A farm water storage tank is a day buffer, not a dry season reserve. One acre of vegetables applying four millimetres a day drinks about 16,200 litres, so the largest tank most Ugandan farms own holds under a day of it. Bought tanks earn their place by smoothing supply for nurseries, greenhouses, livestock and gravity fed drip, and by giving a solar pump somewhere to put the day's water. Season length storage is a different structure entirely.
That is the single most useful thing on this page and it is the thing tank sellers are least likely to say. What follows works through the sizing arithmetic, the materials and how each one fails, the base and the stand, the pressure you buy with height, keeping light out, screening what goes in, and cleaning what settles. Excavated storage, meaning a dug reservoir holding hundreds of cubic metres, sits on farm reservoir construction, and filling either from a roof sits on rainwater harvesting for farms.
How Much Water One Acre of Irrigation Drinks in a Day
Work from crop demand, because every other number follows from it. FAO's irrigation water management training material gives the daily water need of the reference grass crop by climate zone and mean temperature. In a sub humid climate at a mean temperature between 15 and 25 degrees it is 5 to 6 millimetres a day; in a humid climate at the same temperature band it is 3 to 4. Uganda's national annual mean temperature sits at about 23 degrees on the World Bank's climate data, which puts most of the country in that middle temperature band, and Ugandan districts range from humid to sub humid depending where you are. So 3 to 6 millimetres a day is the honest national bracket for reference demand, with your own district deciding where in it you sit.
Most field crops sit close to the reference figure. FAO's crop factor table puts maize, beans, sorghum, soybeans and sunflower about ten per cent above the standard grass at peak, onions, peppers, crucifers, groundnuts and melons level with it, and citrus about thirty per cent below. So for planning a tank you can treat the reference figure as the crop figure and be close enough.
Then convert. One acre is 4,047 square metres, so one millimetre of water spread across it is about 4,047 litres. That single identity does all the work.
| Depth applied | Litres per acre | Tanks of 10,000 L |
|---|---|---|
| 3 mm a day | 12,100 | 1.2 |
| 4 mm a day | 16,200 | 1.6 |
| 5 mm a day | 20,200 | 2.0 |
| 6 mm a day | 24,300 | 2.4 |
Read the third column and the conclusion is unavoidable. A ten thousand litre tank, which is a large and expensive object on a Ugandan smallholding, holds between a third of a day and a day of water for a single acre. Nobody is storing a dry season in bought tanks. What they are storing is a day, or part of a day, and that turns out to be worth a great deal for the right jobs.
Sizing a Farm Storage Tank Against Livestock and Nursery Demand
Once field scale irrigation is off the table, tank sizing gets tractable, because the remaining jobs are small and their demand is published.
FAO's small earth dams manual gives daily livestock water use, on the stated assumption that animals are on dry pastures and good quality water is available: cattle 40 to 80 litres each, with milking cows needing up to 100; young stock 25 to 50; pigs 25; poultry 30 litres a day per hundred adult birds; goats 3 to 8; sheep 2 to 6; and a bee hive 2. It also says to add ten per cent for wild and feral animals drinking from the same source, and another ten per cent if the water is saltier than recommended, since animals drink more to keep their salt balance. For rural domestic use where there is no piped supply it gives 20 to 50 litres a day per person.
Worked through, a mixed smallholding with stock, a tunnel and a household commonly lands between 5,000 and 20,000 litres, and lands there for reasons the owner can explain. That is a far better position than buying the biggest tank affordable and hoping.
Tank Materials Used on Ugandan Farms and What Each One Fails At
Every tank material has a characteristic way of dying, and choosing well means choosing the failure you can live with. The Texas Water Development Board's rainwater harvesting manual sets these out in a form that transfers, because the physics of tanks does not care which country they stand in.
Two practical notes for Ugandan conditions. Movable matters if you farm rented ground, and it is the strongest argument for plastic over anything built in place. And a tank that can be bought locally can be replaced locally, which is worth more over twenty years than a small saving on a tank nobody near you stocks.
Foundation and Base: What Five Tonnes Does to Farm Ground
Water weighs a tonne per cubic metre. A five thousand litre tank full holds five tonnes, and it puts that on a circle a couple of metres across. A ten thousand litre tank puts ten. This is the part of tank buying that gets no attention and causes most of the expensive failures.
The manual's requirement for a plastic tank is that it sits on a smooth, solid, level footing, and each of those three words is load bearing. Smooth, because a stone under a polyethylene base becomes a stress point that the weight above grinds into a crack. Solid, because soft ground under one side tips the tank and a tipped tank stresses its outlet fitting until that fitting leaks. Level, because a tank out of level carries its water unevenly and reads as fuller than it is.
In practice that means a concrete slab or a compacted sand bed retained by blockwork, sized larger than the tank base, with the surface swept clean before the tank goes on. Not bare murram. Not three courses of brick under the rim with a gap in the middle, which is the commonest field bodge and which will crack a plastic tank across the floor. And not ground that becomes a puddle in the rains, because a base that softens seasonally moves seasonally.
Plan the slab before the tank arrives, because the tank cannot be moved once it is full and emptying ten thousand litres to fix a base is a day nobody enjoys.
Building a Tank Stand for Gravity Fed Drip Irrigation
The reason to lift a tank is pressure without fuel, and it is a genuine gain. Water gains about one pound per square inch of pressure for every 2.31 feet it is raised, which the Texas manual gives as the working conversion. The useful way to read that is the other way round: the pressure available at your beds is set by the height of the water surface above them and by nothing else, and it costs nothing to maintain.
What it costs is structure, and this is where the phrase free pressure stops being free. A stand does not just hold the tank up, it holds five or ten tonnes up, at height, in wind, for twenty years, over ground that gets wet. That is a real engineering job with a real bill, and the Wave of farm stands that fail do so because it was treated as carpentry. Three consequences follow.
First, the pressure gain per metre is modest while the structural cost per metre is not, so height should be bought deliberately against a stated requirement rather than maximised. Ask your supplier what operating pressure the dripline you intend to buy is rated for, then build to that and no higher; the emitter specification is the only authority worth trusting and drip irrigation explained covers why standard and low pressure lines differ so much.
Second, height you give the tank you also have to give the pump. Every metre of stand is a metre added to the total head the pump works against, which means a larger pump or a bigger solar array. The stand is not free at the pump either. That trade is set out on choosing an irrigation pump.
Third, a bank beats a stand every time it is available. A tank sitting on levelled ground at the top of a slope, with the beds below it, gets its height from the land for the price of the earthworks. Farmers with any fall on their plot should look at the contour before they look at steel.
Keeping Light Out: Algae, UV and Why a Farm Tank Must Be Opaque
Algae needs light, water and warmth, and a translucent tank in Ugandan sun supplies all three. The Texas manual's rule is unambiguous and it is not about appearance: storage tanks must be opaque, either naturally or by being painted, to inhibit algae growth. It also advises keeping tanks out of direct sunlight where the site allows it.
A covered tank is not the same as an opaque tank, and this is the distinction that gets missed. A lid stops leaves and mosquitoes. It does nothing about light coming through a thin white or translucent wall, and that light is what turns stored irrigation water green. Green water then does two things you will notice: it blocks drip emitters, which is the expensive one, and it feeds a biofilm inside the tank and the pipes that has to be cleaned out.
The same sunlight is working on the tank itself. Ultraviolet breaks down plastics, which is why polyethylene farm tanks are made with stabilisers and why the Texas manual flags fibreglass as needing paint or tint. Uganda sits on the equator at altitude, so the ultraviolet load on a tank standing in the open is at the high end of what these materials are made for. Shade from a tree or a simple roof extends tank life and lowers the water temperature at the same time, and cooler water grows less of everything.
So the specification is three things together: opaque walls, a close fitting lid, and shade if the site offers any. Paint an existing translucent tank rather than living with it.
Inlet Screening and First Flush on a Farm Rainwater Tank
What arrives in the tank decides how often you clean it, so the cheapest maintenance is done at the inlet.
Three layers, in order. A leaf screen or gutter guard keeps the coarse material out before it enters the downpipe. A first flush diverter dumps the initial dirty run off. The Texas manual sets the volume at one to two gallons per hundred square feet of collection area, which in metric terms comes to roughly 0.4 to 0.8 litres per square metre of roof footprint. On a hundred square metre iron sheet roof that is 40 to 80 litres thrown away before anything reaches the tank. Where in the range you sit depends on how long the dry spell was and how much dust and leaf litter the roof has been holding, so a roof standing under trees wants the upper figure. The arithmetic behind roof area and yield is on rainwater harvesting for farms.
The third layer is a fine screen at the point water enters the tank. The manual notes that a screen trap at the water entry point discourages mosquito breeding, and that matters on a farm tank in Uganda for reasons beyond the tank. Screen the overflow and any vent as well, because a mosquito will use whichever opening you forgot.
Cleaning a Storage Tank and Where the Outlet Should Sit
Silt arrives in every tank that is filled from a roof, a river or a shallow well, and it goes to the floor. Two decisions determine whether that becomes a problem.
The first is the height of the draw off. An outlet fitted flush with the floor pulls sludge straight into your filter and your dripline. Fitting the outlet a little above the floor leaves the settled layer where it is and gives you clean water for free, at the cost of a small volume you cannot use without draining. Some designs go further and draw water from just below the surface, on the reasoning the Texas manual gives, that water at that level is cleaner and fresher than water near the bottom. Either way, the principle is the same: do not take water from the dirtiest part of the tank.
The second is a washout. A tank with a low level drain plug can be flushed in ten minutes. A tank without one has to be siphoned, bailed and reached into, which is why tanks without washouts do not get cleaned. Specify one when you buy, and site the tank so the washout can actually discharge somewhere.
The cleaning itself is unglamorous. Drain down, open the washout, scrub the walls and floor with a stiff brush and clean water, flush until the water runs clear, then refill. Do it at the start of the rains when a refill is cheap rather than in the dry season when every litre counts. Inspect the inside while it is empty, because that is the only chance you get to see a crack forming, and check the lid seal, the screens and the overflow at the same time. On a tank supplying a drip system, clean the filter and flush the laterals in the same session.
One safety point that is not optional. A large tank is a confined space. Do not climb into a tank alone, do not go in without somebody watching from outside, and never use a chlorine solution strong enough to produce fumes in an enclosed vessel.
Where a Bought Tank Is the Wrong Storage for a Farm
Four cases where the money should go elsewhere.
You need hundreds of cubic metres. Field scale dry season irrigation means storing tens of thousands of litres a day for weeks, and the cost per cubic metre of manufactured tanks makes that arrangement absurd. That volume is excavated, not bought, and it belongs on farm reservoir construction.
Your source is the problem, not your timing. A tank moves water in time. It does not create any. A farm whose stream dries in January does not need a tank, it needs a source that runs in January, which may mean a well, a borehole or a reservoir. Costing the tank first is the commonest way this money gets wasted.
The plot has fall and you have earth moving available. Where the land gives you height, a lined pond or a tank on a levelled bench may deliver the same pressure for a fraction of the structural cost of a stand.
The land is rented and the tank would be built in place. Ferrocement, block and poured concrete tanks are immovable by nature. On ground you may lose, buy plastic and keep the option of taking it with you.
Farm Water Storage Tanks: Questions Farmers Ask
What does a water tank cost in Uganda? Plastic tanks are priced broadly by volume, so cost per litre falls as the tank gets bigger, which is the main reason to buy one large tank rather than three small ones. The figures move with the shilling, with resin prices and with how far the tank was transported, and a tank is bulky freight, so a district far from the manufacturer pays more for the same object. Get current quotes from at least two sellers for the same volume and the same wall grade, and ask what warranty comes with it. Built in place tanks are quoted as a job rather than a product, so compare them on the finished volume, not on the bags of cement.
What size tank should I buy? Add up the daily demand of the jobs the tank will actually serve using the livestock and domestic figures above, multiply by the number of days you need to bridge, add about a quarter, and round up to a size that is stocked near you. Do not size it against a field crop unless you have run the acre arithmetic and accepted the answer.
Is it cheaper to build a tank or buy one? On a small volume, buying wins on almost every count: speed, warranty, movability and no skilled labour. Building starts to compete at larger volumes, where the cost per litre of a manufactured tank stops falling and a ferrocement or block structure can be made to fit the site. The risk moves with it, because a built tank that cracks is your problem and a bought tank that fails is the maker's.
How high does the tank need to be for drip? That depends entirely on the dripline, so get the rated operating pressure from the product specification and build to it. Purpose made low pressure lines run on a small fraction of what standard emitters want. Build for the line you are buying, not for a general rule, and if you already have a stand height fixed, buy line rated for that height instead.
Can I use one tank for both drinking water and irrigation? Technically yes, but the tank then has to be built and maintained to the drinking water standard, which means lining a concrete tank, keeping the inlet screened and first flushed properly, and cleaning on a schedule rather than when you remember. Two smaller tanks with separate duties is usually the easier arrangement, and it also means a dirty irrigation tank cannot contaminate the household supply.
Why does my tank water smell? Organic material has got in and is decomposing, which usually means a failed screen, a gutter full of leaves, a first flush diverter that is not draining, or a dead animal. Find the entry point, fix it, then drain, wash out and refill. A tank that smells will keep smelling until the inlet is fixed.
How often should a farm tank be cleaned? As often as the silt and the smell say, which depends on the source and the screening, so there is no universal interval. Once a year at the start of the rains is a reasonable default for a screened roof fed tank, and more often for one filled from a river or a shallow well. Inspect it every time it is low enough to see the bottom.
Does a tank need to be full to work? No, and chasing a full tank is a mistake. A tank that is never full is a tank sized to your supply, which is fine. A tank that overflows in every storm is either too small for the roof feeding it or is not being drawn down, and overflow water is the cheapest water on the farm to lose.
Work out your daily demand and your bridging days first, then take those two numbers and your intended dripline pressure to a supplier, and ask them to quote the tank, the base and the stand as three separate lines. The base and the stand are where quotes differ most and where the failures happen. Sellers can be found through where to buy irrigation equipment. What the stored water then serves is laid out across the irrigation and greenhouses guide. For where a tank sits in the overall spend, see irrigation system cost per acre, and for filling it from a borehole rather than a roof, boreholes for agricultural irrigation.