Effect of Water Absorbing Polymer Amendment on Performance of Capacitance Soil Moisture Sensor

Water absorbing polymer (WAP) is emerging as soil amendment material for various soil infrastructures, such as urban green infrastructure, green roofs, landfill covers, and climate-resilient agriculture, to promote vegetation growth and thereby increase the sustainability of the projects. WAP amended soils experience alternating periods of drying and wetting because of their exposure to different climate conditions. Precise determination of volumetric water content (VWC) in vadose zone is required to establish the soil-water retention curve (SWRC) in WAP amended soils. This study aims to evaluate the accuracy of a capacitance sensor (CS) for the purpose of continuous VWC monitoring in WAP amended soils. A controlled laboratory environment was used for the performance evaluation of CS in three different surface soils (sand, silt loam, and clay loam) with four WAP amendment rates (0 %, 0.1 %, 0.2 %, and 0.4 %). The CS completely underestimates the VWC of WAP amended soils because of the bound water inside the WAP network. Two different calibration equations (i.e., third-order polynomial and linear) were recommended to enhance the precision of VWC measurement in WAP amended soils. The linear calibration method is further extended for obtaining a generalized calibration procedure valid for all soil textures and WAP concentrations. The importance of the proposed calibration procedure for a precise SWRC measurement of WAP amended soils was demonstrated. The results indicated that the error in VWC measurement further influences the saturated water content, field capacity, and permanent wilting point, which are essential parameters to estimate the soil-water storage, and irrigation water requirement.