Quantifying Water Release into Soil from Water-Absorbing Polymers under Drought Conditions

The use of water-absorbing polymers (WAPs) is evolving as an amicable solution for preserving soil moisture and promoting vegetation cover under extreme drought conditions associated with changing climate. WAPs can absorb a significant amount of irrigation/rainwater and release water to soil when moisture deficit occurs. As compared to the water-absorbing characteristics, comprehensive information on the water release characteristics of WAPs when interacting with various soil types is scarce in the literature. The objective of this study is to develop an experimental methodology for understanding the water release characteristics (WRCs) of different WAPs in various soils. A horizontal soil column test setup was used for studying the WRCs of two WAPs [fly ash-derived WAP (FA-WAP) and commercially available acrylic-based WAP (Com-WAP)] in three different soils (sand, silt loam, and clay loam). The tests were conducted on dry soils and with the WAPs at two initial conditions, including fully hydrated conditions and limited water availability conditions. A two-parameter kinetic equation was found suitable for quantifying the water release characteristics of the WAPs in both (1) soils; and (2) atmosphere. The study revealed that the dry soils in contact with the FA-WAP able to quickly achieve more than 95% saturation within 12 h, whereas the dry soils in contact with Com-WAP remain well below full saturation after the same time interval. The different combinations of soils and WAPs suggests better efficiency of FA-WAP and clay loam soil for higher WRC if a sufficient amount of water is available in WAP. The water release rate from FA-WAP to dry soil was found to be much higher than the Com-WAP during the initial stage. Under the limited water availability conditions, FA-WAP was able to release 97% of the total absorbed water to the initially dry soils as compared to Com-WAP, where 90% of the stored water was released.