Our bath tubs: case study of stacking functions

The Tubs

I'll continue on the water theme of my recent posts, with this look at our backyard bathtubs. Whenever I give tours, I spend a few minutes discussing these tubs, which I plugged with rubber patches and silicone caulk to form water-tight ponds. They beautifully demonstrate the permaculture principle of "stacking functions" - each element in your design fulfilling multiple roles.

So, take a look at these tubs (click on the image for a larger version), and if you feel so moved, brainstorm for a few minutes about what uses we're making of them... Bonus points if you've already been on a tour here, and you think of some uses I didn't mention!

Some context for the picture above: our passive solar sunspace is directly to the north of the tubs. The ground slopes slightly from the camera position towards the house. The black locust tree post in the center of the picture supports a grape trellis. (Hard to make out the rest of the trellis components in this picture.) The trellis supports two white PVC pipes above the tubs, close to the roofline. These pipes drain the rainwater from half of our house roof system. (There's an easily visible "T" PVC pipe fitting at the left, and a barely visible 90 degree fitting terminating a PVC pipe above the tub on the right.)

OK, brainstorm away!

---

Functions

Ready? Here we go!

Rainwater Harvest

The PVC pipes draining the rainwater from the house roof direct the water into these tubs as an 8' waterfall, which oxygenates the water a bit. (On windy days, some of the water does miss the tubs.) The tubs hold about 50 gallons each, the same as a $10 used rainwater barrel. I "planted" the tubs about 18" deep, tilted slightly away from the house, so the overflow falls over the far edge.

Though we can't gravity feed water from the tubs, we do use this rainwater storage in some similar ways:

• Manual irrigation (fill a bucket or watering can from the tubs, then go dump it somewhere appropriate)
  
• Wash hands
  
• Wash tools
  
• Wash buckets
  
• First rinse of root crops. Sometimes we do this manually by swishing a root around in a tub, or by filling a bucket of roots with water from the tub and whirling it all around to get the dirt off. Recently I've discovered a second function for our water-oxygenating waterfall: I place an open-meshed tray of roots across a bathtub under the waterfall, and let it clean the roots off. This works very well with a couple of interventions to move the roots around so they all get a share of the pummelling.
  

The tubs should help a little with catching nutrient runoff from the ecoroof on the sunspace. Probably some of the nutrients just flow out of the tub as the excess water overflows (feeding the comfrey planted at those spots), but I suspect the plants and other life in the tubs get a shot at some of the nutrients, especially during the active growing months when the aquatic biological systems are in full gear, and we have less rainfall and thus less overflow from the tubs.

Drinking Water & Wildlife Habitat

Seems like everyone in the neighborhood (besides the humans) drops by to sip from our tubs! We do have to top off the tubs in the summer with municipal water. Our visitors include:

• Bees (thousands of them each day in the summer, from our hives and at least one of our neighbors' hives)
  
• Damselflies (we're hoping they can establish breeding populations, but I don't think it's happened yet)
  
• Wasps (and probably many other insects we just haven't noticed)
  
• Chickens (low maintenance system for keeping our hens watered)
  
• Ducks (from time to time, when our neighbors let them free range)
  
• Cats (several from the neighborhood)
  
• Rats (ditto, though we try to shoot or trap them for some stew meat when they get too comfortable sipping during the daytime!)
  
• Birds (taking baths)
  
• Raccoons
  
• Opossums
  
• ...who knows who else comes by in the dark of night?
  

Aquaculture Yields

Many yields for us and our animal friends:

• Wapato - root crop growing in the couple of inches of soil at the bottom of the tubs. Also provides edible leaves and flowers. We prefer to let the leaves grow to pump energy into the root crop. The chickens prefer to eat whatever they can get at right here, right now. No sense of delayed gratification for them. So we have to fence them out from the tub a little bit -- they can get their heads in to drink, but can't extend too far into the middle of the pond.
  
• 
  Fish - I believe the tubs have too little area to support standard aquaculture fish like tilapia (which couldn't overwinter anyway.) Primarily for mosquito control, we have stocked the tubs with Gambusia (mosquito fish) free from Multnomah County Disease & Vector Control, and with 12 cent goldfish from Petsmart. The mosquito fish overwintered at least once, and maybe twice. They did vanish at some point, perhaps during the initial phases of our house project when we moved the tubs all around, draining and refilling them. Our goldfish have mostly survived, including three of them successfully overwintering this year. We did have three go belly-up in a smaller bucket of water two winters ago after a hard freeze. I ate them; they were crunchy and tasted like the oil in which I cooked them. If the future residents at our house don't want to eat tiny little fish, they can certainly toss them to the chickens.
  
• Duckweed - Common aquatic plant, difficult to exclude from our tubs even if we wanted to. Luckily, the neighbors' ducks love it. Our chickens will eat it if they're hungry enough, but seem to prefer other greens. If nothing else, it makes a good mulch, soaking up excess nutrients from the ponds and giving us an easy way to transfer them to our regular garden. (The bees, by the way, prefer duckweed as their landing pads while fetching water.)
  
• Snails - Nothing gourmet here, just tiny 1/8"-1/4" aquatic snails that go 'round and 'round the tubs eating, I presume, algae and decaying vegetation. The snails help keep things in balance. The chickens like eating the snails; I'm guessing they get some calcium from the shells, not to mention that 1/8" worth of protein.
  

Sunlight Reflection for House

On our rare winter days with bright sunshine, I never tire of following the shimmering patches of light as they travel across the walls and ceiling of our sunspace through the day. I placed our bathtubs a little over 4' from the house. This leaves ample path space and hang-out area, and also allows low-angled winter sunlight to reflect off the water surface and into our house, adding heat and light. In the summer months, the reflection of the higher-angled sun will hit the underside of our grapes on their trellis. Plus, the die-back and re-growth of the wapato, which catches the sunlight instead of allowing it to reflect, coincides nicely with when we do and don't want extra sun in the house.

I don't know how to measure the additional gain from the ponds, but they provide about 30 square feet of reflecting surface. I could believe they add nearly as much gain as one of our 3' x 6' windows in our "window wall."

Climate Control

We didn't plan this function; I'm just thinking of it now. The tubs may create a slightly cooler area around them in the warmer months, thanks to their thermal mass and evaporation. Once the grapes grow in fully on the overhead trellis, we might enjoy spending summer (or at least warm spring and autumn) afternoons on the south side of the house, in the shade of grapes and next to the cool water.

The tubs should also provide a little thermal buffering in the wintertime, perhaps helping to protect root systems of immediately adjacent plants. However, we didn't plant anything to take advantage of this possibility; the path layout and occupation of the vine layer by the grapes makes it difficult to plant any frost-sensitive plants against the tubs. (I've had ideas for planting moringa and/or air potato in the front yard, between the more deeply buried graywater ponds.)

The tubs will probably humidify the nearby air in the summer. This may actually negatively impact the grapes, since I think they benefit from good air circulation and not having too much moisture around them. I haven't yet thought of any useful applications for this function.

Terrace Retaining Wall

We don't really have much of a slope going on, so the tubs play a very minor role as retaining walls. The path on the house side lies perhaps 8" lower than the soil on the other side of the tubs.

Slow-Drip Irrigation

We didn't plan this one, but I think we have tiny leaks in one or two of the tubs. Happily, they occur near the grapes, so probably help maintain a constantly moist soil through the summer.

Betchya Didn't Think of This One

Ice skating rink!

Summary

So there you have it: seven main functions, with multiple sub-functions within some of those. I didn't even think of some of these uses until writing up this post, so I may be missing more! Any other ideas out there?