Why rainwater is the best for irrigating crops

Job Tomno (left), Chief Executive Committee in charge of Water and Irrigation in Baringo County, Luke Rotich (right), Chief Executive Committee in charge of Agriculture, Livestock and Fisheries, and officers from the county look on as Richard Chebon, opens furrows to irrigate his farm, during their visit at Ng’enyin Irrigation Scheme in the county on October 04, 2014. PHOTO | JARED NYATAYA | NATION MEDIA GROUP

What you need to know:

  • Several options are available depending on where the irrigation water is going to come from. In my experience, rainwater is the best for irrigation as borehole water can affect crops.
  • The size of the machine and pipe needed has to be calculated in regard to amount of water to be pumped over distance and the elevation.
  • Borehole water might be rich in sodium (salty) or chalky (alkaline) and might also be rich in minerals like iron or fluoride, which will be toxic due to high concentrations.

The long rains season is here and as expected, many farmers are busy planting, hoping that the rains will stay long enough to grow a decent crop.

However, experience has shown that reliance on rain-fed agriculture is not sustainable as the weather becomes erratic. Irrigation, thus, holds the best solution.

Several options are available depending on where the irrigation water is going to come from. In my experience, rainwater is the best for irrigation as borehole water can affect crops.

The investment needed with rainwater is also lower than installing a borehole. All you need to do is fit your buildings with gutters and collect as much water as you can. If one tank is placed high on stilts, the water can be pumped up with a relatively small pump and then flowed to the farm using gravity.

The size of the machine and pipe needed has to be calculated in regard to amount of water to be pumped over distance and the elevation.

On the other hand, the investment needed to sink a borehole to a water-carrying layer in the ground is certainly higher and there is the risk of the well drying.

If you want to irrigate, for example, 5mm per day, that means you need five litres of water per square metre. For an 8 by 30m greenhouse, which has 240 square metres, this would mean you need 1,200 litres per day.

For an acre, which has around 4,000 square metres, you would then need 20,000 litres per day. If you install a pump that can pump 75 litres per minute, it would need to run for 15 minutes per day for the 8 by 30m greenhouse and around four to five hours for an acre of land.

You can run the pump on electricity, solar or generator. However, the water can also be pumped using manual power (money maker pumps) or hydro power, by making use of the water flow with a ram pump. Ram pumps can be used in flowing rivers. They use nine parts of water to pump up a part of water and need no external energy source.

When using borehole or surface water, the question of water quality also must be addressed. Borehole water might be rich in sodium (salty) or chalky (alkaline) and might also be rich in minerals like iron or fluoride, which will be toxic due to high concentrations. Surface water, on the other hand, might be contaminated with environmental toxins or pathogens like bacteria that cause bacterial wilt in tomatoes and potatoes.

And before you plan for irrigation, check the quality of your soils. Do they have the capacity to hold as much rainwater for as long as possible? A soil rich in organic matter forms soil aggregates; crumbs of soil that cluster together. These clusters are formed from sand, clay and organic matter particles and they enclose air-filled spaces in between.

These spaces soak up water like a sponge as soon as it rains and can retain (hold) water for two weeks longer than a soil that does not contain sufficient organic matter. Regular addition of high quality compost to the soil will ensure that it is well-conditioned to receive as much rain as possible and to provide it to the crop for a longer period. Mulching will also help to keep soils cool and to minimise evaporation of water from the soil. Two weeks can make all the difference in losing a crop due to drought or bringing in a harvest.

Therefore, before you invest in an irrigation system; invest in the water retention capacity of your soil.

Ms Weber is the country manager, SoilsCare Ltd