An effective evaporation pan can be constructed from a half 200 litre drum. A 'V' notch 12-15 mm deep is cut in the rim to drain water added to the drum during irrigation or by rain.
The bottom point of the 'V' notch represents the field capacity level. Horizontal lines marked off at 10 mm intervals can be helpful to show the amount of water drawdown.
The drum should be covered with netting to prevent birds, dogs or other animals affecting the water level.
Place the drum in a cropped paddock, making sure it is not sheltered by fences, posts or trees. It should not be placed in a hollow nor on a high spot but freely exposed to the sun and wind. The rim should be at about the same height as the crop at full ground cover.
Every 2-3 days measure the depth of the water below the base of the 'V' notch and top up the pan. This depth in mm is the amount of water used by the crop with full ground cover which needs to be replaced by irrigation. If your crop doesn't have full ground cover, then an estimate of water use is obtained by multiplying the evaporation pan number by the proportion of ground covered by green crop (estimate from the amount of shadow under the mid-day sun).
Add up the daily evaporation since your last irrigation or good soaking rainfall which would have left your soil water storage full (field capacity). This gives you an 'accumulated deficit'. If the evaporation pan has been overtopped by rainfall, then the amount of extra rainfall needs to be deducted from the total deficit.
Evap (day 1) + Evap (day 2) +......... minus rainfall(mm) = soil water deficit(mm). Plot the numbers on a graph to give you a clear visual idea of the accumulating deficit.
Evaporation figures are usually published daily in local regional newspapers for a few locations which record this data. Weekly figures are published in Tasmanian Country. Use the figures from the recording station closest to your farm, together with your own rainfall records, to work out your soil water deficit.
If your farm isn't local to one of the published locations then you are best to install your own evaporation pan in the paddock you are irrigating because of differences in wind exposure for individual sites and local microclimate variations.
When To Start Irrigating
Start to irrigate crops growing on red krasnozem soils when there is a total soil water deficit of 50mm. However, remember that by the time the irrigator has been moved from one side of the paddock to the other, the last run to be irrigated will be much drier than the 50mm deficit. Consequently, to even out the range of deficits across a paddock it is advisable to start your first irrigation when there is only 40mm deficit. If your soil water deficit is 40mm, then this is the amount you will have to apply as irrigation. One millimetre of irrigation equates to 1 litre per square metre or 10,000 litres per hectare.
The amount of soil water deficit allowable before starting to irrigate varies for different crops and different soils. Potatoes grown on sandy soils should be irrigated at a deficit of 35mm. Overseas research has shown that potato yield declines by about 1% for each 10mm of deficit beyond the critical irrigation start up deficit. In other words, an irrigation of 20mm to avoid the soil water deficit going out to 70mm on a red krasnozem soil, is likely to result in a return of 1 t/ha of tubers for a crop with a yield potential of 50 t/ha.
Experience has indicated to some farmers that applying 50mm of irrigation at once is often too much and results in surface water runoff, soil erosion, and incomplete soil water recharge. A better option is to apply less water at each irrigation but to irrigate more frequently.