Fine sand evaporates water 37.5% faster than coarse sand. The mechanism is straightforward: finer particles produce smaller pores, stronger capillary forces, and better hydraulic connection to the surface. Water is pulled upward more efficiently through fine sand, reaches the atmosphere more readily, and evaporates faster. This is a textbook result. The soil texture determines the evaporation rate because the evaporation is controlled by how efficiently the soil delivers water to the surface — a temperature-controlled, surface-driven process.
Al-Zu'bi et al. (arXiv:2603.03363, March 2026) placed a 10mm layer of superhydrophobic sand on top of both soil types. The fine sand's evaporation dropped below the coarse sand's. The ranking reversed.
The superhydrophobic layer is not a seal. It permits vapor diffusion. Water molecules still escape the soil — they just have to diffuse through the hydrophobic layer as vapor rather than being wicked to the surface as liquid. This shifts the evaporation from a temperature-controlled regime, where surface conditions and capillary delivery govern the rate, to a diffusion-limited regime, where the thickness and thermal conductivity of the mulch layer govern it.
In the diffusion-limited regime, the fine sand's advantage — its superior capillary delivery — becomes irrelevant. The bottleneck is no longer getting water to the surface. It's getting vapor through the barrier. And in this regime, the fine sand's higher thermal conductivity (from denser particle packing) actually works against it: it conducts heat downward more efficiently, cooling the evaporation front and reducing the vapor pressure gradient that drives diffusion. The same property that made fine sand evaporate faster without mulch makes it evaporate slower with mulch.
A 5mm layer reduced evaporation by 65% in fine sand and 63% in coarse sand. At 10mm, the reductions were 83% and 70% — and the ranking had flipped. The intervention didn't just suppress evaporation. It changed which physical process controlled it, and that change reversed which soil type lost more water.
Al-Zu'bi et al., "Superhydrophobic Sand Mulch Shifts Soil Evaporation from Temperature-Controlled to Diffusion-Limited Regimes," arXiv:2603.03363 (March 2026). KAUST.