Graduate Thesis Or Dissertation

Make it Rain : A Lagrangian Approach to Investigate Land Surface Controls on Moisture Recycling in the Columbia River Basin

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  • Linkages between land use and its influence on the atmosphere have been a long-standing research area. For example, what is the impact of irrigated agriculture on downwind rainfall? Global-scale models show that moisture recycling is an important source of water for inland regions around the world, but they do not resolve land surface characteristics that may affect recycling ratios. Observational studies at the local scale report both increased and decreased rainfall likelihood as a result of deforestation. I present a Lagrangian framework based on biophysical land - atmosphere interactions to estimate the influence of land cover on the spatial and temporal scale of rainfall recycling. My results demonstrate that convective rainfall conditions occur more frequently over coniferous forests and irrigated crops relative to bare soils and mixed-use land cover. The increased likelihood of rainfall over vegetated surfaces is caused by the response of the atmospheric boundary layer to high water vapor fluxes from evapotranspiration and the partitioning of available energy at the ground surface. The presence of vegetation causes large variability in rainfall likelihood, but free atmosphere conditions are found to be the primary controls on rainfall initiation. Multi-day model simulations suggest that an initially dry atmosphere will experience rainfall recycling along a Lagrangian trajectory every 2 to 3 days over a completely vegetated surface and every 12 days over bare soils. Application of my model focused on the lower Columbia Basin because it is a major source region of continental evaporation and has a history of dramatic change in land use. The increased rainfall trends observed over the 20th century in the lower Columbia Basin cannot be significantly associated with land use changes, but modeled estimates suggest an increase in rainfall likelihood of less than 9% from the change in land cover this region has experienced. The results of this simple physically-based model can be used to quantify the effects of land cover on moisture recycling to better understand processes influencing rainfall patterns at regional scales.
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