Equations for fallow water storage

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Equations for fallow water storage

With the widespread rains throughout early summer and full moisture profiles across much of the north, growers should be thinking about how they are going to convert this moisture to profit.

GRDC validation and integration manager Stuart Kearns says a focus on fallows is an important strategy for helping growers manage the risks associated with high and variable rainfall in the northern region. He says fallow management, including the sequencing of fallows and crops, is the best option for northern growers seeking to increase both water availability to plants and WUE.

“A realistic understanding of the soil’s capacity to store water and assess available water prior to seeding will help in identifying planting opportunities and potential crop yields,” he says. “Good agronomic practices that maximise plant health and growth will allow crops to use water more effectively.”

A measure of a cropping system’s capacity to convert water into plant biomass or grain, WUE includes water stored in the soil and rainfall during the growing season. Mr Kearns says WUE is determined by a range of factors, such as the soil’s ability to capture and store water and a crop’s ability to access stored water in the soil and rainfall in the season.

He says WUE also relies on a crop’s ability to convert water into biomass, which is in turn converted into grain (harvest index).

“Water is the principal limiting factor in rain-fed cropping systems in northern Australia. The objective of rain-fed cropping systems is to maximise the proportion of rainfall that crops use and minimise water losses to weeds and through run-off, drainage and evaporation from the soil surface.”

He says storing water in fallows between crops is the most effective tool for managing the risk of rainfall variability because in-season rainfall alone is rarely sufficient to produce a profitable crop, particularly given high levels of transpiration and evaporation.
“Fortunately, many cropping systems in the northern region have the capacity to store large amounts of water during the fallow,” he says.

More information: GRDC Water Use Efficiency fact sheets can be downloaded at www.grdc.com.au/factsheets

Region North