Introduction

Evapotranspiration (ET) is the combination process of evaporation from surfaces and transpiration from plant tissues and primarily through stomata. The direct measurement of actual ET is very difficult and indirect estimates are usually made as a consequence. Since ET is such a doninant part of the water cycle (~65%) and optimised agriculture require accurate estimates to determine irrigation needs, much work has been performed to determine accurate and practical methodologies for estimating ET.

Much of the development came together with the United Nations Food and Agriculture organization (UN-FAO)as they worked together with researchers for an international standard. As it was most practical to only measure metereological parameters (e.g. temperature, relative humidity, etc), the term reference ET (ETo) was coined to define a specific vegetated surface by which the ET estimation would represent. Different crop coefficients could be applied to convert the reference crop to other types of crops or vegetated surfaces.

The method utilises the Penman-Monteith ET equation and the guildline provides methods to estimate missing metereological parameters. The method and guildlines can handle as little data as minimum and maximum temperature to the full set of metereological parameters. With this standardisation, researchers and water managers can accurate estimate ETo and ultimately ET and be able to compare the results across regions.

This package is meant to assist in efficiently estimating ETo for time series metereological data where instrumentation and consequenty the parameters change over time due to changes in priorities or budgets. Additional historic ETo or potential ET (PET) methods have been added for comparison purposes.