Evaluation of a complementary flux principle and the wind effects of topography-driven micro-climate on evaporation

Danielle Druffel Adams

Evaporation is an important aspect of the hydrologic cycle and significant for crop production. It is pertinent to use appropriate techniques for evaporation estimation and to define spatial variability of evaporation. This study evaluated complementary flux principle formulations for potential, apparent potential, and actual evaporation aimed at evaporation estimation based on measurement at an individual location, and effects of variability in wind on evaporation due to micro-topography. The study site was the Cook Agronomy Farm near Pullman, WA, under different vegetative covers over multiple growing seasons. Measurements at the point location included an eddy covariance flux tower, multiple weather stations, micro-lysimeters, and two evaporation pans. For spatial wind measurements, a two-dimensional array of wind anemometers surrounding the flux tower was used with existing weather stations. The WindNinja model was used to develop wind vector maps and topography-driven wind relationships. Results indicate that our study site has conditions that are not well fit by a single complementary flux relationship. Specifically, evaporation was overestimated under water-limited or arid-like conditions observed during the summer months. Additional data from micro-lysimeters and a nearby evaporation pan provided necessary data to ensure proper calibration and understanding of seasonal changes in surface water availability and surface vapor pressure deficit under energy abundant conditions. Use of the Budyko curve and calibration methodologies show merit in defining the application of the complementary flux principle per climatic conditions based on location and seasonal changes. Variations of wind speed at the study site showed effects of topography and wind direction. Domain-scale wind speed patterns appear to be driven by larger scale wind flows associated with synoptic events rather than diurnal effects of surface heating and cooling. Average observed wind speeds were approximately 2 m s-1. Heterogeneity of wind in the site’s microclimate resulted in relatively insignificant evaporation differences under low average wind speed conditions observed at the CAF, because radiative effects were a more dominant factor in evaporative estimations. Quantification and qualification of the cumulative effects spatially-variable climatic factors affecting evaporation are important to define localized water balances and inform precision farming practices.

Evaluation of a complementary flux principle and the wind effects of topography-driven micro-climate on evaporation

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