: How to encourage a six-year-old to habitually drink more fluids? My six-year-old has never had much interest in drinking fluids, and has recently had problems with passing faeces (i.e. constipation), which will not have been

A very common way to get around this sort of thing is by demonstrating the right behaviours.

Buy some water bottles, for all the family (and make sure she gets to choose the style/design for hers) and always have them with you. A really good design she likes may make her happier to have water at school as well.
Let her have the flavour of choice (Ribena or whatever) but try to avoid making it too concentrated.
Drink from them frequently yourself. Have one with you when watching TV, in the car, anywhere.

It may sound too simple but it works as a general encouragement, without being confrontational, by providing good positive examples of the behaviour you want to see from her.

(Somehow we appear to have 17 water bottles between the 5 of us, and while I get fed up finding them in random places, having them there makes everyone drink.)

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Resist the temptation to allow sugary drinks to solve this problem... you will just trade one problem for another!
Keep a selection of frozen fruits on hand to use as ice cubes. When she gets to the bottom of the drink, she has fruit to eat (usually thawed by then) as a treat. She can devise her own concoctions.
Water must be handy. Carry water bottles with you whenever you leave the house.
Let her shop for glasses or cups with you - whatever would be fun for her to drink out of. One of my kids liked to drink from a coffee cup. Your daughter might choose a wine glass or a character glass that is only hers to use.
Offer tea. It's a grown up kind of drink, and comes in so many herbal flavors now!
Offer high-water content fruits as snacks (watermelon, citrus).

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You could try high-liquid foods, like soups. Fruits like melons or berries provide liquids.
I might also suggest not calling more attention to this because you have provided a battle and that can be a problem... focus on something else.

I had a client whose child would only eat candy and chips. Meals were provided, candy and chips removed from the house and within one day, hunger won. The parents just did not argue.

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Here are some ideas. They might not all be appropriate for your situation, and you don't have to use all of them!

Sometimes it helps to give information to a child about her health situation. One resource you could use for this might be kidshealth.org/en/kids/constipation.html. This is a website aimed at children. If your child doesn't read English, or is not ready to read this with you, you can still use this page as an outline for a conversation in which you provide her with objective medical information.
(As already contributed by Rory) if you and your daughter don't yet have personal water bottles, let your daughter choose a bottle for herself, and buy one for yourself. Keep the bottles handy, and model the desired behavior, without making a big deal about it.

(a) Connected with this -- consider straws and ice cubes to add more fun. You can put food coloring in the ice cubes. There are some new refillable squeeze tubes in the stores where I live now, that can provide more variety.

(b) Related to this, you can set up pouring activities, à la Montessori. You can pour for fun and you can pour as part of measuring activities. In other words, doing things with water can be a wonderful sensory experience, and perhaps these experiences will have a general encouraging effect.
As suggested at the parents' section on kidshealth.org, kidshealth.org/en/parents/constipation.html?ref=search#kha_22, it may be helpful to talk to your child's doctor.
If your daughter is a Michelle Obama fan, tap into her Drink Up campaign. For example, here's a fun, short spot she made about the healthful benefits of drinking water:

Sometimes it helps to buy plain carbonated water (no sugar), or a gadget for making bubble water at home. You can have fun experimenting together with different flavorings, for example unsweetened cranberry juice, lime juice, vanilla, etc.
See if you can partner with your child's teacher so that more opportunities for drinking water can be incorporated into the school day. For example, some parents contribute a box of water bottles to the class, so that each child has a reusable water bottle, labeled with the child's name, to use when there's a snack or water break.

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I also have a child who needs to drink a lot for health reasons, including a tendency towards constipation. What helped for her was gamifying her water intake. We'd send her to school with a large water bottle and challenge her to have finished it by the time we picked her up. We would always ask about it when picking her up from school, and if it wasn't quite done say something like "Uh, oh, you better drink it up quick in the car before we get home and 'officially' check it!" We also put it on her chore chart, so she would get a star or smiley face or whatever she was into at the time for drinking all her water.

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