AbstractWater absorbing polymers (WAP) have shown their potential as vegetation growth promoters in various soil infrastructures by minimizing the downward migration of water and conserving adequate moisture in the vadose zone under semiarid climates. However, the moisture movement mechanism in a soil–WAP system has not been well recognized in previous studies. There is a need to evaluate the moisture diffusivity value of soil in contact with swollen WAP to understand the performance of WAP as soil amendment under water stress conditions. Therefore, the present study demonstrates a laboratory procedure to characterize the moisture sorption and diffusion mechanism from swollen WAP to soil particles. A horizontal diffusion cell was developed to study the moisture transfer and transport mechanism corresponding to three different textured surface soils. The measured moisture profiles with time were used to determine the Boltzmann transformation constant to verify the validity of Fick’s law of diffusion for moisture movement in a WAP–soil system. A tangential relationship between the Boltzmann constant and soil moisture content was proposed to calculate the moisture diffusivity value. The results indicated that the moisture content of the WAP–soil interface was significantly influenced by both soil and WAP characteristics. Moreover, the moisture movement through the soil column (in contact with swollen WAP) follows the Fickian diffusion mechanism, and the proposed laboratory setup is suitable for determination of moisture diffusivity values. The wetting soil–water retention curves (SWRC) of the used soils were measured using suction sensors (T5, TEROS21) and a moisture sensor (ECH2O 5TM) in a PVC column connected to a water reservoir to estimate the moisture diffusivity values. The observed moisture diffusivity curves showed that the addition of WAP serves as an additional water reservoir in soil and can be used for vegetation maintenance on landfill covers, green infrastructures, and bioengineered slopes under semiarid climates.