用等高線來續水

A swale on Zaytuna Farm – © Craig Mackintosh

(Remaining images below © Cam Wilson.)

用等高線來續水

Keyline Swales–a Geoff Lawton/Darren Doherty Hybrid

原文出自Mr. Cambell Wilson, November 30, 2009

譯註:這是這幾年我看過最好的Swales的說明(最小的花費,最大的效益)

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。

圖1.1–基本地形圖

最近我有一個想法，根據各自的優勢，對他們的方法作一個簡單的比較。為了幫助理解，我設計了一組邊界地形圖（圖1.1）。我知道，Geoff和Darren會流口水，因為他們抬頭一看，山上的潛在壩址，但我故意保留公式，讓事情簡單化，並限制滲透率的比較。

圖2.1–地表水窪地滲透表現
詳見動畫

同樣，雖然我還沒有註記，每人都會把在小壩/濕地/淤泥陷阱盡量固住以防止土石流。

Geoff Lawton的做法

Geoff的想法是用景觀蓄水(通常可以連接小水壩,不過這是後話). 他的基本想法是把水留在儘可能的景觀相對最高點,並利用臨界等高線當作水量的最高調控基準。因為水是保持在相對最高點,因此水量有足夠的時間滲透到地底,同時這才能對地底土壤補水,這種做法才可以對樹木及植栽產生最大的效益。(圖2.1).
他經常利用推土機建立大容量的窪地，（例如，以一台推土機的推刀寬，如圖2.2所示-推平行的土壁）。

圖2.2-以推土機建立等高線

這是非常適合亞熱帶，也適用年雨量在50-100mm的乾旱地區，因為雨量不多，因此必須盡量滲透到景觀，對種植的樹木有極大的利益。通常我們可以把水流量當作常數，在90度的連續梯度上，以地表的上下方計算。在圖2.3中的每一個大的紅點，代表著已滲入適量的水沿著窪地的長度。紅線顯示水徑量，因為它是通過土壤剖面中向下移動的水路徑。
景觀上的自然水流

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

Darren不喜歡窪地等高線蓄水法,所做的部份修正

在圖4.1, 我把滲透路徑(圖2.2)跟降雨滲透(圖3.1)重疊。你會發現窪地的蓄水並沒解決山脊稜線缺水的問題。

圖4.1 窪地滲透(紅線)
i參照雨水滲透量 (淺藍)

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。

圖4.2林線空拍圖

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)

簡單的比較分析
Geoff等高線續水 

–可以將大部份的水都囤積在景觀線之上，但在景觀水分佈不太均勻
Darren崚線續水

–土壤水分分佈更均勻，但無法大面積持有和滲透雨水。
等高線蓄水

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

During a large event, the water would fill up along the length like Geoff’s large swale, however the water depth wouldn’t be constant. One possible benefit of having a greater depth of water out on the ridges is that there will be more pressure here, causing water to infiltrate at a faster rate than it will in the valleys (圖5.4 5.5). As the water level drops, it will of course infiltrate the remaining water on the ridge.

圖5.4 – Keyline swale full

If this was a temperate climate where large rainfall events are rare, on this landscape I would go for a keyline swale at the very top of the property, and then use Darren’s keyline mounds parallel to this down the slope. This means you’ll get the benefits of water being infiltrated at the base of each of the tree rows (by the mounds), hydration of the ridgelines, while also capturing any large flows that enter the property, infiltrating them right at the top of the slope.

圖5.5 – 等高線蓄水 (深藍線)
infiltrating during a large rain event

譯註1: 中國的哈尼梯田更是驚人,古人說山有多高,水就有多高,為何要蓋雪山隧道？一個不可逆的生態浩劫

譯註2: 根據統計數據，台灣年平均降雨量有2千多毫米，是世界平均值的3倍之多，但因為降雨時間與­空間的分布不均，因此台灣雖然雨水多，但卻是名列世界第18個缺水的國家，因此在未來­氣候變遷的影響下，水資源愈來愈珍貴，充分利用、留住雨水就相當重要。其實雨水回收的概念相當簡單，不需要耗費太大的成本，­只需要經過簡單的過濾和管線的配合，家家戶戶其實都能有一個雨水的小水庫，在豪雨來襲­水庫源水濁度過高、或是地震中斷自來水供應時，就能有雨水的戰備用水。
期待已久的雨天終於到來，台灣雖然年平均降雨量屬於多雨的國家，但降雨時間與空間分布­的不均，讓台灣始終是缺水國家之一，如何留住這珍貴的雨水，是台灣未來面對氣候變遷不­得不面對的課題。