|Irrigation scheduling |
Centre pivot allows better control of water application than border-check irrigation. While many people achieve good results simply by using their judgement based on observed soil moisture and weather conditions, a centre pivot provides the potential to optimise water efficiency with a more comprehensive irrigation scheduling regime. Irrigation management decisions can be based on climatic information (evapotranspiration less rainfall, or E-R), or soil moisture measurement, or a combination of both.
Over spring and autumn when irrigation requirements are variable, it is appropriate to irrigate on an evaporation less rainfall (E-R) interval, or a given soil moisture deficit. 30 mm is commonly recommended as an appropriate E-R interval for sprinkler irrigated perennial pasture in the SIR, or an equivalent 25 mm soil moisture deficit. That is, 30 mm of E-R is approximately equivalent to 25 mm of pasture water use (0.8 x (E-R) at Tatura), and this is the amount you need to replace when you irrigate. You need to allow for the application efficiency of the pivot - this can normally be assumed to be 95%, particularly when evaporation is low, but may reduced to 90% in hot windy conditions. Hence to replace 25 mm of soil moisture, you need to apply 27 mm.
Over summer when irrigation demand is higher and more constant, regular irrigation (say twice per week may be more convenient, replacing the soil moisture used by the pasture. That is if E-R for the week is say 50 mm (common for January at Tatura), the pivot should apply 50 x 0.8 (40 mm), or the measured soil moisture deficiency, through the week. Allowing for application efficiency of 95%, 42 mm needs to be applied over the week. This may be applied in say two irrigations, or predominantly at the weekend and each night if using off-peak electricity.
Evaporation data or evapotranspiration data can guide your irrigation scheduling