Below Canopy Evaporation Dynamics: Implementing Variable Boundary Conditions in Hydrus 2D/3D.

Accurate understanding of evapotranspiration (ET) is expected to become increasingly important as water resources continue to diminish and pressure on marginal areas increases. Evaporation from the soil surface (E) is generally considered a loss where water permanently leaves the system, as opposed to beneficial water use through transpiration. Quantification of the productive and unproductive allocations of water is highly relevant for water scarce or sparsely vegetated areas. Measurement and modeling of E below canopy is challenging as it is not only a function of soil, climate and irrigation regime/ precipitation patterns but is also altered by plant water uptake and micro-climate. The objective of this research was to quantify E in a drip irrigated vineyard under arid conditions with a specific focus on spatial and temporal variations of E across the inter-row using measurements and models.

An experiment was conducted in an isolated commercial desert vineyard; with a mean annual precipitation of 90 mm. Measurements included ET, actual E and potential E (E_p). In addition E was modeled using HYDRUS 2D/3D, a widely used numerical model for water, heat and solute transport in the soil. For this purpose a new module allowing variable boundary conditions at the soil surface was added to the program. Results show E was affected by available water in the soil and concentrated mostly along the drip line, with an obvious effect of shading on the timing of peak evaporation. The potential to model spatial variation in below canopy E using HYDRUS 2D/3D will be highlighted.