Water issues on Mars

When it comes to deciding where to place a colony on Mars, one issue oft overlooked is water availability.

Mars has water (which is what makes it the best choice for colonization) but it's not evenly distributed. Some areas are akin to the moon, where water can be found only in tiny concentrations. Extracting it via mining and heating would be enormously costly in terms of energy and infrastructure, even if we assume that Curiosity's results of a water concentration of 1.5 to 3% for Martian soil are representative of other areas of Mars. 

What this boils down to is that unless a colony is located near the north polar ice cap, you probably need an aquifer.  There may well be liquid-water aquifers on Mars, and in fact there almost certainly are. However, we don't know where they are yet.

Synthetic aperture radar may be the answer. A technique has been developed, but it has yet to be utilized on Mars.
JPL has an article about it, which highlights the fact that it's proving quite useful here on Earth. (I"ve often said that one of the prime "exports" of a Martian colony will be innovations in technology, and this is an early example; a technique developed for Mars being used on Earth).

However, even if aquifers are located, that alone isn't enough to select a colony site. Even here on Earth, wells still come up dry even when over a well-mapped aquifer. There are also cases where groundwater has unwelcome substances in it, such as arsenic or salt, which would require the water to be distilled (not a deal-breaker, but something you'd need to know in advance). So, what you need is a test well if an aquifer is your source.

Drilling a well on Mars would be a daunting, expensive task. It could be done with automated systems and robotics, but it would require a lot of mass. The drill pipes alone would weigh many tons. You'd need such a rig to drill a well for a colony anyway, but it's still a lot of upmass.

That leaves us with near-polar sites. There, water access is easier and definite; we already know there is water there, and it's on the surface in the form of ice. That saves the up-mass of a drilling rig, and the need to drill pre-site-selection. The only downside to such a high latitude site is that it reduces the available solar power, primarily during winter (Due to Mars' thin atmosphere, sunlight is not hindered when the sun is closer to the horizon, as it is on Earth). You also might need sunlight for greenhouses.

There's a way around that issue, a better way; nuclear power. With it, you don't need energy storage as you do with solar. For farming, you can use LED lighting, which you may well need even in an equatorial location; sunlight on Mars is weaker than on Earth due to distance, and would be further reduced by passing through the needed thicknesses of glass needed to deal with the near-vacuum of the Martian surface. Plants needing full sun on Earth would not do well in Martian sunlit greenhouses (though reflectors could be used to compensate).

Small reactors are closer to being available than is commonly thought; the Department of Energy and several companies are working on small modular reactors now, and those designs could easily be modified to reduce mass via dispensing with some shielding (On Mars, the shielding could be soil.).  

Via exchanging a small, lightweight nuclear reactor for the drilling equipment and solar collectors, you'd actually save net mass, as well as enabling the colony to be in a more advantageous location and be energy-rich. 

  
Edit to add: I'm not saying that a reactor is a prerequisite for a Mars colony; it isn't. However, if one is available, I think it would be preferable to solar-based energy for a colony.