Passive Capillary Sampler for Measuring Soil Water Drainage and Flux in the Vadose Zone: Design, Performance, and Enhancement

Abstract

Various soil water samplers are used to monitor, measure, and estimate drainage water, fluxes, and solute transport in the vadose zone. Passive capillary samplers (PCAPs) have shown potential to provide better measurements and estimates of soil water drainage and fluxes than other lysimeters designs and field sampling methods. Twelve automated PCAPs with sampling surface dimensions of 31 cm width 91 cm long and 87 cm in height were designed, constructed, and tops of the samplers were placed 90 cm below the soil surface in a Lihen sandy loam (sandy, mixed, frigid Entic Haplustoll). The PCAPs were installed to continually quantify the amount of drainage water and fluxes occurring under sugarbeet (Beta vulgaris L.) and malting barley (Hordeum vulgare L.) crops treated with 30 mm (low replacement) and 15 mm (high replacement) irrigation frequencies. Drainage water was extracted, collected, and measured periodically (weekly from May to mid-August, biweekly until late September, and monthly thereafter until mid-November). This design incorporated Bluetooth wireless technology to enable an automated datalogger to transmit drainage water and flux data simultaneously every 15 min to a remote host. Real-time seamless monitoring and measuring of drainage water and fluxes was thus possible without the need for costly time-consuming supportive operations. The mean difference (Md) values between manually extracted and logged drainage water for high frequency (Md = 0.80 mm) and low frequency (Md = 0.26 mm) irrigations were small and not significantly different from zero. The Root Mean Square Error (RMSE) of 2.46 and 7.83 mm for high frequency and low frequency irrigations, respectively, were also small. Despite small variations in drainage water results, our novel PCAP design provided an accurate and convenient way to measure water drainage and flux in the vadose zone. Moreover, it offered a significantly larger coverage area (2700 cm2) than similarly designed vadose zone fluxmeters or PCAPs. In the course of one year's field testing, we incorporated several additional enhancements such as PCAP container, tipping bucket and datalogger unit, all of which we recommend for optimal performance.