A New Contextual Parameterization of Evaporative Fraction to Reduce the Reliance of the Ts − VI Triangle Method on the Dry Edge

Abstract

Abstract:In this study, a new parameterization scheme of evaporative fraction (EF) was developed from the contextual information of remotely sensed radiative surface temperature (T s ) and vegetation index (VI). In the traditional T s − V I triangle methods, the Priestley-Taylor parameter ∅ of each pixel was interpolated for each VI interval; in our proposed new parameterization scheme (NPS), it was performed for each isopiestic line of soil surface moisture. Specifically, ∅ of mixed pixels was determined as the weighted-average value of bare soil ∅ and full-cover vegetation ∅. The maximum T s of bare soil (T smax ) is the sole parameter needed as the constraint of the dry edge. This has not only bypassed the task involved in the determination of the maximum T s of fully vegetated surface (T cmax ), but also made it possible to reduce the reliance of the T s − V I triangle methods on the determination of the dry edge. Ground-based measurements taken during 21 days in 2004 were used to validate the EF retrievals. Results show that the accuracy achieved by the NPS is comparable to that achieved by the traditional T s − V I triangle methods. Therefore, the simplicity of the proposed new parameterization scheme does not compromise its accuracy in monitoring EF.

A Universal Ts‐VI Triangle Method for the Continuous Retrieval of Evaporative Fraction From MODIS Products

“…In summary, although there are good agreements in EF retrieved by these two schemes, the results obtained at both regional and site scale show that the parameterization scheme proposed by Zhu, Lv, Jia, and Yan () provides better estimation of EF in this study. Therefore, only the results produced by the NPS were used thereafter to analyze the spatiotemporal variations of EF over the SGP region.…”

“…Following the squared relationship proposed by Gillies et al (), f c was calculated as follows: where NDVI max and NDVI min are NDVI values of full‐cover vegetation and bare soil, respectively. In consistent with our previous research (Zhu, Lv, Jia, & Yan, ), NDVI max was determined as the maximum NDVI value (0.94) within our study domain, and NDVI min were set as 0.05.…”

“…As Batra et al (), Wang et al (), and Venturini et al () pointed out, the mismatch in spatial scale and the heterogeneity of the surface should be considered during the evaluation. For the purpose of simplicity, the parameterization scheme proposed by Jiang and Islam (, ) is called the traditional parameterization scheme (TPS), and the scheme proposed in our previous research (Zhu, Lv, Jia, & Yan, ) is called the new parameterization scheme (NPS). Figure presents the scatterplots between EF observations and EF estimates retrieved by these two schemes across all sites.…”

“…The second parameterization scheme of ϕ is developed by Zhu, Lv, Jia, and Yan (). Similar to the first one proposed by Jiang and Islam (, ), this parameterization scheme is also conducted through a two‐step interpolation procedure.…”

“…The interpolation procedure was performed for each f c interval in the parameterization scheme proposed by Jiang and Islam (, ). However, in the parameterization scheme proposed by Zhu, Lv, Jia, and Yan () it was performed for each isopiestic line of soil surface moisture.As illustrated in Figure , for each isopiestic line of soil surface moisture within the T s ‐VI feature space, the parameter ϕ for full‐cover vegetation ( ϕ c , i ) is always close to its maximum value ( ϕ c , i = 1.26). In contrast, ϕ of bare soil ( ϕ s , i ) ranges from 0 to its maximum value with soil moisture availability increasing from the dry edge to the wet edge.…”