Supply and Demand
In economy, supply and demand is a model that helps you to determine the price of something in the market considering the level of interest in the product and how scarce that product is.
If many people want the same resource and that resource is not common, the intrinsic value of it becomes considerably higher. If you want to have that, you might have to offer something of great value to its owner.
In today's market, it means that if a product is rare and many people want that, the value in money is going to go up.
Same goes for the example on the question, since the supply is low and the demand for water is high, those with more to offer are going to be the ones who get it. If it goes the other way around (too much water and not much interest in it), the price would decrease considerably.
