Is evapotransporation accounted for in climate models?
Now most models include changes in evapotranspiration, it is usually grouped together with a few other factors (like draining wetlands) under the heading "Land Usage"  

Now the question does "Could additional crops being planted within the last 100 to 50 years account for additional moisture in the air now" the answer is no, not by itself.

To get more moisture into the air long term you need the air to be warmer, otherwise it just rains it out. Another issue is different crops affect evapotranspiration in different ways. For instance irrigation induced evapotranspiration, likely the largest evapotranspiration effect, may actually have a cooling effect, by increasing cloud cover and precipitation. Lastly land usage is only a small impact not counting the increased greenhouse gas emission it can cause.