Abstract
This study illustrates a unified, physically-based framework for mapping landscape parameters of evapotranspiration (ET) using spectral mixture analysis (SMA). The framework integrates two widely used approaches by relating radiometric surface temperature to subpixel fractions of substrate (S), vegetation (V), and dark (D) spectral endmembers (EMs). Spatial and temporal variations in these spectral endmember fractions reflect process-driven variations in soil moisture, vegetation phenology, and illumination. Using all available Landsat 8 scenes from the peak growing season in the agriculturally diverse Sacramento Valley of northern California, we characterize the spatiotemporal relationships between each of the S, V, D land cover fractions and apparent brightness temperature (T) using bivariate distributions in the ET parameter spaces. The dark fraction scales inversely with shortwave broadband albedo (ρ < −0.98), and show a multilinear relationship to T. Substrate fraction estimates show a consistent (ρ ≈ 0.7 to 0.9) linear relationship to T. The vegetation fraction showed the expected triangular relationship to T. However, the bivariate distribution of V and T shows more distinct clustering than the distributions of Normalized Difference Vegetation Index (NDVI)-based proxies and T. Following the Triangle Method, the V fraction is used with T to compute the spatial maps of the ET fraction (EF; the ratio of the actual total ET to the net radiation) and moisture availability (Mo; the ratio of the actual soil surface evaporation to potential ET at the soil surface). EF and Mo estimates derived from the V fraction distinguish among rice growth stages, and between rice and non-rice agriculture, more clearly than those derived from transformed NDVI proxies. Met station-based reference ET & soil temperatures also track vegetation fraction-based estimates of EF & Mo more closely than do NDVI-based estimates of EF & Mo. The proposed approach using S, V, D land cover fractions in conjunction with T (SVD+T) provides a physically-based conceptual framework that unifies two widely-used approaches by simultaneously mapping the effects of albedo and vegetation abundance on the surface temperature field. The additional information provided by the third (Substrate) fraction suggests a potential avenue for ET model improvement by providing an explicit observational constraint on the exposed soil fraction and its moisture-modulated brightness. The structures of the T, EF & Mo vs SVD feature spaces are complementary and that can be interpreted in the context of physical variables that scale linearly and that can be represented directly in process models. Using the structure of the feature spaces to represent the spatiotemporal trajectory of crop phenology is possible in agricultural settings, because variations in the timing of planting and irrigation result in continuous trajectories in the physical parameter spaces that are represented by the feature spaces. The linear scaling properties of the SMA fraction estimates from meter to kilometer scales also facilitate the vicarious validation of ET estimates using multiple resolutions of imagery.
Highlights
Water is critical to life on Earth: metabolic pathways rely on the chemistry of aqueous solutions, plant physiology requires cooling through stomatal water loss, and landscape-scale patterns in ecological communities often develop around the availability of near-surface water
This allows for more direct comparisons with historical studies involving Landsats 4–7, which do not have the benefit of the Landsat Surface Reflectance Code (LaSRC) correction, and are forced to rely on the less accurate Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) correction
All of the images used in this study contain nearly the full range of vegetation abundance, soil exposure, and soil moisture contents
Summary
Water is critical to life on Earth: metabolic pathways rely on the chemistry of aqueous solutions, plant physiology requires cooling through stomatal water loss, and landscape-scale patterns in ecological communities often develop around the availability of near-surface water (or lack thereof). The movement of water between components of the Earth system connects the biosphere with the lithosphere and the atmosphere. Evapotranspiration (ET; the sum of evaporation and transpiration) is a central mechanism in this exchange, describing the directional transfer of water from the Earth’s surface to its atmosphere. In addition to its importance for global biogeochemical cycles, ET plays a major role in Earth’s surface energy balance (SEB). The thermodynamic implications of ET in the SEB result in its fundamental importance in the climate system, where clear global teleconnections are observed between ET and phenomena such as the El Niño–Southern Oscillation [1], in addition to direct relationships between soil moisture and temperature [2]. The sheer variety of biogeophysical systems that are impacted by ET demonstrate the importance of accurate global distributions of the components of ET [3] and characterization of multidecadal trends [4] for our understanding of, and ability to predict changes in, fundamental aspects of the Earth system
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