Large difference in yield per acre between various states?

Hi and thank you for your time.

 

I am very green to Alfalfa, and I am working on a research assignment as part of my learning process (there is so much to learn... I can probably spend the next 20 years learning and still green)

 

My question is about different states' area harvested vs. yield per acre (please see the below charts).  Montana, North Dakota and South Dakota top the chart in 2014 on area harvested, but they lag behind California, Idaho, and Nebraska in yield per acre.  

• Is the yield difference due to the cost of having to irrigate large fields (and states like California earmark water for agriculture use)?  
• Is the yield per acre gap between South Dakota and Nebraska due to warmer weather to the south?
• I always thought mid-western states have more fertile soil than California (please see the last map, showing Dakotas having a lot of (supposedly productive) Mollisol.  Is that not a production advantage?

I wish I have more to contribute to the forum but I hope at least you find the charts interesting.

 

Data source: USDA

Large difference in yield per acre between various states?