Setterguy, Maybe this will help, maybe not. I can semi-accurately predict how much my pond will raise during a rain event. Not exact but usually within an 1" or 2" depending how much rain we get.
First you would need to determine your runoff curve. The curve is based on the composition of the terrain/vegetation. You can calculate this by using Tables 1/2/3/4 on Page 14 of the USDA Pond Construction Guide, probably need to do the weighted curve as explained on the previous page. Then you would need to go to Table 5 on page 18 to get the rainfall curve. Rainfall curve is important as it accounts for ground saturation but only on specific rain events. If the ground is saturated from a 2 inch rain 2 days prior, I would calculate using a 3 or 4 inch rainfall. With those numbers you can then calculate number of gallons that will theoretically flow into your pond after any rain event. Due to the fact that most pond banks slope the number of gallons for every inch will be variable.
I do NOT run these calculations every time. After doing the first few major rains, I determined 43% of the rainfall reached my pond during the spring rain events. It takes a little more guesswork for me during the first part of spring due to the 2 ponds in the upper watershed not flowing out. In late summer and fall this holds true if we get 1.5" or more of rain in a day. I also have a table that tells me how many gallons per 1/2 foot of water in my pond.
It took a while to do this but if you use google maps (or whatever) to look at the terrain and complete measurements, it goes pretty quick. You also only have to calculate it once. The harder part to me is calculating how many gallons you need to come up 1 inch in the pond based on the current depth, thank you AutoCAD!
Hope it helps, first and foremost I hope it makes sense.
