Let’s talk about some of the things I learned during my searches.
First, what is onsen?
The general understanding is it’s hot spring water, whether naturally existing or drawn up through wells. The legal definition is a lot more specific, first of all, the water needs to be higher than 25 degrees C, and must contain specific quantity of minerals. The concentration of the minerals required depends on the type of minerals, some less, some more.
So onsen is hot mineral water? Kinda, when used in a bathing context it became a lot murkier. As onsen is such a selling point for hotels and ryokans, all of them try to offer onsens and stick all kind of impressive descriptions before them. Some are just fluff, others misleading.
- Natural vs Artificial 天然vs人工
First thing to knock out is natural vs artificial onsen. Natural simple enough means the onsen water is drawn from natural sources.
Artificial onsen water is created from normal water by adding onsen powder, grounded mineral stones and other additives to increase mineral content of water. These are most often used in city baths, or when a place wishes to offer nigori にごり, murky onsen and the natural sources are not concentrated enough or contain the right mix of minerals to have that oxidation colour effect. Whether artificial onsen has same effect as natural onsen is another matter.
- Recycled vs Kakenagashi 循環vs掛け流し
Recycled is easy enough to understand, think of a swimming pool, the water is sucked down through inlets, filtered and then flows back out. In the recycling process chemicals may also be added to prevent the growth of bacteria and clear out dirty particles. Supposedly technology has advanced enough that the process does not affect the mineral content of the onsen water, won’t go into discussing whether this holds true or not.
Kakenagashi supposedly means letting it flow continuously from the source without cycling, in actuality it has a devious definition, for the sake of sanity for now it’s best to simply think of it is the opposite of recycled. We will go into this in depth later.
So why recycle at all? The answer is because it takes a heck of a lot more water than one would think to achieve kakenagashi.
Onsen water needs to be constantly replaced to keep the bath clean and avoid bacteria growth, like legionnaire’s disease which sometimes break out when an onsen bath isn’t kept up to standards.
Just how much water is needed? A general sum of rule is the water of a bath need to be able to be completely replaced every hour (ie, inflow volume in an hour = total volume of bath). Take a decent sized public bath, 3 by 5 metres and 0.6 metres deep, 9 cubic metres = 9000 litres every hour = 150 litres every minute (150L/m). Then double the bath for both genders, 300L/m, suddenly we’re looking at a lot of water. For larger hotels there’s a need for even more baths to prevent crowding, a hotel with 100 rooms is going to need several pair of public baths.
掛流需要多少水呢？原則上一個溫泉池的水必須一小時汰換一次 (一小時流入的水 = 池子的容積)。拿一個還算寬廣的公共池，3公尺長 5公尺寬 0.6公尺深，容積的9立方 = 每小時9000公升 = 每分鐘150公升 (150L/m)。男女池都要所以x2，300L/m，這樣消耗的水量是很可觀的。大一點的旅館避免擁擠必須多幾個池，一個數百房的旅館可能需要數對溫泉池。
Although this number seems to be mostly refer to public baths where it’s expected to be in constant use. Private baths or baths that sees less usage may require less water to maintain cleanliness.
Most places simply don’t have this much onsen water available, the larger a hotel or ryokan the larger its bath needs to be and the less likely it’s going to be able to source enough water for it.
Larger hotels either recycle their water, or sometimes have one bath that is kakenagashi then recycle the water from that bath to feed other baths. Keep an eye out on a hotel’s onsen description to see which ones are kakenagashi.
On a similar note, if the hotel offers onsen baths in their rooms, keep an eye out to see if they specify it is kakenagashi, as private baths uses up way more water per guest. Or worse, sometimes the room’s open air baths aren’t onsen water at all, be sure to double check this, don’t go to a onsen hotel and bath in plain piped water.
If the size of baths seems too hard to estimate, another rough rule is 1 litres per minute for every staying guest, this doesn’t take into account private baths, sizes and other factors so don’t rely on it.
Now consider that Yufuin, one of the largest onsen in Japan by onsen water amount range about 40~60000L/m. By comparison Hakone, probably the most visited onsen by foreigners, has about ~20000L/m. It is then not difficult to see why it is quite rare for hotels to offer kakenagashi.
現在來看看溫泉地的水量，拿由布院來說，算是日本湧泉量中名列前茅的，每分鐘40~60000L/m. 相較下外國人最愛的箱根大概是20000L/m. 這樣就不難理解為何提供掛流的旅館不多。
If look look at Taiwan, the most famous Beitou onsen, has blue and white spring combined output of 6000~8000m3 a day, 6000×1000/24/60 = ~4166~5555L/m, a quarter of Hakone and a tenth of Yufuin. The supply demand situation of onsen water by Beitou’s onsen hotels is clear, and so are the implications.
拿台灣來說，最有名的北投溫泉每天供給青磺白磺一天共6000~8000立方, 6000×1000/24/60 = ~4166~5555L/m，是箱根的1/4，由布院的1/10。北投旅館的溫泉供需狀況和引發現象可想而知。
If we’re looking for the kakenagashi experience, focus on hotels and ryokans with less than 30 rooms as they are more likely to have them.
Though less common, temperature can also affect whether kakenagashi is possible. Onsen water needs to be about 40 degrees, less in summer slightly more in winter. If the onsen source is not at the right temperature then they will need to be adjusted. Keep in mind the water will have cooled between the source, the holding tank and finally piped to the bath.
If the source is below desired temperature, either the hotel will let it be as is and warn the guests that the onsen may be too cold (especially in winter), or the hotel will opt to heat it up. But heating up water costs money, if the outflow is let off as in the case of kakenagashi, all that heated water essentially goes to waste and continuously heating up fresh water will cost a fortune, so the hotel is forced to recycle the hot outflow to reduce heating bills.
On the other end, if the onsen source is too hot, then the water will need to be cooled down before it can be fed into the baths. Cooling down may sound simple, but natural cooling may not cool down water fast enough nor in sufficient quantity. A heat exchanger may be used but that itself requires a huge amount of fresh cold water that may not be cheaply available.
There’s also the option of adding cold water directly, but that can only be done to a small degree before the water stop qualifying as onsen.
If the onsen source is not of the right temperature it may not be possible for the hotel to do kakenagashi.
Bonus: Kusatsu uses hot water that comes out of their heat exchanger to supply household hot water and pipe it beneath roads to melt snow.
- Gensen Kakenagashi, Gensen 100% Kakenagashi 源泉掛け流し, 源泉100%掛け流し
We’ve already determined kakenagashi means no recycling so it’s the best there is, right?
Well, not quite. If we look at various descriptions onsens hotels use to describe their onsen water, we’ll come some labels like “gensen kakenagashi” (源泉かけ流し), or goodness forbid, “gensen 100% kakenagashi” (源泉100%かけ流し).
It’s a way for hotels to promote themselves by sticking more labels to make things seem more impressive.
Let us look at “gensen” first. Gensen means local source, so “gensen kakenagashi” means directly from local source with no recycling.
Technically the term gensen is to separate it from indou (引湯) where onsen water is trucked in to a hotel from elsewhere. Hotels and onsen baths in city do this often to provide the busy city dwellers a chance to relax and enjoy onsen bath.
Sometimes onsen hotels secretly resort to this too when they’re short and desperate for more onsen water, like what happened in Arima (有馬温泉) a few years ago, causing quite a scandal.
Except in the case of kakenagashi this is largely a redundant qualifier since no one is going to be capable of trucking in that much water to allow it to flow continuously without recycling. Adding gensen before kakenagashi is just fluff.
Next let’s see about that 100%. The first thought would be, it means 100% directly from the source without treating it. And that would be correct***. Yes, that’s what it should mean but with several fine prints.
There is some vagueness around for what qualifies for 100%. The gensen kakenagashi association considers an onsen to be 100% given the “added amount of water for purpose of adjusting temperature does not affect quality of onsen water”. ¯\_(ツ)_/¯
The onsen association considers 100% to have no added water but heating is allowed. (As mentioned heating is a rare case anyway due to cost involved).
For the Japanese Fair Trade Commission, they stipulates that 100% must be 100% with no water or heating. But we can still see some advertising as 100% with exception in footnotes, such as “100% gensen kakenagashi*. *Sometimes water may be added for cooling”.
If that is all getting very confusing, it is, especially when a hotel’s website may not be very clear or even mention what kind of onsen water it offers, sometimes they put that description in the room’s booking plans, othertimes it might even be in the description of a photo in their gallery.
There is a quick way to help find out though. That’s Jalan. Jalan is quite clear in how hotels should label their onsen. Unfortunately whether a hotel follow the guidelines is another matter, I find it to be correct most of the time at least.
Bonus: Jalan is very detailed about everything about its listings. One of the other detail which I love is it lists the number of rooms and the number of public baths a place has, I use it a lot to get a feel for how big a ryokan is. Jalan for info of a hotel, Rakuten for pretty pictures of a hotel.
On Jalan there is a field showing what onsen a hotel has. It goes in the format of <Onsen Name> (Qualifier) [Kakenagashi tag if any]
The qualifier can be:
100% natural (no added water, no heating)
The last 3 can be combined, eg “Recycled, added water”.
A hotel gets the kakenagashi label as long as it does not recycle (adding water or heating is allowed, but that in theory disqualify them from 100% natural). The hotel may add in footnote explaining they’re 100% most of the time and when water would be added.
- Onsen analysis certificate 溫泉分析書
Every hotel is supposed to put up in the bath area a certificate of their onsen source showing their mineral breakdowns and certifying they are indeed using onsen water. Though not required, many certificate will also indicate the flow rate of the onsen source so we can learn about the L/m this way. We can also learn about the onsen sources’ temperature (which may give some indication whether it’ suitable for bathing in winter, summer), and the kind of onsen water it is, whether strong or weak, scent, acidic or alkaline. For first time onsen bathers it’s not recommended that they try out strong acidic (such as some in Kusatsu) as they may cause skin agitation.