The Irrigation Season Has Started

The irrigation season has started across the south east. Crop water use has increased steadily in the past few weeks, particularly in the upper south east, and there has been little to no effective rainfall since the first week of September.

Keep an eye on daily ETo figures via the link to the SENRMB weather station network at left (nrmWEATHER). Suggestions on how to use this information can be found on the PIRSA website, but regular updates will be provided on this site. You can also look at the soil water graphs posted regularly on this site. 

It is important at this time of the year to inspect all aspects of your irrigation system and ensure that it is operating according to design - do not allow maintenance problems to cause you to fall behind in your irrigation schedule.

To start with, make use of a fundamental system component - your water meter. A water meter provides simple information, but it is information that can be used to assess a variety of things. So it is important that you are familiar with your meter and know how to read it properly.

For example, to calculate the depth of irrigation applied (mm) you can use the following formula:

                    Depth Applied (mm)     =     Total Volume Applied (KL)
                                                                               Area (ha)   x   10

To find the Total Volume Applied simply subtract the start meter reading from the end meter reading - the time taken doesn't matter, just as long as you know the area irrigated between readings.

This calculation does not account for under or over-irrigation caused by poor system uniformity, but is useful for considering the level of water input against crop water use (i.e. daily ETc is given in mm). Particularly for centre pivot applications, this calculation will indicate whether the actual depth of irrigation applied is consistent with the designed application depth for the rotation speed used. 

Keep a record of meter readings through the season and come back to this site for more ways in which to use this information.

Michael Zerk
Rural Solutions SA

Broadacre Application of Soil Water Monitoring Devices (SWMDs) - updated October 5

Some of the tools that are used to assist irrigation management are starting to be used in broader applications. In particular, an assortment of Soil Water Monitoring Devices (SWMDs) - commonly used for irrigation scheduling - have been trialled in rainfed crop rotations to help determine crop water use efficiency and the effect of different soil management strategies on rainfall capture and storage.


Datalogger in wheat crop - Bordertown, October 5. 

source: M Zerk






The following graphs show changes in soil moisture status for the past month in two wheat crops. Graphs 1 and 3 display soil moisture content (i.e. how much water is in the soil, expressed in %), whereas Graphs 2 and 4 display soil moisture tension (i.e. how much effort must be applied to extract the water, expressed in -kPa). Content is recorded using SM200 sensors connected to MEA t-Bug loggers and tension is recorded using Watermark sensors connected to MEA g-Bug loggers. At both sites, sensors were installed within a week of sowing at positions 10, 20, 30 and 40 cm below the surface.

Graph 1 (soil water content, %) - Bordertown wheat. Early signs of flowering near the monitored site. Crop water use at 10 cm (red) has slowed further and the trace is now quite flat. Moisture content at the remaining depths is little changed, apart from the small increase at 20 cm (blue trace) that may be a result of capillary rise.



Graph 2 (soil water tension, -kPa) - Bordertown wheat. The 10 cm (red) trace is at approximately -80 kPa - the saw-tooth pattern perhaps a result of daily water use being partially replenished by capillary movement in the evening. Soil water tension at all other depths suggests that, at this stage, the crop has ready access to water. 



Graph 3 (soil water content, %) - Wolseley wheat. The rate of water use has declined at 10 (red), 20 (blue) and 30 (green) cm, but increased at 40 cm (purple). It will be interesting to see if the crop can draw to below 30 % moisture at 20 and 30 cm if no rain is received in the coming weeks.



Graph 4 (soil water tension, -kPa) - Wolseley wheat. Soil water tension at the first three depths have passed the nominal -80 kPa threshold, with the 40 cm (purple) trace approaching -60 kPa. The decline in crop water use observed in Graph 3 is clearly explained by the degree of tension observed here.  

 

To give some perspective of current moisture levels, the following graph displays 3-months of data from the site at Bordertown.

Graph 5 (soil water content, %) - Bordertown wheat.  



Return to this site in the next fortnight to view the most recent data. Both sites are near to flowering - a period of high water demand - so we are expecting to see dramatic changes in the graphs.

Meanwhile, please post your comments and questions or give me a call:

Michael Zerk

Irrigation Consultant

Rural Solutions SA

Office:  (08) 8762 9176

Mobile:  0428 101 539