الملخص الإنجليزي
Title: Comparative Performance of a Hydrophobic Siloxane Polymer and a Hydrophilic Gel-Forming
Copolymer on Intermittent Evaporation, Water Distribution and Salt Leaching Through Soil Columns
Conserving water resources is very important for the Sultanate of Oman, since about 90 % of fresh water is used for agriculture. A major loss of water applied to soils is through evaporation. If evaporative losses could be reduced, then more of the irrigation water would be available for plant growth and possible of salts leaching from the root zone. A large number of methods have been described to limit evaporation, including tillage, mulching, and the use of soil amendments or synthetic polymers. Both hydrophobic and hydrophilic polymers may be useful to reduce evaporative losses from soils. The objective of this study was to evaluate the performances of a hydrophobic Siloxane Polymer (Guilspare) and a hydrophilic Gel Forming Copolymer (Waterworks Gels) on intermittent evaporation, water distribution, and salt leaching through soil profiles. Thirty-two PVC columns, 60 cm long (10 cm in diameter), were assembled each having 6 sections (10 cm long) held with adhesive tape. The bottom of each column was closed with a firmly held piece of cloth. Columns were uniformly packed with sandy loam soil. In 8 columns, Guilspare (2 % solution) was applied to the soil surface. Eight columns were treated with Waterworks Gels (0.3%) at the top 15 cm soil depth. A combined treatment of Guilspare plus Waterworks Gels was applied in other eight columns. The remaining eight columns were left untreated. Moderately saline water (EC = 3.80 dS m) was applied at two rates of 3 and 6 mm day and two intervals of 4 and 8 days. Soil columns were kept outside during summer weather conditions. The columns were weighed daily to monitor water loss. After 36 days, volumetric soil water content and salinity (EC, S m) were measured in each column section. The hydrophobic layer created by Guilspare was an effective barrier against soil water evaporation. Waterworks Gels alone were ineffective in reducing evaporation. During the series of wetting and drying cycles, the pattern of water loss from Guilspare, and Guilspare plus Waterworks Gels treatments was linear over time, while for Waterworks Gels and the controls, the pattern of water loss was cyclic. Regression analysis showed a linear relationship between cumulative evaporation and square root of time from all treatments. Evaporation constants were in the following order: Guilspare plus Waterworks Gels < Guilspare < Waterworks Gels < Control. Averaged over all watering rates and intervals, Guilspare plus Waterworks Gels, Guilspare, and Waterworks Gels treatments reduced cumulative evaporation by 58, 54 and 1.7 % and increased the percent in depth of water conserved by 69,64, and 2%, respectively, relative to the control. Guilspare application substantially reduced constant rate stage (stage I) of soil drying. The rates of stage I drying from Waterworks Gels treated columns and controls were higher than during falling rate stage (stage II) of soil drying, at both rates of water application. No significant differences between treatments were found during stage II of soil drying. Guilspare application proved to be an effective technique in improving water contents and distribution in soils. Guilspare application showed an increase in soil water contents and distribution through all depths of soil columns. Waterworks Gels showed an increase in soil water contents only between 5-15 cm depths. The combined treatment had the highest water storage between 15 to 25 cm depths. The study showed that polymers application were very effective in preventing surface salt accumulation and improving the efficiency of salt leaching, Guilspare treatment increased salt distribution with depth and leached the salts towards the bottom of soil columns. Waterworks Gels accumulated salts in the middle of soil columns. The order of treatments in promoting salt leaching was as follows: Guilspare > Guilspare plus Waterworks Gels > Waterworks Gels > Control. In comparison, a Hydrophobic Siloxane Polymer proved to be more effective, under the experimental conditions, in reducing evaporation, increasing water conservation, improving water distribution and consequently enhancing salt leaching than a Hydrophilic Gel-Forming Copolymer.
