A swale on Zaytuna Farm – © Craig Mackintosh(Remaining images below © Cam Wilson.)
Keyline Swales–a Geoff Lawton/Darren Doherty Hybrid
原文出自Mr. Cambell Wilson, November 30, 2009
Geoff Lawton 和Darren Doherty是澳洲永續農業的兩位大老，尤其在涉及到等高線續水的領域。他們都在一些非常艱難的的環境中有所表現，但有趣的是，他們的風格很不同，尤其是對水滲透的思考。
A natural pattern in the landscape is that valleys are moist whereas ridges are dry. You can see this in the vegetation in any undulating National Park you go walking in, with lush, moisture loving plants in the valleys, and dry sclerophyll forest on the ridges.
In figure 3.1, each large blue dot represents an even amount of rainwater that has infiltrated into the land above our boundary. The dotted lines show the path that the water takes (90 degrees to contour) as it moves down through the soil profile. This image clearly illustrating why it is that the ridges are much drier than the valleys.
|圖3.1 – Movement of soil moisture|
體認到這個問題後, Darren使用了林線當作等高線。在George Howson的造林地Dalpura空拍圖中 (圖 4.2),林木聳立地並沒有輪廓,而是輕輕地從山谷（自然濕潤地區）對脊（自然乾燥的地區傾斜） ，因此，甚至代表了在景觀中的水含量。
He creates his tree lines using a ripper and mounder, common in forestry plantings, which have a small gutter on the upper and lower sides which help to direct the water. This is a cheaper and more fuel efficient option than a bulldozer or excavator, and works well in climates where rainfall events are generally consistent but small, such as in many temperate landscapes.
The green dots and arrows in figure 4.3 indicate the infiltration of the keyline mound during a small event. Water has been directed away from the valleys and encouraged to infiltrate on the ridge instead. You’ll notice that when combined with the water naturally moving down through the landscape from above, the moisture distribution is far more even than in the swale in figure 4.1
|圖4.3 – Keyline mound infiltration|
(green) in a small rain event
Despite the obvious benefits, one downside I see to this approach is that the gutters on the sides of the tree mounds have a relatively small water holding capacity. If the landscape has dried out significantly, for instance during a long drought, it’s highly possible that the soils will become hydrophobic, and therefore there will be little water infiltrating as it travels along the gutters. During a large rain event, which occasionally come during the summer when moisture is most needed, due to the small capacity of the gutters, only a small amount of water will be held and given time to infiltrate. The rest will spill over the mound and down the ridge (figure 4.4). This would particularly be the case where there is a large catchment above as in the example used.
|圖4.4 – Keyline mound overflow|
during a large rain event
(Note: At this point, I should mention that despite Darren’s mounds being smaller than Geoff’s swales, he places one for every line of trees, meaning that water infiltrates right at the base of each tree. Also, in the widescale forestry example of figure 4.2, the pasture in between the rows has been ripped using a keyline plow, which further increases the infiltration capacity. Similarly, when water does spill, it is in the best place possible – right up on the ridge where the water will fan out and have further opportunity to infiltrate)
|圖5.1 – Side section view of a bulldozer|
building a keyline swale
With the benefits of each in mind, I came up with a hybrid, which you could call a keyline-swale.
It’s built just like a swale, set out on contour, except that the base of the swale isn’t level, rather it slopes from the valley out towards the ridges.
To build the keyline-swale, pegs are set out on contour. Starting at the ridge, a mark is made on each peg, rising at 1 in 500 towards the valleys. This is the guide for the blade depth (figure 5.1).
|圖5.2 Side section of a keyline swale|
during a small rain event
During a small rainfall event (figures 5.2 & 5.3), water is directed along the trench from the valleys to the ridges, where it infiltrates in a very similar pattern to Darren’s keyline mound.
|圖5.3 – Keyline swale (dark blue)|
infiltrating during a small rain event
|圖5.4 – Keyline swale full|
|圖5.5 – 等高線蓄水 (深藍線)|
infiltrating during a large rain event