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Improving evapotranspiration estimates in the arid west using multi-platform remote sensing
Knipper, Kyle R.
Knipper, Kyle R.
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2016
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2018-01-10
Abstract
Evapotranspiration (ET) is a key component of the water balance, especially in arid and semiarid regions, yet it remains one of the most difficult hydrologic components to estimate. This dissertation research investigates and develops methodologies to estimate ET while minimizing the need for ground-based observations so as to make the final product easily transferable to ungauged basins located in arid and semiarid regions. Initial work investigates an application of a modified Moderate Resolution Imaging Spectroradiometer (MODIS) triangle-based method to estimate ET in a sub-alpine environment in northern California. Results show positive bias at three of the four sites when compared to ground-based measurements, indicating issues related to water stressed conditions on overall ET estimation. This triangle concept is further utilized by exploring a downscaling approach through the combination of higher spatial resolution MODIS and lower spatial resolution AMSR2 (Advanced Microwave Scanning Radiometer 2) and SMOS (Soil Moisture Ocean Salinity). Evaluation occurred in southern Arizona using AmeriFlux stations providing observed data with Version 2 of the National Land Data Assimilation Systems (NLDAS2) model providing additional comparisons. Results indicate a much improved spatial representation of soil moisture at the watershed scale. Downscaled soil moisture estimates were then used to scale potential evapotranspiration (PET) to ET. Derived ET estimates (MOD-SMET) are validated using four ground-based flux tower sites in southern Arizona USA, while also being compared to a calibrated empirical ET model as well as output from NLDAS2. Validation against observed ET indicates high correlations, with positive bias at upland sites and negative bias at a riparian site. MOD-SMET estimates compare well to the calibrated empirical ET model, while outperforming NLDAS2 simulations. MOD-SMET proves to be an effective alternative to more complex surface-atmosphere models for estimating actual ET. Moreover, the proposed methodologies used in all sections of the dissertation require no ancillary ground-based data, site specific calibration, or subjective specifications, allowing them to be transferable to ungauged basins located in water limited regions. Results of the dissertation contribute to better understanding of ET and soil moisture variability in semi-arid regions through development and application of improved spatial and temporal resolution remotely-sensed observations.
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