I was listening to Jack Spirko’s most recent interview with Geoff Lawton (Episode #923 ) and I was inspired to look up some info for myself as I begin to review pieces of property for our homeplace. Goeff made the statement that the 3 key pieces of information to consider when evaluating a property for Permaculture design are 1. Aspect (which way does it face) 2. Water and 3. Slope.
Where we are looking in N. Alabama the first two are pretty easy. Look for a piece of land that mostly faces South, insuring shelter from winter winds and access to winter sun. Likewise, water is everywhere. The whole area is mostly a sandstone cap on top of limestone on top of shale. This keeps the water table fairly shallow, the water soft and clear and natural springs and seeps are everywhere, as are caves. Slope, though, can be a problem. The area is the dot at the end of the exclamation point that is the Appalachians. The Appalachians dip underneath the Cumberland Plateau and Sand Mountain and re-emerge in an area that covers several counties in N. Alabama, making them much more similar to E. Tennessee than anywhere else.
Slope plays a key role, because too much precludes easy use of machinery to do initial landscaping like swales, ponds, terraces etc. Not that you can’t, but it makes it much harder. For example, John Deere’s 450J Dozer is only rated to operate on a slope of 26.6 degrees or 2:1 ratio or 50%. Likewise, too little slope can cause you to expend energy that you can get for free from gravity with the right amount of slope. For example, if your watersource is above your house, gravity can preassurize your water for you, eliminating the need for a pump. Residential water pressure is between 30 and 80 psi, with the norm being between 45 and 60. In order to get that kind of pressure without a pump we simply calculate how high above the house the head of water (the tank) needs to be. Without sufficient slope, the only way to achieve that height will include a lengthy pipe from the head to the house, so a steep slope behind the house makes for a shorter pipe to achieve a decent pressure. On a side note, the amount of water in the head, 5 gallons or 5000 gallons, doesn’t make a difference in the amount of psi, so long as the pipe starts out full. It will just produce that psi for less time the smaller the tank.
According to THIS site we can calculate psi by multiplying the height of the tank above the outlet times .433 to get psi. So, height difference x .433 = Pounds per square inch (PSI). So, a head of 70 feet would give us a nominal psi of 30.31 and there would be some additional loss to pipe friction.
So, we can see that too much slope, or too large a percentage of the property with too much slope, is detrimental to most agricultural practices, even things like terraces, swales and steep land grazing. And, too little results in higher energy expenditure. The perfect piece of land then, for me, will be over 70% south facing, less than 10% with a slope over 30%, and with a homesite backing up to a steep, south facing slope.
Here are some pics from the area we are looking at, from real estate ads for specific pieces we are considering.