Zhiwei Wang

The photometric behavior of the lunar surface provides important constraints on regolith scattering processes and serves as a key tool for interpreting remote sensing observations and understanding the physical mechanisms responsible for the opposition effect. In this study, lunar observations acquired by the Advanced Geostationary Radiation Imager (AGRI) onboard the Fengyun-4B (FY-4B) geostationary meteorological satellite are processed to construct a photometric dataset spanning wavelengths from 470 nm to 2230 nm and phase angles from −133.7° to 122.7°. From this dataset, disk-integrated reflectance is derived, and phase curves in five spectral bands are constructed and modeled using the Hapke photometric formulation. The opposition surge amplitude exhibits weak wavelength dependence within the sampled range, suggesting that shadow-hiding effects dominate over coherent backscatter at disk-integrated scale. While most band ratios increase monotonically with phase angle, ratios involving the 2230 nm band exhibit a distinct arch-shaped dependence whose peak phase angle increases systematically with wavelength separation, indicating a transition in scattering regimes toward the shortwave infrared. Photometric correction tests demonstrate that the retrieved Hapke parameters provide effective reflectance normalization, with cross-phase reflectance stability better than ∼8%. These results demonstrate that geostationary disk-integrated observations provide robust constraints on lunar photometric properties and contribute to improved characterization of regolith scattering at global scales.

Fengyun-4 (FY-4) is a series of Chinese operational geostationary meteorological satellites, providing crucial data for weather forecasting, climate prediction, and environmental monitoring. Advanced Geostationary Radiation Imagers (AGRI) onboard the FY-4A and FY-4B satellites play a key role in observing the Earth’s surface, oceans, and atmosphere. However, their calibration stability is still uncertain, which clearly limits corresponding downstream applications. This study evaluates the long-term radiometric stability of AGRI reflective solar bands (RSBs) using a general cloud target (CT) calibration method, covering the periods from March 2018 to December 2024 for FY-4A/AGRI and from June 2022 to December 2024 for FY-4B/AGRI. By utilizing MODIS cloud products as references for cloud properties, we simulate the top-of-atmosphere (TOA) reflectances of CTs through the Discrete Ordinates Radiative Transfer (DISORT) model and compare results with observed reflectances to infer the instrumental calibration stability. Our results indicate that the radiometric responses of AGRI exhibit significant degradation in visible bands, while showing relatively smaller degradation rates in near- and shortwave-infrared bands. Specifically, the annual degradation rates for band 1 (0.47 μm) of FY-4A/AGRI and FY-4B/AGRI are 4.3% and 8.8% respectively. Both AGRIs demonstrate comparable degradation rates of approximately 3.5% for band 2 (0.65 μm). In contrast, bands 3 (0.83 μm), 5 (1.61 μm), and 6 (2.25 μm) show annual degradation rates around −1.0%, despite they exhibit notable fluctuations. The operational calibration of FY-4B/AGRI is more accurate than that of FY-4A/AGRI and with smaller fluctuations. By fitting the time series of relative errors between simulated and current calibrated reflectances, we calculate daily recalibration coefficients and effectively recalibrated the long-term data, with a calibration accuracy within ±3%. This study demonstrates that the CT-based calibration method can successfully track the radiometric stability of AGRI and provide a robust calibration solution to ensure data stability and accuracy.

The Advanced Geostationary Radiation Imager (AGRI) of the FY-4B satellite employs a set of reflective solar band (RSB) onboard calibration components, with a solar diffuser (SD) and a solar diffuser reflectance degradation monitor (SDRDM). The SDRDM is primarily utilized to monitor the degradation of the SD bidirectional reflectance distribution function (BRDF) during the satellite’s on-orbit period and to provide degradation correction coefficients. In this work, the degradation coefficients of the SD BRDF in the visible near-infrared (NIR) band are calculated on the basis of the observation dataset obtained with the SDRDM from August 2021 to March 2023. The time series of calculated SD degradation coefficients exhibit notable anomalous fluctuations due to the cosine error in the incidence angle associated with the placement of the calibration device and the variation in stray light with the illumination angle. A systematic degradation coefficient evaluation method based on the optical mechanical structure characteristics and working principle of the SDRDM is proposed. The SD BRDF degradation coefficient is obtained by combining time series data with the same geometric lighting conditions and interband degradation coefficient ratios and removing the relative angular changes and associated effects. With the stable CH3 band as the reference band, geometric factors are reconstructed for other bands for subsequent SD degradation monitoring. Finally, the monitoring results are applied to observed SD data for verification, and from August 2021 to March 2023, the SD BRDF degradation was 2.3% at 0.45-0.55 μm and almost no degradation at 0.55-0.90 μm.

The payload Geostationary High-speed Imager (GHI) onboard the Fengyun-4B satellite employs the technology of the long linear array detectors. The long-wave infrared channel of GHI exhibits significant non-uniformity, manifested as brightness temperature discontinuities at the edges of adjacent swaths and abnormal signal value drops at the edges of the full-disk targets. To address this issue, ground tests were conducted on the GHI backup unit, identifying channel crosstalk within the infrared bands, which further impacted the radiometric calibration coefficients. Based on these ground tests, a linear crosstalk model was established along with a solution method. Subsequently, a crosstalk parameter solving algorithm suitable for long linear array detectors was proposed. This algorithm can solve the crosstalk coefficients using data from the edge regions of the full-disk in orbit and use them for correction. After applying this algorithm, the crosstalk phenomenon was well corrected, reducing non-uniformity by more than 90%. Furthermore, a fixed-scene processing method based on the generalized inverse matrix of least squares was proposed, enabling automated operation of the crosstalk correction algorithm. Currently, this algorithm has been deployed in the Data Acquisition & Tele-command System (DTS) of the Fengyun satellite, where it is used for real-time correction of GHI’s data.

Inter-basin water transfer projects are designed to relieve water scarcity around the world. However, ecological problems relating to reductions in protection zone functions can occur during inter-basin transfers. This paper uses the largest inter-basin water transfer project in the world, namely, the South-to-North Water Transfer Project (SNWTP) in China, as an example to analyze the variation of Miyun Reservoir’s inner protection zone functions when water is transferred. Specifically, a riparian model (RIPAM) coupled with remote sensing data were used to calculate the nitrogen (N) and phosphorus (P) losses due to plant uptake, and these results were validated by in situ survey data. Then, correlations between water levels and N and P removal were analyzed. The results show that water table disturbances resulting from elevated water levels strongly influence the growth of plants and have obvious negative impacts on N and P removal in the inner protection zone. With the implementation of the middle route of the SNWTP, the water level of Miyun will rise to 150 m in 2020, and subsequently, the total net primary productivity (NPP) could decline by more than 40.90% from the level in 2015, while the N and P uptake could decline by more than 53.03% and 43.49%, respectively, from the levels in 2015, according to the modeling results. This will lead to declines in the inner protection zone’s defense effectiveness for N and P interception and increases in risks to the security of water resources. The results of this study provide useful knowledge for managing the defense function of the terminal reservoir’s inner protection zone and for ensuring that water quality is maintained during the diversion process.

Vegetation deterioration and soil loss are the main causes of more precipitation leakages and surface water shortages in degraded karst areas. In order to improve the utilization of water resources in such regions, water storage engineering has been considered; however, site selection and cost associated with the special karstic geological structure have made this difficult. According to the principle of the Soil Plant Atmosphere Continuum, increasing both vegetation cover and soil thickness would change water cycle process, resulting in a transformation from leaked blue water (liquid form) into green water (gas or saturated water form) for terrestrial plant ecosystems, thereby improving the utilization of water resources. Using the Soil Vegetation Atmosphere Transfer model and the geographical distributed approach, this study simulated the conversion from leaked blue water (leakage) into green water in the environs of Guiyang, a typical degraded karst area. The primary results were as follows: (1) Green water in the area accounted for normal year > rainy year. Thus, increased soil thickness was shown effective in improving the utilization ratio of water resources and in raising the amount of plant ecological water use; (4) The transformation of blue water into green water, which avoids constructions of hydraulic engineering, could provide an alternative solution for the improvement of the utilization of water resources in degraded karst area. Although there are inevitable uncertainties in simulation process, it has important significance for overcoming similar problems.

The transport characteristics of phosphorus in soil and the assessment of its environmental risk have become hot topics in the environmental and agricultural fields. The Sanjiang Plain is an important grain production base in China, and it is characterised by serious land use change caused by large-scale agricultural exploitation. Agricultural inputs and tillage management have destroyed the soil nutrient balance formed over long-term conditions. There are few studies on non-point source phosphorus pollution in the Sanjiang Plain, which is the largest swampy low plain in a mid-high-latitude region in China. Most studies have focused on the water quality of rivers in marsh areas, or the export mechanism of phosphorus from specific land uses. They were conducted using experimental methods or empirical models, and need further development towards mechanism models and the macro-scale. The question is how to find a way to couple processes in phosphorus cycling and a distributed hydrological model considering local hydrological features. In this study, we report an attempt to use a distributed phosphorus transport model to analyse non-point source total phosphorus pollution from different land uses and soil types on the Sanjiang Plain. The total phosphorus concentration generally shows an annually increasing trend in the study area. The total phosphorus load intensity is heterogeneous in different land use types and different soil types. The average total phosphorus load intensity of different land use types can be ranked in descending order from paddy field, dry land, wetlands, grassland, and forestland. The average total phosphorus load intensity of different soil types can be ranked in descending order: paddy soil, bog soil, planosol, meadow soil, black soil, and dark brown earth. The dry land and paddy fields account for the majority of total phosphorus load in the study area. This is mainly caused by extensive use of phosphate fertilizer on the cultivated land. This has important implications for future agricultural management and non-point source control in this agricultural area of the mid-high latitude region.