Erica asked me to share my experiences with the water holding capacity of my HK's. I'll be happy to. However, unlike most people on the forums, the primary reason I build HKs is to help control seasonal flood waters. My secondary intention was to use the HKs as a permie habitat and cropping system. I will talk most about my first HK because this will be it's third season in "production." The second is a variation that was installed this past fall.
The shift in primary purpose caused me to choose woody inputs that are different than most HKs. My goal was to increase the surface area of the wood in hopes of *really* getting the most water retention for my efforts. In my first HK, I have some very nice fungal logs as the very base but they make-up maybe 20% of the total woody mass. The remainder of the wood is well-aged ground pine bedding (200+ yards) that I recycled from horse farms . I topped this off with grass clippings from untreated lawns, leaf mold, 100 yards of finished compost (homemade), 8 -12 inches of topsoil and wood chips. The size was 4.5 feet tall 6 feet wide. The HK settled to 3.5 feet tall. I forget the exact length but it is roughly 350 feet long. This was build on-grade vs. dug-in. In the past 2 years, flooding withing 40 feet of the HK has been minimal (if any).
I do not have any good moisture sample statistics to provide but, I have not watered any of the plants in my HK (aside from an initial dowsing of transplants) and have not had any production problems. The pine layer does not appear to have had a negative effect on any of the crops I've planted (melons, pumpkins, tomatoes, broccoli, tomatillos, garlic, scallions, etc) or in any native weeds that have emerged. I've pulled a few tap-roots and found them to be more than 24 inches long (so they are in the pine). Last year we had a record heat wave and drought in Michigan. Despite that, my HK was vibrantly green and no plant showed the slightest sign of wilt. I have some voles and mice in the pile and I've noticed many more Garters and Blue Racers are joining the party, too. Nasty, crop-eating bugs have been few.
My second HK, I wanted to test the retention capacity of mixed woodchips provided by local landscapers. The HK is 220 feet long, is dug in 2 feet into the ground, 3 feet wide and 3 feet above grade. There are fungal logs at the bottom, then woodchips for the next 3 feet then 18 inches of finished compost and, finally, 4 inches of additional woodchips on top. NO topsoil because I didn't have any to spare.
Since this is just a "baby" I'll have to let you know how it performs.
As I type this message we are receiving a massive amount of rain from a storm system that is not slated for departure until another 36 hours have passed. I believe my first HK is, already, at saturation point because the water on it's southern face (my neighbors side) is already 2-4 inches deep and their barn (another 50 feet south) is 8 inches underwater. My side of the pile, the north face, is starting to shed water from beneath (slow percolation) but is only 1/8 to 1/4 inch deep. My water retention pond is full and my spillway and southern pastures are under 4-8 inches of water (an area encompassing 1 acre+/-). I expect some amazing pictures will be forthcoming.
Joy to all,