An automated microinfiltrometer to measure small-scale soil water infiltration properties

We developed an automated miniature constant-head tension infiltrometer that measures very small infiltration rates at millimetre resolution with minimal demands on the operator. The infiltrometer is made of 2.9 mm internal radius glass tube, with an integrated bubbling tower to maintain constant negative head and a porous mesh tip to avoid air-entry. In the bubbling tower, bubble formation and release changes the electrical resistance between two electrodes at the air-inlet. Tests were conducted on repacked sieved sands, sandy loam soil and clay loam soil, packed to a soil bulk density ρd of 1200 kg m^-3or 1400 kg m^-3and tested either air-dried or at a water potential ψ of-50 kPa. The change in water volume in the infiltrometer had a linear relationship with the number of bubbles, allowing bubble rate to be converted to infiltration rate. Sorptivity measured with the infiltrometer was similar between replicates and showed expected differences from soil texture and ρd, varying from 0.15 ± 0.01 (s.e.) mm s^-1/2for 1400 kg m^-3clay loam at ψ =-50 kPa to 0.65 ± 0.06 mm s^-1/2for 1200 kg m^-3air dry sandy loam soil. An array of infiltrometers is currently being developed so many measurements can be taken simultaneously.