Measurement of evapotranspiration during sprinkler irrigation using a precision energy budget (Bowen ratio, eddy covariance) methodology

Abstract

Eddy covariance (ECV) is a micrometeorological mass transfer method for measuring evapotranspiration (ET) which has been used successfully for the last decade to measure evapotranspiration from natural and agricultural plant communities. However, no such usage has been reported in relation to evaporation losses occurring during sprinkler irrigation. Instead, sprinkler irrigation research has focused on using traditional methods acknowledged to have many limitations. In this field study, the feasibility of precision energy budget measurement, utilising ECV, to measure the total ET during sprinkler irrigation over different surfaces has been established.The trials were carried out at small scale using a low-pressure impact type sprinkler irrigation system over bare soil, grass and cotton during the Australian summer of 2010–11. All radiation components plus soil heat flux were measured. The sensible and latent heat fluxes measured using ECV were used to deduce instantaneous Bowen ratio values and hence partition the energy available for sensible plus latent heat (ET) transport. From the measurements of energy fluxes, the effect of advection on irrigation was inferred. Non-dimensionalisation of measured ET permitted comparison of the rate of change of ET in different phases of measurement, i.e. pre-, during and post-irrigation.Results indicated that the technique was clearly able to measure the different rates of total evaporation over the short crops before, during and after sprinkler irrigation and a relative ET discrimination of order 0.06mmh−1 was achieved, equivalent to a discrimination in the difference between irrigation and non-irrigation evaporation rates of ±11%. In addition, the results showed that nondimensional ET ranged from 1.4 to 1.6 times the reference (non-irrigating) ET value for mature cotton of height approaching 1m.