One thing to keep in mind is that you will likely need to be sure your dam does not back up water on your neighbor's land. As a plus, your neighbor's pond will act as a settling pond. If watershed is limiting, sealing up the pond well will be critical. My new 1/2 acre pond has very little watershed, but is sealed up tight as a drum and dropped very little last summer. We also built a wetland/sump that has less watershed than water surface area but still held good water the last couple years (since we built it). We have another wetland with great inflow, but it was not sealed up well by the government mitigation folks that built it, and it goes dry most Summers.