Laboratory studies on hydraulic properties of wick materials

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RATIONALE

Irrigated agriculture needs to play an important role in meeting the projected increase in food demand from a growing population (Howell, 2001). It is particularly important in the arid and semi-arid environments where water is the limiting factor for crop production, and where water use efficiency must be increased through better scheduling of irrigation. The most commonly used objective irrigation scheduling methods include soil water monitoring tools, atmospheric based quantification of evapotranspiration, plant based monitoring and integrated water balance models (Stevens et al., 2005). The use of these methods, however, is limited among farmers due to the cost and complexity in use and interpretation (Shearer and Vomocil, 1981; Stevens et al., 2005; Stirzaker, 2006). In the face of this poor adoption, Stirzaker (2003) tried to find simpler ways to improve irrigation on-farm. This materialized in the development of a simple and affordable tool called the FullStopTM wetting front detector (FS WFD) for making irrigation decisions from the depth of a wetting front. This tool is a funnel-shaped device buried in the root zone with a mechanically operated indicator visible above the soil surface (see Section 1.3). It has shown promise in the field for water, nutrient and salt management in the root zone (Fessehazion et al., 2011; Tesfamariam et al., 2010; van der Laan et al., 2010). The FS WFD responds well to relatively “strong” wetting fronts (30 cm tension or wetter), which are usually present in the upper soil layers during or in the few hours following irrigation. During the redistribution phase, however, wetting fronts become more diffuse (or “weak”) with depth and time. In this case wetting fronts below the detection limit of the FS could pass without activating the indicator may cause substantial drainage water losses if this low flux persists (Stirzaker et al., 2004a). Therefore further research is needed to (i) specify the sensitivity of a WFD design in terms of the minimum water flux it can detect, and ii) evaluate new design options that respond to weak redistributing fronts. The Water Research Commission (WRC) of South Africa initiated a project entitled „Cheap and simple irrigation scheduling using wetting front detectors‟ (WRC Project No. 1135). Based on a survey of 54 irrigators and their advisors who used the WFDs on their own farms for at least one season during the above project, 82% found that it conferred a relative advantage over their current practice. Some farmers who use centre pivot and furrow irrigation, however, reported that detectors did not respond to irrigation, hence their current practice was not challenged (Stirzaker et al., 2004a). This signalled that further work was required to develop a modified design of wetting front detector (WFD) that could be used, especially in furrow irrigation. A second WRC project was initiated in 2005 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‟. Among others, two important objectives of this project were (i), to develop and test a modified WFD, dedicated to the needs of small-scale furrow irrigators, and (ii), to define the sensitivity of a WFD (the amount of water that could pass a detector without activating it). The research presented in this thesis on “Design features determining the sensitivity of wetting front detectors for managing irrigation water in the root zone” was conducted within the context of these two objectives of the latter WRC project.

A BRIEF REVIEW OF DRAINAGE LYSIMETRY

A WFD is essentially a type of passive lysimeter. In order to build a WFD, which can measure a weak, wetting front based on a sound theoretical principle governing water flux measurement, it is essential to understand the uses, designs and operating principles and factors affecting performance of a drainage lysimeters.

CONTENTS :

• List of figures
• List of tables
• Acknowledgements
• Declaration
• Abstract
• Chapter 1: Introduction
  • 1.1Rationale
  • 1.2 A brief review of drainage lysimetry
  • 1.2.1 General
  • 1.2.2 Drainage lysimeter
    • 1.2.2.1 Pan (Zero tension) lysimeter
    • 1.2.2.2 Passive wick (fixed tension) lysimeter
    • 1.2.3 Factors influencing lysimeter performance
    • 1.2.3.1Lysimeter size and construction material
    • 1.2.3.2Hydraulic characteristics of wick and soil materials
  • 1.3 Link between lysimeter and wetting front detector
    • 1.3.1 Length of divergence barrier (extension tube)
    • 1.3.2 Length of a wick material
    • 1.3.3 Hydraulic characteristics of wick and soil materials
    • 1.3.4 Appropriate drainage system
  • 1.4 Thesis objectives
  • 1.5 Approach
    • 1.5.1 A brief review on lysimetry
    • 1.5.2 Analysis of design features and building of WFD prototypes
    • 1.5.3 Measurement of hydraulic characteristics of soil materials
    • 1.5.4 Measurement of hydraulic characteristics of wick materials
    • 1.5.5 Prototype testing and evaluation

• Chapter 2: Analysis of design features of wetting front detectors
  • 2.1 Design considerations
    • 2.1.1 Theoretical and practical design considerations
    • 2.1.2 Boundary conditions and analytical solutions as the wick-soil layer
    • 2.1.3 Pressure head and gravitational head distribution
    • 2.1.4 Specific yield of field soils
  • 2.2 Design features of wetting front detectors
    • 2.2.1 Design types
    • 2.2.1.1 Tube wetting front detector design and operation
    • 2.2.1.2 Hybrid wetting front detector design and operation
    • 2.2.2 Wick material characteristics
• Chapter 3: Comparison of methods for determining unsaturated hydraulic conductivity in the wet range to evaluate the sensitivity of wetting front detectors
  • 3.1 Introduction
  • 3.2 Materials and methods
    • 3.2.1 Soil data measurements
    • 3.2.1.1 In-situ drainage experiment
    • 3.2.1.2 Sample preparation and laboratory measurements
    • 3.2.1.3 Water retention characteristics
    • 3.2.1.4 Saturated hydraulic conductivity
    • 3.2.1.5 Rosetta Pedotransfer Function
    • 3.2.1.6 Bruce-Klute test
  • 3.2.2 Data analysis
    • 3.2.2.1 The instantaneous profile method (IPM)
    • 3.2.2.2 The in-situ inverse model
    • 3.2.2.3 Bruce-Klute test
    • 3.2.2.4 Theoretical methods
    • 3.2.2.5 Evaluation procedures
  • 3.3 Results and discussion
    • 3.3.1 Inverse method
    • 3.3.2 Instantaneous profile method, Bruce-Klute test and Theoretical methods
    • 3.3.3 Comparison of hydraulic conductivity estimation methods
  • 3.4 Conclusions
• Chapter 4: Laboratory studies on hydraulic properties of wick materials
  • 4.1 Introduction
  • 4.2 Analysis of result and discussions
  • 4.2.1 Particle size distribution (PSD)
  • 4.2.2 Water retention characteristics
  • 4.2.3 Specific yield
  • 4.2.4 Response time of wick materials
  • 4.2.5 Unsaturated hydraulic conductivity
    • 4.2.6 Use of hydraulic properties in the application of the Tube Detector
    • 4.2.6.1 Determine the maximum height of the Tube Detector
    • 4.2.6.2 Select a wick material with a rapid response time
    • 4.2.6.3 Select a material that validates the assumption of equilibrium state
  • 4.3 Conclusions
• Chapter 5: Empirical characteristics of the wick materials
  • 5.1 Introduction
  • 5.2 Materials and methods
  • 5.2.1 Tube Detector: Indoor wick drying test
  • 5.2.2 Tube Detector: Buried in the soil
  • 5.3 Result and discussions
  • 5.3.1 Tube detector: Indoor wick drying test
    • 5.3.1.1 Water level in a tube and contact tension
    • 5.3.1.2 Contact tension and water level as a function of time
  • 5.3.2 Tube Detector: Buried in the soil
    • 5.3.2.1 Water level in a tube and contact tension
    • 5.3.2.2 Contact tension and water level as a function of time
  • 5.4 Conclusions
• Chapter 6: Field evaluation of wetting front detectors
  • 6.1 Introduction
  • 6.2 Field experimental materials and methods
  • 6.2.1 Field description, Calibration and Installation
  • 6.2.1.1 Experimental site
  • 6.2.1.2 Calibration
  • 6.2.1.3 Installation procedures
  • 6.2.2 Field data measurements and analyses
  • 6.2.2.1 Water applications
  • 6.2.2.2 Measurements of responses of wetting front detectors
  • 6.2.3 Estimation of water fluxes
  • 6.3 Result and discussions
  • 6.4 Evaluation of wetting front detectors under sprinkler irrigation
  • 6.4.1 Irrigation, responses of tensiometers and wetting front detectors
    • 6.4.1.1 General overview of irrigation and wetting front movement
    • 6.5 Evaluation of the performance of wetting front detectors
    • 6.5.1 First approach: “Yes” type of response as a measure of performance
    • 6.5.2 Second approach: Performance against the sensitivity ranges of WFD
    • 6.5.2.1 Response types of wetting front detectors
    • 6.5.2.2 Tension sensitivities of wetting front detectors
    • 6.5.2.3 Performance evaluations with respect to tension sensitivities of WFD
  • 6.6 Conclusions
• Chapter 7: Evaluation of wetting front detectors under rainfall conditions
  • 7.1 Rainfall, responses of tensiometers and wetting front detectors
  • 7.1.1 General overview of rainfall and wetting front movement
  • 7.2 Evaluation of the performance of wetting front detectors
  • 7.2.1 First approach: “Yes” type of response as a measure of performance
  • 7.2.2 Second approach: Performance against a theoretical sensitivity ranges of WFD
    • 7.2.2.1 Response types of wetting front detectors
    • 7.2.2.2 Tension sensitivities of wetting front detectors
    • 7.2.2.3 Performance evaluations with respect to tension sensitivities of WFD
  • 7.3 Conclusions
• Chapter 8: Analyses of hydraulic characteristics of wick and soil, wick-soil equilibrium conditions and water flux sensitivities of wetting front detectors
  • 8.1 General
  • 8.2 Hydraulic characteristics of wick and soil material
  • 8.2.1 Hydraulic conductivity characteristics of a wick and soil material
  • 8.2.2 Specific yield of a wick material
  • 8.3 Wick-soil equilibrium under field conditions
  • 8.3.1 Effect of wick type and installation disturbance on a contact tension
  • 8.4 Sensitivities of wetting front detectors and water flux profiles
• • 8.5 Drainage and response of wetting front detectors
  • 8.5.1 Sprinkler irrigation
  • 8.5.2 Rainfall
• Chapter 9: Synthesis of the thesis
  • 9.1 Introduction
  • 9.2 Structure of Synthesis
  • 9.3 Summary of thesis results
    • 9.3.1 Purpose of the thesis
    • 9.3.2 A summary of thesis findings
    • 9.3.2.1 Design features and performance of wetting front detectors
    • 9.3.2.2 Hydraulic properties of wick and soil materials and performance
    • evaluation of WFDs
  • 9.4 Model Simulations
  • 9.4.1 Numerical model validation (Hydrus 2D/3D model)
    • 9.4.1.1 Comparison between model simulations and field experimental data
    • 9.4.1.2 Predicting theoretical sensitivity thresholds of WFD
  • 9.4.2 Determining FS placement depth by analytical approach (control mode)
  • 9.4.3 Evaluating irrigation amounts and intervals by FS (mechanical mode)
  • 9.4.4 Sensitivity of WFDs under flood irrigation
  • 9.5 Guidelines for using wetting front detectors to schedule irrigation
  • 9.5.1 Use of the FullStop wetting front detector
  • 9.5.2 Use of the 90-cm-long Tube Detector (90TD)
  • 9.6 Summary and conclusions
  • 9.7 Future research needs
  • References
  • Appendix

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