Rangelands NRM Blog, News & Resources

Arid Rangeland Rainfall Use Efficiency. Are you giving away your water?

[July-August 2016]

By Jason Hastie, Chairman, Upper Gascoyne LCDC 

We all want to see best use of the small amount of rain that falls on our stations. In that sense we all want better Rainfall Use Efficiency (RUE).

RUE is the amount of dry matter (DM) produced in a given area over a given period of time per unit of rain, usually expressed in Kg DM/ha/yr/mm. Arid rangeland ecosystems in good condition have a RUE in the range of 3-6 kg DM/ha/yr/mm. Degraded rangelands may have a RUE below 1.

RUE is directly related to rangeland condition, primarily ground cover. Ground cover allows your rain to infiltrate where plants (read grass) can use it. Without ground cover you lose the rain, it runs off. Tests done in the Alice Springs district found rain could infiltrate under ground cover up to 1.5 meters (1,500 mm) deep and on bare ground only 30 mm.

Capturing 1 mm more rain per year means:

  • 10,000 litres more water per hectare
  • 1,000,000 litres more water per square kilometre
  • More forage, because plants use that water.

Though it’s not that simple. Arid rangelands are characterised by run-off and run-on areas. The run-on areas are where nutrients gather and vegetation grows that drive production. Run-off and run-on areas in rangeland in good condition are close together. Most rainfall runs off and is caught within a few tens of metres or hundreds of metres at the most.

As rangeland becomes degraded, by the removal of vegetative cover, water runs further. It is not caught by local vegetation. Soil over large areas becomes poorly structured due to a lack of organic matter. It caps over and the rain runs off.

Once the run off increases there are fewer obstacles to slow it down. This leads further degradation as water concentrates and accelerates leading to scouring. Thus, run-off and erosion and turns us into exporters of soil and water as opposed to exporters of animal product.

But, if you can manage grazing pressure to keep ground cover in place you can begin a chain reaction of improvement being:

  1. production of above and below ground dry matter
  2. production of litter
  3. incorporation of litter into the soil
  4. increase of organic matter content in soil
  5. increased permeability to air and water
  6. increased water-holding capacity due to increased organic matter (each 1% of organic matter in the soil increases water-holding capacity by 12%)
  7. decreased runoff
  8. speeding-up of the turnover of minerals, hence increased fertility and primary productivity
  9. increased growth of perennial plant species, enhanced ecosystem functioning, increased productivity and production.

Rainfall is our biggest limiting factor to production and we don’t get a lot of it. Therefore, to maximise production we must seek to maximise the effect of rainfall we do receive.

Image: Mt Boggola from North East (J.Hastie)