by Giovanni Castaldo, Santa Cruz Permaculture Design Course participant
This series of blog posts about Rainwater Harvesting provides an overview of some of the key practices. It’s informed and guided by the book Rainwater Harvesting for Drylands and Beyond, one of Brad Lancaster’s approachable and inspiring works.
As discussed in the prior post in this series, the first principle of rainwater harvesting is to begin with long and thoughtful observation. The success of any water-harvesting earthwork depends on a good understanding of the water resources of the site and the slopes that determine the direction and magnitude of water flow.
Assessing Water Resources
To assess the water resources of any site, define the boundaries of the site itself and of the larger watershed in which it sits. To gain insight on the extent of the watershed and the slopes within the site, walk the land in the rain to see where the water runs. When it’s not raining, it is possible to get a good sense of where water flows and with what intensity by looking at erosion patterns. Erosion occurs in places where runoff flows rapidly over the landscape, washing away soil and leaving vegetation stunted. Also, where vegetation and topsoil is thicker and healthier, rainwater flows slowly and soaks into the ground. A topographic map gives a general idea of the watershed’s ridgelines, so getting your hands on one could be very helpful.
Breaklines and Keylines
Breaklines and keylines are two contour lines of particular interest in understanding where landscape sediments are eroding and depositing. Identifying them is important in planning for water-harvesting earthworks.
First, we must define a contour line: “a line on a map representing an imaginary line on the land surface, all points of which are at the same elevation” above sea level.
Breaklines are contour lines at locations in a landscape where grades change from gradual convex slopes to steeper concave slopes. Runoff more readily infiltrates convex slopes, and this is where sediment tends to deposit. On concave slopes, water runs off more readily and sediments erode.
In contrast, keylines are the contour lines at locations in the landscapes where grades change from steeper concave slopes to more gradual convex slopes. The key point is found along the contour line at the steepest part of the slope where the land changes from convex to concave.
On a micro-level you may see a breakline where a slope covered with leaves and silts changes to a steeper patch of naked sloping dirt. The keyline would be downhill where that naked slope changes to a more gradual slope, where collected fines, silts, and organic matter accumulate.
Concentrate initial water-harvesting and revegetation efforts above breaklines and below keylines. These are the locations where water more readily infiltrates, sediment more readily accumulates, and vegetation more readily germinates.
It might be more advantageous to focus exclusively below the keyline to start. Here, water and eroded sediment that have crossed the breakline above slowly erode the steeper section of the slope between the break line and the keyline. Water can easily be harvested below the keyline to create a more gradual, stable angle of repose with higher moisture content.
Slopes are described in three ways:
- As a proportion: the ratio of the slope’s horizontal distance to its vertical distance (e.g. 1.5:1)
- As degrees from the horizontal (e.g. 30°)
- As a percent: calculated by multiplying the distance of vertical rise of a slope by 100, then dividing by the horizontal distance covered by the slope.
A quick way to understand an area’s slope is to bend over and look at the landscape between one’s legs. This altered perspective gets the eyes close to the ground and sometimes makes subtle slopes more obvious.
A-frames and water levels called “bunyips” are inexpensive and easy-to-build instruments. They can measure slope, find contours, and define how to shape the land.
A bunyip consists of a long clear vinyl tube, with each end attached to a tall stake that is marked in inches or centimeters. The tube makes a “U” when the two stakes stand vertically. Being careful to avoid air bubbles, pour enough water into the tube so that the water surface reaches halfway up each vertically held stake.
The bunyip works on the principle that still, standing water is level across its entire surface, like in a lake. Therefore, if the two stakes give out different measurement readings, they signal the elevation difference of the land.
The A-frame level is even simpler to construct, does not require tubing and water, and can be operated by one person alone. It is used to find contour lines on the landscape. Unlike the bunyip it cannot be used to find elevation differences and slopes.
An A-frame is made of three poles or sticks tied or fastened together to form a capital “A.” Hang a weighted string from the top of the “A.” When both feet of the “A” are level with one another, the weighted string will hang alongside a centerline marked on the horizontal stick of the A-frame. When they are not level, the string will fall on either side of the centerline mark.
To make the centerline mark, calibrate the A-frame by setting the feet with one a little higher than the other. When the weighted string comes to rest, make a sign on the horizontal stick. Mark the points where the two feet are standing on the ground. Then rotate the A-frame and set it back down so that the feet now switched places. Mark the spot where the weighted string comes to rest. Permanently mark the midpoint between the two marked spots as the A-frame centerline mark.
Calculating Rain Volume
Once the boundaries are mapped and defined, use simple calculations to determine average rainfall volume on the site. You can calculate the volume of rainfall in cubic feet that falls in an average year on a specific catchment area. Catchment areas could include your roof, yard, neighborhood, or other subwatershed. The calculation is:
catchment area (ft^2) x average annual rainfall (ft) = total rainwater on catchment in average year (ft^3)
To get a ballpark estimate of runoff volume from any sloped surface, multiply the total rainfall that falls on the catchment by the surface’s runoff coefficient. The coefficient is influenced by the specific surface’s composition and the rainfall intensity (the higher the intensity the higher the runoff coefficient).
Once you’ve followed the steps outline above, you can identify which methods to implement to capture or slow your water. To sink water into the soil, you may build a berm and swale. We learn how to build berms and swales in our Permaculture Design Certificate course. To capture the rainwater for future use in your home or garden, explore options like tanks and cisterns.
Up Next: Tanks & Cisterns
Now that we’ve outlined these basic techniques for rainwater harvesting surveying, we’ll explore structures for capturing and storing rainwater in the next post.
Want to go deeper with all of this? Join us for our Permaculture Design Certificate Course! We spend an entire weekend focused on Regenerating Watersheds & Soils, and part of your design project includes assessing a landscape for rainwater harvesting. Learn more and register today at santacruzpermaculture.com/permaculture-design-course/