Author: Idit Sofer

Abstract

Whole canopy evapotranspiration (ET) measurements remain the backbone of modern irrigation and are crucial for efficient and sustainable water management. In agricultural structures such as screenhouses and greenhouses, evapotranspiration is currently measured by using lysimeters or sap flow gauges. These measurements provide ET estimates at the single plant scale that must then be extrapolated, often statistically or empirically, to the whole canopy for irrigation scheduling purposes. On the other hand, turbulent transport techniques, like the eddy covariance, have become the standard for measuring whole canopy evapotranspiration in open field conditions, but their applicability to agricultural structures has not yet been established. The subject of this research thesis is the application of the eddy covariance technique to measure ET and other scalar fluxes within large agricultural screenhouses.
The specific purpose of this study is to examine several criteria, commonly used for canopies in the open, to assess the suitability of the eddy covariance technique for reliable flux measurements in screenhouses.
The research combined field experiments and modeling. A comprehensive and detailed experimental study was conducted during the years 2007-2009 to investigate the turbulent fluxes and turbulence characteristics in large screenhouses. Flux measurements were conducted using eddy covariance systems simultaneously deployed at either different vertical levels or different horizontal positions, within the air gap between the top of the plants and the horizontal screened roof. Measurements were conducted in different types of screenhouses, 10% shading screens and 50 mesh insect-exclusion screens, and in different climatic regions in Israel. Banana and pepper plantations were examined at different canopy development stages. The modeling was conducted using the Penman-Monteith equation adjusted to the screenhouse environment.
Results are based on 9 field campaigns. For mature plants, energy balance closure slopes were mostly larger than 0.8, turbulence intensity was about 0.5 or smaller and the ITCσ test was mostly lower than 30% (and never exceeded 40%). Average friction velocity for all normalized heights was 0.25±0.04 ms-1. Spectral energy density functions for the total energy above the crop were analyzed as a function of frequency for all campaigns. The obtained slopes were very close to -5/3 (= -1.67), the value typical of the inertial subrange, suggesting that close to the canopy top, where the EC system was positioned, steady state turbulent boundary layer flow prevailed. The ratio between latent heat fluxes measured at two different vertical levels was closer to one for larger than for smaller plants. The ratio between latent heat fluxes measured under and above the screen, was 1.16. The ratio between sensible heat fluxes measured at two different levels under the screen resulted with ratios close to one (0.9-1.14). However, sensible heat flux measurements under and above the screen resulted with a ratio of 0.75.
Measurements of fluxes (LE, H and CO2) were also analyzed separately for small and large plants. Better energy balance closure was obtained for larger than smaller plants. A modified Penman-Monteith evapotranspiration model adapted to the screenhouse environment was applied here for the first time for a banana screenhouse; the model was in better agreement with the measurements than the conventional Penman-Monteith model. Both evapotranspiration models provided more accurate predictions for larger than smaller plants. This was due to inadequate treatment of the partial cover of young plants, which could be overcome by using a crop coefficient.
The major conclusion of this study is that under most conditions, and provided sufficient fetch is available, turbulence characteristics in the air gap between the top of the plants and the horizontal screen are adequate for reliable eddy covariance flux measurements in screenhouses.