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Extending forecasts of global river flow out to seasonal timescales

5 June 2018
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Rebecca Emerton

Rebecca Emerton, PhD Student at University of Reading and Visiting Scientist at ECMWF

My PhD research at the University of Reading and ECMWF looks into how we can provide earlier indications of flood hazard at the global scale. One way of doing this is through seasonal forecasts of high (or low) river flow. Seasonal forecasts are designed to provide an early indication that a given variable, such as temperature, rainfall or even river flow, will differ from normal in the coming weeks or months.

While many operational centres produce seasonal forecasts of meteorological variables, operational seasonal forecasts of hydrological variables, particularly at large or global scales, are few and far between. Over the past year or so, I’ve worked with scientists at ECMWF and the University of Reading to develop the first global-scale seasonal hydro-meteorological forecasting system; this blog post talks about why we implemented such a system, how it works, and the new forecast products it provides.  

Existing global-scale flood and drought forecasts

Global overviews of upcoming flood and drought events are key for many water-related sectors, from agriculture and water resources management, to disaster risk reduction. The Global Flood Awareness System, GloFAS, is produced by ECMWF and the European Commission Joint Research Centre (JRC) as part of the Copernicus Emergency Management Services. Since 2011, GloFAS has been providing openly available, medium-range flood forecasts up to 30 days ahead for the global river network.

GloFAS forecasts are used by international organisations such as the Red Cross and the Regional Integrated Multi-Hazard Early Warning System (RIMES) for Asia; by national hydro-meteorological forecasting services around the world, and for research purposes. Recently, GloFAS was used by the Red Cross to distribute aid ahead of a flood event, for example.

Providing even earlier indicators of potential flood or drought events, many weeks or even months in advance, could be hugely beneficial not only for disaster preparedness initiatives, but for organisations working at the global scale, and for regions where no other forecasting system exists.

In the absence of global-scale seasonal forecasts of hydrological variables, seasonal forecasts of precipitation are often used as a proxy for flood hazard. But recent research has shown that seasonal precipitation forecasts are not necessarily a good indicator of seasonal floodiness, and called for investment in better forecasts of seasonal flood risk using hydrological models.

Advances in the science and techniques of global forecasting

While it may seem like a natural next step to produce global-scale seasonal hydro-meteorological forecasts, coupling seasonal meteorological forecasts with a hydrological model, this is not a simple task. 

GloFAS-Seasonal facts and figuresNot only are there complex geographical variations in rainfall-runoff and river regimes across the globe, but extensive computing resources are required to run the models and efficiently process and store huge volumes of data.

Global-scale medium-range forecasts of river flow have only become possible in recent years due to the integration of meteorological and hydrological modelling capabilities, alongside improvements in data, land-surface hydrology modelling and increased computer power. The recent move towards the development of coupled atmosphere-ocean-land models means that it is now becoming possible to produce seasonal hydro-meteorological forecasts for the global river network.

How does the new GloFAS-Seasonal forecast system work?

GloFAS-Seasonal uses the runoff forecasts from ECMWF’s latest seasonal forecasting system, SEAS5, which includes the HTESSEL land surface scheme. HTESSEL computes the land surface’s response to atmospheric forcing, but doesn’t simulate the movement of water through the river network. So, we use the surface and sub-surface runoff variables as input to the Lisflood river routing model, which simulates groundwater processes and routes the water through the rivers. We also ran ECMWF’s latest reanalysis product, ERA5, through the Lisflood model to produce a river flow reanalysis product – which we use as the initial conditions to start the Lisflood model for the forecasts.

GloFAS-Seasonal forecasts are designed to provide the likelihood of high or low river flow in the coming months, and as such for each forecast, we calculate the probability of the weekly-averaged river flow forecast exceeding a high or low flow threshold for each week out to 4 months, for every grid point with an upstream area >1500 km2. The high and low flow thresholds come from the 80th and 20th percentile, respectively, of the reference climatology – for which we use 18 years of GloFAS-Seasonal reforecasts.

GloFAS seasonal forecast components

Figure 1: The GloFAS-Seasonal forecasting system components. From Emerton et al. (2018).

New GloFAS-Seasonal forecast products

The GloFAS-Seasonal forecasts are provided as three new forecast layers in the GloFAS interface, and are free to view and use at www.globalfloods.eu. The new layers are designed to provide a global overview of the likelihood of high or low flow. The basin overview layer indicates the average probability of high or low flow for 306 major world river basins – where orange indicates that low flows are likely, and blue high flows – and the darker the colour, the higher the probability. The river network layer provides further detail at the sub-basin scale, colour-coding the model river network according to the probability of high or low flow. Finally, the reporting points layer provides points throughout the river network at which additional information is available, showing an ensemble hydrograph for the 4-month forecast horizon, and persistence diagrams indicating the weekly probabilities for the current and previous 3 forecasts. More detail on these new layers is available via the GloFAS website.

GloFAS basin overview and river network layers

Figure 2: Screenshot of the new basin overview and river network layers in the GloFAS interface (www.globalfloods.eu).

 

GloFAS hydrograph

Figure 3: Screenshot of the new river network and reporting point layers, including an example hydrograph, in the GloFAS interface (www.globalfloods.eu).

Further reading

To find out more about the development of GloFAS-Seasonal, the system components and the initial evaluation of the forecasts, head to our recent paper, under review (open access) in Geoscientific Model Development:

Emerton, R., Zsoter, E., Arnal, L., Cloke, H. L., Muraro, D., Prudhomme, C., Stephens, E. M., Salamon, P. and Pappenberger, F., 2018: Developing a global operational seasonal hydro-meteorological forecasting system: GloFAS v2.2 Seasonal v1.0, Geoscientific Model Development Discussions, in review