Tensiometers are commonly used for measuring soil water matric pressures. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately 20.085 MPa. Tensiometers filled with a polymer solution instead of water are able to measure a larger range of soil water matric pressures. We designed and constructed six prototype polymer tensiometers (previously called osmotic tensiometers) consisting of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane preventing polymer leakage. A polymer chamber (0.1-2.2 cm 3 ) was located between the pressure transducer and the plate. We tested the polymer tensiometers for long-term operation, the effects of temperature, response times, and performance in a repacked sandy loam under laboratory conditions. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which we developed a suitable correction. The osmotic potential of polymer solutions is temperature dependent, and requires calibration before installation. The response times to sudden and gradual changes in ambient temperature were found to be affected by polymer chamber height and polymer type. Practically useful response times (,0.2 d) are feasible, particularly for chambers shorter than 0.20 cm. We demonstrated the ability of the instrument to measure the range of soil water pressures in which plant roots are able to take up water (from 0 to 21.6 MPa), to regain pressure without user interference and to function properly for time periods of up to 1 yr.

Core Ideas Increasing macropore volume may decrease the final cumulative outflow. Heterogeneous macroporous geometries can be represented explicitly or implicitly. HYDRUS‐1D and SWAP performed well in soils with heterogeneous macroporous geometries. Dead‐end macropores are included by changing lateral transfer parameters with depth. Heterogeneous macroporous geometries (HMGs) comprise unevenly distributed macropores with depth. A large variety of macropore distributions produce fast water flow and chemical transport that deviate from uniform flow. We analyzed the measured pressure head and outflow in column experiments with a uniform matrix (Exp. I), one central macropore (main bypass) (Exp. II), and HMG (Exp. III) and evaluated the performance of the models HYDRUS‐1D and SWAP under these conditions. Two replicate soil columns were prepared with a 62‐cm silty loam layer above a 5‐cm sandy loam layer. Well‐defined infiltration and drainage conditions were applied to top and bottom boundaries, respectively. Pressure head and outflow were measured at short time intervals, and calibration was performed by PEST. Experiment I was conducted to calibrate the matrix parameters and Exp. II to calibrate macropore parameters. In Exp. III, four dead‐end macropores were created around the main bypass, and the models were run using the previously calibrated parameters, updating only the macropore geometry parameters. The results indicated that HMGs increased total macropore influx, especially in the internal catchment domain. Interaction between the internal catchment, main bypass, and matrix domains was necessary for explaining the change in cumulative outflow and outflow onset observations. The simulations with both models were accurate for HMG regarding pressure head and outflow. The implicit representation of HMGs by HYDRUS‐1D improved outcomes for cumulative outflow, whereas the explicit representation by SWAP improved results for lateral mass transfer. The ability to model the effects of HMGs is essential for environmental and agricultural studies.

In many regions of the world, plant growth and productivity are limited by water deficits. As a result of more frequent and intense droughts, the area of land characterized as very dry has more than doubled since the 1970s. Consequently, understanding root water uptake under water‐stressed conditions is gaining importance. The performance of a recently developed polymer tensiometer (POT) designed to measure matric potentials down to −1.6 MPa was evaluated and compared with volumetric moisture content measurements in dry soil. Three irrigation intensities created severe, intermediate, and no water stress conditions in lysimeters with growing maize (Zea mays L.) plants. By monitoring matric potentials using POTs, levels of local water stress in our experiments were better defined. When the defined stress levels were reached, volumetric moisture measurements for this particular loam soil were below 0.1, thus less informative compared with matric potential measurements. The observed matric potential profiles indicate significant root water uptake between 0.3‐ and 0.5‐m depth in the later growth stages under water‐stressed conditions. The temporal pattern of matric potential profiles reflected changing root water uptake behavior under dry conditions. As the total soil water potential is a direct indication of the amount of energy required by plants to take up water, POTs may contribute to quantifying root water uptake in dry soils.

Abstract. Measuring soil water potentials is crucial to characterize vadose zone processes. Conventional tensiometers only measure until approximately −0.09 MPa, and indirect methods may suffer from the non-uniqueness in the relationship between matric potential and measured properties. Recently developed polymer tensiometers (POTs) are able to directly measure soil matric potentials until the theoretical wilting point (−1.6 MPa). By minimizing the volume of polymer solution inside the POT while maximizing the ceramic area in contact with that polymer solution, response times drop to acceptable ranges for laboratory and field conditions. Contact with the soil is drastically improved with the use of coneshaped solid ceramics instead of flat ceramics. The comparison between measured potentials by polymer tensiometers and indirectly obtained potentials with time domain reflectometry highlights the risk of using the latter method at low water contents. By combining POT and time domain reflectometry readings in situ moisture retention curves can be measured over the range permitted by the measurement range of both POT and time domain reflectometry.

Abstract:In recent years, a polymer tensiometer (POT) was developed and tested to directly measure matric potentials in dry soils. By extending the measurement range to wilting point (a 20-fold increase compared to conventional, waterfilled tensiometers), a myriad of previously unapproachable research questions are now open to experimental exploration. Furthermore, the instrument may well allow the development of more water-efficient irrigation strategies by recording water potential rather than soil water content. The principle of the sensor is to fill it with a polymer solution instead of water, thereby building up osmotic pressure inside the sensor. A high-quality ceramic allows the exchange of water with the soil while retaining the polymer. The ceramic has pores sufficiently small to remain saturated even under very negative matric potentials. Installing the sensor in an unsaturated soil causes the high pressure of the polymer solution to drop as the water potentials in the soil and in the POT equilibrate. As long as the pressure inside the polymer chamber remains sufficiently large to prevent cavitation, the sensor will function properly. If the osmotic potential in the polymer chamber can produce a pressure of approximately 2.0 MPa when the sensor is placed in water, proper readings down to wilting point are secured. Various tests in disturbed soil, including an experiment with root water uptake, demonstrate the operation and performance of the new polymer tensiometer and illustrate how processes such as root water uptake can be studied in more detail than before. The paper discusses the available data and explores the long term perspectives offered by the instrument.