An Apple-Specific ET Model

A common method for estimating crop water use for irrigation is to use a crop coefficient, Kc, that is a fraction at any time of the estimated potential evapotranspiration (ETo), often modeled for a reference grass. The modified Penman-Monteith (PM) equation used for the standard reference grass adopts average values typical for healthy short grass. These include a relatively heavy boundary layer common for grass that tends to uncouple the grass from the bulk air. This leads to estimated ETo values that are overwhelmingly controlled by net radiation (Rn) and relatively insensitive to vapor pressure deficit (VPD). Conversely, due to the tall irregular structure of an apple orchard, the canopies of apple trees are highly coupled to the bulk air. This means that apple trees are dependent on VPD and stomatal conductance as well as Rn. We have developed an apple-specific ET model based on the PM equation. The model is a "big leaf" model and estimates the total transpiration of the tree as a distinct product of sunlit and shaded leaves. The model has a much smaller boundary layer resistance than the PM grass model. Stomatal conductance has a unique dependence on VPD with limitations imposed by stomatal coupling to leaf photosynthesis rate. The model has been very promising in explaining variations in apple tree transpiration and Kc measured in the humid climate of NY.