The interaction between radio waves and vegetation has been extensively studied for efficient wireless network planning. Both empirical and analytical models have been developed for this purpose. However, radio transmission within a wireless sensor network could be used for sensing the medium in the signal path in addition to data transmission within the network. Prior work suggested that radio waves interact strongly with water contained in vegetation. The dependence of radio waves on water is through its dielectric constant. In this thesis, the relationship between RF loss and water content in vegetation in the path of a radio communications link was modelled to study the feasibility of monitoring plant health using wireless communication networks.

A model to calculate the RF loss through packed vegetation with different moisture content was developed. This model only considers the dielectric constant of the vegetation and does not include the air component of the tree canopy. The model was further extended to calculate RF loss through tree canopies. Both the models were verified against experimental measurements acquired using Eucalyptus leaves and trees. The results show that there is a positive non-linear relationship between RF loss in dB and water content expressed as effective water path (EWP) in mm. Vegetation thickness is also a factor in RF loss, however the contribution was minor compared to water content of vegetation. This work has quantified the relationship between RF loss and water within vegetation. It may lead to wireless sensor networks being repurposed to monitor, over periods of many years, changes in plant water status.

Detection of plant water status is important for monitoring plant physiology. Previous studies showed that radio waves are attenuated when passing through vegetation such as trees, and models (both empirical and analytical) were developed. However, for models to be more broadly applicable across a broad range of vegetation types and constructs, basic electrical properties of the vegetation need to be characterised. In our previous work, a model was developed to calculate the RF loss through vegetation with varying water content. In this paper, the model was extended to calculate RF loss through tree canopies with or without an air gap. When the model was compared with the actual RF loss acquired using Eucalyptus blakelyi trees (with and without leaves), there was a systematic offset equivalent to a residual moisture content of 13% that was attributed to bound water. When the model was adjusted for the additional water content, the effective water path (EWP) was found to explain 72% of the variance in the measured RF loss.

Assessing plant water status is important for monitoring plant physiology. Previous studies showed that radio waves are attenuated when passing through vegetation such as trees. The degree of radio frequency (RF) loss has previously been measured for various tree types but the relationship between water content and RF loss has not been quantified. In this study, the amount of water inside leaves was expressed as an effective water path (EWP), the thickness of a hypothetical sheet of 100% water with the same mass. A 2.4331 GHz radio wave was transmitted through a wooden frame covered on both sides with 5 mm clear acrylic sheets and filled with Eucalyptus laevopinea leaves. The RF loss through the leaves was measured for different stages of drying. The results showed that there is a nonlinear relationship between effective water path (EWP) in mm and RF loss in dB. It can be concluded that 2.4 GHz frequency radio waves can be used to predict the water content inside eucalyptus leaves (0 < EWP < 14 mm; RMSE ± 0.87 mm) and demonstrates the potential to measure the water content of whole trees.

Previous studies have investigated the attenuation of 2.4 GHz radio waves and vegetation, and generated a model to predict RF loss in response to the effective water path (EWP) of both packed leaves and whole tree canopies. Owing to the absence of phase information in the radio signal strength (RSS) it is not possible to directly invert the model to elicit EWP. This paper builds upon previous work and proposes a two-point iteration methodology to predict actual gravimetric water content (M_g) of single tree canopies from RF loss. An investigation for sample tree canopies (including two trees in series) demonstrated M_g explained 72% of the variance in measured RF loss.

Assessing plant water status is important for monitoring plant physiology. Radio signals are attenuated when passing through vegetation. Both analytical and empirical models developed for radio frequency (RF) loss through vegetation have been dependent on experimental measurements and those measurements have been completed in specific situations. However, for models to be more broadly applicable across a broad range of vegetation types and constructs, basic electrical properties of the vegetation need to be characterised. Radio waves are affected especially by water and the relationship between water content in vegetation expressed as effective water path (EWP) in mm and measured RF loss (dB) at 2.4 GHz was investigated in this work. The EWP of eucalyptus leaves of varying amounts of leaf moisture (0% - 41.5%) ranged from 0 - 14 mm, respectively. When the model was compared with the actual RF loss there was a systematic offset equivalent to a residual leaf moisture content of 6.5% that was unaccounted for in the leaf moisture content determination (oven drying). This was attributed to bound water. When the model was adjusted for this amount of additional leaf water, the average RMSE in predicted RF loss was ±2.2 dB and was found to explain 89% of the variance in measured RF loss.

Water evaporation rates can be reduced by covering the surface of the water resource with a monomolecular layer of material. This surface layer is a one molecule layer at the air/water interface, and effective monomolecular layers form from amphiphilic long-chain carbon molecules with a polar head group and long-chain hydrocarbon tail. Such surface-active molecules self-organise on the water's surface with the polar head group adsorbed to the surface of the water and the non-polar tail extending away from the surface. Previous studies have proposed that these systems operate via a barrier mechanism, i.e., a hydrophobic barrier to diffusion that limits evaporation by slowing the movement of molecules away from the surface. Evaporation reduction is correlated to wave damping capacity and an alternative mechanism proposed is that wave suppression reduces the circulation of air on the surface of the water, and hence the rate of removal of water-saturated air away from the boundary region. The hypothesis of this work is that wave suppression is the main mechanism by which monomolecular surface layers work. Commercial hollow polystyrene nanoparticles (Ropaque) were applied to test this hypothesis. Ropaque nanoparticles are round, rigid, and of approximately 400 nm diameter, and so they should not be able to form a continuous barrier on the surface because they are around the shape. Thus, they are not able to provide an impermeable barrier at the surface and cannot be acting by the barrier mechanism. This research evaluated the evaporation reduction and wave damping capacity of Ropaque nanoparticles under different conditions and compared the behaviour with hexadecanol and octadecanol monolayers. The detailed results of evaporation reduction tests with different wind speeds of pure water both in the water tank and with the dam water in water pans showed that the Ropaque exhibited a greater increase in saving water through the prevention of evaporation than the hexadecanol and octadecanol monolayers. In practical terms, the Ropaque nanoparticles gave better evaporation control than the chemical monolayers investigated in this study.

It was determined that Ropaque nanoparticles were less easily dispersed by high winds speeds and remained effective for longer. Furthermore, in wave damping studies, Ropaque nanoparticles increased the wave damping capacity at the frequency range between 1 and 2 Hz, which is most important for damping capillary waves to a greater extent than hexadecanol and octadecanol.