Irrigation consumes a lot of water because the individual plants can be very many
By Eng. Kant Ateenyi Kanyarusoke
PART 9: IRRIGATION WATER SOURCING IN UGANDA: d) Rain water harvesting
When I used to teach fluid mechanics to mechanical engineering undergraduates, my favourite midterm project assignment to students was always ‘design of a home rain water harvesting system'. Particularly in South Africa, where some African compatriots pose as expert rain makers (among other things), from some unknown Islamic islands in the Pacific speaking a ‘strange' language having words like ‘Jajja', ‘Kojja', ‘Munyambe', and sentences like ‘Laba emaali bweyingiraKyaddondo', etc. You won't believe it - but my main objective was to liberate minds of most of our young ‘science-educated' siblings in Mandela land from the bondage of superstition. In today's article, I won't be fighting witch doctors (Sangomas). Rather, I want to contribute in a small way to empowering the peasant Ugandan and his/her middle class adult child to take a legitimate share of rain water with minimal effect on our far off cousins in North Sudan and Egypt.
The last three articles showed how water can be extracted from underground, rivers and lakes for use in irrigation. All those sources have one problem: when we use them, we actually more adversely affect the volume that reaches Egypt because the withdrawal is on the portion of rain water already on its journey to Egypt. To reduce this impact, it is essential to harvest the water well before it starts the journey - i.e. as it falls from the sky. To understand this, imagine in your village there are 20 children, about to start school this year. Ideally, you would want as many of them as possible to pursue studies up to university level. However, the realities are that some will drop out in P7; others at S4 and S6. May be 3 or 4 will go to university. But then, you get a prophesy that any time in future, there will be a war in which definitely two to four of the children in school at the time will be called to join one of the fighting groups. Question is when would you wish that prophesy to happen? The answer is: as soon as possible - so that you can possibly save some or all of those good enough to reach university!
In the same way, when rain falls, some water (may be 5%) washes off the ground to rivers and lakes; In most of Uganda, about 70 - 75% gets absorbed by plants and is retained in the country. The remainder sinks beyond where roots can reach it with some, may be 5-10% coming out as spring water eventually forming rivers. The other 10-15% enters aquifers as previously explained. There is also evaporation on the lakes as explained in part 8. Therefore, the journey (education) starts with entry into the ground. Roots (P7) pull back most. Aquifers (S4) grab some. Evaporation on lakes and river surfaces (S6) retains some. Only that which leaves the country (about 10-15%) matriculates to university (Egypt). Whether it will graduate (reach Mediterranean sea) or not is not our business in this article. The point with harvesting rain water is that you draw mainly on the 75% (P7 dropouts) which in any case would still have remained in the country.
Rainwater can be harvested in several ways. The easiest and commonest is to ‘kulembeka' (Luganda) or ‘kulegya' (Runyakitara). A gutter or its improvisation is fixed outside below the roof to receive water running off and collect it in some container, usually a clay pot or a big pan or even a basin. For irrigation or even for long term drinking purposes, this rudimentary system has to be replaced as exemplified by Figure 9.
The requirements are a permanent material roof - such as galvanised iron sheets or tiles - to collect the water; gutters to convey it from the roof; a screening/cleaning system to remove rubbish before storage; piping to the main tanks and finally the tanks themselves. The tanks may be above or underground. If the latter, a pump will generally be required to draw the water out and send it to the gardens.
The most important (and most expensive) component is the tank system because it dictates how much water you can hold and hence, how much will be available for irrigation. There should never be a compromise on volume, if it can be afforded because it is very difficult to store all the water from the roof in a given rainy season. Taking Moroto in Karamoja (one of the driest areas in the country) for example, a 15 X 8 m2 house, during the months of April to July, could give you quantities averaging 13,000 litres every month. This being a wet season, you need to store all that 52,000 litres - which can be a problem. In the early 1990s when I used to rear chicken - with no running water in one Kampala suburb, I had 4 X 10,000 litre GS tanks from Kireka. Two to three rains on my main and chicken houses would fill them up, leaving the rest of the rain water to attempt the Egypt journey.
In light of the above, I suggest underground storage for every home that can afford it. What is required is unskilled labour to dig them up, say to a depth of between two and three metres followed by a good mason to brick and seal them. They should be covered with a concrete slab to prevent evaporation, plant growth, mosquito breeding etc. Properly constructed and looked after, the slab can itself be an additional water collection surface. The Karamoja house above together with a similar concrete surface covering a two metre deep storage would give a storage capacity in neighbourhood of 200,000 litres, more than enough to store the rainy season's water. The downside to underground storage is inability to quickly detect leakages and also, susceptibility to damage by earthquakes. Hence, construction in the Rwenzori region would have to be thorough.
Irrigation consumes a lot of water because the individual plants can be very many. Therefore, it is strongly recommended that whenever harvested rainwater is used, consideration should be given to using drip irrigation. In fact, all middle class farmers hoping to irrigate (with or without harvested rain water) ought to give it serious thought.
This closes the engineering section of the series. We will, however, constantly refer to it in the next articles on Economics.
Kant Ateenyi is a pan Africanist Engineering Don and CEO of Progressive Africa Solar Engineering Pty Ltd. in Cape Town