Evaluation and soil-specific calibration of the EnviroSCAN soil moisture sensor…

technique. Therefore the equipment and techniques developed by the manufacturers of EnviroSCAN that are claimed to eliminate this problem were used.

If care is taken in the installation of the access tubes and the sensors are carefully calibrated and sealed inside the access tube, these potential problems become insignificant (Paltineanu and Starr, 1997). Moreover, the calibration equation used in the software of the EnviroSCAN system is deemed "universal". The system is shipped with a default (uncalibrated) equation to the user. Due to the large variability in soil types found in samples from the soil around the tubes, which were taken in order to determine the volumetric water content in wet, moist and dry soil, there is some evidence that this is not so. In addition, there is concern about the influence of soil salinity and soil temperature on the sensor readings. Thus, if the uncalibrated equations are used to determine the amount and time of irrigation for crops, there is a strong likelihood that the underestimation or overestimation can seriously impact crop yields. However, EnviroSCAN sensors do not automatically produce an accurate estimate of individual soil water content measurements for all soils. Therefore a simple soil calibration of these sensors is required to obtain accurate soil water content. Leib et al. (2003), Fares et al. (2004) and Jabro et al. (2005) found that the site-specific calibration of the EnviroSCAN sensor is essential for the most precise soil moisture content measurements as well as to improve the sensor’s accuracy and performance, because statistical analysis supported considerable discrepancies between soil water contents estimated by the site-calibration and uncalibrated equations. Meanwhile, the software provided with the EnviroSCAN allows the users to enter their own calibration constants/equations.

Soil moisture and its variation within IMZs were measured daily by the EnviroSCAN sensor, and soil samples were taken during the irrigation season for soil-specific calibration and the evaluation of the possibility of using EnviroSCAN sensor as a continuous, multiple depth and reliable method. In this study, soil moisture sensors were located at multiple depths of 10, 20, 30, 50 and 70 cm as shown in the Figure 3.11. The sensors were connected by cable to an ISM-modem,

which powers the probes with a solar panel. All sensors in the same access tube shared one electronic measuring circuit, located at the top of each tube. Three EnviroSCAN soil moisture sensors were placed in three IMZs where the locational coordinates of soil moisture sensors (number of solenoid valve, angle) were (8,108 °), (3,127 °) and (13,176 °) as shown in Figure 3.10.

Care has to be taken during installation as air gaps can dramatically alter the response. The sensor installation process and operational procedures were carried out according to the manufacturer`s recommendations and instructions (Sentek, 1995; www.sentek.com.au). Soil moisture data at different depths were received on a mobile phone by sending an SMS-message to PLC.

Field evaluation and soil-specific calibration of EnviroSCAN were done at the study site by taking soil samples (as shown in Figure 3.13) and calculating the water contents by mass. Then water contents by mass were converted to volumetric values using soil bulk density values available at the FAL (ρb = 1.42 gr/cm3). Two replications of soil samples were taken at the multiple layers of 0 to 10 cm, 10 to 20 cm, 20 to 30 cm, 40 to 50 cm and 60 to 70 cm. The distance between the EnviroSCAN access tube and the auger sampling points was between 1 and 2 m.

Immediately after the soil samples had been e talen, the EnviroSCAN readings were done by sending a mobile-SMS to the control unit and receiving a text message from PLC on mobile phone.

Given the nearly uniform soil texture at depths of 0 to 40 cm and 40 to 70 cm at the study site as shown in Figure 3.2, it was decided to find two soil-specific calibration equations for these two layers.

Figure 3.13: Soil sampling for irrigation scheduling and soil-specific calibration of the EnviroSCAN soil moisture sensor

3.2.3.2 The field tests of data transmission and power supply

Wireless communication was tested for different distances between the data transmission unit and the central ISM-modem, such as 100, 200, 300, 350 and 400 m. Given battery voltage dissipation, the minimum battery operation voltage needed to start battery recharging and to ensure that the the solar panel is large enough for the self-recharge of the batteries, battery voltage was measured daily.

3.2.3.3 Validation of the AMBAV model

Obtaining soil moisture information through field practice like soil sampling and using soil moisture sensors is time-consumin, difficult and expensive. Also the validation of soil water balance models and the evaluation of the quality of the model predictions at field-scale and in the active root zone of grass require time-series of in situ measured model outputs. In this case, the validation of the AMBAV CWB_model as a cheap and reliable method to measure the soil moisture content was evaluated by comparing the soil moisture simulated by the AMBAV model with the moisture of soil samples (observed data) which were taken during the measuring period.

In order to validate the model, comparisons were made between the simulated and observed values and three statistical tests were performed. These tests are the coefficient of determination (R²), Mean Absolute Relative Error (MARE) and prediction efficiency (PE) index. The index MARE is computed as:

(3.8) Where SWCi, observed and SWCi, simulated are the observed and simulated soil water content in the active root zone of crops on i^th time, i the index of the time that is taken as one time in the study and N is the total number of times for which observations are taken. The index PE is computed as:

(3.9) Where SWC observed is the arithmetic mean of the individual observations of SWC in the active root zone of the crop.

3.2.4 Irrigation system and its modification

A two-span and commercial centre pivot system with an overhang, located at the FAL research field and with a total length of 90 m was used to irrigate an area of 2.54 ha during summer 2006 (Figure 3.10). The irrigation system could be operated in forward or in reverse, with and without applying water, which is pumped from an underlying network. The pressure at the pivot was regulated to 220 kPa. The first step taken was the modification of this present commercial system to a site-specific or Precision Mobile Drip Irrigation (PMDI) system by making some modifications. With due attention to the effect of CP speed on the water application rate, linear CP speed at end of 2^nd span that is appropriate to the CP speed stated in percent on the CP control box was measured to calculate an increase and a decrease in the water application rate with CP speed.

The irrigation system had to be modified so that the desired water level could be applied to IMZ (Camp et al., 1998). The basic requirements established for the modified water application system are that the system must apply water depths needed to replace crop evapotranspiration, while it was being moved, to the management zones with different TAWC, based on data stored in a database. The variable-rate application system would be achieved by modifying this commercial centre pivot irrigation system equipped with a computer-aided management system.

A variable rate MDI system was designed and installed on an existing 38-m 2^nd CP span. The VRI system used the pulse technique described by Perry et al, (2003) by solenoid valve to apply the desired water application rate. Irrigation system modification was divided into six parts as:

- Programmable Logic Control (PLC) - Position Encoder

- Solenoid valves (SV)

- Irrigation segments and drop tubes

- Calculating number of emitters required on the drop tubes and length of drop tubes - Evaluation of emitter performance

3.2.4.1 Programmable logic control

The 2^nd span of CP was controlled by the pulsing technique using PLC and SV as DIC by EIB-BUS (Europäischer Installationsbus) for variable-rate water application. EIB-EIB-BUS is a free-cost and simple communication protocol that can control many SV together with one cable. All electrical output devices including SV, position encoder, etc., were controlled by a prototype PLC and EIB-Bus communication which were developed by Büro für Steuerungstechnik und

Schaltanlagen (www.schudzich.de). The control unit was mounted on the CP about 3 m from pivot point (Figure 3.9). PLC was programmed to control 64 SV including 16 boxes (every box to control 4 SV). But in this study only four boxes installed on 2^nd CP span including fifteen SV were used. The integrative PLC had an on-board PC as data logger, which can read a saved data file and allows changes in the system information and can convert the map of control to on/off setting in the directly-addressable solenoid control registers of the PLC. The main features and flowchart of this PLC are shown in Figure 3.14. When the location had been determined and a zone boundary was crossed, the program checked the expected application map, the appropriate table lookup was performed and the solenoid registers set accordingly. The application rate was varied by different SV pulsing levels from 0 to 100 % of 100 seconds intervals as inserting and removing the pin provided a time-averaged application rate ranging from about 0 to 100 % of maximum sprinkler flow rate. For example for pulsing level of 70 %, SV were 70 seconds opened and 30 seconds closed (1 second for every 1 % pulsing level).

Field tests of PLC validation and uniformity performance: Tests of water distribution were conducted using catch-cups in the direction of system travel (vertical distribution) and along the length of CP (horizontal distribution) a) to examine and evaluate the validation of the PLC and system modifications, b) to ensure that the pulsing technique produces the desired amount of irrigation under different pulsing levels and c) to examine water uniformity over the entire separate IMZ. This water distribution will be used as the comparison baseline for the evaluation of the effectiveness of the PLC for variable-rate water application. To better visualize a comparison of different pulsing conditions, tests were conducted using a sprinkler (NELSON R3000 rotator, U4-8

°, blue plate) before installing drop tubes and under relatively light wind. Seven pulsing levels and two CP speed levels were considered to test PLC validation. The tests were run while the machine was operating under 15 and 30 % of CP speed and programmed on three different pulsing settings of 10-40-70 %, 30-60-90 % and 100-100-100 % of pulsing levels. Three pulsing levels within each setting were considered for IMZ1, IMZ2 and IMZ3, respectively. Uniformity tests are currently being conducted to ensure that the irrigation system is applying an even distribution of water over the entire span of the CP lateral. In this case average uniformity of horizontal and vertical distribution at different pulsing levels and CP speeds were calculated. Uniformity tests were conducted based on a new ASAE standard (2003), which has been updated, and DIN EN ISO 11545 (2001) using the formula developed by Heermann et al (1992):

(3.10)

Figure 3.14: Flow-chart of the PLC

where CUHH is the Heermann and Hein uniformity coefficient, n is number of collectors used in data analysis, i is a number assigned to identify a particular collector beginning with i = 1 for the catch cup located nearest to the pivot point and ending with i = n for the most remote catch cup from the pivot point, Vi is the volume (or alternately the mass or depth) of water collected in the i^th catch cup; Si is the distance of the i^th catch cup and is the weighted average of the volume of water caught by all collectors. The tests were conducted during early morning hours, the wind speed during test time was less than 1.5 m/s and considered to have an insignificant effect on distribution. Water was collected in two horizontal and three vertical rows of 12 catch-cups placed 40 cm from the ground and spaced 1 m between catch-cups with two replications as shown in Figure 3.15. Three catch cup collection arrangements were simultaneously placed on the field as every collection collected water under the different conditions of three zones. Tests were conducted between 90° and 130° where all three IMZ had enough width in both horizontal and vertical distribution.