1 Trangie Agricultural Research Centre, NSW Department of Primary Industries, Mitchell Highway, Trangie, NSW, 2830
Evaporation and transpiration are the central components of the water balance in irrigated and rain-fed cropping systems. Their dynamics and the energy budget at pre, during- and post-overhead sprinkler are unique from other irrigation methods such as surface and drip. The photosynthesis process and net exchange of carbon dioxide (CO2) are also different in wet canopy conditions compared with dry canopy conditions. However, these phenomena are not well understood in water and energy budget during the different phases of overhead irrigation events. A study was conducted in a cotton field using eddy covariance and sap flow systems to observe the dynamics of evaporation, transpiration, energy fluxes and exchange of (CO2) in different phases of overhead irrigation. Data showed a marked increase in evapotranspiration rate during irrigation, a consequence of the high evaporation rate of water intercepted by the canopy. Canopy development has a distinct effect on evaporation rates. Evapotranspiration rates declined after irrigation as the canopy dried. Transpiration rates considerably reduced during irrigation but quickly recovered in the post-irrigation phase. The exchange of CO2 decreased substantially during irrigation, possible due to a reduction in photosynthesis and closure of stomatal openings under wet canopy conditions. The energy fluxes show that sensible and latent heat act in opposite ways. Both may contribute to the evaporation of canopy intercepted water.