Predicting Water-Use Efficiency Canopy at level under Climatic Change

Quantifying changes in plant water use efficiency (WUE) under elevated CO₂ and other environmental variable is essential for predicting and modeling productivity in water-limited areas under climatic change scenarios. Hsiao (1993) proposed a simple model to predict changes in WUE as atmospheric CO₂ and other environmental factors vary. Defining WUE as the ratio of CO₂ assimilation to transpiration, the model is based on fundamentals of leaf gas exchange and the ratio of intercellular CO₂ concentration (C_i) to ambient CO₂ concentration (C_a) characteristic of each species. This ratio (a) tends to be constant as environmental conditions vary, or changes in a simple manner in accordance with a particular environmental variable. The model in the final form can be written as:

(1)

where the new condition is denoted by the subscript n and the original condition is denoted by the subscript o, and DW is leaf-to-air vapor pressure difference driving transpiration. For species like maize, with stomata not responsive to humidity, a remains relative constant under different DW. For situation where a remains constant, Eq. 1 can be further simplified to:

(2)

Inputs necessary for making predictions with the model are C_a, a, DW and a WUE_o chosen as the reference. Validity of the prediction is obvious for gas exchange at the single leaf level. But is the prediction valid for crop fields without up-scaling, assuming the whole canopy is a "big-leaf"? Data from profile measurements in a cotton and a maize field show pronounced diurnal variation in canopy CO₂ as well as water vapor under vary calm weather. For instances, air CO₂ concentration in fully closed canopies is often as high as 450 mmol mol^-1 in the early morning due to temperature inversion and accumulated respired CO₂, then slowly decrease to as low as about 300 mmol mol^-1 in the early afternoon. Air vapor pressure deficit (VPD) in the summer at Davis also shows large day to day variations. The large diurnal and day to day natural variations in VPD and CO₂ concentration provide us an opportunity to test the model in the open fields without free-air CO₂ enrichment . Canopy CO₂ assimilation, evapotranspiration, and C_a and vapor pressure surrounding the canopy of cotton and maize were monitored continuously by Bowen ratio/energy balance instruments and a profile-air sampling apparatus. DW was calculated based on canopy surface temperature measured by an infrared thermometer and vapor pressure of canopies. The data were used to compare model-predicted changes in WUE as conditions varied with the measured changes. Remarkably close predictions were obtained for cotton, when the dependence of a on DW was accounted for. Reasonably good predictions were obtained for maize. Possible causes of deviation between predicted and measured WUE will be discussed.