Taiwan is rainy — sometimes

I haven’t written a lot on Frozen Garlic in the past year and a half. Part of this is that this is the dead time in the election cycle, so there really isn’t a whole lot of news to write about. More importantly, this has been a difficult year and a half for me. I’ve been preoccupied with other things, of which the Covid pandemic is merely somewhere in the top ten. Anyway, this blog has always been a fun thing for me rather than a responsibility, so when I don’t want to write anything, I just don’t write anything. And that has described most of the last year and a half.

Today’s topic is going to be strange and uninteresting to many readers, and I imagine many people will not read all the way to the end. That’s ok with me.

I’ve been distracting myself over the past few months by thinking about water. Water is one of those topics that people interested in politics only think about when something goes wrong. It’s always important, but usually it fades into the background. However, when water becomes a problem, it has the potential to shift political outcomes. Taichung experienced a few months of water rationing this spring. When no water comes out of your faucet two days a week, the pain can build up. The train accident earlier this year got more headlines, but that was a one-time event that didn’t affect many people and was quickly forgotten. Water rationing had the potential to be a much more demoralizing event.

I don’t think water will turn out to be a critical factor in the next elections. It started raining the last two days in May, and it has been basically raining ever since. The reservoirs were all dangerously low then, but now they are all full. I might be wrong, but I think the pain of water rationing will fade from memory, even for Taichung voters. So the rest of this post will be discussing something that I don’t think will matter much for the 2022 or 2024 elections.

I’ve never really looked at rain and reservoirs before this year, so I’m probably going to “discover” a lot of things that experts already know. It’s also possible I will be making all the obvious mistakes that they learned to avoid long ago.

Let’s start with a data point. In the 16 days from July 25 to August 9, the Pingtung weather station recorded 1496.5mm of rain. 1.5 meters!!! I’m a pretty average height, and that is nearly to the top of my shoulders. And remember, this is the official Pingtung weather station. There are lots of rain gauges in Pingtung, and many of them were considerably higher.

But wait a minute. The first thing that you learn in dealing with data is that you have to ask, “Is that a lot?” At first glance, it seems like a lot, but is it? Is that more than normal? If so, is it a lot more than normal? After all, Taiwan is a rainy place. And when typhoons hit, we can see unfathomable amounts of rain in just a day or two. Anyone who has traveled through Taiwan has seen enormous river beds with just a trickle of water in them. Once or twice every few years, those enormous river beds are needed for a few hours to transport stupendous quantities of water back into the ocean. So maybe this year has just been – ordinary?

While we’re asking these questions, what about last year? This spring when all the reservoirs were nearly empty, we were reminded that last year no typhoons hit Taiwan so the rainfall had been abnormally low. It seems like a good time to check that assertion. Was rainfall markedly lower last year?

The Central Weather Bureau website provides daily rainfall data for 35 weather stations starting in 2009. However, some of these are missing chunks of data and some of them are rocks in the middle of the ocean that I don’t care about. Also, daily data would have been more than just a time-wasting project. I ended up collecting monthly rain data from 2009 to 2020 for 26 weather stations. (If it had been a real research project, I would have gotten daily data for the hundreds (thousands?) of rain measurement stations. But I study politics, not rain. Gotta keep time-wasting side projects under control.)

How much rain does Taiwan get? Most places get somewhere between 1500 and 2500mm annually, though there are some pretty big discrepancies. Central Taiwan (in red) gets a bit less than most places, while the Taipei area (in dark blue) gets a bit more. The islands (in diamonds) in the Taiwan Strait (light blue) all get considerably less than everywhere else, with Kinmen not even averaging 1000mm a year. Meanwhile, the northeastern corner (in yellow) of Taiwan is very rainy, with Keelung getting over 3500mm a year. I live in Keelung, so that didn’t surprise me. What did surprise me was Su-ao, a little port town in southern Ilan. Su-ao is unbelievably rainy, averaging a staggering 4482mm a year! The mountains (with horizontal stripes) generally get more rain than the coastal areas. The two weather stations on Yangmingshan* get twice as much rain as downtown Taipei, and the station atop Alishan gets twice as much as Chiayi City. What’s crazy about Su-ao is that it gets mountainous quantities of rain at sea level.

*Why do we need two weather stations right next each other on Yangmingshan? Wouldn’t it be better to have a mountainous weather station somewhere in, say, Hsinchu or Miaoli? I’m guessing this has something to do with bureaucrats who want to be stationed in Yangmingshan and not deep in the Central Mountain Range.

How much rain is this? Is it a lot or a little? I did a quick internet search of annual rainfall in major cities around the world. For comparison, I used the Taiwan scale (0-5000mm). Taiwan’s rainfall is pretty standard for Southeast Asia (in red) and the two coastal South Asian cities (in green), but this is the rainiest region on earth. West Asia is very dry, with most cities getting somewhere around 500mm a year. In most of the rest of the world, cities get somewhere around 1000mm a year. In the USA, the southeast gets a bit more and the west gets quite a bit less. (Contrary to popular belief, it doesn’t rain very much in Seattle.) Europe is quite a bit dryer than the USA, with very few cities even getting 1000mm a year. British comedians moan incessantly about how much it rains in Scotland and Wales, but Glasgow would be considered a dry spot here in Taiwan (and Cardiff is even dryer). In global perspective, Kinmen doesn’t seem arid at all.

I live in Keelung, so I was well aware that I live in the rainiest corner of the island. What I didn’t realize is that the rain falls at different times during the year here.

There are two distinct rain patterns in Taiwan. The dominant pattern covers central and southern Taiwan. In this pattern, there isn’t much rain at all between October and April. Almost all the rain comes from May to September, especially in June and August. The chart shows the rainfall for Tainan, but everything from Miaoli to Pingtung looks about the same. The second pattern is the northeastern pattern, covering Keelung and Ilan. Here, it rains all year round, and there is actually more rain in the winter than in the summer. The rest of northern Taiwan is a transition zone between these two patterns, though it is generally closer to the southern pattern than the northeastern pattern. In Taipei, for example, May-October is the rainy season, but it isn’t quite as rainy as in the south then. And the winter is the dry season, but it isn’t nearly as dry as the south.

The summer spike for Tainan makes it clear just how seasonal rain is in central and southern Taiwan. All the rain comes either in late spring/early summer plum rains or in late summer typhoons. Even more importantly, when the rain comes, it comes all at once. On average, most places in central and southern Taiwan get a fourth to a third of their rain in August, but there is tremendous variation in that average. Tainan, for example, averages 576mm in August, but that has ranged from 91mm in 2016 to 1301mm in 2018. Remember, this is only a 12-year dataset; if we went back 50 or 100 years, we’d almost certainly find even more extreme Tainan Augusts. Remember that 1.5m of rain in Pingtung? Is that extreme? Well, it’s a lot, but it’s not unheard of. There are 48 months in my dataset with at least 1000mm, and 11 months with at least 1500mm. This Pingtung storm was split between July and August and didn’t reach 1000mm in either month, so it is not part of the 48 1000mm months, much less the 11 1500mm months. The heaviest rainfall in a single month since 2009 was August 2009, when Typhoon Morakot dumped 3346mm of rain on the Alishan weather station. For those of you who don’t understand metric, that is almost exactly 11 feet. Think about eleven feet of rain. It’s hard to even imagine.

The reservoirs were all empty in mid-May. Was 2020 really an extremely dry year? It turns out that 2020 wasn’t the worst year for a lot of places. It was the lowest year (from 2009 to 2020) for 11 of the 26 weather stations that I had data on. That means that the other 15 had at least one dryer year. In fact, the three northeastern areas got more rain than normal.

2020 wasn’t a historically dry year everywhere, but it was in central Taiwan. 2020 was the worst year for Hsinchu, Taichung, Wuqi, Sun Moon Lake, Chiay, Alishan, and Yushan. I’m missing some data for Miaoli, Changhua, and Yunlin, but from the data I have, 2020 was the worst year for them too. It just didn’t rain much in central Taiwan.

Every ranked list has to have a first and last place, so maybe it’s better to use a threshold. It is very rare for a place to get less than 60% of its normal yearly rainfall. Of the 312 place-years, only 8 fell below this 60% threshold. Four of those occurred in 2020, including three in central Taiwan. (Kinmen was the other.) Miaoli and Changhua may have also fallen below this threshold, but I don’t have complete data for the other years. Remember, people don’t get first claim to rainfall. The trees drink first. If central Taiwan only saw 60% of the normal rainfall, that implies that the reservoirs probably got quite a bit less than 60% of their normal supply. Central Taiwan was dry, dry, dry in 2020.

Ok, so it didn’t rain much in central Taiwan in 2020. Why were all the reservoirs empty? Why was there a water crisis stretching from New Taipei City all the way to Kaohsiung?

I learned something new this year staring at maps of rain and reservoirs: most of the reservoirs are fed by central Taiwan. The Shimen Reservoir is located in Taoyuan and supplies most of Taoyuan and New Taipei City, so I have always thought of it as a northern reservoir. However, its watershed covers the mountainous areas of Miaoli, Hsinchu, and Taoyuan. That stretches pretty far into central Taiwan. If it doesn’t rain in Miaoli and Hsinchu (like in 2020), the Taipei suburbs are going to run out of water (as almost happened in May 2021). Likewise, the Tsengwen reservoir, which supplies most of southern Taiwan, is fed by the Tsengwen River, which empties out into the ocean in Tainan City. I think of it as southern. However, the water in the Tsengwen Reservoir comes mostly from Alishan. While most of us think of Chiayi as politically and culturally southern, geographically it looks pretty central. If it doesn’t rain in Alishan (as in 2020), the water supply in Kaohsiung is going to run low (as happened in 2021).

In May, the only big reservoir that was more than half full was Feitsui, which supplies Taipei City. Feitsui is in Shiding, southeast of the city. More to the point, the rainfall pattern in Shiding is closer to the Keelung pattern of steady rain all year long than the central Taiwan feast or famine pattern. Strangely, this is one place where the Japanese did not plan well. It might seem obvious to build a reservoir where there is a steady supply of water, but Feitsui was not built until the 1970s.

We are all very aware that power plants in Taichung ship electricity north. I never realized that central Taiwan also supplies most of the water for the island. Except Taipei. CKS – always wary of angry mobs in the capital – was careful to ensure a steady water supply for Taipei.

To conclude, I am very happy to say that I assume none of this will have much impact on any elections coming up over the next few years. We (mostly) dodged a bullet this time.