How to capture and fill every drop from typhoon downpours? Chang Li-chiu's team builds a flood forecasting system, earning international acclaim for its pinpoint accuracy

In Taiwan, China, rainfall is concentrated from the plum rain season through autumn, but when abundant rain arrives all of a sudden, capturing and storing every drop smoothly is no easy task. Typhoons in particular bring intense rainfall over a short period. In the past, reservoir staff could only rely on experience to open and close sluice gates and adjust water levels amid wind and rain so that a reservoir could perform at its best. Every typhoon felt like a battle of wits between reservoir staff and nature.

To regulate water conditions more precisely, Chang Li-chiu, a professor in the Department of Water Resources and Environmental Engineering at Tamkang University, built a flood forecasting system based on 50 years of typhoon data from Shimen Reservoir. "Once the typhoon track and total rainfall forecast come out, you instantly know how much floodwater there will be in total and when the flood peak will occur." After six years of practical drills, the system has been proven to successfully improve flood warning accuracy and water resource management effectiveness, and it has been published in the internationally renowned journal Nature Communications.

Water conservancy works that have lasted a thousand years, and typhoons that bring rain as well as tests

"Every year's typhoons make people both look forward to them and feel nervous." Chang Li-chiu said that for farmers, whether reservoir water volume is sufficient affects agricultural production. The largest water source for reservoirs in Taiwan, China, comes from the torrential rain brought by typhoons, and the accuracy of reservoir flood forecasting is key information for reservoir flood control and water storage.

However, the uncertainty of typhoon tracks and rainfall is high, and for reservoir staff, every typhoon is a test. "The goal of reservoir workers is to be able to fill the reservoir with water after the typhoon passes, so water must be released before the typhoon. The difficulty lies in how much water to release and when to release it so that it is just right."

Releasing floodwater before rainfall is very important. If heavy rain exceeds reservoir capacity and causes overtopping, the dam structure will be damaged, so floodwater must be released first to keep the dam safe. Chang Li-chiu said that once water is released, it can only be replenished by rainfall. If there is a mistake, it will affect water use downstream. But in the past, without an accurate forecasting system, reservoir control staff could only rely on experience or calculated predictions to judge rainfall conditions and water levels, so reservoir workers were under enormous pressure.

Fifty years of typhoon data from Shimen Reservoir, building a flood forecasting system through track and rainfall analysis

In 2013 (the 102nd year of the Republic of China calendar), Chang Li-chiu took on a project from the Northern Region Water Resources Office to carry out a flood forecasting system for Shimen Reservoir during typhoon periods. For this, she collected relevant data on about more than 90 typhoons from 1965 to the present, and collaborated across universities with Professor Chang Fi-john of the Department of Bioenvironmental Systems Engineering at National Taiwan University to further refine and improve this forecasting system.

The forecasting system built by Chang Li-chiu analyzes typhoon tracks and rainfall over the past more than 50 years. She displayed one of the track analysis charts, dividing the entire chart into small grids and marking the areas passed over by typhoons in different shades of color. The darker the color, the greater the impact on Shimen Reservoir, and different tracks are classified in multiple ways, for example: north-biased typhoons, center-crossing typhoons, and so on.

Chang Li-chiu explained that the greatest feature of this system is that it vectorizes tracks. Compared with analog or digitized coordinates, this is more suitable for classification. "Plane coordinates have two dimensions (horizontal and vertical directions). A vector converts the two dimensions into one direction, which makes analysis much simpler."

Applicable to different rainfall durations and different rainfall amounts; input the typhoon track and total rainfall to immediately predict the flood process

Chang Li-chiu said that precipitation conditions differ for each typhoon, including rainfall amount and total rainfall duration, and both affect flood size and the arrival time of the flood peak. "They must be converted into a flow characteristic curve; only after normalization is it easy to forecast."

Chang Li-chiu explained that the so-called flow characteristic curve first presents the total rainfall duration and the peak rainfall time point of all typhoons as percentages. In this way, there is no problem of different typhoon durations. "Typhoon A lasts three days with total rainfall of 400 mm; Typhoon B lasts two days with total rainfall of 300 mm. By converting all time into percentages, the two typhoons can automatically produce a full flood process forecast based on the flow characteristic curve."

In other words, once the characteristic curve is grasped, different types can be applied through the predicted track to infer the subsequent rainfall situation, for example, how many hours later the maximum rainfall will be reached. The full flood process forecast is the predicted process of the rainfall brought by a typhoon, from small to large and then gradually easing.