Stream and river water is exposed ‘running’ water while lake water generally appears ‘stationary’.
PART 7: IRRIGATION WATER SOURCING IN UGANDA: b) Surface water: Rivers
By Eng. Kant Ateenyi Kanyarusoke
In the last article, we saw that most rain water sinks into the ground and a portion gets trapped there as ground water which we can abstract in boreholes or in spring wells. We also said that there tends to exist some natural balance between ground water, wetlands and plant life on hills.
Our activities on any of these can offset that balance to our own detriment. Ground water abstraction, especially by boreholes precisely does this. The better approach would be to first let nature take its course: that is, let the water emerge out of the wet lands into streams, rivers and lakes then extract some of it from these exposed surfaces. This is the subject of this and the next article.
Surface Water - In Uganda, apart from those originating in the glaciers of Rwenzori (Semliki, Mpanga, Mubuku, etc.), rivers formed from streams which in turn came from several springs joining together.
Our lakes were initially empty big valleys (Victoria, Kioga, Wamala, etc.) and holes (Albert, Edward, George, Bunyonyi and a myriad of crater lakes) in the ground which were filled by rivers and ground water from surrounding wetlands. Thus, most of our river and lake water passed underground and, therefore, would be expected to have dissolved salts from rocks it passed through. But during rain, there is water which does not seep into the ground - running on the surface until it joins a stream or river or lake.
This, together with that directly falling on the river/lake, creates a beneficial diluting effect. This is why most of our surface water is called ‘fresh water', making it much more superior for irrigation than ground water.
Stream and river water is exposed ‘running' water while lake water generally appears ‘stationary'. Sourcing of irrigation water from streams and rivers depends on the flow which in turn varies from month to month. How the water is extracted and used depends on whether the irrigated land is in a valley below the river or on a hill above it. In the first case, flood irrigation is used as described in Part 2 and explained in Part 4 of this series.
In the second case, however, the water must be pumped from the river. It may directly be used as in high flow spray irrigation or it may be stored first - to be used later using any of the irrigation methods of part 4.
Although use of river water looks simple, it is very controversial because its effects on downstream people and life can be immediate and spectacular. To appreciate this - particularly for Uganda - the reader could refer to the New Vision of July 15 - 28, 2013 Nile waters series.
Care must, therefore, be taken not to adversely affect downstream dwellers. Needless to say, river water extraction is only feasible for those farming on land adjacent the rivers/streams. In Figure 8, we summarise the possible actions on river water and their implications. Society needs to be prepared to handle those implications because all of them will somehow manifest. We will come back to them in later articles on ‘opportunities'.
Apart from digging ditches and possibly building storage wells/tanks, many of the activities in Figure 8 may be beyond the capabilities of Uganda's typical peasants. In that case, either the Government or a co-operative effort on their part could come in handy. Otherwise, they have to contend with manual fetching of water to irrigate seedlings only.
Pumping requires technical skills and a source of energy. In the villages, we could use solar energy in form of wind mills and solar panels. In some cases, where flow is fast, we could even use a modification of the ancient Romans' water wheel: please do not laugh it off - because this is feasible and infinitely better than ‘giving up' and continuing to procrastinate as some of Museveni-act critics are doing.
A radical way not shown in Figure 7 is to dam the river, essentially forming a ‘mini lake' from which water can be drawn as explained in the next article on lakes. This can only be done by government.
Figure 8: Using River water for irrigation - some actions and implications
I cannot leave rivers without talking about flood control. In Uganda, annual floods are common in Bugisu high lands, Teso-Tororo low lands and in Semuliki and Mubuku river systems of Rwenzori region. Osuret et al (2016) attributed Eastern Uganda floods to population pressure, poverty and poor farming practices.
An engineering way to control river floods is to construct raised barriers across the river without blocking it (we call them weirs - not dams).
These raise the upstream river level but reduce the flow rate over them. Hence the river level after the submerged weir is kept almost within its normal banks, thus controlling the flooding. During the dry season, either the river can be diverted around the weir to sustain its normal flow or the raised level could be used as a ‘mini' lake water source to irrigate surrounding fields.
In Uganda's case, where building can - and should - be done in the dry seasons. In a later article, I advance an approach to doing this cheaply and gainfully. For now, we will leave rivers at that. In the next article, we look at surface water from lakes and mini lakes.
The writer is a pan Africanist Engineering Don and CEO of Progressive Africa Solar Engineering Pty Ltd. in Cape Town