Passive irrigation is the best way to make sure your garden is making the most efficient use of the free water that falls from the sky. By applying our understanding of the basic properties of water to the design of our gardens, we can all have healthier and more productive green spaces. The important concepts to learn in order to capture rainwater before it runs off of our land are Slope, Contour, Swale, and Berm. Slope is the description of a plot of land that identifies where the higher and lower points are. It is more simply known as a hill, though even land that looks flat does have a slope, however gradual. It is important to identify the slope of your plot because as rain drops hit the land they merge together and will be pulled by gravity down to the lowest point possible. A very simple concept, but also very powerful. All rivers start as small rivulets on the high points the landscape. The job of passive irrigation is to slow the water as it descends the slope. The best way to slow water is to interrupt its flow with an impediment placed on the contour of the hill. Contour can be considered the opposite of slope. If the slope is a straight line, the contour line would be perpendicular to the slope line. Of course there are no straight lines in nature and so when we mark the contours of a hill, we see it curve in sometimes unexpected ways. Finding the contour of a hill can be done in simple or complex ways, depending on how big the project you are working on is. For the purposes of the amateur gardener, it is enough just to idenitfy the higher and lower point of your land and lay your impediment long ways between these points. The impediment that works best in passive irrigation is the swale and berm. Swale is another word for trench, though the word implies a gentleness that is more appropriate for the garden. The depression in the earth that is the swale is placed along the contour line of the slope. The swales purpose is to catch the water on its way down the hill, slowing its descent and spreading it along one of the hills contours. One of the great properties of water is that when its descent has been stopped, but its mass continues to be added to, it will fill a space while its surface is entirely level. Therefore, during a rain event water in a level swale will spread along the contour of the hill. The purpose of swale is to allow the water time to soak into the earth delivering water directly to your plants roots. The place where most water will be available the soonest is on the downhill side of the swale, where you will place your berm. A berm is the opposite of a swale, a gentle mounding of the earth taken from the swale and placed on the downhill side. This long, low mound will follow the same contour as its swale and will add more volume to the space created by the swale. It is also the perfect place to plant water loving plants! This concept is very useful for capturing rainwater in the earth, where it is most useful. It is also a concept that can be applied in a variety of ways. For example, when planning their gardens many gardeners choose to build raise beds. When it comes to placing those beds, most will use arbitrary boundary lines line sidewalks or fences as a means or orientation. With our new knowledge, however, we orient our rectangular raised bed along the contour of the slope, thereby allowing our bed to get the full benefit of the rain before it runs off the land. Similarly, if we see that valuable topsoil is being eroded out of our garden, we can start thinking about where to place a small swale and berm to best stop the loss.
Posted on: Tue, 12 Nov 2013 16:58:21 +0000
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