Comparison of methods for determining unsaturated hydraulic conductivity in the wet range to evaluate the sensitivity of detectors

The design of passive lysimeters or wetting front detectors determines the tensions at which they collect a water sample from an unsaturated soil. When deployed in the field to help manage irrigation, it is necessary to know the minimum flux of water that can be sampled by a passive lysimeter and how this relates to the drainage flux at field capacity. This requires a good estimate of the unsaturated hydraulic conductivity characteristic, K(h), in the wet range (< 10 kPa). We compared various field, laboratory and theoretical approaches for obtaining the K(h) function and compared these to a reference K(h) function derived by applying inverse modelling approaches to field drainage experimental data. The Van Genuchten model and three of the pedotransfer models produced K(h) functions with a root mean square error of less than 5% compared to the reference, and appear to be simple methods of obtaining a reasonable estimate of unsaturated hydraulic conductivity. However, despite the goodness of fit, there can be a 10-fold difference in conductivity at a given tension < 10 kPa estimated from the different methods. Moreover, water content at field capacity depends entirely on whether field capacity is defined as time elapsed after saturation, a set tension or a minimum flux.Funding for this research was provided by the Water Research Commission (WRC), South Africa, as part of the WRC research project entitled ‘Adapting the wetting front detector to the needs of small-scale furrow irrigators and providing a basis for the interpretation of salt and nutrient measurements from the water sample’.http://www.wrc.org.zanf201

Design features determining the sensitivity of wetting front detectors for managing irrigation water in the root zone

Current irrigation scheduling technologies are limited to refilling the root zone based on measured or predicted amount of water stored within the root zone. This needs measurement of soil-water status and specifying soil field capacity that make this approach expensive and challenging. The FullStopTM wetting front detector (FS) was specifically developed to be a simple and affordable technology to help farmers manage water, nutrients and salts in the root zone. This device responds to a strong wetting front, but research has shown it is less sensitive to weak redistributing wetting fronts, and this may compromise its efficacy in certain situations. The objectives of this study were to recommend a modified version of the FS that responds to weak redistributing wetting fronts and to develop guidelines for the deployment of these detectors to schedule irrigation. The research described herein comprises of two phases: the first phase focused on literature review, field evaluation of wetting front detector of varying sensitivities (WFD) and laboratory measurements of hydraulic properties of soil and wick materials. The second phase validates the HYDRUS-2D/3D for the development of guidelines on how to use WFD to schedule irrigation. The first phase includes: i) a literature review on passive lysimetry that relates design features to the sensitivity of WFD and how prototypes of WFD operate; ii) hydraulic characterization of soil and wick materials to describe the functioning of the different WFD designs; iii) an empirical investigation to determine whether the wick characteristics limits the attainment of equilibrium between the opening of the outer tube and the water table in the inner tube; iv) field evaluations of five types of WFD under sprinkler and natural rainfall to examine the accuracy and sensitivity of the different WFD designs; and v) analysis of the equilibrium between the WFD and the surrounding soil, and recommendations for the best design options based on the sensitivity requirement for different situations. The second phase of the study used observed data sets to validate the Hydrus-2D/3D model. After validation, the model was used to simulate different irrigation scenarios to develop guidelines for the deployment of WFD to schedule irrigation. Field evaluations of various WFD designs showed that length has significant effect on the sensitivity of WFD (P ≤ 0.05). The 90-cm-long Tube wetting front detector (90TD) was significantly more sensitive than the original FS design. The hydraulic conductivity function of two wick materials (Diatomaceous Earth and Fine sand) were not limiting for the attainment of the equilibrium between the Tube Detector and the surrounding soil, and the opening of the Tube Detector and the water level in the inner tube. The Hydrus-2D/3D model performed well in simulating the measured responses of FS or 90TD and the experimental sensitivity thresholds of these detectors. This model was deployed to link WFD responses to different simulated irrigation scenarios to generate monitoring protocol such as detector placement depth, irrigation amount or interval. The model simulations showed that FS can be used to schedule irrigation objectively for sprinkler or drip irrigations, i.e. adjusting irrigation amount or interval based on the response of a detector. Though further study is warranted, model simulation has indicated that 90TD can be used to improving furrow irrigation management. It is envisaged that WFD technology can guide farmers to make informed irrigation decisions and alerting farmers to percolation losses below the root zone.Thesis (PhD)--University of Pretoria, 2014gm2015Plant Production and Soil SciencePhDUnrestricte