The Reversed Loss

Fine sand evaporates more water than coarse sand. The finer grains expose more surface area to solar irradiation, and evaporation in unmulched soil is controlled by surface temperature. This is textbook.

Al-Zu'bi, Sadullah, Zheng, Exposito, Gallo, and Mishra (arXiv:2603.03363) coat sand grains with a superhydrophobic layer and spread it as mulch over irrigated soil. A 10mm layer of this superhydrophobic sand reduces evaporative loss by 83% in fine sand. But it also reverses the ranking: fine-sand evaporation drops to 40% below coarse sand. The soil that lost more without the mulch now loses less with it.

The mechanism is a regime change. Without mulch, evaporation is temperature-controlled — surface heat drives vapor from the wet soil into the air, and fine sand's larger heated surface dominates. With mulch, the bottleneck shifts to diffusion through the porous dry layer. Now what matters is how quickly heat conducts through the mulch-soil system and how steep the vapor gradient is across the barrier. Fine sand's higher thermal conductivity, which accelerated temperature-controlled evaporation, changes the temperature profile in the diffusion-limited regime in a way that reduces the driving gradient.

Same soil. Same water. Same sunlight. Different bottleneck, opposite answer.