The subject of this paper is the explicit momentum construction of complete Einstein metrics by ODE methods. Using the Calabi ansatz, further generalized by Hwang-Singer, we show that there are ...non-trivial complete conformally Kähler–Einstein metrics on certain Hermitian holomorphic vector bundles and their subbundles over complete Kähler–Einstein manifolds. In special cases, we give the explicit expressions of these metrics. These examples show that there are a compact Kähler manifold
M
and its subvariety
N
whose codimension is greater than 1 such that there is a complete conformally Kähler–Einstein metric on
M
–
N
.
Landsat-7 Enhanced Thematic Mapper Plus (ETM+) and Landsat-8 Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) are currently operational for routine Earth observation. There are ...substantial differences between instruments onboard both satellites. The enhancements achieved with Landsat-8 refer to the scanning technology (replacing of whisk-broom scanners with two separate push-broom OLI and TIRS scanners), an extended number of spectral bands (two additional bands provided) and narrower bandwidths. Therefore, cross-comparative analysis is very necessary for the combined use of multi-decadal Landsat imagery. In this study, 3,311 independent sample points of four major land cover types (primary forest, unplanted cropland, swidden cultivation and water body) were used to compare the spectral bands of ETM+ and OLI. Eight sample plots with different land cover types were manually selected for comparison with the Normalized Difference Vegetation Index (NDVI), the Modified Normalized Difference Water Index (MNDWI), the Land Surface Water Index (LSWI) and the Normalized Burn Ratio (NBR). These indices were calculated with six pairs of ETM+ and OLI cloud-free images, which were acquired over the border area of Myanmar, Laos and Thailand just two days apart, when Landsat-8 achieved operational obit. Comparative results showed that: (1) the average surface reflectance of each band differed slightly, but with a high degree of similarities between both sensors. In comparison with ETM+, the OLI had higher values for the near-infrared band for vegetative land cover types, but lower values for non-vegetative types. The new sensor had lower values for the shortwave infrared (2.11-2.29 mu m) band for all land cover types. In addition, it also basically had higher values for the shortwave infrared (1.57-1.65 mu m) band for non-water land cover types. (2) The subtle differences of vegetation indices derived from both sensors and their high linear correlation coefficient (R2 > 0.96) demonstrated that ETM+ and OLI imagery can be used as complementary data. (3) LSWI and NBR performed better than NDVI and MNDWI for cross-comparison analysis of satellite sensors, due to the spectral band difference effects.
•Three categories of remotely-sensed methods at different scales for swidden agriculture are reviewed.•Landsat has promoted the remote sensing of swidden agriculture since the 1970s.•Spectral ...feature-based algorithms dominate the remote sensing of swidden agriculture.•The slash and burn signature lays the foundation for remote sensing of swidden agriculture.•New data and algorithms are the directions for remote sensing of swidden agriculture.
The evolution and transformations of swidden agriculture closely relate to ongoing studies of carbon budget, biological diversity, ecosystem services, indigenous wellbeing, and ethnic identities. Remote sensing has become an important tool to quantify and display swidden agriculture in transition. Due to its diversity, dynamics, and complexity, however, the development of universal remote sensing algorithms for mapping swidden agriculture faces major challenges. Although swidden agriculture has increasingly become the focus of climate change and other sustainable initiatives, a systematic review on remote sensing of this farming system in the tropics is seldom reported. With 89 peer-reviewed journal articles related to the identification, mapping, monitoring or forecast of swidden agriculture since the 1970s, the methodological progresses and shortcomings of remote sensing of swidden agriculture were compared and summarized, followed by providing several perspectives on near-future research directions. The conclusions were drawn as follows. Firstly, the limited remote sensing studies of swidden agriculture were mostly reported in Southeast Asia (66%), followed by Latin America and Central Africa. Secondly, Landsat is the most (59%) widely applied to monitor swidden agriculture and also holds huge potential in delineating its trajectories. Thirdly, the spectral feature-based algorithms (76%) are more used than the counterparts of geometric-based and structure-based ones. Finally, several aspects including exploring new data and algorithms for the enhancement of remote sensing of swidden agriculture have been proposed. This review paper might be served as a long-delayed appeal for strengthening remote sensing of swidden agriculture in the tropics for accurately assessing the carbon budget of forest change and other ecological and social effects caused by the evolution and transformations of swidden agriculture.
Currently, fires (e.g., biomass burning and/or straw burning) are still prevailing and serious globally. However, the issue of the characteristics, types, and drives of fire occurrence is always a ...challenge and varies distinctively worldwide. Using Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 (C6) active fire products during 2001–2020, here, we analyzed the occurrence frequencies and spatiotemporal characteristics of active fires at the provincial and regional to national scales and at the monthly and annual scales in China. The accumulated occurrence frequencies of MODIS C6 active fires in China were up to 184.91 × 104 in the past two decades, and the average annual level was 9.25 × 104, especially in 2014 (15.20 × 104). The overall trend of active fires was rising and then falling, but with significant spatial and temporal differences in the last 20-years. Temporally, nearly 61% of active fires occurred in spring (36%) and autumn (25%), particularly in August (16%), April (14%), and October (13%). Spatially, about 90% of active fires occurred in the east of the Hu Huanyong Line, particularly in Northeast China (25%), South China (23%), and East China (20%). In China, the most active fires were concentrated in the Northeast Plain, the North China Plain, the southeast hills, and the Yunnan–Kweichow Plateau. In terms of temporal differences across regions, active fires in Northeast China, North China, and Northwest China were concentrated in spring and autumn, especially in March, April, and October; in East China, they were concentrated in summer, especially in June; and in South China and Southwest China, they were concentrated in winter and spring, especially from December to April of the following year. Our study provides a full analysis of spatio–temporal characteristics and changes of active fires in China, and it can also assist in supplying a beneficial reference for higher monitoring and controlling of fires such as straw burning.
•Globally, national borderlands are undergoing rapid and widespread land use and land cover changes.•About 20% of global population growth occurred in borderlands, leading to over one-third of forest ...loss.•Land use changes due to agricultural expansion and forest retreat characterize the landscape of the borderlands.•We appeal a border-related scientific platform of Global Border Watch, to investigate border land use change and its consequences.
National borderlands as the forefront of geo-cooperation, are experiencing rapid and widespread land use and land cover changes, such as rubber boom in Southeast Asia. The booming agro-industries in the borderlands have improved the livelihoods of indigenous people and cross-border dwellers but may have ecological and environmental consequences and controversies, such as cross-border land grabbing. Although previous research has significantly enhanced our knowledge of land use/cover changes and geo-cooperative mechanisms, the relationship between border geography (via land use changes) and geopolitical economics, as well as their processes, mechanisms, and models for measuring cause–effect and impact–response relationships, has been seldom reported. We thus propose that a joint action, such as setting up a scheme or strategy of a supranational level (e.g., United Nations (UN)) Global Border Watch (GBW), to be initiated to focus on inter- and multi-disciplinary studies involving geography, land system science, remote sensing science, and international relations. In particular, studies should be enhanced to investigate the relationship between border geography and geopolitical economics, as well as the processes, mechanisms, and effects under the umbrella of the UN GBW.
•A red-edge spectral indices (RESI) was firstly applied to detect rubber plots.•The 20-m Sentinel-based map of the rubber was first generated in northern Laos.•A twelfth of rubber plantations were ...planted in the protected area of Luang Namtha.
Rubber (Hevea brasiliensis) tree cultivation is being continuously expanded northward by replacing evergreen forests and swidden-related regenerated vegetation across the uplands of mainland Southeast Asia (MSEA), e.g., Laos, a landlocked mountainous country. The non-native tree establishment in the northern tropical edge, or the non-traditional suitable planting area, provides stable supplies of natural latex, yet also leads to severe ecological degradation and environmental effects in water conservation, soil quality, rainforest fragmentation and biodiversity. Rubber plantations in the northern part of MSEA are normally characterized by periodic deciduous during the dry season, along with a lengthy defoliation-foliation duration, because of seasonal variations in temperature and precipitation. It thus lays a phenological and physiological base for dynamics monitoring with common multispectral (e.g., near-infrared and short-wave infrared bands) satellites, particularly Landsat. However, whether Sentinel-2 red-edge based algorithms are suitable for discriminating rubber plantations is not yet exclusively reported. Here, we developed a red-edge spectral indices (RESI) method through the normalization of three red-edge bands and applied it to identify and map rubber plantations in Luang Namtha Province of northern Laos, where a rubber boom begun in the mid-2000s. The RESI algorithm highlights the sensitivity of red-edge bands to the changes in moisture content and canopy density of rubber plantations. The area of mature rubber plantations was estimated to be 771.2 km2 in this province bordering southwest China in 2018, which was nearly twice as much as that of 2011, with the overall accuracy and kappa coefficient up to 92.50% and 0.91, respectively. Our phenology-based RESI approach not only indicates that Sentinel-2 imagery holds significant potential for monitoring rubber plantations, but also improves the remotely-sensed methods of rubber boom mapping via introducing the red-edge channel.
(1) Background: Fire affects global agricultural and/or forest ecosystems with high biomass accumulation. However, the delineation of fire-hazardous areas based on satellite-derived active fire ...intensity is not well-studied. Therefore, examining the characteristics of fire occurrence and development plays an important role in zoning fire-hazardous areas and promoting fire management. (2) Methods: A fire intensity (FI) index was developed with Visible Infrared Imaging Radiometer Suite (VIIRS) active fires and then applied to identify fire-hazardous areas in Northeast China. Combined with terrain, land cover and net primary productivity (NPP), the spatial and temporal characteristics of active fire occurrence were consistently analyzed. Next, a conceptual decision tree model was constructed for delineating fire-induced patterns impacted by varied factors in Northeast China. (3) Results: The accidental, frequent, prone and high-incidence areas of active fires defined by the FI index accounted for 31.62%, 30.97%, 26.23% and 11.18%, respectively. More than 90% of active fires occurred in areas with altitude <350 m above sea level (asl), slope <3° and NPP between 2500–5000 kg·C/m2. Similarly, about 75% occurred in cropland and forest. Then, four fire-induced conceptual patterns driven by different factors were classified, including the agricultural and forest active fire-induced patterns (i.e., the Agri-pattern and FRST pattern) with NPP ranging 2500–5000 kg·C/m2, and two others related to settlements and unused land with an altitude <350 m asl. The Agri-pattern dominates in Northeast China because of agricultural straw burning. (4) Conclusions: Despite the national bans of open burning of straws, active fires due to agricultural production have occurred frequently in Northeast China in the last decade, followed by small and sporadic forest fires. The approach for defining fire-hazardous areas and varied fire occurrence patterns is of significance for fire management and risk prediction at continental to global scales.
In this paper, we use the the well-known Calabi ansatz, further generalized by Hwang–Singer and Apostolov–Calderbank–Gauduchon, to study the existence of constant scalar curvature Kähler (cscK for ...short) metrics and balanced metrics on certain holomorphic polyball bundles
M
which are locally expressed as
M
=
{
(
z
1
,
…
,
z
N
,
u
1
,
…
,
u
ℓ
)
∈
∏
j
=
1
N
Ω
j
×
∏
i
=
1
ℓ
C
r
i
:
e
∑
j
=
1
N
λ
ij
ϕ
j
(
z
j
)
‖
u
i
‖
2
<
1
,
1
≤
i
≤
ℓ
}
.
Let
g
F
be the natural Kähler metrics on
M
associated with the Kähler forms locally expressed as
ω
F
=
-
1
∂
∂
¯
(
∑
j
=
1
N
ν
j
ϕ
j
(
z
j
)
+
∑
i
=
1
ℓ
f
i
(
∑
j
=
1
N
λ
ij
ϕ
j
(
z
j
)
+
log
‖
u
i
‖
2
)
)
. Firstly, we obtain sufficient and necessary conditions for
g
F
to be cscK metrics. Secondly, using this result, we obtain necessary and sufficient conditions for
m
g
F
to be balanced metrics for all sufficiently large positive integer numbers
m
. Finally, we obtain complete cscK metrics and balanced metrics on the polyball bundles over simply connected Riemann surfaces, or products of simply connected Riemannian surfaces and unit balls. The main contribution of this paper is the explicit construction of complete, non-compact cscK metrics and balanced metrics.
The study of urban spatial interaction is closely linked to that of economic geography, urban planning, regional development, and so on. Currently, this topic is generating a great deal of interest ...among researchers who are striving to find accurate ways to measure urban spatial interaction. Classical spatial interaction models lack theoretical guidance and require complicated parameter-adjusting processes. The radiation model, however, as proposed by Simini et al. with rigorous formula derivation, can simulate directional urban spatial interaction. We applied the radiation model in China to simulate the directional migration number among 337 nationwide research units, comprising 4 municipalities and 333 prefecture-level cities. We then analyzed the overall situation in Chinese cities, the interaction intensity hierarchy, and the prime urban agglomerations from the perspective of migration. This was done to ascertain China's urban spatial interaction and regional development from 2000 to 2010 to reveal ground realities.
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