Farm Dam Yield & Reliability Calculator (South Africa)
Runs your catchment, soil and dam depth against ten years of real daily rainfall to show how reliably your farm dam actually refills, not just how big it is.
Most farm dam advice stops at "how big should it be." This tool answers the harder question: given your catchment, soil and dam size, will it actually stay full enough over a normal year? It runs your inputs against ten years of real day-by-day rainfall for 68 South African towns, not a single annual rainfall figure, so it catches the years a dam nearly empties even though the annual total looked fine.
Rainfall varies by region, so the simulation runs on the closest of 68 towns covering every South African rainfall regime.
The slope draining into your dam, usually 10 to 50 times the dam's own surface area.
Sets how much of the rain that falls on your catchment actually runs off, using your town's own rainfall via -.
1 m³ = 1 000 litres = 1 kilolitre. From the Volume calculator above, or your own figure.
Matters more than it sounds: a shallow dam has more surface per litre stored, so it loses proportionally more to evaporation than a deep one holding the same volume.
Reliability against dam capacity, your own capacity marked. The curve flattens where a bigger dam stops earning its keep.
Same catchment, soil, dam and demand, simulated against each town's own decade of rainfall.
💬 Discuss this tool, or check the numbers against your own dam, on the forum →
Why "how big should my dam be" isn't the whole question
Most farm dam guidance stops at volume: length, width, depth, done. But a dam's capacity only tells you the ceiling, not whether it actually fills. This calculator works out your dam's catchment yield, the runoff your catchment area, soil type and dam depth actually generate, run against ten real years of daily rainfall for your nearest of 68 South African towns, the same decade used for this site's Rainwater Tank Calculator, to answer the harder, more useful question: given a normal year, how reliably does this dam actually meet your demand?
Already worked out your dam's volume? The Farm Dam Volume & Water Security Calculator gets you a capacity figure using proper shape-correction formulas. Sizing water for livestock first? The Stocking Rate Calculator works out how many animals your veld can carry. You can also browse all our free South African tools and calculators.
Frequently asked questions
How is this different from the Farm Dam Volume calculator?
The Volume calculator works out how big your dam is and how many rain-free days it covers, assuming zero rain or catchment inflow in the meantime, a deliberate dry-spell planning number. This calculator assumes the opposite: it runs your catchment, soil and dam against ten years of real day-by-day rainfall for your nearest of 68 South African towns to see how reliably the dam actually refills over a normal year, not a worst case.
Why does dam depth matter if the volume is the same?
A shallow, wide dam has more surface area for every litre it stores than a deep, narrow one holding the exact same volume, and evaporation acts on surface area, not volume. Two dams of identical capacity can have meaningfully different reliability purely because of shape, which is why this calculator asks for an average depth rather than capacity alone.
Where does the rainfall data come from?
Open-Meteo's archive API, 2015 to 2024, the same decade used for the Rainwater Tank Calculator, for 68 South African towns spanning every rainfall regime from winter-rainfall Western Cape to the arid Karoo and Kalahari.
How is the runoff coefficient worked out?
From a published soil-runoff table (NSW Local Land Services' Farm Dams Technical Guide), cross-tabulated by rainfall zone and soil type, applied against your town's own measured annual rainfall rather than asking you to guess an Australian climate zone. No South African equivalent table could be sourced, the same honest-disclosure approach this site's Farm Dam Volume calculator already takes with its own borrowed Australian shape-correction formulas.
Why isn't my exact soil type on the list?
The four categories, sandy loam, sandy clay, cracking (elastic) clay and hardpan or shale, are the ones the published runoff table gives numbers for. Pick the closest match, a knowledgeable neighbour or your local agricultural extension office can help if you're unsure.
Why is there no rand cost for building a dam?
Dam excavation cost depends on soil, access and machinery day-rates in a way that varies enormously by site, with no reliable national reference price the way retail tank prices exist for the Rainwater Tank Calculator. This tool reports reliability, not an invented rand figure, and a proper quote is the only honest way to get a construction cost.
What counts as a reliable dam?
There's no universal number, but 80 percent or higher is a common planning target for supplementary stock water, while irrigation users needing more certainty typically look for 90 percent or above. A lower figure isn't necessarily wrong, it depends on how bad a partial shortfall would actually be for your operation.
Does this account for silting?
No. Sediment build-up reduces a dam's effective capacity over time, sometimes significantly in badly eroded catchments, the same issue flagged in the Farm Dam Volume calculator's own FAQ. Re-run this tool with a reduced capacity figure if your dam is known to be heavily silted, rather than trusting its nameplate capacity.
Does a bigger dam always mean better reliability?
Almost always, but not with total certainty in a shallow dam. Since a bigger dam at the same average depth has a proportionally bigger surface area, and therefore loses proportionally more to evaporation, the model can occasionally show a very small dip (a few percentage points at most) rather than a rise as capacity increases at an unusually shallow depth. It's a real, if minor, side effect of modelling evaporation honestly rather than ignoring it.
This calculator gives planning estimates from real daily rainfall records, not a survey or an engineering assessment. Runoff coefficients are a borrowed, disclosed Australian table (no South African equivalent could be sourced); the evaporation factor is a first-pass cross-check pending fuller per-region sourcing. Last reviewed July 2026.