Irrigation and Agriculture
Food security is a critical issue in Malawi. Drought and crop failure occurs with alarming regularity in the single consumption crop culture of Malawi. Sufficient water supply to sustain villages of people as well as for conventional irrigation, is not often available throughout the year. From our first visit to Malawi in 2009, we quickly became convinced that an effective solution might be found in the use of drip irrigation. Drip irrigation has high water utilization efficiency compared to conventional flood irrigation, lowering water requirement by up to 70%, resulting much lower demand as well as installation and operating costs. Our objective was to develop systems to reliably provide water to community gardens, seeking to supplement the traditional, non-nutritional diet for the children with more nutritious vegetables. Our further hope was to enable two, and possibly even three crops per year, as typically there is virtually no rain at all for 8 months out of the year.
We were familiar with irrigation drip tapes often employed in 3rd world countries to create household gardens, gravity fed by an 5 gallon bucket reservoir, elevated 3 to 5 feet above the ground. These flat tapes have specialized slits, calibrated to deliver a specific water rate to each plant, and are available in rolls. After further research we learned that these same tapes are commonly used in full scale commercial farming, and set out to scale-up such an arrangement at the Matapila center, where a vegetable garden was needed. Lacking sufficient electricity for pumping, we had to rely upon gravity. The first irrigation plot was constructed in 2010 at which time we built a heavy wooden platform to elevate a surplus 1000 liter (25o gallon) polyethylene tank 5 feet to provide appropriate water pressure to the tapes, that we purchased locally, to feed a 10 meter x 10 meter garden “square”, with 20 rows per square.
Unfortunately we later learned that the Matapila gardeners were experiencing difficulty keeping the irrigation running, requiring frequent and painstaking cleaning of the drip tapes. This dilemma was unexpected as drip tapes have been extensively used worldwide for many years. While periodically replacing the clogging tapes, we sent tape samples to the manufacturer and had the water analyzed by a laboratory in Lilongwe. We discovered the local well water had a high chemical mineral content (hardness), causing the tape slits to clog. Confirmed by the manufacturer, it was suggested that the lines be periodically flushed with concentrated acid to dissolve the accumulated minerals. We concluded this not to be an acceptable operation considering the risk to workers unfamiliar with such chemical operations.
We were pleased when we quickly began receiving photos of the gardeners successfully growing tomatoes, cabbages, and other vegetables, and began planning to expand the number of irrigation "squares" the following year.
During this time we became acquainted with "flag emitters", which we confirmed to have a greater tolerance to water hardness by parallel testing with the tapes. Furthermore, flag emitters can be disassembled if cleaning is needed. Another significant advantage of these "drippers" is that we don't have to ship proprietary tapes to Malawi but rather can locally source rolls of polyethylene tubing, into which the flag emitters are inserted. It is much easier to simply ship boxes of the emitters.
In addition to storing water, the tank has the very important function of pressure control. The elevation of the tank above the ground provides the water pressure upon the drippers, (although dropping somewhat as the water level is lowered). The "drip rate" is a function of water pressure and must remain as consistent as possible
Drip irrigation tape is rated in flow capacity; e.g. the standard we were using was 0.083 GPH per 100 feet. This equates to 3 GPM for our 100 sq meter "squares". The Matapila gardeners found that watering for 20-30 minutes per day was adequate to sustain good vegetable growth. Assuming a 25 minute daily watering cycle, a single garden square will consume 75 gallons of water, approx 1/3 of the tank. Hence one tank filling per day was adequate for watering 3 garden squares.
With the well 100 meters from the garden, our original scheme required the gardener to fill 5 gallon cans at the well, carry them 100 meters, up a ramp, to pour the water into the tank . . . 50 times a day! A considerable effort.
Flag emitters are available in nominal sizes: we selected the 1 GPH size as they produce approx. the same water rate as the drip tapes at the same tank elevation.
With a functioning prototype garden, of course we wished to expand capacity. Our strategy was to add "garden squares," connected by a manifold of 50 mm PVC piping. Using PVC ball valves, each square can be irrigated separately.
But if carrying 75 gallons of water to a single square was laborious, adding multiple sections made the task overwhelming! When we added the second garden square, we also buried a 50 mm PVC pipeline between the well and the irrigation tank, 100 yards away.
One end ran to the top of the elevated supply tank. The other end connected to a solar powered portable pump used to pump water out of a 30 gallon polyethylene "charge tank," dug into the ground next to the borehole.
The gardener filled the charge tank by pumping the borehole, and once full, switching on the pump to transfer the charge volume to the primary storage tank. It took about 8 "charges" to fill the primary tank, saving many water carrying trips.
While the portable pump, with it's solar charged internal battery, saved a good deal of water carrying to fill the irrigation tank, it was not a feasible long term solution if the garden were to be expanded in future years. For one reason, at 3 1/2 gpm, it's battery capacity would only allow less than 2 hours of operation, filling the primary tank once to irrigate 3 squares. Labor was still a concern: the AFRIDEV borehole pumps so prevalent throughout Africa, when new are capable of pumping 0.4 liters from a well depth of 45 meters, per stroke. Filling the charge tank the 8 times required to in turn fill the primary tank once . . . requires on the order of 2500 pump strokes! And for only 3 squares . . . every day! Furthermore because these village pumps usually are in service for many years without maintenance, they are likely delivering less than design performance. We can not jeopardize access to life sustaining water, via borehole capacity, or wearing out the pump, for large scale irrigation projects. Clearly the only long term solution is to drill a second borehole dedicated to irrigation, hydraulically independent of the villager's water supply