3. UKULINGA FIELD TRIAL
3.4 Trial Management and Monitoring
3.4.2 Soil moisture monitoring
The third project goal was to assess the performance of ASFI compared to a reference drip system. One of the means of evaluation was to assess the trends in soil moisture over the crop
cycle. Therefore, as part of the trial monitoring, soil water monitors were required at specific areas in the trial to obtain soil moisture readings to be used in the soil moisture evaluation.
The soil water monitors used in this trial were watermark sensors (Thomson and Ross, 1994).
To compare the soil water in absolute terms was difficult because of spatial variation of the soil and crop rooting structure and because there were only a few sensors available for the trial as a result of budget constraints. Moving the watermark sensors a few centimetres up or down the furrow or drip line could significantly alter the results. The goal, therefore, was to rather assess trends where increasing wetness indicates over-watering or decreasing wetness indicates under-watering and to compare these trends to trends obtained from SAsched. A direct comparison of soil moisture of the two irrigation systems would therefore not be accurate. A number of factors were considered in the placing of the watermark sensors. It was decided that the watermark sensors would be placed in three different sets along a set furrow:
one near the start, another near the middle and another near the end of the furrow. The three sets along the furrow would allow for analysis of the wetting pattern at various points along the furrow. One set of watermark sensors were installed in the drip plot for comparative reasons.
The next important issue was selecting a representative furrow and drip line which were near each other to help minimise the effects of soil variability. The first factor that was assessed was the furrow slope. Due to the furrow slopes being steeper than the optimal value, the furrows with the least slope were selected as potential monitoring furrows. A field check was then conducted on these furrows to ensure that the crop growth for the furrow and accompanying drip line were representative of the area, to eliminate biasing the results of either irrigation system. These checks ensured there was consistency in crop growth between the selected drip line and furrow. The fifth furrow from the top in Blot B on the first ASFI lateral and the fifth drip line on the first drip irrigation lateral, were selected as the monitoring furrows and drip lines respectively.
Due to the different response times of the three channels of the hobo logger, each channel was individually calibrated in the laboratory using watermark sensors. The watermark sensors were also calibrated in the laboratory. The calibration resulted in the exponential relationship, represented in Figure 3.21, with the resultant exponential equation used to calculate the soil water tension (mm). In the very dry soil range, a small change in voltage (e.g. between 1.5 volts and 2 volts) results in a large change in the tension. For example, 1.5 volts represents 17
182 mm (or 171.82 kPa), 1.7 volts represents 40 860 mm and 1.9 volts represents 347 176 mm. Thus, watermark sensors are only valid for a tension between 1000 mm and 20 000 mm (Thomson and Ross, 1994).
0 5000 10000 15000 20000 25000 30000 35000 40000
0 0.5 1 1.5 2 2.5
Response (V)
Soil Water Tension (mm)
Figure 3.21 Watermark calibration
The next step was to identify regions where a variation is soil moisture was likely, using a model called Hydrus 2D (Šimůnek et al., 1999). Hydrus 2D is a numerical model used to predict the water transfer processes connecting the soil surface and the groundwater table.
Hydrus 2D also has a sink term to account for water uptake by the plant roots (Šimůnek et al., 1999). Initially, assumptions such as root infrastructure were made and parameters were input into the Hydrus 2D model. This gave rough guidelines as to where water activity and changes in water content could be recorded by the watermark sensors. The results from Hydrus 2D were used to determine where there would be a response to the irrigation event between the furrow/dripper line and the root system. However, moving past the root system and further away from the furrow/dripper line, the irrigation event had little to no impact on the soil moisture. It was therefore decided to adopt the watermark arrangements in Figure 3.22. The arrangement would give an indication of the vertical and the horizontal flux. It would hopefully also give an indication of the deep percolation and compare this for the SSD and ASFI system. However, both the water flux and deep percolation are highly dependent on the localised soil conditions and variations in the plant root structure. The soil depth is approximately 55 cm, therefore the lower watermark would give a good indication as to the deep percolation.
Y = 192.67e3.0909x R2 = 0.9834
The watermark sensors were installed as illustrated in the furrow cross-section in Figure 3.22 in three positions along the length of the furrow, one near the start, one near the middle and one near the end of the furrow. A 6 cm diameter hole was augured directly beneath the furrow to a depth of 48 cm below the natural surface level. This was done by placing a rod across the top of the furrow and auguring until the 48 cm marker on the furrow was parallel with the rod.
The watermark sensor was then placed in the hole using a PVC pipe with the same diameter as the top of the watermark sensor. The cables for the sensor were run through the pipe. A portion of the soil that was augured was then wet slightly and made into a paste. This paste was placed around the watermark sensor and compacted to ensure that there was contact around the entire sensor. Small amounts of soil were then placed on top of the sensor and then compacted until the depth of the hole was 33 cm below the natural surface level. The second watermark was then placed in the hole, with the hole being covered with the above-mentioned procedure. A hole was then augured 20 cm to the side at a depth of 33 cm and also covered in the above procedure. The cables from the watermark sensors were then connected to a Hobo logger, which is placed in a watertight electronic box. The Hobo logger was used to log the results from the watermark sensors. The Hobo logger has four ports, three of which are used in this experiment for the watermark sensors. The logging interval was set at two hours which was selected as a shorter logging interval may result in polarisation of the loggers as described in Allen (1999). The configuration of the Water mark sensors in the drip treatment is shown in Figure 3.22b. The installation procedure was the same as for the furrow system. The first hole was augured next to the drip line, 5 cm away from the dripper. The watermark was placed in this position and not directly beneath the emitter so as to give representative soil moisture parallel to the drip line. Ideally, the wetting pattern of the emitters would overlap, resulting in a relatively consistent wetting profile down the drip line.
Figure 3.22 Watermark sensor installations for a) ASFI and b) SSD irrigation