Es posible que se acabe la reserva de agua.

Because es possible que triggers the subjunctive.

In Spanish, when you say something is possible, doubtful, uncertain, desirable, etc., the verb in the following clause often goes in the subjunctive:

• Es posible que se acabe...
• Es probable que llueva.
• Es dudoso que venga.

So:

• se acaba = indicative, more like a fact or statement
• se acabe = subjunctive, used here because the speaker is talking about a possibility, not a certainty

That is why Es posible que se acabe la reserva de agua uses acabe.

Here, acabarse means to run out, to be used up, or to come to an end.

So se acabe la reserva de agua means something like:

• the water supply may run out
• the reserve of water may be exhausted

This is a very common use of acabarse with things like:

• Se acabó el café. = The coffee ran out.
• Se me acabó el tiempo. = I ran out of time.
• Se están acabando las opciones. = The options are running out.

In this sentence, se is part of the verb acabarse.

Compare:

• acabar = to finish, to end something
• acabarse = to finish up, to run out, to come to an end

So:

• Acabaron la comida. = They finished the food.
• Se acabó la comida. = The food ran out / the food was gone.

In your sentence, la reserva de agua is not finishing something else; rather, it itself may come to an end. That is why Spanish uses acabarse.

The subject is la reserva de agua.

So the structure is:

• Es posible = It is possible
• que se acabe = that it may run out / be used up
• la reserva de agua = the water reserve / water supply

The verb is singular because la reserva is singular:

• Es posible que se acabe la reserva de agua.
• Es posible que se acaben las reservas de agua. = if it were plural

Spanish often places the subject after the verb, especially when the subject is the thing that appears, disappears, begins, ends, runs out, etc.

So se acabe la reserva de agua sounds very natural.

You could think of it as similar to structures like:

• Llegó el tren.
• Se terminó la película.
• Se acabó el agua.

This does not change the basic meaning. It is just a common and natural word order in Spanish.

Not exactly.

• el agua = the water
• la reserva de agua = the water reserve, water supply, stored water

So this sentence is not necessarily saying that all water everywhere will run out. It refers to a specific reserve or supply of water.

Depending on context, reserva de agua could mean:

• a stored supply of water
• a reservoir of available water
• a reserve for future use

Yes, that is possible, and it would sound natural.

Both acabarse and terminarse can express the idea of something being used up or coming to an end.

However, acabarse is especially common for running out of things like:

• water
• food
• money
• time

So Es posible que se acabe la reserva de agua is very idiomatic.

Because the correct structure is es posible que + subjunctive.

In Spanish, de is not used here.

Correct:

• Es posible que venga.
• Es posible que se acabe la reserva de agua.

Incorrect:

• Es posible de que...

This is just the normal construction you need to memorize.

Yes. Puede que is a very common alternative, and it also requires the subjunctive.

So these are both correct:

• Es posible que se acabe la reserva de agua.
• Puede que se acabe la reserva de agua.

They are very similar in meaning:

• Es posible que... = It is possible that...
• Puede que... = It may be that... / Maybe...

Yes, it sounds natural and correct.

Depending on the situation, they might also say more everyday versions such as:

• Es posible que se acabe el agua.
• Puede que se nos acabe el agua.
• Puede que se agote la reserva de agua.

But your sentence is perfectly good Spanish, especially in a formal, written, or neutral context.