4. CONCLUSIONS AND RECOMMENDATIONS
2.1 Soil Moisture Monitoring and Characterisation
In order to assess the effect of different innovations on crop water availability, a range of instruments designed to acquire information relating to water balance components were placed in the gardens of the six participating farmers. These complemented a detailed Potshini Catchment monitoring network which also provided supplemental data
#
#
3 0 3 6 Kilometers
Potshini sub-catchment in Emmaus
N
Commercial farmer's boundary Roads
Rivers Dams
# Gauging sites
Commercial farmers
Lindequespruit river Smallholder farmers
Potshini Area
where necessary (Kongo et al., 2006). The instruments installed included manual reading rain gauges, Wetting Front Detectors (WFDs), nested Watermark® sensors (WMS), and Capacitance Probe tubes. Descriptions of each instrument’s function are presented in Table 3.1 and a diagram showing instrument configuration is presented in Figure 3.2. An explanation of the innovations chosen by farmers for comparison to a control and the instrumentation used for individual trials is presented in section 2.3.
TABLE 3.1
Description of technical instrumentation function.
INSTRUMENT FUNCTION Monitoring Strategy
Manual Rain
Gauge Used to measure rainfall (mm). Monitored daily by farmers.
WFD
Provides visual signal when “wetting front” has reached a certain depth. Works on the principle of flow line convergence. Water moving from surface downwards through soil is concentrated when water molecules enter the wide end of WFD funnel. Soil in funnel becomes wetter as funnel narrows. Funnel shape is designed such that soil at the base of the funnel's bowl reaches saturation when wetting front outside funnel is at a similar depth. After saturation, free water flows through a sand filter into a reservoir where a float is activated, causing a plastic indicator to pop up above ground. As wetting front dissipates, water is withdrawn from the funnel through capillary action. WFDs are placed as a pair, one about half way down the managed root zone and a deeper one near the bottom of the managed root zone (Stirzaker et al., 2004).
Recorded by farmers when activated by rainfall
or irrigation.
WMS
WMSs measure water tension in the soil, which increases with decreasing water content. WMSs consist of a fine aggregate mixed with a gypsum buffer, held inside a permeable membrane and a perforated stainless steel sleeve. The sensors are buried in contact with the soil and attain equilibrium with the soil moisture. Electrodes are embedded in the aggregate/gypsum matrix and the electrical resistance between them is measured to determine soil moisture. The varied resistance is calibrated against known values and reported in terms of soil water tension. Resistance decreases with decreasing soil moisture. Signal may fluctuate with soil
temperature changes (Irrometer Co., 2006).
Electronically recorded every 30 Minutes. Data
downloaded for post processing by researcher
every 4 weeks.
Capacitance Probe
The Capacitance Probe measures near surface changes in soil water content. It functions by lowering the probe to certain soil depths through a tube, and sending very high frequency (~GHz) electrical signals into the soil. The reflected signal is a function of soil water content. Facilitates water content profiling by recording water content information from multiple depths (Wallace, 1996).
Weekly recording at depths 15, 30, 45, 60 and 75 cm undertaken by researcher and local
field assistant.
20 cm
40 cm
80 cm DEPTH 0 cm
= WMS
Capacitance Probe access tube
= WMS data logger 20 cm
40 cm
80 cm DEPTH 0 cm
= WMS
Capacitance Probe access tube
= WMS data logger
FIGURE 3.2: Idealized diagram of Wetting Front Detector activation process (altered from Stirzaker et al., 2004). Idealized placement of Watermark® sensors and Capacitance Probe access tube shown in far left diagram. Soil temperature sensor at 20 cm depth not shown.
Drawing not to scale.
The WFD is a mechanical instrument which activates a pop-up signal when water in the form of a “wetting front” resulting from rainfall or irrigation passes a certain depth, as illustrated in Figure 3.2 (Stirzaker et al., 2004). WFDs can be useful in terms of participatory learning because they are a technical tool that is not electronic and they provide an immediate visual signal, allowing farmers to see when the soil has become saturated to certain depths. Ideally this knowledge can be used to adjust irrigation amounts and timing to maximize Water Use Efficiency by determining the amount of water required to reach rooting depth without infiltrating beyond depths accessible to crop roots. The WFD can also be used to monitor solutes and nitrates in irrigation water at specified depths, which can indicate consistent under irrigation (resulting in saline water) or over irrigation, indicated by dramatic changes in nitrate concentration (Stirzaker et al., 2007). However, WFD data collection in this study focused solely on WFD activation events and irrigation water sampling was not incorporated into the learning process.
While the WFD was originally developed as an irrigation control tool, a Water Research Commission project using participatory research showed that its primary value was the role it played as a learning tool (Stirzaker et al., 2004). Stirzaker et al. (2004) reported that by treating the instruments as learning tools, WFDs were successfully used to create a dialogue between farmers and researchers, challenge the perceptions of both parties and stimulate changes in irrigation practices. During that study, WFDs did not answer all of the farmers’ questions, but they helped them formulate their next set of questions.
Changes reported by farmers included irrigation timing as well as the quantity of water applied. In some instances farmers’ acceptance of the WFD as a valid decision support aid in irrigation management required three years of experimentation, while other farmers used the WFDs to change their practices soon after their introduction. Stirzaker et al. (2004) concluded that on-going support in the use of WFDs is important for ensuring that the technology not be discontinued after its introduction. This is especially true when working with small scale farmers because substantial input is required for them to acquire skills needed to make effective use of the WFD
WMSs, used to measure soil water tension, and the Capacitance Probe, which measures water content, provide digital signals in response to different soil water characteristics at different depths which can be stored with electronic data loggers. Data collected with the Capacitance Probe was incomplete due to equipment malfunction throughout the season, and was not used for detailed data interpretations. Information from technical tools was supplemented with laboratory generated soil analyses (hydraulic conductivity and nutrient analyses) and in-field soil hydraulic characterizations (double ring and tension disc infiltrometer tests, Lorentz et al., 2001) in two of the six gardens. Curves were fitted to hydraulic characterization data using the van Genuchten equation and used to strengthen interpretations of soil water tension data.