Feature article

Calculating irrigation need

Modelled water deficit at Winchmore for 2005–2006 (magenta curve).

Estimating likely irrigation need is an important aspect of efficient farm management and feed budgeting.

Last month’s issue of The Climate Update illustrated the close relationship between measured and modelled soil moisture content at Winchmore. A soil moisture model, once calibrated from measured water content, can easily be run on a computer spreadsheet to track ongoing moisture status in the pasture root zone on a daily basis. This is a particular advantage when subsequent soil moisture measurements are not available or are costly to obtain.

The adjacent figure shows modelled water deficit at Winchmore for 2005–2006 (magenta curve), and the same model (green curve) adjusted for irrigation applications (illustrated by the dark blue bars shown at each irrigation date). The model assumes that, from November to April, when a trigger level deficit of 25% moisture content by volume is reached (orange dashed line), a 30 mm irrigation is applied.

Here we assume that a 30 mm irrigation will raise the soil moisture content by a little over 5%. The model adjusts the level of soil water by this amount, while also taking into account evapotranspiration, rainfall, and irrigation efficiency where this is important.

In this example the model shows that 11 irrigation applications would have been needed, adding to a total depth of 330 mm of applied water, or 3300 cubic metres per hectare. Running the model over several seasons would quickly show the typical range of water volumes required.

Daily water balance data and 15 day rainfall predictions are updated daily on NIWA’s ClimateExplorer webpage climate-explorer.niwa.co.nz. For more information, and to obtain an annual subscription for all routinely updated products, please contact [email protected]. Take advantage of the three month trial subscription by contacting Andrew Tait, [email protected].