Estimation of wet canopy bulk stomatal resistance from energy flux measurements during sprinkler irrigation

• An experimental technique to estimate wet crop canopy bulk stomatal resistance (rs).
• rs was zero in wet canopy condition, when advected energy is taken into account.
• Ignoring advected energy resulted overestimated rs and underestimated ET.
• Theory can be extended to P–M Equation estimating potential evaporation during rain.

Bulk stomatal resistance, also known as surface resistance, is typically assumed to be zero during rainfall or other circumstances when a foliage canopy is wet, such as during sprinkler irrigation. However, some recent studies have suggested that resistance does not necessarily fall completely to zero. Although the assumption of zero bulk stomatal resistance for a wet canopy condition may be reasonable, estimation of actual evaporation as well as bulk stomatal resistance during wet conditions via the Penman–Montieth (P–M) equation is still problematic due to the difficulties in measuring the various energy fluxes in the energy balance. It has recently been demonstrated that eddy covariance (ECV) can be used to estimate the actual evapotranspiration during both irrigation and non-irrigation periods. It has also been shown that advection is important in sprinkler irrigation. This paper demonstrates how the same technique can also provide an estimate of the bulk stomatal resistance for a wet crop canopy. It is shown that when all significant energy terms (including advected energy) are taken into account, the bulk stomatal resistance was effectively zero, in contrast to dry canopy values for the same crop of order 30 s m−1, both determined under midday, open sky conditions. The study also shows that ignoring advected energy can lead to an overestimation of bulk stomatal resistance and underestimation of ET when the canopy is wet.