Rachel, Here are my rough ...

Rachel,

Here are my rough answers to your questions:

a) to bring soil from wilting point to field capacity, for medium texture soil (silty soils) you need  8 - 10 inches. However, if you let soil moisture to reach wilting point, then the irrigation of corn will be useless. Crop will be already dead at wilting point. You have to start irrigation when in soil there are at least 4 - 5 inches of water to avoid too much reduction of the planned yield. 

b) your reservoir should keep 62500 cubic meters (51 acre-foot) to apply irrigation one time to entire area within 10 days interval. For this volume the area occupied by the reservoir would be 430 x 430 feet. After 10 days, the reservoir should be replenished. If you can replenish it daily, then the reservoir can be 140 x 140 feet.

c) to answer this question much more information is needed.

d) to answer this question more information is needed such as: flow of collected drainage as a function of calendar days (or at least flow in January, March... December). Usually, when drainage is working, removing the excess of water, the irrigation is not needed. When the irrigation is most needed, then the drainage will supply very little or no water. At point b) above you have the size of the reservoir needed for one day and for 10 days.  

I wish you success,