Novel techniques to determine soil evaporation rates: Heat pulse probe and automated microlysimeter

Soil water evaporation is a critical component of both the surface energy balance and the hydrologic cycle, coupling heat and water transfer between land and atmosphere. Bare-soil evaporation and plant-soil-atmospheric interactions are important components of the water balance, especially in semiarid and arid regions. Soil evaporation has been thoroughly studied during the past century, yielding many methods and models. However, none of the methods have adequately addressed the needs for in situ and real-time monitoring of soil evaporation. The objectives of this research project were to track soil water evaporation losses using two different methods: a heat pulse probe (HPP) array and a fully automated microlysimeter (FAML). The HPP consists of a heater needle and five thermistor needles; when rotated to an angle of 27.3o from a vertical orientation, it yielded temperature measurements every 3 mm within the soil profile. On the application of heat input to a resistance wire in the heater needle, the remaining thermistor needles measured the temperature response at a fixed distance of 6.5 mm from the heater. Results from our study demonstrate application of the sensible heat balance approach that provided reasonable estimates of subsurface evaporation rates. Inconsistencies due to the inability of the HPP to estimate evaporation rates in the near-surface "undetectable zone" are also reported in comparison to actual stage-2 evaporation based on the mass balance method. Additionally, deviations from the prescribed installation angle introduced errors when calculating the temperature gradient; hence, a vertical spacing algorithm was developed to resolve spacing errors. In the third chapter, a fully automated design is discussed based on the microlysimeter concept with the enhancement of an 80 cm deep lysimeter that was mounted on a 10 kg load cell for real-time monitoring of diurnal evaporation rates from bare soil. The comparison with HYDRUS-1D simulation validated the FAML measured instantaneous evaporation rates with slight disparity towards the end of the experiment. Overall, this study shows two feasible methods for estimating real time evaporation rates in situ over prolonged periods with the aid of the HPP or the FAML. These tools can assist researchers with improved assessment of soil evaporation while taking into account proper correction methods.