Evaporation suppressants: a chemical approach to water conservation in rice fields and reservoirs

The use of evaporation suppressants was demonstrated to be quite effective for retarding the evaporation of water from rice fields and reservoirs. All of the chemical evaporation suppressants had molecular structures containing a polar, hydrophilic head and a long hydrophobic tail. The compounds were of two types: coconut fatty alcohols like hexadecanol and octadecanol, and monoethers of diols and triols. Optimization studies using coconut fatty alcohols, the traditional chemical suppressants, indicated that a 2:3 (w/w) mixture of hexadecanol and octadecanol performed better than pure hexadecanol or octadecanol. The ability of hexadecanol/octadecanol formulations to retard the evaporation of water was found to be dependent on solar radiation, wind velocity and relative humidity. It was noted that water availability in fields treated with hexadecanol/octadecanol films was one to five days longer than in untreated fields, with no significant effects on floodwater, pH, temperature and volatilization rate of ammonia. However, the film forming material had to be replenished every 3 days due to losses from volatilization and microbial degradation of the films. Three types of hydroxyether were prepared in fields of 50-80% from locally available coconut fatty alcohols for use as evaporation suppressants. These compounds were found to spread on the water surface and form a continuous film more readily than hexadecanol or octadecanol because of their greater affinity for water due to the presence of a more polar head. Among the compounds tested, glyceryl monostearyl ether applied either as a paste or as an alcoholic solution showed the highest reduction rate. The best formulations were the solution formulations using ethanol or isopropanol as the solvent and the micronized powder formulations. The cost of the alternative suppressants was found to be higher than that of hexadecanol/octadecanol because these compounds are derived from hexadecanol or octadecanol. Using the traditional suppressant compounds, hexadecanol and octadecanol, the average increase in yield was found to be roughly 7x the cost of the applied suppressants (22 cavans/ha increase in yield vs. the equivalent of 3 cavans of palay in application costs). It has been estimated that a decrease in water evaporation rate by 50% may result in water savings that can irrigate as much as 21% more rice land during the dry season. With this technology, it is possible that rainfed ricefields may produce yields comparable to irrigated fields. With the use of chemical suppressants, all these economic benefits may soon become reality