Cushioning Climate Change with Farm Water Catchments  REMIX 

The ability to build moderate-sized reservoirs at low-cost for irrigation water - that will fill-in for rains missing from traditional growing seasons - can reduce the affects of changes in rain patterns cause by global warming and immediately boost farm production.  By using local animal power to construct the reservoirs, international support can pump a dramatic economic stimulus into farm communities while preparing them to weather the greenhouse effects that global industrialization seems to be causing. 

(This entry corrects for transfer errors in the earlier submission, "Low-Cost Water..." .)

Climate change seems to be making African rainfall even less predictable.  And in the best of times, African farmers suffer much greater variation in rainfall than farmers in the Midwest of the United States.  Adequate and predictable moisture through irrigation of crops would provide significant assurance against rainfall reductions and variations predicted by climatologists.  Irrigation within the rainy season alone could frequently increase yields by over 100% by raising both the best yields and most significantly the worst yields. 

Catching surface water high in the watersheds would be much more environmentally appropriate and economically competitive than irrigating from borehole wells.   When catchment reservoirs are carefully planned and well-situated, no pumps will be required.   Most people seriously underestimate the amount of water needed for irrigation.  A corn plant needs about 250 liters.  Plants thrive at 20 to 30 mm of water per week depending on humidity and other factors.    That is about 300 cubic meters of water per hectare - much more than can be collected casually from the roof of a modest house.  Indeed, to be able to collect it from rainfall in an environment with 800 mm per year and a 5% runoff rate, will require about 6 ha. of watershed for each ha to be irrigated over an in-fill period of just 10 weeks.  This would be an amount adequate to stretch the growing season a little and fill-in for mid-season droughts.   A reservoir of 4,800 cubic meters of water would provide enough water for 10 weeks on 2 hectares (5 acres).  That much irrigated land would provide substantial food security for a farm family.  By assuring a more reliable production of food, this system would reduce surplus stocking ("hoarding" beyond needs of the year) and increase the willingness to sell crops for profit.

To build a robust earthen dam for a reservoir of 4,800 cubic meters would use about 2,400 cubic meters of appropriate soil.  In excavating the pond, the topsoil removed could be separated and added to fields below.  Probably a total of 3,200 cubic meters of total soil would need to be moved.  At Tillers International, we think animal power can do this most effectively in areas with low wages.  Oxen or donkeys can move 10-15 cubic meters of soil per day.  At a pay rate of $10 per team and slipscraper, the basic labor cost would be about $1.00 per cubic meter, or $3,200 for the base work.  Added costs would need to be calculated for materials and for management.  Modern plastic culverts could reduce the need for concrete.  After the first few reservoirs constructed, management costs should be reduced.  Field tests would tell us if an expectation for construction of $1 per cubic meter of water capacity, or about $5,000, would be reasonable.

These reservoirs would increase yields from about 900 kg to 2,500 kg per ha.  With values of at least $.50 per kg even for low value grains, farmers could expect $1,400 per rainy season from two hectares.  That would pay off the investment in about 3 years or less. Tillers would like to test this system in northern Uganda which has two rain seasons each calendar year.  With bimodal rains, pay back would be twice as fast.  High value crops would also speed this payback.  After the first few systems, the subsidy incentive could hopefully be limited to 1/3 to ½ of the cost.  In developed countries we still provide significant subsidies for soil and water conservation improvements.

The greatest advantage of using animal power is that almost all of the construction money would be circulated through the local rural economy instead of to heavy equipment suppliers and fuel companies.    The farmer/teamsters doing the animal-powered excavation would likely re-invest their earnings in tools for farming.  Others might re-invested in additional, locally-funded irrigation capacity.  Ideally, the reservoir construction would be contracted by local crews operating as entrepreneurs.  They would quickly be able to diversify their services to cover other needs such as building rural roads.

Application of the water to the fields will also be more reasonable in small applications of 1 or 2 hectares.  Avoiding costly and inefficient sprinklers and pumps in preference to low-cost gravity channel systems or highly efficient, low-pressure drip irrigation will assure greater success and faster payback.  A more detailed plan would cover the water applications but is beyond the conversation we want to stimulate here. 

The case can be made that, even though these reservoirs are a good local investment since they have a short payback, they are in the interest of the global commmuity.   Investing in the reservoirs would be a highly effective stimulus to targeted communities.  With many replications, the environmental effects would become significant.

• 1) Intensifying production per hectare could take the pressure off of surviving forest lands,
• 2) Retaining water high in the watershed would slow water's movement downstream, reduce erosion and help recharge groundwater,
• 3) Increasing bio-diversity at the water's edge,
• 4) Extending the growing season would increase opportunities to use cover crops in conservation tillage, and
• 5) Encouraging perennial cropping and trees.
• 6) There may be some negatives in increased malarial mosquitoes and tsetse fly carried diseases. This would need to be monitored with an eye to controlling disadvantages.

In conclusion, the low cost of excavating with animal power and the use of locally built tools would permit irrigation at a reasonable cost that could even be initiated locally.  Subsidizing the investment from outside would provide a powerful economic stimulus.  Most importantly, each water catchment reservoir would assure tons of locally produced food in all but the most extreme failures of rainfall.  Families would be protected against the worst effects of global warming and while greatly increasing their season-to-season farm production.