Polymer tensiometers to characterize unsaturated zone processes in dry soils

More frequent and intense droughts due to global climate change, together with an increasing agricultural water use emphasize the importance of understanding root water uptake by plants under water-stressed conditions. Root water uptake is driven by potential gradients between water in the soil and in the root. In unsaturated soil, the soil water matric potential is often the largest component of the total soil water potential. Tensiometers are commonly used to measure the pressure-equivalent of the matric potential. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately –0.09 MPa. Using tensiometers filled with a polymer solution instead of water extends the measurement range beyond a matric pressure of –1.6 MPa (almost twenty times more then water-filled tensiometers). This thesis deals with the development of such polymer tensiometers, which consists of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane to prevent leakage of the polymer solution. The polymer tensiometers were tested under laboratory conditions for long-term operation, the effects of temperature, response times, and performance in repacked sandy loam, sand, and loam. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which a suitable correction was developed. The osmotic potential of polymer solutions is temperature-dependent, and requires calibration before installation in the soil. The response times to ambient temperature variations were found to be affected by polymer chamber height and polymer type. By minimizing the volume of polymer solution inside the tensiometer while at the same time maximizing the ceramic area in contact with that polymer solution, response times dropped to acceptable ranges for laboratory and field conditions. Contact with the soil has been improved by using cone-shaped solid ceramics instead of flat ceramics. By combining polymer tensiometer and time domain reflectometry readings, in situ moisture retention curves could be measured over the range permitted by both instruments. Independently determined soil moisture retention curves were used to convert soil moisture content measurements from time domain reflectometry probes to matric potentials. It was shown that at low moisture contents, the accuracy of the time domain reflectometry probes, and the accuracy of the conversion had a large influence. The comparison between matric potentials measured by polymer tensiometers and potentials obtained indirectly by time domain reflectometry thus highlights the risk of using the latter method at low soil moisture contents. Subsequently, the suitability of polymer tensiometers to monitor soil matric potentials in the presence of root water uptake was evaluated in a cropped lysimeter experiment. Three irrigation intensities created severe, intermediate, and no water stress conditions in lysimeters with growing maize (Zea mays, L.) plants. In the lysimeter experiment soil water matric potentials measured by polymer tensiometers yielded more accurate levels of local water stress than would have resulted using conventional methods. The predefined stress levels were located at the steep dry end of the moisture retention curve, where volumetric moisture measurements for this particular loam soil were less informative compared to matric potential measurements. Observation of matric potentials by polymer tensiometers showed the ability of maize plants to take up water under extremely dry conditions, and to shift water uptake areas to lower, relatively wetter soil layers. This shift in water uptake to deeper layers seems to occur when the steep dry end of the retention curve is reached at a shallower depth. Observations made in rhizotubes during the experiment showed that water stress provoked root growth deeper in the soil profile, and showed the dynamic response of root growth during periods of water stress and resumed irrigation. Polymer tensiometers are currently the only field instruments that can reliably measure the steep dry end of the soil moisture retention curve. The results presented in this thesis demonstrate that polymer tensiometers are an important instrumental addition when characterizing soil physical processes in dry soils.