Should I... Dig Swales to Harvest Rainwater Surface Run-off?
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- Опубліковано 10 вер 2024
- Not sure whether swales could help you... or even what they are? Does your land suffer long periods of low or no rainfall and include poorly vegetated slopes? Or would swales only make things worse? Aranya offers his thoughts - from Dartmoor - a place where you definitely wouldn't need them!
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A good video by Geoff Lawton where he explains how to make swales on heavy clay soils: ua-cam.com/video/P1HN5gsJL7k/v-deo.html
Hi Aranya, assuming that they do not cause a problem with too much water retention, could a swale be built to capture what would otherwise be lost such as organic matter and soil? Thanks
Wherever water slows it will drop some of what it carries - the heaviest material first. This is why Earth ponds (with a stream flowing through them) silt up. Water flowing into a swale is no different - material will accumulate there. As you say though we need to be careful not to over-saturate soil in wetter climates and create anaerobic conditions. Also, with certain types of clay we run the risk of creating landslides - best case scenario embarrassing, worst case dangerous...
What about places that get alternating El Niño excess rain and La Niña drought? How would you manage these multiyear events? Both can be quite destructive
Tree cover? Their canopies slow rainfall, allowing it to gently trickle to the ground and also provide shade to keep soils moist and cool. Of course plough agriculture is going to create problems (sadly introduced to regions where it's unsuited by those from cooler more temperate lands). Unvegetated slopes are going to be vulnerable to both sheet erosion and severe drying.
Can ridges function same as swells
A ridge as in 'ridge and furrow' ploughing? If on-contour each furrow would act like a mini-swale but in drylands on significant slope (where swales can make the most difference) modern tractors can't operate without the risk of rolling. Bare soil exposed to the Sun will also be hot and dry, so the warm air rising from the soil will oppose the infiltration of any rain. Mulching swales are possible if you can find the organic matter but ploughed soil is intended to be uncovered.
So are you saying that less have no overflow system?
Swales are useful for wet and dry climates as you should know that often the solutions for drought are the same as those for flooding. Etc, etc, etc.
A dam should have at least one spillway for overflow management as it's intended to be a sealed system. A swale, by contrast, should need no overflow as it's for temporarily holding water to allow it to percolate into the soil (it will do this more quickly if the soil is kept cool with a covering of mulch). If a swale is overflowing (which could lead to the swale bank washing away) it's either not big enough or it's spaced too far from the next swale down the slope. A solution would be to add more swales in between to reduce the catchment area above each one, make the overflowing swale(s) bigger, or mulch the swale to increase the rate of infiltration (probably the easiest). I'm not sure how swales would be useful in a situation where the soil was already receiving plenty of rainfall. Drainage ditches are another thing (and we need to manage their outflows carefully to avoid creating erosion).