An Inside Look at Flood Forecasting

Sella Nevo, Senior Software Engineer, Google Research, Tel Aviv

Several years ago, we identified flood forecasts as a unique opportunity to improve people’s lives, and began looking into how Google’s infrastructure and machine learning expertise can help in this field. Last year, we started our flood forecasting pilot in the Patna region, and since then we have expanded our flood forecasting coverage, as part of our larger AI for Social Good efforts. In this post, we discuss some of the technology and methodology behind this effort.

The Inundation Model
A critical step in developing an accurate flood forecasting system is to develop inundation models, which use either a measurement or a forecast of the water level in a river as an input, and simulate the water behavior across the floodplain.

A 3D visualization of a hydraulic model simulating various river conditions.

This allows us to translate current or future river conditions, to highly spatially accurate risk maps - which tell us what areas will be flooded and what areas will be safe. Inundation models depend on four major components, each with its own challenges and innovations:

Real-time Water Level Measurements
To run these models operationally, we need to know what is happening on the ground in real-time, and thus we rely on partnerships with the relevant government agencies to receive timely and accurate information. Our first governmental partner is the Indian Central Water Commission (CWC), which measures water levels hourly in over a thousand stream gauges across all of India, aggregates this data, and produces forecasts based on upstream measurements. The CWC provides these real-time river measurements and forecasts, which are then used as inputs for our models.

CWC employees measuring water level and discharge near Lucknow, India.

Elevation Map Creation
Once we know how much water is in a river, it is critical that the models have a good map of the terrain. High-resolution digital elevation models (DEMs) are incredibly useful for a wide range of applications in the earth sciences, but are still difficult to acquire in most of the world, especially for flood forecasting. This is because meter-wide features of the ground conditions can create a critical difference in the resulting flooding (embankments are one exceptionally important example), but publicly accessible global DEMs have resolutions of tens of meters. To help address this challenge, we’ve developed a novel methodology to produce high resolution DEMs based on completely standard optical imagery.

We start with the large and varied collection of satellite images used in Google Maps. Correlating and aligning the images in large batches, we simultaneously optimize for satellite camera model corrections (for orientation errors, etc.) and for coarse terrain elevation. We then use the corrected camera models to create a depth map for each image. To make the elevation map, we optimally fuse the depth maps together at each location. Finally, we remove objects such as trees and bridges so that they don’t block water flow in our simulations. This can be done manually or by training convolutional neural networks that can identify where the terrain elevations need to be interpolated. The result is a roughly 1 meter DEM, which can be used to run hydraulic models.

Hydraulic Modeling
Once we have both these inputs - the riverine measurements and forecasts, and the elevation map - we can begin the modeling itself, which can be divided into two main components. The first and most substantial component is the physics-based hydraulic model, which updates the location and velocity of the water through time based on (an approximated) computation of the laws of physics. Specifically, we’ve implemented a solver for the 2D form of the shallow-water Saint-Venant equations. These models are suitably accurate when given accurate inputs and run at high resolutions, but their computational complexity creates challenges - it is proportional to the cube of the resolution desired. That is, if you double the resolution, you’ll need roughly 8 times as much processing time. Since we’re committed to the high-resolution required for highly accurate forecasts, this can lead to unscalable computational costs, even for Google!

To help address this problem, we’ve created a unique implementation of our hydraulic model, optimized for Tensor Processing Units (TPUs). While TPUs were optimized for neural networks (rather than differential equation solvers like our hydraulic model), their highly parallelized nature leads to the performance per TPU core being 85x times faster than the performance per CPU core. For additional efficiency improvements, we’re also looking at using machine learning to replace some of the physics-based algorithmics, extending data-driven discretization to two-dimensional hydraulic models, so we can support even larger grids and cover even more people.

A snapshot of a TPU-based simulation of flooding in Goalpara, mid-event.

As mentioned earlier, the hydraulic model is only one component of our inundation forecasts. We’ve repeatedly found locations where our hydraulic models are not sufficiently accurate - whether that’s due to inaccuracies in the DEM, breaches in embankments, or unexpected water sources. Our goal is to find effective ways to reduce these errors. For this purpose, we added a predictive inundation model, based on historical measurements. Since 2014, the European Space Agency has been operating a satellite constellation named Sentinel-1 with C-band Synthetic-Aperture Radar (SAR) instruments. SAR imagery is great at identifying inundation, and can do so regardless of weather conditions and clouds. Based on this valuable data set, we correlate historical water level measurements with historical inundations, allowing us to identify consistent corrections to our hydraulic model. Based on the outputs of both components, we can estimate which disagreements are due to genuine ground condition changes, and which are due to modeling inaccuracies.

Flood warnings across Google’s interfaces.

Looking Forward
We still have a lot to do to fully realize the benefits of our inundation models. First and foremost, we’re working hard to expand the coverage of our operational systems, both within India and to new countries. There’s also a lot more information we want to be able to provide in real time, including forecasted flood depth, temporal information and more. Additionally, we’re researching how to best convey this information to individuals to maximize clarity and encourage them to take the necessary protective actions.

Computationally, while the inundation model is a good tool for improving the spatial resolution (and therefore the accuracy and reliability) of existing flood forecasts, multiple governmental agencies and international organizations we’ve spoken to are concerned about areas that do not have access to effective flood forecasts at all, or whose forecasts don’t provide enough lead time for effective response. In parallel to our work on the inundation model, we’re working on some basic research into improved hydrologic models, which we hope will allow governments not only to produce more spatially accurate forecasts, but also achieve longer preparation time.

Hydrologic models accept as inputs things like precipitation, solar radiation, soil moisture and the like, and produce a forecast for the river discharge (among other things), days into the future. These models are traditionally implemented using a combination of conceptual models approximating different core processes such as snowmelt, surface runoff, evapotranspiration and more.

The core processes of a hydrologic model. Designed by Daniel Klotz, JKU Institute for Machine Learning.

These models also traditionally require a large amount of manual calibration, and tend to underperform in data scarce regions. We are exploring how multi-task learning can be used to address both of these problems — making hydrologic models both more scalable, and more accurate. In research collaboration with JKU Institute For Machine Learning group under Sepp Hochreiter on developing ML-based hydrologic models, Kratzert et al. show how LSTMs perform better than all benchmarked classic hydrologic models.

The distribution of NSE scores on basins across the United States for various models, showing the proposed EA-LSTM consistently outperforming a wide range of commonly used models.

Though this work is still in the basic research stage and not yet operational, we think it is an important first step, and hope it can already be useful for other researchers and hydrologists. It’s an incredible privilege to take part in the large eco-system of researchers, governments, and NGOs working to reduce the harms of flooding. We’re excited about the potential impact this type of research can provide, and look forward to where research in this field will go.

Acknowledgements
There are many people who contributed to this large effort, and we’d like to highlight some of the key contributors: Aaron Yonas, Adi Mano, Ajai Tirumali, Avinatan Hassidim, Carla Bromberg, Damien Pierce, Gal Elidan, Guy Shalev, John Anderson, Karan Agarwal, Kartik Murthy, Manan Singhi, Mor Schlesinger, Ofir Reich, Oleg Zlydenko, Pete Giencke, Piyush Poddar, Ruha Devanesan, Slava Salasin, Varun Gulshan, Vova Anisimov, Yossi Matias, Yi-fan Chen, Yotam Gigi, Yusef Shafi, Zach Moshe and Zvika Ben-Haim.