In last month's newsletter, we discussed subsoiling, and its benefits in allowing water to infiltrate deep into the soil column, beginning a virtuous cycle of improved water capacity, deeper roots, more organic matter deep in the soil, and hence more water holding capacity.
This month, we'll be discussing another water-management tool, called swales.
As we all know, when rain falls onto an area, it flows from regions of higher elevation, to regions of lower elevation, and this flow comes in many forms. Sheet flow is the slow movement of a sheet of water across a wide area. As it flows, some of the water infiltrates the soil, and some of the water flows away. Eventually, much of this water accumulates in natural or human-made channels that then make their way into brooks, streams, and rivers.
The essential function of swales is to interrupt this flow, and prevent the water from flowing downstream. By slowing or preventing this downward flow of water across the soil, we allow the water to infiltrate higher up in the landscape, and utilize water that would otherwise flow into streams and rivers.
A swale is an irrigation channel that is dug along a contour line in the landscape. Contour lines are lines of equal elevation, such as you see on a topographic map:
A swale is a channel dug along contour lines such as these, and they serve to capture water that is sheeting off of an area, and preventing water from leaving a the area:
Over time, water will infiltrate into the soil, generally deep into the soil column. The soil then saturates more rapidly with less total rainfall, since all rainfall on the area is captured, and none of it sheets off. Since the water is deeper in the soil, there is less loss due to evaporation. Some of this water can eventually go on to recharge groundwater stores, raising the water table in a given area. By increasing organic matter content locally, the soil is more receptive to rain water infiltration, promoting a virtuous cycle similar to the effect of subsoiling.
Because swales store water deep within the soil column, the water they store is not available to grass, forbs, and other pasture species. Rather, swales are inherently tree planting systems, as trees have the deep roots necessary to reach through the soil column to access this water. The ongoing tree planting at Turning Leaf Ranch is being coupled with small swales to provide our trees water even during the dry summer months. In this way, we can produce fodder for the animals all summer (even into the fall), without the use of extensive irrigation.
In addition to the benefits of swales at the site of installation, there are many other benefits to the downstream landscape. In large rain events, the soil cannot always absorb the large amount of rain at a high enough rate, so water inevitably begins to flow down hill. As small streams of water turn into larger streams, the concentrated flow of water can quickly erode the landscape, forming small rivulets at first, that widen into rills and gullies. As the water becomes more and more concentrated, its erosive capacity increases, and thousands of years of soil deposits can be stripped in minutes by these concentrated flows.
By capturing and storing water higher up on the landscape, it prevents water from becoming concentrated, and reduces the erosive power of the flow. Topsoil erosion, flooding, and property damage are all reduced, and drought tolerance is increased.
Although the benefits of swales are many, there are also challenges and difficulties with swale technology. While swales can alleviate the concentration of water downhill, they do so by concentrating water uphill. Any time water is concentrated, it brings with it the potential for catastrophic events. Dams and levees bursting, rivers overflowing their banks, ocean waves rupturing dikes... These events show the power of concentrated water. Due to the high-stakes nature of these installations, great care must be taken in the planning and design of swales. Most installations require site analysis, extensive calculations, safety features, heavy equipment, etc. With these barriers, the installation of swales is slow-going, time, energy, and financially intensive process. Nevertheless, given the dismal outlook for rain in the west, they are well worth the investment.
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