Good luck on your brevet attempt!

200km, or 124 miles, is a long ride. For reference, on the road, average riders might complete a century ride (160km/100mi) in 5-7 hours, maybe less if one is with a fast group.

Note: I initially recommended that the OP drink ahead of thirst. In comments, @whatisname contended that this advice is based on a myth. On further research, I modified it.

Thirst perception

Modifying my initial argument a bit: on a brevet, I recommend the OP drink at least until their thirst is slaked, but also consider electrolyte intake (described further below). Thirst alone can be a good guide for most people in most situations. However, the OP is not in a normal situation based on his proposed ride length. If it's hot, and especially if it's hot and humid, he may want to drink before perceiving thirst, or attempt to do so. Athletes should be aware that our thirst perception declines with age, so athletes over about age 65 should err on the side of drinking more as well. Athletes in cooler conditions should be fine drinking to thirst alone. I don't believe empirically measuring your fluid consumption is necessary, but it can be done, described in the next section.

Individual fluid consumption can vary a lot, but one can obviously start from a rule of thumb, e.g. one bottle per hour as proposed by @DavidW, and adjust based on your perception of thirst. (NB: cyclists also start with a rule of thumb and adjust as we go with bike fit parameters, e.g. saddle height and stem length.) With experience, you will find your own equilibrium. For example, in temperatures of 85F/30C and higher, I find that while I'm only 133 lbs/60.3 kg, I can drink as much as a 25oz/750ml bottle per hour. In temperatures around 70F/21C, my intake is more like 25oz/750ml per 1.5 hours.

British Cycling recommended that cyclists drink ahead of thirst, rather than just to thirst. They argued that thirst is a lagging indicator of your hydration status; when you feel thirsty, you are in a significant deficit. However, Allen Lim, who has worked extensively with professional road cyclists, countered on a podcast that these athletes were good at listening to their perception of thirst. He felt that perceived thirst was enough for them, and that they would generally hydrate themselves enough based on drinking to thirst. That said, these are professional athletes, and they train very extensively, so their perceptions of their bodies are well-developed.

Andy Blow, an exercise scientist writing for the TrainerRoad blog, argued that thirst alone is a good general guide in most exercise conditions. He argues that heat and exercise duration may modify this. I based my argument above on his post.

Measuring your fluid intake

Some of us might want to consider actually measuring our intake. It appears that individual perspiration rates vary a lot, thus you may want a more accurate measure of your intake (alternatively, you might notice with experience that you need more fluid than your peers, or you might notice that you perform fine with less fluid).

Alternatively, a more empirical method like weighing onself before and after a ride of known duration (preferably in similar conditions as the event!), as the OP wrote, is also fine. This was proposed in an article by British Cycling, although they recommended a test period of 60 minutes. Naturally, you would need to adjust your intake up and down based on the weather and your own perception. Per this podcast interview with Allen Lim (referenced later), we would expect that most of the weight you lose should be through perspiration, rather than through burning stored glycogen or fat. If you eat, you then need to net out the weight of the food you ate.

The role of electrolytes, particularly sodium

The OP clearly seems to recognize the need for sodium already, but the discussion is incomplete without a mention of electrolytes, particularly sodium. You can take sodium in your fluid and/or your food.

Per the podcast interview cited earlier, we excrete sodium as we perspire. The rate at which we do this is highly variable, like sweat rates. If you don't replenish sodium, you will compromise your athletic performance. Thus, your drinks should contain some sodium.

As Lim discussed on the podcast, we may also develop a sense for how much salt we need. When I started cycling, I'd just stick to plain water, only to find that I'd crash at the end of long rides in the heat. This changed considerably when I started using electrolyte drinks, and when I paid attention to my diet's overall salt content. How much you crave salty food should be an indicator of low sodium. If you have very salty sweat, you will see salt deposits on your exercise gear after a long workout, or dogs may lick you after a workout. However, these may only indicate that you are a very salty sweater.

This ties into the role of carbohydrates in sports drinks

Carbohydrate in your drink may affect how well you absorb the fluid, but you do not want your drinks too sweet. Lim argued that a drink that is too concentrated (i.e. has too much sugar and other material dissolved) actually inhibited how well athletes could absorb the water content. Basically, the sports drinks had a higher concentration than our blood, and due to chemistry, this tends to pull water into your digestive tract rather than into your blood. Lim reported that many of his athletes developed bloating or other gastric distress from too-concentrated drinks. While you need to replenish carbohydrate stores while riding, I would not rely on sports drinks to do this.

Lim runs a company called Skratch Labs, and their drink mix contains about 20g of carbs (i.e. 80 calories) in a 16oz / 500ml drink. I'm unable to find consistent nutrition information for Gatorade on the web, but I believe this is half or less the amount in an equivalent Gatorade serving.

Specialist cycling hydration mixes are available, but they can be pricey, and you can brew your own. The section at the end of the answer has some suggestions.

What about fueling?

While the OP didn't ask directly, the discussion about carbs does naturally tie into the role of fueling during exercise. Cycling for very long distances burns a huge number of calories, and even Gatorade alone wouldn't be able to replenish them adequately. For example, during a solo century ride that took about 5h 30m of riding time, my power meter estimated that I burned about 2,700 calories. I weigh about 133 lbs. That's about 490 calories per hour. This was a fast ride (i.e. I was attempting to go as fast as I could; my intensity factor was 79% for those of you into power metrics). A slower pace will obviously burn less energy per unit time, and the OP should go at a slower pace for his first brevet. However, heavier individuals will naturally burn more energy.

Your body stores carbohydrates in the form of glycogen. You will deplete your glycogen stores after about 3 hours of cycling. If you don't replenish them, your performance will suffer (i.e. you are likely to bonk). Note that you also burn fat, and as exercise intensity increases, the ratio of fat to glycogen burned will increase (i.e. you burn proportionately less fat). However, you will still burn a lot of glycogen if you ride at an easy pace for 5 hours or more. For almost all athletes, our body's fat stores are adequate on their own. (NB: Ben Greenfield and Allen Lim discussed the potential for adding medium chain triglycerides, i.e. a form of fat, to food as a supplemental energy source; most of us should be fine with less sophisticated methods.)

You obviously don't need to replenish all the carbs you burn. In fact, in professional road cycling, riders physically couldn't digest enough food to accomplish that, even if they could eat that much in the heat of battle. Lim proposed that in his context, athletes could aim to eat about half the calories they actually burned per hour.

The OP may be trying to lose weight, but he will nonetheless want to have some simple carbs while riding. I would recommend at least 200 calories per hour. Fresh fruit, if available, contains water as well as carbohydrates, and it can make for a good healthy snack at rest stops (although take note that too much fiber on a long ride can upset your stomach, so it may be worth experimenting to see if you can tolerate this). Otherwise, energy gels and chews were developed for this purpose. If not that, then confections like chocolate bars or cookies will also work, although note that their fat content may also cause gastric upset (so, again, experiment if you're able).

(Side discussion: In some competitions, tactical considerations might make it hard to eat consistently. There are some highly-concentrated drinks that contain a lot of carbs in gel form that you might consider. Few of us are likely to need these. An example situation is a professional road race, where there may be numerous attacks throughout the race that riders have to respond to. In this context, respond means to get into the breakaway, or to ride with the peloton to retrieve the breakaway or to limit its time gap.)

---

A counterargument on drinking ahead of thirst

This answer initially advised to drink ahead of your thirst. @whatisname countered, in comments and in a separate answer, that this advice stems from a myth, and that one should focus on drinking solely to thirst.

Tim Noakes, interviewed in Outside magazine, is an exercise scientist who advised on the dangers of over-hydrating during long endurance events. Indeed, if you take in too much water without also taking in sodium, your blood sodium levels can drop too low (i.e. hyponatremia). This can be dangerous. In fact, a number of marathoners have died from hyponatremia.

Blow, on the TrainerRoad blog cited earlier, responded that Noakes may be exaggerating the evidence. Blow countered that there is significant evidence that many successful athletes take on supplemental sodium during exercise. Blow did agree that drinking plain water (i.e. no electrolytes added) was generally sufficient, emphasis mine:

And for many people training or racing shorter events in cool to moderate conditions, drinking water to thirst will be sufficient most of the time.

He also added that we have some room for error; if you under-hydrate on a two hour ride, you can push through and just drink more afterward.

However, he cautioned that if you're in a long event on a hot day, that's not really a normal situation. You may also have trained hard up to that event, and thus depleted your existing reserves. In this case, drinking plain water alone to thirst is not likely to be sufficient. Thus, I would advise electrolyte drinks in long events like brevets. Be aware that hyponatremia is possible, but you will need to drink lots of plain water while perspiring a lot.

For those of us who need low sodium intake due to blood pressure, I would advise consulting your physician. However, you will still need to replenish your electrolytes during long exercise. On the podcast, Lim discussed that there is a lab test that measures your sweat rate. It is normally done to detect cystic fibrosis, because people with CF excrete salt at abnormally high rates. For reference, Lim said that healthy individuals can excrete between 300 and 2,000 mg of sodium per liter of salt, whereas people with CF may excrete 3,500mg or more. This test could be worth considering if you really must closely calibrate your sodium intake. I believe that in most cases, you would need to approach your physician to take one.

---

Bonus: Low-cost electrolyte drink mixes

Cycling-specific drink mixes can be very good, but they also can be pricey per serving. My favorite mix, Skratch Labs, is about US$1 or more per serving depending on how concentrated you mix it. I reserve it for significant events.

You can brew your own electrolyte drinks. Abby Mickey, a former professional road cyclist and now a cycling journalist with Cyclingtips, provided one recipe using freeze-dried fruit or vegetable powder, salt, and maple syrup or sugar. Dave Everett, another Cyclingtips writer, provided recipes for drinks as well as an energy gel. He also provided a recipe for energy chews, but he advised that his attempt turned out "pretty damn terrible." Last, I have taken powdered Gatorade, mixed it at half the recommended concentration, and added a pinch of table salt.

When I first started doing longer distance rides, the rule-of-thumb I was told was "drink a bottle every hour."

Obviously, that's a very rough figure, and it's going to depend on a lot of factors, including how much you're sweating (a combination of workload and heat+humidity), how much you weigh, and just generally how much fluid you personally need. The first time I did a century (160km/100mi) I was loaded touring and it was very hot; that day I drank 9l of water on the road, and still needed to keep rehydrating that evening.

But to get to my point. After many years of riding, I've found one of the best indicators that I'm getting adequate hydration is that I keep regularly needing to urinate; if I find I'm going more than 2.5h without needing to relieve myself, I'm getting dehydrated. Again, there's a certain amount of knowing your own body here, and my numbers won't be the same as yours. But if you're doing a long ride, you're going to need nutrition; metabolizing nutrients (especially protein) produces water and metabolic waste products you need to get rid of. If you're not doing so regularly, it means that your body is starting to hoard water and you need to drink more.

You need more water when it's hot—-particularly on the days your body isn't you yet adapted to the heat. You need less water if the effort is lower. Some days that's a lot of water per hour and some days it's almost none.

I hate to answer with a product when you're asking for a rule of thumb, but I've been really impressed with the hydration tracking feature on my Garmin 530. It monitors temperature, elevation gain, speed, duration, and heart rate and power data (if applicable) and then alerts you when it's time to drink as well as milestones for how much you should have consumed by now.

https://www8.garmin.com/manuals/webhelp/edge530/EN-US/GUID-E5AEF3C5-2139-42FF-8E5E-069D09D1D80D.html

The answer is simple:

Drink when you're thirsty.

And, it has a similar counterpart: Eat when you're hungry.

Yes, it doesn't have to be any more complicated than that.* We're creatures that are the result of millions of years of evolution, constantly tweaking the biochemical pathways that make us work. "Sports medicine" and practices more complicated than "drink when thirsty" are barely a century old.

Human beings have lived for millions of years doing strenuous activities without complicated formulas and schedules indicating when they should drink. Wild animals are born able to know when to drink, they don't need to be instructed. We don't either.

I understand that modern life has caused many of use to learn to ignore those signs. such as jobs where we have limited breaks, or we've learned to ignore them because we want to "power through it". And if you think you are having a problem hydrating, maybe that's what you've done. But you can listen to those signals again.

*Unless you are in a competition where you are in it to win it, or you are riding in extreme heat. You've probably seen numerous advice saying things along the lines of "if you're thirsty, you're already dehydrated". That is good advice if you are a serious competitor, but it is totally untrue for normal activities. Unless one of those conditions is satisfied, your own biological feedback systems are more than capable of signaling when you need to drink.

This is just some additional tips.

One thing I do on really long rides (first on a 400km brevet) is set a periodic reminder on my phone to drink every 15 minutes, also to stretch, and to consume some calories every half hour. Also a proper drink before and after each snack.

On a 200 a proper stop is a good idea, and popular: walking around, sitting on something other than a saddle for a few minutes, and having a big drink and something light but proper to eat. While it's only a couple of hours longer than a century, this stop is much more likely to be needed.

It's important at this distance to know where you can get water. This may mean a tap or a cafe that will refill bottles, or it may mean buying a big bottle. Normal facilities may still not be operating normally at the moment. What you don't want to do is set out onto a dry stretch without much water, then struggle.

If you're drinking several litres in a day, and mostly losing that to sweat, you should consider your electrolyte intake too.

Added later: Your need for hydration depends on your energy source. Specifically as you switch to burning more fat later in the ride, your need will go up, as well as any reserve drunk just before setting off being depleted. This means you can't easily extrapolate from shorter rides even in the same weather. As an example I forgot my bottle on a 50km ride last week, and was thirsty but fine even at the end. A few days later, in similar but wetter conditions, I did 83km and drank about 0.7l, nearly all of that in the 2nd half.

Coincidentally this got bumped just as I got back from a 200km in early winter (7C at the start, 3C at the end). I went fully self sufficient, carrying 2 litres of water, 0.75 litres of electrolyte drink, and 0.5 litres of coffee in a vacuum flask. I drank nearly all of it. (shops weren't completely ruled out, but our localised restrictions are complex and I was trying to stick to the spirit of minimising unnecessary contact; I'd also done something similar a couple of weeks ago, passing very near home just in case).That would never work in summer. Note that if you're cold on the surface, vasoconstriction leads to urination.

Medically, dehydration is usually detected/defined as a 5% increase in the concentration of solutes in your blood. Thirst sets in at about 2%, so you'll always feel strong thirst setting in long before you're dehydrated. This is basically a way of measuring the fact that your blood volume is low. Body weight is not a reliable way of measuring this. Your body weight changes for a lot of reasons, some of which have nothing to do with hydration. For example, when you exhale CO2, you're getting rid of the mass of those carbon atoms. It also changes when you eat, drink, sweat, urinate, or poop.

Fortunately we don't need a doctor with a syringe to tell us when we're dehydrated. Thirst is a powerful sensation. People who are actually dehydrated know it because they're extremely thirsty. Folk wisdom about hydration has diverged from the scientific evidence, especially with some of the silly or even dangerous bro-advice about drinking very large amounts of water (which can cause hyponatremia). Drink if you're thirsty.

Pop culture would have us believe that dehydration sneaks up on us like a silent killer. Actually this is a pretty good description of heat illness, which can be insidious and very deadly, and can happen regardless of water consumption. Heat illness can fry your brain and make you behave irrationally or fail to do important, obvious things like ceasing exercise or seeking shade.

gschenk asks in a comment:

would you clarify the following: (a) at which amounts of water intake one might begin to worry about hyponatremia? (b) how does direct Na loss through perspiration factor in.

For a complete answer, with references to scientific papers, see this SE answer. In the case described there resulting in death, an army recruit drank 20 liters of water in one morning because his drill instructor told him he had to.

Ivan McA says in a comment:

I think your (and @whatsisname)'s "you only need to drink when you are thirsty" is the bro science here.

This is a valid complaint, since I didn't provide any references to scientific evidence in the original version of this answer. I've added some references below.

References

Almond et al., Hyponatremia among Runners in the Boston Marathon, 2005, NEJM, DOI: 10.1056/NEJMoa043901

Noakes et al., "The danger of an inadequate water intake during prolonged exercise," European Journal of Applied Physiology and Occupational Physiology 57 (1988) 210

Heinz Valtin, "'Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8x8'?," Am J Physiol Regul Integr Comp Physiol 283: R993-R1004, 2002. http://ajpregu.physiology.org/cgi/content/full/283/5/R993

I have been doing some experiments to see if I'm hydrating enough during rides of moderate length and intensity.

It only matters for rides of high length or high intensity. For rides of moderate length and intensity, hydration is not required during the ride as the human body has enough spare water for a 50 km ride built in.

Long time ago, I used to do a daily 50 km ride. I never once needed water during the ride.

if I'm hydrating enough, should I expect my body weight to stay approximately the same before and after a ride?

For long-distance riding, this is the rule. The body weight is reduced by exhaled carbon dioxide and evaporated water. Of these, the evaporated water explains most of the difference.

However, for moderate-distance riding, you have to allow some flexibility. Nobody cares whether you weight one or two kilograms or half a kilogram less, or zero kilograms less for that matter, after the ride.

There is spare water built in to the human body. I suspect the reserve of spare water is around 2 liters (2 kg). A healthy human that was not thirsty before the ride can lose this amount of water (weight) during the ride without any ill effects.

Or would I expect to lose some measurable amount of weight independent of water loss (i.e. from respiration of CO2).

Yes, as I explained CO₂ removes some carbon from your body.

A human producing moderate power can produce 200 watts of power, burning 800 watts of hydrocarbons to do that. A 2-hour ride burns thus at most 1.6 kWh = 5760 kJ of energy.

If we assume half of the energy comes from hydrogen and half from carbon, 2880 kJ of carbon is burned. Carbon has 32.8 MJ = 32800 kJ of chemical energy per kilogram. You thus need 2880 / 32800 = 0.0878 kg of additional weight reduction for those 2 hours.

You said you were 0.45 kg lighter and consumed 44oz of water. If I calculated correctly, 44 oz is 1.23 kg so your weight reduced by 1.68 kg. Of this, 0.0878 kg was due to carbon burning to carbon dioxide and the rest 1.5922 kg was due to evaporated water. Carbon dioxide exhaling therefore explains only 5.5% of your weight loss. The rest, 94.5%, is explained solely by water evaporation.

By the way, the 1.5922 kg evaporated water is entirely within the safe limits, so the length of your ride does not need you to carry 1.5922 kg of water with you. 1.5922 kg is the difference between weight of an expensive bike and a cheap bike. Save some weight and drink only after the ride.

For longer rides, let's say 70-100 km, you may need to drink during the ride.

And if so, should I be drinking an extra half liter of water on similar rides?

No. As I explained, a human can lose 2 kilograms of water easily with no ill effects.

Or is it possible this weight difference was mostly from calorie burn, and my hydration level was adequate?

No. As I explained, only about 0.0878 kg is explained by calorie burn.

Obviously it isn't too much trouble to make up a small water deficit post ride, but I'm training for my first 200k brevet, and trying to establish a good baseline for staying hydrated over a substantially longer time-frame.

For this your experiments are useful. While a healthy human can do a 50 km ride without drinking any extra water during the ride, 200 km is way above the limits for riding water bottle free. In fact, I would argue that during a 200 km ride, you need to not only drink but also eat.