Evaporation studies have been conducted in a controlled environment chamber. Three variables were introduced: Treatment rate, relative humidity, and wind velocity. Air temperature was held constant at 100°F. Two evaporation media were employed: washed masonry sand, and Amarillo fine sandy loam soil.
The fatty alcohol used in the study was a commercial mixture of hexa- and octadecanol, which was uniformly mixed with the top 3-inch layer of soil. The rates of application were: 100, 200, and 400 pounds per acre with the masonry sand, and 200 and 800 pounds per acre with the sandy loam soil. A third treatment with the soil consisted of irrigating the column with a 0.1% solution of a nonionic surface active chemical. The control in each case consisted of untreated sand or soil. All columns were irrigated and drained to approximately field capacity at the start of each drying cycle. Evaporative loss was measured by weighing.
Fatty alcohol effectively reduced evaporation from the sand surface up to a maximum of 87% at 400 pounds per acre, at 20% relative humidity, and 2 miles per hour wind velocity. The greatest reduction with soil was 24% at 800 pounds per acre, at 20% relative humidity, and 2 miles per hour wind velocity. The nonionic surfactant reduced evaporation from the soil surface up to a maximum of 52% at 60% relative humidity and 8 miles per hour wind velocity.
Increased relative humidity reduced evaporation rates in all cases. The higher wind velocity accelerated the evaporation process, causing the earlier formation of a dry diffusion barrier at the surface.
The mechanism by which the fatty alcohol suppressed evaporation from the masonry sand appeared to be essentially the same as that which acts to reduce evaporation from an open water surface. With the soil, the mechanism appeared to be that of reducing capillary flow of moisture to the soil surface. The treated soil formed a dry layer at the surface sooner than the control.
The action of the nonionic surfactant in reducing evaporation was probably due to decreased surface tension at the solid-liquid interface, thereby reducing capillary flow to the surface layer of the soil, and causing the formation of a dry diffusion barrier.