Underlined text are links to expanded information.
Rain Water Harvesting Earthworks
One of the main purposes of Earthworks in rain water harvesting is to replenish ground water storage aquifers in areas where large numbers of wells are drilled. Another purpose is to capture rainwater in terraces and directly water crops planted on them.
Excessive drilling and harvesting of underground aquifers can lead to drought and unsustainable situations for people dependent on wells for their drinking water. The must sustainable approach involves a combination of sensible and educated planning of crops and rotation, well drilling and ground water replenishment. A water harvesting Earthwork without associated vegetation is dead. It can quickly erode, clog with slit, breed mosquitos and excessively evaporate water. Vegetation stabilizes soil; roots expand, canopies grow, leaves fall and new top soil is formed. Insects attract birds and plants attract bees and the fresh water attracts wildlife completing the cycle.
Some Types of Earthworks;
Infiltration basins help prevent flooding and downstream erosion, and improve water quality in adjacent waterways. It is essentially a shallow artificial pond that is designed to infiltrate storm water though permeable soils into the groundwater aquifer. Infiltration basins do not discharge to a surface water body under most storm conditions, but are designed with overflow structures (pipes, weirs, etc.) that operate during flood conditions.
For in depth explanations of infiltration basins click here.
A terrace, for the purposes of rain water harvesting Earthworks are relatively flat shelves built parallel to contour on a slope. The purpose is to create a level planting area to intercept direct rainfall and runoff from a slope and is suitable for up to a 48.8% grade but are not suitable to areas prone to waterlogging.
For in depth explanation of terraces in rain water harvesting context and beyond click here.
|Radial planted terrace in Rwanda|
Water-Harvesting Swales, Soil Conservation Swales and Diversion Ditches
Diversion Swales and soil imprinting are more likely used in areas where preventing or reversing desertification is the issue.
A 'swale' is simply a long, shallow depression in the ground, designed to collect or redirect water. In general, permaculture swales are used to mimic the water-collecting and -holding abilities of a thick forest mulch.
Swales are most useful in reforestation of degraded, mostly-bare, arid or semi-arid hillsides, to direct water to trees (this water would otherwise run off the bare soil and be lost to the local landscape). In a healthy forest with a thick mulch of leaves or needles covering the ground, very little runoff occurs and swales would usually be unnecessary. A healthy forest is very good at managing its own water resources, and it is usually only Earth that is stripped of vegetation that needs to be rehabilitated.
For permacultural purposes, there are three main types of swale used in water-management earthworks:
- on-contour, water-harvesting permaculture swales;
- gently sloped water-transporting swales (or diversion ditches); and
- soil conservation swales (a modified form of diversion ditch).
- For an in depth explanation of bio-swales click here.
A check dam is a small dam, which can be either temporary or permanent, built across a minor channel, swale, bioswale, or drainage ditch. Similar to drop structures in purpose, they reduce erosion and gullying in the channel and allow sediments and pollutants to settle. They also lower the speed of water flow during storm events. These features help the root systems of plants upslope of the check dam by charging the water into the soil.
For a expert explanation of a check dam click here.
Desertification in Rwanda?
The U.N. Plan
The Government of Rwanda , in 2010- guided by a UN pilot project that mapped and developed a comprehensive plan for land suitability and use - has allocated US$25 million to relocate human settlements from the Gishwati Forest. Those resources were expended to correct years of deforestation that is causing erosion, landslides, deaths and poor water quality.
A knowledge base about the importance of vegetation and rain water harvesting Earthworks would possibly have rendered these millions available for use elsewhere. Application of these concepts may save hundreds of millions of dollars going forward.
The NASA study