The water balance of irrigated forages in northern Victoria, Australia

Knowledge of the components of the water balance – evaporation, transpiration and deep drainage – would be beneficial for targeting productivity improvements for irrigated forages in northern Victoria. We aimed to estimate these components using a simple water balance and the dual crop coefficients provided in FAO-56. Soil water deficits from a field experiment, comparing the water use of six border-check and one spray irrigated forage system, agreed well with the modelled values, except for alfalfa where irrigation intake was restricted. About 85% of the water applied to perennial forages (perennial ryegrass/white clover, tall fescue/white clover and alfalfa) was used for transpiration, 10% for evaporation and 5% was lost as drainage below the root zone. Evaporation was highest from the double-cropped (oats/millet) system (30%) and was 5–25% of the water used by winter-growing annual pastures (Persian clover/Italian ryegrass and both border-check and spray irrigated subterranean clover/Italian ryegrass). The high proportion of water used as transpiration by the perennial forages was due to their high ground cover maintained throughout the year. When compared over similar seasonal conditions, actively growing forages used similar amounts of water, indicating that any increases in water productivity will be mainly due to higher production and/or to matching the growing season of the forage to periods of lower potential evapotranspiration.