Most Popular Books by M. Barr

Precision irrigation has made spatially explicit water management a reality. However, detailed knowledge about the spatially variable evapotranspiration is often unavailable. One example location where we can begin to understand the impact of spatial variability on evapotranspiration is at the field edge, which contains a sharp discontinuity in surface conditions. Theoretical work on this issue has a long history: an analytical solution to the two-dimensional advection-dispersion equation for a step-change in water vapor concentrations was developed in 1934. This analytical solution predicts a boundary layer growth in water vapor, a result that has been observed. The solution also predicts an evaporation maximum at the upwind edge of the irrigated field; this evaporation maximum has not been observed. This presents an interesting paradox whereby the boundary layer structure is predicted by theory but the surface flux is not. One possibility for why the evaporation maximum is not present at the field edge is that the plants in the irrigated area modulate the latent heat flux through transpiration. To test this hypothesis, a field experiment was performed to measure water vapor and carbon dioxide concentrations from an irrigated field. From this experiment, vertical flux divergence, which is linearly proportional to advection, was determined. The experiment provided direct support that the plants are modulating the fluxes at the field edge. A model for plant behavior was then linked to the two-dimensional advection-dispersion equation. The plant behavior model served as a flux boundary condition. This simulation showed boundary layer growth in water vapor as expected and corroborated the field observations. The simulation was also extended to include carbon dioxide because water vapor and carbon dioxide are linked through photosynthesis. Finally, the fluxes of water vapor and carbon dioxide were simulated, providing a possible explanation for the field edge evaporation paradox.