Monday 26 March 2018

RAINWATER: THE UNTAPPED GOLD OF DRYLAND

RAINWATER IS THE GOLD WE WASTE IN DRYLANDS.



For dryland farmers, harvesting water is as important as harvesting their crop. They need tools and techniques to more efficiently capture, store and manage water to remain productive during dry seasons and to mitigate the impacts of climate change.

Note:

Rain is the embodiment of life. It infuses water into our springs, rivers, and aquifers. It cools us, greens the land, and nourishes the plants that feed us. It cleans the air, washes salts from the soil, and makes the animals fly around in happiness.

Yet, the world's supply of fresh water is finite.
Less than one half of one percent of all the water on Earth is fresh and available. 
The rest is seawater, or frozen. Our supply is renewed only through precipitation, a precious gift from the sky that falls as droplets, hail, or snow, and then flows over the landscape as runoff.

RUNOFF: THE GOLD WE SHOULDN'T LET GO TO WASTE.

Rainwater harvesting captures precipitation and uses it as close as possible to where it falls. The process mimics intact and healthy ecosystems, which naturally infiltrate rainwater into the soil and cycle it through myriad life forms.


Instead of sealing and dehydrating the landscape with impervious pavement and convex shapes that drain this gold away, as most modern cities, suburbs, and home landscapes do, harvesting accepts rain and allows it to follow its natural path to productivity.


PUTTING THE ASSUMPTION INTO MATHEMATICAL PERSPECTIVE:

Consider a house in a suburb of Nairobi, with a roof area of 450 m2 ( 15mx30m). The house's roof is the catchment/harvesting area.
Av. Annual Rainfall for Nairobi is about 900mm( 250mm in Oct-Nov-Dec and 400mm in Mar-Apri-May)

NB: 1mm of rain on 1 m2 surface area = 1ltr of water.

Let's now use this season's rainfall, i.e.400mm
If 1mm of rainwater on 1 m2 = 1ltr,
then 400mm of rainwater on 15mx30m(450 m2) = 180,000 litres
So for Oct-Dec: 250 ltr x 450 m2 = 112,500 litres

TOTAL AMOUNT OF WATER THAT CAN BE COLLECTED FROM ONE (1) HOUSE IN THESE TWO SEASONS USING SIMPLE GUTTERS IS:

292,500 Ltrs

Allow 10% loss for evaporation, splashes or leaking gutters i.e.
292,000 ltrs minus 10% of 292,000ltrs = 262,000litres

Net Amount of water we lose in the runoff is 262,000 litres.

Now suppose that a family of six people live in the house, with a fairly high water consumption of, say 500 ltr per day.
In a month the total consumption is 15,000 litres.

In a year the total consumption is 180,000 litres

Surplus of water in that year is 262000ltrs - 180,000 ltrs = 82,000 ltrs.

This 82, 000 litres can be used to grow trees or irrigate vegetable garden.

HOW ABOUT IN DRYLAND, ASALs?

We can now take a nother example: a house in semi arid place like Kitui, with a simple mabati (iron sheet) roof of 10m x 20m ( 200m2)

We do the same calculation as above

Av. Annual Rainfall for is  700mm( 250mm in Nov-Dec and 200mm in Mar-Apri-May)
Potential water collected is then respectively:
350 litre x 200m2= 70,000 litres for Nov-Dec rains
200litre x 200m2 = 40,000 litre for April-May rains.

Total raifall collected is 110,000 litre of water.

If you give an allowance for loss of 10% or 11,000 litres, we are left with 99, 000 litres.

Now get this:

A family of six with a smaller water consumption (say 200litre per day) will use 6000 litres on a monthly basis and 72,000 litres in a year. 

Surplus water = (99,000 - 72,000)ltrs = 27,000 litres.

As you and I can see, water is there in surplus, but it has to be captures and stored.

Data Source: Betterglobe Forestry


METHODS OF CAPTURING/HARVESTING RAINWATER IN DRYLANDS:

1. Roof catchments: Using gutters around roofs as discussed above

2. Road catchment: Channelling paved roadside runoffs into shambas by the road.

Nb: A 3m wide, and 300m long road, receiving 20mm amount of rainfall for a couple of hours collects 14,400(20mmx300mx3mx0.8) litres of water, with 20% loss.

3. Micro- catchments: Applying technology of zai pits system. “Zai” is a term that farmers in northern Burkina Faso use to refer to small planting pits that typically measure 20-30 cm in width, are 10-20 cm deep and spaced 60-80 cm apart.



Make a decision. We must start by conserving, then preserve.

RAINWATER: THE UNTAPPED GOLD OF DRYLAND

RAINWATER IS THE GOLD WE WASTE IN DRYLANDS. For dryland farmers, harvesting water is as important as harvesting their crop. They need ...