS data. It enable us to conduct 1) monthly or daily assessments of global water supply and demand, 2) numerical experiments concerning the interaction of human activity and land hydrology, 3) analyses on global water balance components in which energy and water balances are perfectly closed. In the former part of this report, the structure of model and the simulation setting are described. In the latter part, the performance and limitation of the model are discussed using the results of a preliminary global simulation.

River basin hydrology in a global off‐line land‐surface model

Land‐surface hydrology has been examined for the Off‐line Land‐surface GEOS Assimilation (OLGA) system and Goddard Earth Observing System (GEOS‐1) data assimilation system using a river routing model. The GEOS‐1 land‐surface parameterization is very simple, using an energy balance prediction of surface temperature and prescribed soil water. OLGA uses atmospheric data from GEOS‐1 to drive a more comprehensive parameterization of the land‐surface processes. Using a global river routing model, OLGA's hydrology is evaluated against GEOS‐1 (which serves as a control case) and observations. The river routing model moves monthly mean climatologic source runoff through river networks to simulate the river discharge from many river basins around the world, which can be compared to observed climatologic river discharge. Because of the soil hydrology, the OLGA system shows a general improvement in the simulation of river discharge, compared to the GEOS‐1, by slowing the discharge of water. Snowmelt processes included in OLGA also have a positive effect on the annual cycle of river discharge and source runoff. The timing of the snowmelt in river discharge, however, still needs improvement in the annual cycle. Preliminary tests of a coupled land‐atmosphere model indicate improvements to the hydrologic cycle compared to the uncoupled system. The river routing model has provided a useful tool in the evaluation of the GCM hydrologic cycle and has helped quantify the influence of the more advanced land‐surface model.