Rainfall is a huge resource that is generally underused. Lots of rain just runs off or evaporates without productive use. In fact, after a big rainstorm, we mostly just want to get rid of it as quick as possible. It is one of those paradoxes in water resources management: fearing (and sometimes complaining about) water scarcity but wasting most of the principal water source (rainfall). Thus, increasing the productivity of rainfall is a key objective—on paper—of many water resources management programs. There are plenty of strategies to achieve this. Rainwater harvesting is one of them.
For most people, rainwater harvesting concerns collecting rain from rooftops and storing it in a tank or underground cistern. While certainly useful and capable of providing water security for domestic purposes, the volumes of water involved are generally small.
Something bigger is needed to make an impact on the water balance at catchment scale. To this effect, rainwater can also be ‘harvested’ from paved areas, grasslands, croplands, and even forests.
This post will present a few examples of ‘water harvesting at scale’. The objective in all cases is to make better use of rainfall—to increase its productivity and reduce wasteful (and sometimes destructive) runoff.
In a wider context, these examples should be perceived as ‘options for adaptive management’. They are part of the toolbox to achieve water security in a specific environment. How, and if, these options are employed will depend on the specific local conditions and socio-economic circumstances. At the right time and under the right circumstances—it is unclear when this constellation may occur, if ever—a long dormant option could become a very useful and practical solution. Thus, it is always useful to ‘collect options’.
Since agriculture is, overall, by far the largest water user, it makes sense to start with having a look at water harvesting options in the agricultural sector.
The above imaginary conversation is explained in the below video.
No-till agriculture
The video vividly demonstrates the beneficial impact of no-till agriculture on making better use of rainfall. Only a fraction of rainfall on soils that were not tilled for 22-months (or longer) ran off. Instead, most rainfall either infiltrated or was absorbed by the soil, thus improving water security at plot level. By contrast, the runoff coefficient for soils recently tilled approached 50%. It represents a massive waste of scarce water resources.
In fact, the impact of agricultural practices on the productive use of rainfall is just stunning. Not only does the infiltration rate increase when moving from tillage to no-till practices—almost threefold in the above video—but rainfall absorbed by the soil is mostly used for transpiration (creating biomass) rather than for non-productive evaporation. Thus, if occasional moisture deficits during the growing season are a recurrent and critical concern—which is almost always the case in the (sub) tropics—no-till agriculture offers a practical option to improve water security at little cost and without the need for external inputs such as irrigation water, which often needs to be provided by a water management agency or requires a permit and all kind of infrastructure if used at scale.
Note that there are trade-offs associated with no-till agriculture. The main disadvantage is concerned with the inability to mechanically control weeds (through tillage). Several strategies exist for non-mechanical weed control—some quite OK, and some not so good. But yields will go down quite substantially if you opt not to use herbicides (in excessive quantities) for weed control. On the other side, there are other major benefits associated with no-till agriculture such as erosion control, topsoil creation, and nutrient recycling because of the undisturbed soil-food-web. Whether no-till agriculture is feasible will depend on the farming/livelihood system and needs to be decided on a case-by-case basis.
Last comment on this video: there is an obvious analogy between no-till agriculture and grazing practices that avoid overgrazing and focus on maintaining a healthy ground cover.
| BOX: The Five Landscape Functions Charles Massy—in his book “Call of the Reed Warbler”—lists five crucial landscape functions (p 48), which have been copied (almost) verbatim below: 1. The solar-energy function, which is focused on maximizing the capture of solar energy by fixing as many plant sugars, through photosynthesis, as possible. 2. The water cycle, which is focused on maximizing water infiltration, storage, and recycling in the soil. 3. The soil-mineral cycle, which is focused on inculcating biologically alive and healthy soils that contain and recycle a rich lode of diverse minerals and chemicals. 4. Dynamic ecosystems, which focus on maximum biodiversity and health of integrated ecosystems at all levels. 5. The human-social aspect, which focuses on human agency triggering landscape regeneration by working in harmony with natural systems. Many strategies to strengthen water harvesting at scale will positively impact on functions 1-5 and thus contribute—very substantially—to a healthy landscape; their impact goes far beyond just reducing direct runoff. By the way, the overlap between these landscape functions and what we aim to achieve with permaculture is obvious. |
Dense urban environment
A typical urban landscape represents another example of a massive waste of rainwater. In fact, huge investments are often required to manage storm runoff.
Options for water harvesting at scale in a hyper dense urban environment are presented by Andrew Millison in his video “rehydrating the concrete jungle”.
The basic function of all features—which, by the way, are beautiful—in the above video is to capture rainfall on streets and roofs, and slowdown runoff. It leads to beautification, green spaces, reduced storm runoff, flood mitigation, urban habitats, groundwater recharge, micro-climates, cooling of the urban environment, etc. This is a long list of beneficial functions that generally improve the quality of urban life—just by making a bit better use of rainfall.
“From wastewater to resource water (Brad Lancaster)”
Below is another video—by Brad Lancaster—on using free onsite rainwater in the urban landscape. Many features focus on collecting rainwater on paved areas or roofs. Preferably, rainwater collects in infiltration basins (sponges) where it infiltrates rapidly and therefore cannot breed mosquitos…. These sponges are built from natural vegetation that has been grown with the harvested water and provide green spaces, urban habitats, and micro-climates.
Suburbia
Yet another video by Brad Lancaster on rainwater harvesting in suburbia, in semi-arid climate conditions. An obvious conclusion is that all urban and sub-urban vegetation in a semi-arid environment (and wetter) should not be irrigated but rely on captured rainwater only. The benefits are obvious.
Encore!
Brad Lancaster has been coined “the prophet of rainwater harvesting”. He focuses on rehydrating the urban landscape by mimicking the original hydrological cycle. This insightful and inspirational video presents a profile of Brad and his work.