Evapotranspiration (ET) is the second largest component of the water cycle in arid and semiarid environments, and, in fact, more than 60% of the precipitation on earth is returned to the atmosphere through it. MOD16 represents an operational source of ET estimates with adequate spatial resolution for several applications, such as water resources planning, at a regional scale. However, the use of these estimates in routine applications will require MOD16 evaluation and validation using accurate ground-based measurements. The main objective of this study was to evaluate the performance of the MOD16A2 product by comparing it with eddy covariance (EC) systems. Additional objectives were the analysis of the limitations, uncertainties, and possible improvements of the MOD16-estimated ET. The EC measurements were acquired for five sites and for a variety of land covers in northwestern Mexico. The indicators used for the comparison were: root mean square error (RMSE), bias (BIAS), concordance index (d), and determination coefficient (R 2) of the correlation, comparing measured and modelled ET. The best performance was observed in Rayón (RMSE = 0.77 mm•day −1 , BIAS = −0.46 mm•day −1 , d = 0.88, and R 2 = 0.86); El Mogor and La Paz showed errors and coefficients of determination comparable to each other (RMSE = 0.39 mm•day −1 , BIAS = −0.04 mm•day −1 , R 2 = 0.46 and RMSE = 0.42 mm•day −1 , BIAS = −0.18 mm•day −1 , R 2 = 0.45, respectively). In most cases, MOD16 underestimated the ET values.
Classification of vegetation using multi-angular spectral information has been approximated using restricted modeling schemes or with homogeneity or stationarity hypotheses. This paper presents a global modeling scheme of zenith and azimuthal angles in vegetation reflectance, extending the developments to the case of effects of changes on optical properties of vegetation substrates (soils) and plant density (foliage). This synthesis is performed in an upper space of compaction that depends essentially on two parameters and encompasses all the variations analyzed. The global modeling scheme developed was applied to the case of plant arrangements, different densities, of eight species with contrasting archetypes and bare soils. Modeling was applied to measurements of bi-conical reflectance factors, where adjustments were excellent (generally with R2 ≥ 0.99 and mean relative errors less than 7%). Analysis of the results showed that multi-angular spectral information for species classification under generalized conditions still has problems in discriminating species. This can be explained by the analysis synthesis of variations associated with discrimination.
La información multi-angular de las reflectancias de las clases de la vegetación natural ha sido planteada, para su discriminación, bajo diferentes enfoques de caracterización. Aunque en apariencia este enfoque permite def inir en forma conf iable f irmas espectrales multi-angulares típicas, su uso generalizado no ha sido analizado. En este trabajo se presenta un modelo de los efectos del ángulo cenital de visión e iluminación en las reflectancias y se valida en un conjunto de experimentos de ocho especies vegetales con arquetipos contrastantes más suelo desnudo, bajo condiciones de iluminación-visión bi-cónicas. Los resultados muestran que el modelo propuesto es robusto y conf iable, por lo que puede usarse en forma operacional. Los análisis realizados para el efecto del suelo y la densidad de plantas muestran que hay importantes zonas de confusión (traslape de información) entre las especies, al considerar efectos combinados para diferentes ángulos acimutales. Estos resultados ponen en duda el uso en forma generalizada de la información espectral multi-angular y solo justif ican este enfoque de clasif icación si se dejan f ijos el fondo de la vegetación y la densidad de plantas (cantidad de follaje).
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