Phenology and Evapotranspiration

Many phenological models use ambient air temperature to estimate phenological stages during current and projected future climate conditions. However, the difference between ambient air temperature and plant-canopy temperature biases such estimates. Evapotranspiration (ET) has a big impact on the relationship between plant-canopy and air temperature, so awareness of ET facilitates understanding of temperature-based phenological models, their limitations, and possible changes in response to climate change. This chapter presents information on the estimation of reference ET (ETo), applying crop coefficient (Kc) values to determine well-watered crop ET (ETc), and assessing water stress effects on crop ET. It also discusses how to account for water stress effects to determine actual crop ET (ETa) and it presents some of the problems associated with estimating the ET of natural ecosystems. Rising CO2 concentration in the atmosphere is known to increase plant canopy resistance, which reduces transpiration and increases the daytime canopy temperature relative to the air temperature. Consequently, degree-days-based canopy temperature are also projected to rise more than degree-days based on air temperature as CO2 concentrations increase. In this chapter, we show that air temperature degreeday models will probably need periodic updating as the canopy temperature rises relative to air temperature, or canopy-temperature-based degree-days are needed to improve phenological models.