No abstract

No abstract

Coastal zones are inhabited by three-quarters of the global population and contribute to more than half of the global gross domestic product (GDP) despite accounting for only 5% of the Earth's land mass. Within coastal zones, river deltas are alluvial plains formed by the deposition of sediment and often occur where inland water flow leaves river estuaries and meets salty, stagnant ocean water. River deltas are home to more than half-a-billion people and are a crux of human civilization. The morphology of river deltas highly depends on the oceanographic forces of tides and waves and is also regularly reshaped by deltaic sediment cycling, which can be influenced by anthropogenic activities, such as damming and deforestation, resulting in the gain or loss of land area (Nienhuis et al., 2020;Syvitski et al., 2009). Quantifying the individual importance of tides, waves, and fluvial sediment supply in river delta formation is critical for policymakers to understand potential threats to deltas and develop adaptation strategies.As one of the largest deltas in Asia, the Pearl River Delta (see Figure 1a) is known for its high population density and economic level. The GDP of the Pearl River Delta reached 1,084.5 billion USD by 2017, accounting for nearly 9.1% of the GDP of mainland China (Statistics Bureau of Guangdong Province, 2019).

Abstract:Nitrogen is an essential nutrient in many terrestrial ecosystems because it affects vegetation's primary production. Due to the variety of nitrogen-containing substances and the differences in their composition across species, statistical approaches are now dominant in remote sensing retrieval of leaf nitrogen content. Many studies remove spectral regions characterized by strong water absorptions before retrieving nitrogen content, because water is believed to mask the absorption features of nitrogen. The objectives of this study are to discuss the necessity of this practice and to explore how water absorption affects leaf nitrogen estimation. Spectral measurements and chemical analyses for Maize, Sawtooth Oak, and Sweetgum leaves were carried out in 2014. The leaf optical properties model PROSPECT5 was used to eliminate the influences of water on the measured reflectance spectra. The inversion accuracy of PROPECT5 for chlorophyll, carotenoid, water, and dry matter of Maize was also discussed. Measured, simulated, and water-removed spectra were used to: (1) find the optimal nitrogen-related spectral index; and (2) regress with the area-based leaf nitrogen concentration (LNC) using the partial least square regression technique (PLSR). Two types of spectral indices were selected in this study: Normalized Difference Spectral Index (NDSI) and Ratio Spectral Index (RSI). Additionally, first-order derivative forms of measured, simulated, and water-removed spectra were devised to search for the optimal spectral indices. Finally, species-specific optimal indices and cross-species optimal indices, as well as their root mean square errors (RMSE) and coefficients of determination (R 2 ), were obtained. The Ending Top Percentile (ETP), an indicator of the performance of cross-species optimal indices, was also calculated. PLSR was combined with leave-one-out cross validation (LOOCV) for each species. The predicted root mean square errors (RMSEP) and predicted R 2 were finally calculated. The results showed that chlorophyll, carotenoid, and water contents could be estimated with R 2 of 0.75, 0.59, and 0.69, respectively, which were acceptable for fresh leaves. The dry matter was retrieved with a relatively lower accuracy because of the fixed absorption coefficients adopted by PROSPECT5. The performances of species-specific optimal indices using water-free spectra were comparable to or worse than the corresponding indices derived with measured or simulated spectra. Compared with measured spectra, ETP did not change much after the effects of water were removed, and the R 2 between cross-species optimal spectral indices and area-based LNC for Sawtooth Oak and Sweetgum decreased while it remained almost the same for Maize, suggesting that the water-removed cross-species optimal indices were inferior to the corresponding optimal indices found without water removal. ETP was larger than 30% for all spectra, demonstrating the non-existence of common optimal NDSI or RSI for the three species. After water removal, the accuracy...

The directional-hemispherical reflectance and transmittance of needle-shaped leaves are redefined in this study. We suggest that the reflected and transmitted radiation of a leaf should be distinguished by the illuminated and shaded leaf surfaces rather than the usual separation of the two hemispheres by a plane perpendicular to the incoming radiation. Through theoretical analysis, we found that needle directional-hemispherical reflectance and transmittance measured by two existing techniques, namely Daughtry's method and Harron's method, could be significantly biased. This finding was proved by ray-tracing simulations intuitively as well as by inversions of the PROSPECT model indirectly. We propose the following requirements for needle spectral measurement in an integrating sphere: needles should be fully exposed to the light source, the interfusion of reflected and transmitted radiation on convex needle surfaces should be avoided, and multiple scattering of radiation among needles should be minimized.

Leaf Incorporating Biochemistry Exhibiting Reflectance and Transmittance Yields (LIBERTY) models the effects of leaf biochemical concentrations on reflectance spectra on the basis of Melamed theory, which has several limitations. These are: (1) the radiation components are not treated satisfactorily; (2) the directional changes of both particle and sublayer scattering ratios are not considered; and (3) the boundary constraint which makes needle leaves different from broadleaves is not included. Proofs of these limitations as well as theoretical improvements are given in this study. Global sensitivity analysis (SA) of three models: the original LIBERTY, our improved LIBERTY (LIBERTY im ) and The optical PROperties SPECTra model (PROSPECT) suggests that compared with LIBERTY, the global reflectance and transmittance of LIBERTY im are more sensitive to diametrical absorbance αd-a parameter related to leaf biochemistry. Moreover, the global reflectance and transmittance of LIBERTY im and PROSPECT had similar sensitivity patterns to the input variables, demonstrating indirectly the validity of our improvements over LIBERTY. However, neither LIBERTY nor LIBERTY im considers boundary constraints, which limits their applications in modelling needle leaf optical properties. We introduced a particle string model, which might be used to simulate needle leaf optical properties in the future.