Every year more and more boreholes are being dug around the country to extend groundwater access to areas previously reliant on seasonal or distant surface water sources. This is a very good thing, but it is important to remember that groundwater is only a reflection of surface absorption further uphill. Every drop that comes up out of a well first fell as rain and was absorbed into the soil before descending to a layer of clay or rock that carries it laterally across the landscape or enables it to pool underground into aquifers. When the borehole creates an empty space in the saturated layer of earth, then the water naturally flows in to fill the available space up to the level of the rest of the underground water. The only way to keep all of these new boreholes viable is to recharge the groundwater each year at the highest elevations possible within each watershed, and not to take out more than gets filtered in at the top . Zambia's coastal neighbors may be able to believe in water that comes from rivers or springs from the ground, but that luxury is not available at the top of the watershed, and we know that much of the water in their rivers and springs is really Zambian rain.
The basic concept in both groundwater recharging and flood mitigation is to slow the progress of rainwater across the landscape and down slope. There are several things that Zambians can do to encourage this process, which will have the added benefit of reducing the impact and severity of annual floods over time. This is by no means an exhaustive treatment of the subject, but floods are an annual fact of life, and the calendar is rolling around that way again. Encouraging surface absorption of rainwater can go against our instincts, after all from an animal perspective only surface water is accessible, that which seeps away under the ground appears to be lost forever. On the other hand, when flood waters rise, we naturally want the excess to move away downstream as quickly as possible once local reservoirs are topped up, and development along waterways causes us to discourage rivers from exceeding their banks or changing course. Urban sprawl can cause us to forget that a seasonal wetland which is dry for most of the year is still a wetland, which absorbs large quantities of rainwater (place a damp sponge and a dry sponge under a tap and observe the result), and expensive to replace with man-made systems from a water management perspective .
One way to assist this process is with small "seepage" dams arranged in upstream canyons which flood during rains but are dry for much of the year. Such dams are of low height with gentle slopes, more like speed bumps than walls, and designed to be smoothly overtopped by water levels above their capacity. The lowest portions extend below the level of the canyon floor and are composed of porous materials such as gravel stabilised by boulders and rock shards. The upper portion can be composed of compacted earth or other less absorbent material, and encouraging deep rooting plant growth can help reduce erosion of the structure over time. When the peak of the flood falls below the level of the dam, the continued inflow from uphill will pool behind it and seep through the gravel and stones underneath where much will be absorbed before the remainder returns to the surface to flow on down the hill to the next seepage dam. The pool eventually runs dry when the last of the water seeps underneath. Experience with such structures in rural India have both replenished wells that were running dry and are also causing seasonal streams to continue running for months longer, easing seasonal water shortages and strengthening local ecosystems. Closer to farmland or habitation, such dams can be scaled up into "check" dams that retain small reservoirs behind them while allowing the bulk of the flood to pass over them .
For farmers with mildly sloping plots, "Keyline" trench systems apply a similar seepage model of groundwater charging which can both absorb a significant portion of local heavy rainfall as well as provide reasonably efficient longer term irrigation when combined with an upstream reservoir. Cover cropping and no-till planting methods can help to reduce topsoil erosion, as well as improving soil health and reducing weeds. This will also significantly increase the percentage of organic matter in the content of the topsoil with each season which in turn dramatically increases the moisture retention of the soil itself, thus increasing the effectiveness of irrigation in general. Livestock pasturage should be rotated regularly, before patches of bare soil become exposed to rain and other elements. Goats and other animals which will eat plant roots as well as the top greens should be restrained from doing so unless the plants are known weeds or invasive species. Cattle wades and watering from natural sources should be done with advance planning to issues of erosion, defoliation and soil compaction along stream banks, and water contamination issues.
Seepage can also be applied in urban areas to supplement the ordinary surface drainage system. First, some or all of the water collected by roof gutters can be diverted into storage tanks, or into gravel beds or landscape foliage to encourage absorption, or even temporarily collected in surface pools for release after the drainage system demand has peaked. Large non-porous surfaces such as parking lots are generally designed for safety and comfort reasons to drain rapidly, however a simple trench along the downhill side of the lot filled to the top with gravel and stones can help mitigate both the rate and amount of rainfall the lot contributes to the drainage system (a deeper and/or wider trench adds surface area and storage volume thus increasing mitigation and absorption; coarser gravel or stones at the bottom will leave more spaces for water; finer gravel at the surface will trap less loose garbage and debris). Such trenches can also accommodate some plants or trees, and can be appropriate, even decorative for city parks, squares, and marketplaces. These steps will also help protect urban streams from degradation when combined with a comprehensive but distributed low-impact water catchment design.
Watershed boundaries are at the high points, where adjacent drops of rainwater flow downhill in opposite directions. Traditional political boundaries are often drawn through the centre of watersheds, often making use of rivers themselves as boundary lines between constituencies and jurisdictions. Water is becoming an increasingly scarce and contentious resource both globally and regionally, and it helps when all the people within a given watershed are working together, living in the same "polder" as the Dutch who live below sea level behind dikes call it. When the flood alarms ring, it doesn't matter if you are rich or poor, everyone inside the polder will die together if the dikes fail, so they all contribute to maintaining them and it brings them together as a community. As Zambia undertakes its current governmental and Constitutional restructuring process, some thought towards realignment of political boundaries to account for geographical natural resource distribution may serve to reduce future conflict and confusion over watershed boundaries as well as access to and control over water resources. From the water management perspective, drawing boundaries along ridges makes far more sense than dividing river valleys down the middle.
The above post was written by our resident special contributor - L Yakima.
Zambian Economist encourages special contributions from leading thinkers on matters relevant to Zambia's national development. The purpose of these notes is to stimulate discussion and ensure logic and impartial critique plays a leading role in shaping public debate.
Some Further Readings and Sources:
 "Global synthesis of groundwater recharge in semiarid and arid regions", B. R. Scanlon et al., Hydrological Processes 20, 3335–3370 (2006) (1.2MB)
 "The value of wetlands: importance of scale and landscape setting", William J. Mitsch & James G. Gosselink, Ecological Economics 35 (2000) 25–33 (176KB)
 "The Check-Dam Route to Mitigate India's Water Shortages", Govindasamy Agoramoorthy, Sunita Chaudhary & Minna J. Hsu', Natural Resources Journal [Vol. 48, pp. 565-583]: (2.0MB)
 Yeomans Keyline Designs, Australia, [HT: MrK]: (commercial website)
 "Farming Systems Trial 30-Year Final Report", Rodale Institute (1.3MB)
 "Erosion Control on Watersheds and Pond Embankments", Alabama Aquaculture Best Management Practice [BMP No. 3](113KB)
"Stream restoration in urban catchments through redesigning stormwater systems: looking to the catchment to save the stream", Christopher J. Walsh et al., J. N. Am. Benthol. Soc., 2005, 24(3):690–705: (578KB)