It is hoped that the explanation below will enable users of the Across the Fenceline site to interpret the data for their own purposes. If anything is unclear, or if the data is behaving in unexpected ways, please contact the fenceline webmaster, who will do his best to clarify or provide further explanation. See below for tips on using the charts and web browser compatibility.
As explained on the Measurements page, the sensors measure soil water content on a volume basis. Making the assumption that the water content measured at any particular depth is representative of that for the whole layer, then multiplying the water content by the layer thickness (in mm) results in an estimate of the water stored in that layer (in mm). The depths of the measurements and the thickness of the layers they are assumed to represent are illustrated in the diagram below.
[Note: highlighted terms, such as water content, have clickable links to their definition in the glossary.]
Total Soil Water Stored
The measured water content
includes all the water in the soil, including that which plants cannot extract because it is very tightly held by the soil particles. In soils with a high clay content, the amount of water than cannot be extracted can be very large. Therefore, in a typical Australian duplex soil, such as those around Harden, the amount of water that plants can extract from the soil decreases with increasing depth.
The following chart gives an example of total stored water in each layer and how it has changed over time since our current measurements began.
The soil water stored in the deepest layer (1.5 to 2 m) is graphed on the bottom. The water stored in each shallower layer is then added to that, and so on. The line bounding the top of the shallowest layer also indicates, on the left hand axis, the total amount of water stored in the soil between 0.05 and 2.0 m. [Note: in the live charts it is possible to hover over the chart and read off the amount of water in each layer.]
As would be expected, deeper layers show less change through the season, because less water reaches them to be stored and fewer crop roots reach them to extract water.
At the Taralgon site, we have a reasonable estimate of the soil's lower limit for each measurement depth, based on previous measurements (although because of variation from site to site in a paddock the estimates are likely to have some uncertainty associated with them). The estimated lower limits, converted to soil water storage for the layers used here, are:
|Layer depth (m) :
||0.05 - 0.35
||0.35 - 0.65
||0.65 - 1.0
||1.0 - 1.5
||1.5 - 2.0
|Lower limit water content (mm/mm) :
|Lower limit storage (mm) :
Subtracting the lower limit storage value from the total storage for each layer, results in an estimate of the available water stored in each layer, as shown in the next chart. This is our current best estimate of the amount of water available to plants in each of the soil layers.
Note that the total available water in the soil (0.05 to 2.0 m) is about half of the total stored water shown in the previous chart for this soil, showing that in soils with a high clay content there is considerable water unable to be extracted by crops.
Change in Stored Water
Because the lower limits are not known with sufficient certainty for all measurement sites, we have chosen to present the data not as available water as traditionally defined, but as the amount of water that is in each soil layer relative to how much was there at the end of the 2013 growing season. The amount left by the previous crop could be larger that the lower limit for a range of reasons: in deeper layers, the crops roots may have not reached them, so the water was left behind; in shallow layers, rainfall after senescence may have topped up the soil water stored after crop uptake ceased but before our first measurements were made.
The following chart shows the effect of treating the data in this way for our example site.
Note that the total water that has been stored (0.05 to 2.0 m) at this site since the lowest values were observed in December 2013 is about 100 mm less than the estimate above of available water. The differences are mainly in the deeper layers, where the roots of the 2013 crop (wheat) may not have been able to reach. The estimated lower limits given in the table above were established when the crop being grown was lucerne, which, because of its perennial nature, is able to send roots deeper and extract more water than an annual crop.
The advantage of presenting the data this way is that we can say with certainty that, for a crop that performs as well as the wheat crop in this paddock in 2013, there is at least 80 mm of water available to it at the time of the last measurement shown above (3 September 2014). If this year's crop performs better than the 2013 crop, or if its physiology allows its roots to explore a greater depth of soil, there may be up to another 100 mm available to it.