Hints and tips

Except for the Sentry 200AP, only access tubing from the manufacturer should be used. The diameter and wall thickness of the tubing are not found from other suppliers. The inside diameter of the tubing is carefully controlled so that the sensors will self-centre when inserted in the tube. The wall thickness is also controlled to minimize variations along a tube and between tubes. Because the measurement volume is small, deviations from precise centring or in distance from the sensor to the soil will cause variability in the measurements. The Sentry 200AP is designed to work with Schedule 40 PVC plastic water pipe of the kind commonly found in the United States of America.

5.3.2. Number of access tubes needed for a given precision

Studies of the number of access tubes required to determine the soil profile water storage to a given precision have shown that at least 75 times more Sentek access tubes would be needed to determine a profile water content to a given precision than would be needed to determine the storage to the same precision using the neutron moisture meter (Table 5.2). Thus, if six NMM access tubes were sufficient to determine a field plot water storage to a given precision,

it would require at least 450 Sentek system access tubes to deliver the same precision. Data from the PR1/6 were even more prone to noise, resulting in a requirement for thousands of access tubes to reach the same measurement precision. Results from Evett and Steiner (1995) indicate that the number of access tubes needed for the Sentry 200AP would also be large. However, Table 5.2 should not be interpreted to mean that a large field, catena or watershed could be adequately represented by only one or two NMM access tubes. The large scale variation of soil properties, slope, aspect and vegetation implies that a representative sample over a larger area would require access tubes in each identifiable representative subarea. Discussion of sampling strategies for areas beyond the field plot size is well beyond the scope of this work.

Table 5.2. Calculation of the number of access tubes (N) needed to find the mean profile water storage in a field to a precision d (cm) at the (1 – α) probability level (μα/2 is the value of the standard normal distribution at α/2) for a given field measure standard deviation (S, cm) of profile storage

α = 0.05 0.10

μα/2 = 1.96 1.64

d (cm) = 1 0.1

Method Soil condition S (cm) N N

Diviner 2000 Irrigated 1.31 6.6 464 Dryland 2.42 22.5 1584 EnviroSCAN Irrigated 1.52 8.9 625 Dryland 2.66 27.2 1914 Delta-T PR1/6 Irrigated 2.72 28.4 2002 Dryland 12.16 568.0 40006 Trime T3 Irrigated 0.75 2.2 152 Dryland 2.38 21.8 1533

Sentry 200APa _{Overall 3.78 54.9 3866}

Gravimetric Irrigated 0.45 0.8 55

Dryland 0.70 1.9 133

NMM Irrigated 0.15 0.1 6

Dryland 0.27 0.3 20

a _{Estimated from data of Evett and Steiner (1995).}

5.3.3. Tube installation in problem soils

Access tubes may be installed in gravelly, stony or very hard soils by drilling an oversized hole and using the slurry technique as described in the chapter on the neutron moisture meter. However, this is not recommended, due to the small volume of measurement of the capacitance devices. The slurry material, even after drying and soil water potential equilibration with the surrounding soil, may have a considerably different water content than the surrounding field soil. This will bias the water content readings.

5.3.4. Customizing reading depths

For the Sentek systems, depths at which readings are taken are determined by the elevation of the top cap. If the bottom skirt of the cap is flush with the soil surface, then readings are centred at the 10 cm depth and at increments of 10 cm below that. With the EnviroSCAN system, sensors may be placed on the backbone at each 10 cm interval, or some intervals may be skipped, although this is not recommended. Another way to customize reading depths is to

place a spacer between the bottom part of the top cap and the top of the sensor backbone (for the EnviroSCAN) or the Diviner 2000 cap, thus elevating all reading depths by a distance equal to the length of the spacer. For instance, there is a limitation of 16 sensors per EnviroSCAN backbone, which would ordinarily allow readings at 10 cm intervals to only 1.6 m depth. To get around this limitation, ten sensors may be placed at 20 cm intervals on the backbone, beginning at the 20 cm depth position (e.g. 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 cm positions). The backbone is inserted into the access tube and readings are taken. Then the backbone is raised by 10 cm, using a spacer to hold it in position, and readings are repeated, resulting in readings at intermediate depths (e.g. 10, 30, 50, 70, 90, 110, 130, 140, 170 and 190 cm). A similar procedure may be used to change reading depths of the PR1/6. In all cases, the user should be aware that sensors that are elevated to or above the soil surface will not provide useful readings.

5.3.5. Moisture in access tubes

Liquid moisture in access tubes will have a strong effect on readings due to the small sensed volume and the nearness of moisture on tube side walls to the sensor. The access tube system is designed to avoid moisture buildup by using a bottom plug and a top cap sealed with an O- ring. However, if there is any question that a tube might contain liquid, it should be wiped dry. For long term installations using the EnviroSCAN sensors, moisture buildup in tubes can be problematic. The electronic circuit boards in the sensors and the communications circuit board at the head of the sensor string are not completely sealed and may develop corrosion. Also, the sensors are connected via a ribbon cable using a press fit pin connector whose pins push through the cable insulation to make contact with the wires within. Corrosion may also develop at this connection. Therefore, careful attention to sealing of the access tube is important, as is periodic maintenance and checking for moisture. The use of water absorbent gel packs can retard the buildup of moisture.

5.3.6. Salinity (bulk electrical conductivity) effects

Soils irrigated with brackish or saline water, other salt affected soils, and soils irrigated non- uniformly (e.g. most drip irrigated soils) exhibit large variations in bulk electrical conductivity in both time and space. Typically, values of bulk electrical conductivity (BEC) will increase during an irrigation or crop growth season. Because all of the capacitance systems are sensitive to variations in BEC, and none of them provide for corrections for this problem, none can be recommended for use under such conditions.

5.4. TAKING READINGS