A companion to the Farm Dam Volume calculator I posted about a while back, this one answers a different question.
The Volume calculator works out how big your dam is and how many rain-free days it covers, deliberately assuming zero rain or inflow in the meantime, a worst-case planning number. This one assumes the opposite. Give it your catchment area, soil type, dam depth and demand, and it runs all of that against ten real years of daily rainfall for your nearest of 68 South African towns, the same decade this site’s Rainwater Tank Calculator uses, to show how reliably the dam actually refills over a normal year.
The thing that genuinely surprised me while building it is how much dam depth matters even when volume stays 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 eats at surface area, not volume. I ran the numbers and a shallow dam can lose meaningfully more to evaporation than a deep one at identical capacity, enough that I ended up building a whole extra dimension into the simulation just for depth rather than assuming one typical figure would do.
I’d planned to also show a breakdown of where your catchment’s water actually goes, how much gets used, how much overflows, how much evaporates, since that felt like the most interesting output. I built it, tested it hard, and ended up cutting it before launch. The evaporation share specifically wouldn’t interpolate cleanly enough across the full range of inputs to trust, and I wasn’t willing to ship a confident-looking pie chart with real error hiding in one slice. So v1 just reports reliability, which I trust, rather than reliability plus a breakdown I don’t fully trust yet. Might revisit that properly in a future update.
Also worth flagging honestly: the runoff coefficients (how much of the rain that falls on your catchment actually reaches the dam) come from a published Australian soil-runoff table, since I couldn’t find a South African equivalent, same disclosure approach as the Volume calculator’s own borrowed shape-correction formulas. If anyone knows of a proper South African source for catchment runoff by soil type, I’d genuinely like to hear about it and update the tool.