Solar evaporation ponds concentrate borate brine until it crosses its solubility curve and crystals nucleate — the process used to produce borax at real evaporite operations. Water leaves the pond at a rate set by temperature, wind and exposed surface area:
evapRate = k · area · max(0, T−8) · (0.4 + 0.035·wind)
dV/dt = −evapRate · (1 − crustRisk/150)
Because the dissolved solute mass is conserved while the water volume V shrinks, concentration C = dissolvedMass / V climbs. Solubility S(T) rises with temperature (a real borax property — hot brine holds far more dissolved solute than cold brine):
S(T) = 5 + 1.1·T + 0.01·T² (g/L)
supersaturation = C − S(T)
Whenever concentration exceeds the solubility limit, solute precipitates out as crystal mass proportional to the excess (supersaturation) and to the remaining volume — a standard first-order precipitation kinetics model. That removes dissolved mass, which pulls concentration back toward the solubility line: a self-limiting feedback, same as in a real crystallizer.
- Crystal yield — the share of the pond's total solute that has precipitated so far.
- Crust risk — when evaporation runs too fast, a salt skin forms on the surface and throttles further evaporation; shown here as a rising risk percentage that feeds back into the evaporation rate itself.
- Temperature is a genuine trade-off: it speeds evaporation but also raises the solubility limit, so a brine that is too hot can evaporate quickly yet still under-crystallize.