By: Amulya Chevuturi
Figure 1: Photograph of the Negro River and the Amazon rainforest.
The Amazon is the largest river basin in the world, with large free-flowing rivers, draining about one-sixth of global freshwater to the ocean. The Amazonian floodplains have been long settled and used by indigenous populations, providing essential ecosystem services and natural resources for human needs (Junk et al., 2014). Increasing frequency and magnitude of floods in the last two decades has caused considerable environmental and socio-economic losses in many regions of the Amazon basin (Marengo and Espinoza, 2016). Although some studies have estimated flood risk for the Amazon basin (de Andrade et al., 2017), most towns and cities in this region still lack operational flood forecasts and integrated flood risk management plans.
The main aim of the PEACFLOW (Predicting the Evolution of the Amazon Catchment to Forecast the Level Of Water) project was to develop skilful forecasting systems for high water levels of Amazonian rivers, at sufficiently long lead time, for effective implementation of disaster risk management actions. In this project, we focused on developing forecast models for annual maximum water level for the Negro River at Manaus, Brazil (Figure 1), as a pilot case study, using a multiple linear regression approach. We used various potential predictors from preceding months: rainfall, water level, Pacific and Atlantic Ocean conditions and linear trend, all of which strongly influence the water levels in the Amazon basin. Flood levels in the Negro River occur between May and July and are strongly influenced by the rainfall during November to February, as its large floodplains delay the flood wave by months (Schöngart and Junk, 2007). This delay and the regularity between the rainfall and peak water level allows for the development of skilful statistical forecast models that can issue forecasts by March or earlier.
Figure 2: The Negro, Solimões and Madeira Rivers (blue lines) and their catchment basins (regions bounded by black lines) contributing to the river water level at Manaus (yellow circle; 3.14°S, 60.03°W).
In collaboration with Brazilian scientists, from various partner institutes, our team developed forecast models of the annual maximum water level (flood level) for the Negro River at Manaus by finding the best model fit over the training period of 1903 to 2004. For our models, rainfall over the catchment of the Negro River as well as from the catchment of nearby Solimões and Madeira Rivers (Figure 2) is the predominant predictor. We developed three models in this project, which use observations as input and can be implemented operationally to provide flood forecasts for Manaus. We compared the models developed in this project against current operational forecasts, provided by Brazilian agencies (CPRM and INPA), for the period of 2005 to 2019. The three PEACFLOW models issue forecasts of flood levels in the middle of January, February and March each year, but the skill of the models increase with decreasing lead-time (Figure 3a). Our results show that the models developed in this study can provide forecasts with the same skill as existing operational models one month in advance.
We also gained an additional month of lead-time when we relaced the observed input data with the ECMWF seasonal ensemble forecast. We developed two operational models using this data, which provide probabilistic forecasts at the beginning of January and February (Figure 3b). The probabilistic forecasts for the maximum water level, using ECMWF input, show good skill for extreme flood likelihood.
Figure 3: Comparison of models developed in PEACFLOW project and existing models (CPRM and INPA) and observed values for annual maximum water level at Manaus for models using (a) observations and (b) seasonal forecasts as input.
The methods developed in this project can also be used to develop forecast models for flood and drought levels over other regions of the Amazon basin. We provide the fully automated PEACFLOW models in a GitHub repository at https://github.com/achevuturi/PEACFLOW_Manaus-flood-forecasting. We retrospectively forecasted the annual maximum water levels at Manaus for 2020, and we are actively forecasting for 2021 (Table 1). Our forecasts this year show the maximum water levels crossing 29m, which is the extreme flood threshold for Manaus, at which the government declares emergency conditions.
Table 1: Forecasts for 2020 and 2021 using PEACFLOW models at different lead-times. Observed annual maximum water level at Manaus for 2020 was 28.52m.