What to call a small, open stone or cement reservoir that supplies fresh water from a spring or other natural source?
In Spanish, this is called a pila or pilón. It's built out of stone or cement near a natural source of water (a spring or a stream) to hold the naturally-arriving water to make it convenient for people to fetch some water for personal use.
I have also seen them in the Italian alps, but I don't know what they're called there.
What can they be called in English? (either a single word or an attested phrase)
That would be a stone trough.
[Yes, I know, a trough in Spanish, for animals is an abrevadero, which is the same term in English: trough, like a pig or animal trough]
Here are all the images one might wish for:
images of stone troughs
Here is one:
From the Real Academia Dictionary:
- f. Pieza grande de piedra o de otra materia, cóncava y profunda, donde cae o se echa el agua para varios usos.
A large piece made of stone or other material, that is concave and deep, where water falls or is put for various uses. I say trough for pieza because in English, trough is about the way it is made more than who drinks from it.
They can also be referred to as stone water tanks as in the British House and Garden publication about an Italian estate:
I just remembered that in Portuguese, a pila is a tanque, and that is tank for the same meaning in English.
First, I'll point out that several on-line "English" dictionaries include an entry for pila, so you have the option of just using that word, with equal fluency in both languages.
M-W says: https://www.merriam-webster.com/dictionary/pila
Beyond that, word choice would be influenced by shape and intended usage. Some dictionaries define pila as a "stone horse-trough."
So if your stone pila is round, or square-ish, you might use basin. If the shape is much longer than wide, trough is the way to go. For something shaped like a chest (rectangular but not extreme), cistern is perfect as it derives from chest.
If the pila is elevated and shallow, consider font. If the source of the water is inside the pila, or captured so it feeds exclusively through the pila, you might use fount or fountain.
Sometimes it is called a basin as described here.
Although basin is often used to mean a sanitary appliance, that is a specific use of its more general meaning.
Cistern is precisely the right word. The house I formerly lived in had a similar tank in the basement to hold water piped in from a nearby spring, covered with a wooden lid to prevent mice from falling into it.
Because the spring was only a few feet higher than the level of the house, it didn't provide enough water pressure to reach the upper floors, so we had a water pump to pump the water from the cistern into the house. During a power outage, we could lift the lid and scoop out water for drinking and washing.