Development of Heat Pulse Sensors to Measure Vadose Zone Thermal Properties, Water Content, and Water Flux Density.

摘要

The vadose zone is extremely dynamic with an assortment of complex, coupled, nonlinear, and time-dependent processes. The upper part of the vadose zone contains the soil and is subjected to fluctuations in water content, temperature, and chemical concentrations. These are driven by processes such as infiltration and leaching, water and nutrient uptake by plant roots, and by evaporation and energy exchange at the soil surface. Atmospheric conditions at the soil surface are the main driver for these processes, forcing exchange of energy and mass across that interface. The soil properties control the rate of these processes and the capacity of the vadose zone to support their mass and energy. The extent of these processes is controlled by the thickness of the vadose zone, which is determined by the depth of groundwater table.;Despite the vast importance of real-time data to augment the general understanding of vadose zone processes, measurement capabilities are limited, posing a pressing need for improved instrumentation. Specifically, innovative integrated measurement techniques are required to measure and monitor multiple environmental parameters using a single sensor, applying the measurements at the same spatial and temporal scales to minimize the effects of sampling-volume and heterogeneity on the measurements. Improved capabilities for measuring thermal properties, water content, and water flux density in the vadose zone are achieved through development of the heat pulse method, as presented in this dissertation.;Heat-pulse sensors are useful for measuring thermal properties, water content, water flux density, and electrical conductivity. Measurements with this sensor do not entail soil-specific calibration and data acquisition requirements are relatively simple. The sensor consists of a heater probe for generating heat and one or more temperature probes for measuring temperature at a certain distance. The conventional heat-pulse sensor design is limited in its application because the small-diameter probes can deflect during field installation. However, thin probes are required satisfy the assumptions of the line-source heat transfer model.;The studies presented in this dissertation were conducted to remove the main limitations associated with the conventional heat-pulse method. The dissertation is divided into four research chapters. The first research chapter presents the numerical tools that were fundamental for this dissertation and some preliminary findings about the conventional sensor. In the following three chapters different sensor prototypes are studied. Each of these chapters is distinct from another, as it addresses a specific measurement niche with a unique sensor-prototype, accompanied with an exclusive heat transfer model. The prototype development-process is a combination of numerical modeling and experimental studies. The first prototype is made using a heater probe with a larger diameter that enables longer heat pulse durations, providing for measurement of water flux density down to 1 cm d−1, typical for vadose zone environments. The second is a button heat pulse probe. This sensor uses a ring heater that is attached to the sensor body with the temperature measurement taken at the center of the ring. The probe type removes the probe deflection issue because the probes are attached to the body. Due to the geometrical arrangement of the heater and the location of the temperature measurement, the button sensor is more responsive to changes in soil properties. The last prototype has longer and larger-diameter probes compared to the conventional sensor, such that the added stainless steel material makes its probes about five times more resistant to probe deflection. The newly developed sensor-prototypes with their accompanied heat transfer models are an important step towards improved heat-pulse sensors for soil water measurements.