Factsheet 13: Evapotranspiration
Evaporation refers to water turning from a liquid on a surface or in a container into water vapour in air, and transpiration is the process of water moving through a plant and evaporating from aerial parts (such as leaves and flowers). Evapotranspiration (ET) captures both processes and refers to the loss of liquid water from land and vegetation. It provides a metric for the rate of water loss from an area which can be useful, for example, for farmers to efficiently manage the available water for their crops. Evapotranspiration plays a key role in the regional water cycle, water resource management, vegetation productivity and climate change.
Three measures of ET are widely used in environmental and water resource management, hydrology, irrigation, crop yield estimation, and forestry - actual ET (ETa), potential ET (ETp), and reference ET (ET0).
On the Queensland Future Climate Dashboard and Regional Explorer, we provide estimates of pan evaporation (a standard measure of potential evaporation) and a special case of ETp used by the Food and Agriculture Organization (FAO) of the United Nations – FAO reference evapotranspiration (crop ET0).
Evapotranspiration variables and models
Calculating ET requires solar radiation, vapour pressure, maximum and minimum temperature, mean sea level pressure, and wind speed. These variables were obtained from our CMIP6 downscaled simulations (refer to factsheet #4).
Potential ET is the ET from a sizable, vegetated land surface without restrictions on moisture availability. It is used for water resource planning, irrigation scheduling, and determining water requirements of crops and plants. Crop reference ET is the ETp from a crop with specific characteristics. Here, we have used short crops as the reference, based on the FAO. FAO Reference Evapotranspiration is based on Penman-Monteith FAO56 model for short crop reference evapotranspiration. It assumes a hypothetical reference crop with crop height of 0.12 m, a fixed surface resistance (surface resistance describes the resistance of vapour flow through stomata openings, total leaf area and soil surface) of 70 sec/m and an albedo (albedo is an expression of the ability of surfaces to reflect sunlight) of 0.23, closely resembling the evapotranspiration from an extensive surface of green grass of uniform height, actively growing, well-watered, and completely shading the ground (Allan et al., 1998).
Queensland Future Climate also provides data for Pan evaporation, which is the amount of water evaporating from bare ground under the assumption of unlimited water supply. It can be used to estimate evapotranspiration over land or evaporation over water using conversion factors. However, the conversion factors are not constant, varying with local conditions.
The potential ET datasets are used to derive Standardized Precipitation Evapotranspiration Index (SPEI, see factsheet #9). The SPEI is designed to take into account both precipitation and potential ET in determining drought and wetness. It encompasses phenomena on the supply (precipitation) and demand (evapotranspiration) sides. SPEI is widely used owing to its ability to capture dry and wet extremes, to capture both precipitation and evaporative demand, and to characterize hydroclimate volatility at the timescales that are most broadly relevant to societal and ecological hazards.
Evapotranspiration Metrics on Queensland Future Climate
The main tools for viewing climate projections data on the Queensland Future Climate website are the Queensland Future Climate Dashboard and Regional Explorer. Please refer to the user guide for detailed information on how to access and interpret information available from these resources.
Examples of the evapotranspiration metrics available on the Queensland Future Climate Dashboard and Regional Explorer are shown below in Figures 1 and 2.
References
Allan R., Pereira L., Smith M. 1998. Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56
