Fifth tool in the Farming section, this one for working out what’s actually in your dam, and how long it’ll last you.
The thing that got me building this one is that length times width times depth, the calculation most people reach for, is genuinely wrong for a real farm dam. It describes a rectangular tank with vertical walls, and an actual dam has sloped earth banks, so that simple sum overstates what you’re holding, often by a fair margin. I couldn’t find a South African source that publishes a proper correction for this, so I used the same shape formulas two Australian agricultural authorities, DPIRD in Western Australia and Agriculture Victoria, publish and use themselves, and checked my implementation against their own worked examples before trusting it. There are three shape options, a simple dam with ordinary sloped banks, a valley or gully dam built across a drainage line, which is measured differently, and a multiple-soundings mode if you’ve actually measured depth at a few points yourself, which is the most accurate option going.
The part I think is actually the most useful, though, is what happens after the volume number. South Africa’s open-water evaporation is genuinely severe, and on a wide, shallow dam it can be a bigger loss than everything you’re pumping out for stock or irrigation put together. The calculator works that loss out from your dam’s actual surface area and a regional evaporation figure, not a generic assumption, adds up whatever you’re drawing off, livestock, irrigation, the house, and gives you an honest answer to the question that actually matters for planning a dry spell: how many days does this water last if nothing tops it up. I’ve deliberately kept that assumption of no rain or inflow, since it’s meant as a worst-case runway, not a forecast.
One more thing I added after some reading I wasn’t expecting to need: if your dam is large, over 50 000 cubic metres and has a wall over 5 metres, South Africa’s National Water Act classes it as a “dam with a safety risk” and it needs to be registered with the Department of Water and Sanitation. Most farm dams are well under that, but the calculator flags it if you’re close, worth checking properly with DWS or a dam engineer rather than relying on the tool’s word for it.
If you use a farm dam, I’d genuinely value a sanity check on the numbers, particularly the evaporation regional bands, which are honest indicative ranges rather than a precise map, so if you’ve got your own pan reading or a strong sense that your area runs hotter or cooler than what’s shown, that feedback is useful.