The moisture in the ground is constantly evaporating, unless it is sealed in by something. Don't believe this? Put a sheet of plastic down on dry ground, seal the edges with dirt and leave it for a few days. Condensation will soon form on the bottom of the plastic, even from seemingly dry dirt in the middle of the summer. The only thing that keeps moisture from evaporating over time is either when #1, the area within a "sealed" container reaches equilibrium, i.e. the air spaces are at the same humdity level as the relative moisture in the soil or #2, the humidity in the air is higher then the relative soil moisture level in which case the process is reversed and the soil will absorb moisture.
And like all natural things that move because of pressure differences (all natural things ALWAYS move from a high pressure area to a lower pressure area, which is also what causes the wind to blow), moisture will always migrate from a damper area to dryer area. It doesn't have to evaporate right away, but as the moisture migrates through capilary action towards the surface, the dryer area, it will evaporate as the soil temperature rises, at which point it moves very quickly into the air. The only thing that will stop it at that point is a vapor barrier.
This is obviously what causes the top of the ground to dry quickly after it rains, and the longer it goes betweens rains, such as in the summer, the deeper the ground will become dry, especially if the dirt is bare and there isn't any ground cover, which speeds up the evaporation rates.
The reason dams don't leak is because they are sealed with a vapor barrier of some sort on the water side, and it is also why all concrete basement walls MUST be sealed with a foundation coating before the dirt is backfilled. If this isn't done I 100% guarantee you that the concrete on the inside of an 8" thick wall will be damp a lot, especially during spring rains. It is one of the main reasons that old houses have damp basements, even if they used poured concrete walls, and especially if they used concrete block walls which are even more porous.
There are a lot of misconceptions, outright falsehoods and old wives tales out there regarding things like vapor barriers, humidity, etc. It really isn't that complicated, if you understand things like gas laws and dew point. Suffice it to say, any house that is heated in the winter and doesn't have a vapor barrier will will tend to have a very low interior humidity unless additional humidity is added.
Adding humidity isn't necessarily a good thing if your house doesn't have a vapor barrier, especially as you move further north. The reason for this is the dew point, which is the temperature at which a given humidty level at a given temperature reaches saturation, or 100% relative humdity. As the moisture inside the house travels through the walls and insulation, somewhere within the insulation the dewpoint will be reached and the moisture will condense out of the air and make the insulation wet. Obviously, wet insulation isn't a good thing.
The reason this isn't a bigger problem in most areas is becuase there are enough days in most areas of the U.S. when the weather warms up enough during the day to raise the dewpoint level within the insulation enough that the water will evaporate again and move on out through the walls to the outside without causing serious longterm damage. But as you move further north, especially into Canada the weather is colder longer, and wet insulation causes more problems.
One way to understand dew point is to watch the news. The weatherman will give you the dewpoint temperature, which is the temperature at which moisture condenses at the current humidity levels. If you are wondering if there will be dew in the morning, there will be if the overnight temperatures drop to or below the dewpoint temperature, but there won't be if the temperatures remain above the dewpoint temperature. This is assuming that the relative humidity level doesn't change meanwhile due to other factors.
