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Interpreting the Measurements

Gypsum block measurements

Gypsum blocks measure soil water potential. At this stage, because the sensors being used are a new design, they have not been calibrated. The measurements are therefore related to soil water potential, but are not presented in the normal units of soil water potential (namely kPa). Nevertheless, the measurements can be interpreted qualitatively in the same way.

The measurements are presented for each individual depth at which measurements are being made (0.3, 0.6, 0.9, 1.2, and 1.5 m below the soil surface).

Soil water potential is related to soil water content, but not uniquely; the relationship is not linear and varies with soil type and from place to place within a given soil type as individual soil properties vary. Unlike soil water content measurements, water potential measurements cannot, by themselves, indicate how much water is stored in the soil and available for crops. Soil water potential measurements are, however, much easier and cheaper to make that soil water content measurements and still provide much useful information.

Soil water potential provides an absolute measure of how wet or dry the soil is:

• a value of zero indicates that the soil is saturated - ie. water will ooze from the soil;

• a value of ~10 kPa indicates that the soil is at field capacity or drained upper limit - this is the practical upper limit of wetness; a well-drained soil, would rarely spend more than a day or two wetter than this ;

• a value of 1,500 kPa indicates that the soil is at wilting point - ie. so dry that plants cannot extract more water from it.

In contrast, a measurement of soil water content can only be interpreted in this way if the values of water content at these three limits (saturation, drained upper limit, and wilting point) are known in advance, which is usually not the case.

A complication in interpreting the output from gypsum blocks, and most other soil water sensors (including the very expensive ones) is that they are temperature dependent. Their output is affected not only by water content, but also by the soil temperature at the time of the measurement. For more details, see Temperature Effects below.

Fiberglass block measurements

Like gypsum blocks, fiberglass blocks also measure soil water potential. They have been calibrated and the measurements presented here are expressed as kPa.

The interpretation is the same as that described above for gypsum blocks.

Because the fiberglass blocks are part of a separate project aimed specifically at measurements around the bottom of the annual crop rooting zone (see brief project description under Supporting projects), measurements are only made at 0.9, 1.2 and 1.5 m below the soil surface.

Neutron moisture meter measurements

The neutron moisture meter measurements have been aggregated into 3 layers:

• 0 to 0.6 m below the soil surface
  This is the depth interval in which the soil moisture content is most dynamic; it responds quickest to rainfall, and is also dried quickly by plants as well as by evaporation even when crops are not present.

• 0.6 to 1.4 m below the soil surface
  This second layer extends to the depth below which the roots of annual crops are unlikely to grow. The two top layers are therefore where the water available to annual crops resides. Water that passes below 1.4 m will be lost to annual crops, and only likely to be extracted by lucerne or other deep-rooted perennials.

• 1.4 to 3.0 m below the soil surface
  Wetting in this zone represents water that is no longer accessible to annual crops; drying is only likely by deep rooted perennials such as lucerne.

The aggregated data are presented as the total amount of water stored in each of these layers, in mm of water. This is obtained by adding the water contents measured in each layer and multiplying the result by the depth of each layer (in mm).

The neutron measurements are therefore able to be interpreted directly as amounts of water stored in the soil. For example, if neutron measurements were made immediately before and after a rainfall event of, say, 20 mm, on a dry soil, before there was time for any evaporation, then the change in water storage in the first layer should be 20 mm.

Temperature effects

A complication in interpreting the output from gypsum and fiberglass blocks, and most other soil water sensors (except for the neutron moisture meter) is that they are temperature dependent. Their output is affected not only by water content, but also by the soil temperature at the time of the measurement. Soil temperature changes in response to air temperature, although the magnitude of the change in soil temperature decreases with depth.

At shallow depths (less than about 200 mm), soil temperature reaches a peak in early afternoon and a low around dawn. Because of the time it takes for the soil to heat up and cool down, these daily variations are not seen below 200 mm, and so aren't a concern for the measurements at this site. However, the seasonal oscillation in air temperature shows up at much greater depths, because in 6 months heat can travel down as far as 2 meters into the soil and cause a temperature rise. The seasonal oscillation of gypsum block output caused by temperature can be quite large, and is evident in the data from the Condobolin sites.

The consequence of the temperature effect on gypsum block measurements is that some extra care is required when interpreting their output:

• in spring and summer, a gradual wetting in gypsum block output is more likely to be a result of increasing soil temperature than an increase in soil wetness

• in autumn and winter, a gradual drying in the gypsum block output is more likely to be a result of decreasing soil temperature than a decrease in soil wetness