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Estimating plant-accessible water storage through evaluating evapotranspiration in the semi-arid western United States using eddy-covariance, remote sensing, and spatially distributed data

  • Author(s): Rungee, Joseph
  • Advisor(s): Bales, Roger
  • et al.
Abstract

The studies within this dissertation use a suite of long-term flux-tower, remotely sensed, and spatially distributed data to more accurately assess the withdrawal of subsurface plant-accessible water storage during multi-year dry periods, more accurately represent measurements of evapotranspiration across the landscape, and examine how vegetation use of plant-accessible water storage varies along latitudinal and elevation gradients, and with time. First, a suite of flux towers from across the arid and semi-arid western United States were used to assess the response of evapotranspiration under varying climates and vegetation types to drought. Here we found that regions experiencing a Mediterranean climate are substantially more dependent on subsurface storage than those receiving a summer monsoon, but available plant-accessible subsurface water storage in the Mediterranean climates can support evapotranspiration for the entirety of a multi-year dry period at some locations. It was also discovered that a transition from snow to rain could increase dependency vegetation on plant-accessible subsurface water storage by as much as 20\% at energy-limited, snow-dominated sites. Next, measurements of evapotranspiration were distributed across the 14 river basins draining into California$'$s Central Valley. This was performed by expanding on current remotely sensed-based methods to include climatic data and consider vegetation type. This novel approach decreased the root-mean-square error by 31-50\% when compared to methods only using NDVI and was insensitive to the spatial resolution of data used. This product showed that evapotranspiration was greatest in the northern basins, peaking at lower elevations, and decreased in magnitude while peaking at higher elevations as latitude decreased. It was also revealed that runoff was derived in primarily one of two ways in this region, the rain-dominated north where annual rainfall grossly exceeds annual evapotranspiration; and the snowmelt-driven south where most precipitation contributes to high-elevation snowpack in energy-limited areas. Finally, the 14 basins draining into California$'$s Central Valley could be binned into four groups based upon what water-balance components and climatic variables were most highly correlated with changes in subsurface water storage, the northernmost, northern, mid-range and southern basins. The results showed that the southern basins may have already reached a critical threshold in storage drawdown, explaining why tree mortality is so widespread in the region, and that the northern and northernmost basins will likely follow a similar path if measures are not taken to reduce evapotranspiration. The studies in this dissertation provided comprehensive analyses of how evapotranspiration spatially varies and how its response to climate extremes alters the hydrologic cycle. Spatial products are in high demand for water resources and forest management applications, and although quantifying uncertainties remain a challenge, these products provide substantial value to improving our understanding of the water cycle.

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