Field measurements of bare soil evaporation and crop transpiration, and transpiration efficiency, for rainfed grain crops in Australia - A review

Australian agriculture is dominated by rainfed cropping in environments where evaporative demand greatly exceeds annual rainfall. In this paper we review field measurements of crop transpiration and bare soil evaporation under rainfed grain crops, and crop transpiration efficiencies. Crop transpiration is typically calculated from the difference between evapotranspiration and bare soil evaporation, however, while the former is readily measured, the latter is difficult to obtain. For wheat we found only 19 studies which measured the critical water balance parameters of bare soil evaporation and crop transpiration in Australia, and very many fewer for other crops. From the studies reported for wheat, on average 38% of evapotranspiration was lost to direct soil evaporation. Data for other crops are insufficient to ascertain whether they are similar or different to wheat in terms of the relative contributions of Es and T to the water balance. Although it may have occurred in practice, we can find no field measurements of the crop water balance to demonstrate an increase in crop transpiration at the expense of bare soil evaporation as a function of improvements in agronomic practices in recent decades. Although it is thought that crop transpiration efficiencies are primarily a function of vapour pressure deficit, transpiration efficiencies reported in the literature vary considerably within crops, even after accounting for vapour pressure deficit. We conclude that more reliable estimates of crop transpiration efficiency would be highly valuable for calculating seasonal transpiration of field grown crops from shoot biomass measurement, and provide an fruitful avenue for exploring water use efficiency of grain crops.