Design of an automated tension infiltrometer for unsaturated hydraulic conductivity measurement

A tension infiltrometer for field use, where both water level changes measurement and tension settings could be automated, was built in-house. Differential pressure transducers were used to automate the water level measurement in the reservoir. The Mariotte bottle was automated by a set of solenoid valves which were connected via tubing to pre-defined depths in the Mariotte bottle. Based on design parameters tested in the laboratory (sensor sensitivity, water reservoir diameter, and bubbling rate) three identical tension infiltrometers connected to a single Mariotte bottle were built. A new reservoir system made of two plexiglass tubes of different diameter slotted in each other was found to reduce the measurement fluctuations caused by the disturbance created to rising bubbles in the reservoir. The new system was tested on a uniform sandy loam profile prepared in the soil bin laboratory and different analytical and numerical data analysis methods were compared. The measured steady state data were used to determine K(h) at different suctions using the analytical method proposed by Reynolds and Elrick (1991). The K(h) points obtained were fitted with the van Genuchten’s equation (van Genuchten, 1980) using the RETC program to calculate the best fit parameters Ks,  and n. These parameters were used as initial estimates of the soil hydraulic parameters in the numerical models HYDRUS 1D and 2D, in which transient cumulative flow data was used to determine the soil hydraulic functions via inverse modelling. The analysis of variance determined significantly higher K(h) values calculated by HYDRUS 1D while the other methods did not differ from each other. Finally, the tension infiltrometer was used in the field on a sandy loam soil to characterise five different tillage treatments (conventional plough, shallow plough, minimum tillage, direct drill, and no-tillage). The effect of wheel traffic was also evaluated by measuring the infiltration rates in the wheel-marks. The fully automated system allowed the measurement of infiltration rates for 8 tensions in triplicate per day with hardly any human intervention apart for refilling the reservoir. The results show that the tillage practices and wheel-traffic have a significant influence on the soil hydraulic function K(h).