Xi Nan

Land Cover Mapping in Southwestern China Using the HC-MMK Approach

Land cover mapping in mountainous areas is a notoriously challenging task due to the rugged terrain and high spatial heterogeneity of land surfaces as well as the frequent cloud contamination of satellite imagery. Taking Southwestern China (a typical mountainous region) as an example, this paper established a new HC-MMK approach (Hierarchical Classification based on Multi-source and Multi-temporal data and geo-Knowledge), which was especially designed for land cover mapping in mountainous areas. This approach was taken in order to generate a 30 m-resolution land cover product in Southwestern China in 2010 (hereinafter referred to as CLC-SW2010). The multi-temporal native HJ (HuanJing, small satellite constellation for disaster and environmental monitoring) CCD (Charge-Coupled Device) images, Landsat TM (Thematic Mapper) images and topographical data (including elevation, aspect, slope, etc.) were taken as the main input data sources. Hierarchical classification tree construction and a five-step knowledge-based interactive quality control were the major components of this proposed approach. The CLC-SW2010 product contained six primary categories and 38 secondary categories, which covered about 2.33 million km(2) (accounting for about a quarter of the land area of China). The accuracies of primary and secondary categories for CLC-SW2010 reached 95.09% and 87.14%, respectively, which were assessed independently by a third-party group. This product has so far been used to estimate the terrestrial carbon stocks and assess the quality of the ecological environments. The proposed HC-MMK approach could be used not only in mountainous areas, but also for plains, hills and other regions. Meanwhile, this study could also be used as a reference for other land cover mapping projects over large areas or even the entire globe.

A Cost-Constrained Sampling Strategy in Support of LAI Product Validation in Mountainous Areas

Increasing attention is being paid on leaf area index (LAI) retrieval in mountainous areas. Mountainous areas present extreme topographic variability, and are characterized by more spatial heterogeneity and inaccessibility compared with flat terrain. It is difficult to collect representative ground-truth measurements, and the validation of LAI in mountainous areas is still problematic. A cost-constrained sampling strategy (CSS) in support of LAI validation was presented in this study. To account for the influence of rugged terrain on implementation cost, a cost-objective function was incorporated to traditional conditioned Latin hypercube (CLH) sampling strategy. A case study in Hailuogou, Sichuan province, China was used to assess the efficiency of CSS. Normalized difference vegetation index (NDVI), land cover type, and slope were selected as auxiliary variables to present the variability of LAI in the study area. Results show that CSS can satisfactorily capture the variability across the site extent, while minimizing field efforts. One appealing feature of CSS is that the compromise between representativeness and implementation cost can be regulated according to actual surface heterogeneity and budget constraints, and this makes CSS flexible. Although the proposed method was only validated for the auxiliary variables rather than the LAI measurements, it serves as a starting point for establishing the locations of field plots and facilitates the preparation of field campaigns in mountainous areas.

Coupling stability-enhancing mechanism with compact self-recirculating injection in an axial flow compressor

Compact self-recirculating injection that bleeds air from the casing downstream of a rotor blade row and injects the air as a wall jet upstream of the same rotor blade row is experimentally studied after the elaborated design of its structure. The bleed ports, injection ports, and recirculating channels are circumferentially discrete and occupy only 38% of the circumference. Separate tip air injection and outlet bleed air are simultaneously selected for comparison with the self-recirculating injection. Results show that the compact self-recirculating injection can improve the most stall margin by 6.12% among all the three cases on the premise of no efficiency penalty and can also enhance the efficiency (maximum of 1%) for only 0.47% of the total injected momentum ratio recirculated near stall. The details of the flow field are obtained using a multihole probe, a time-resolved Kiel probe, and pressure transducers. The detailed comparative analysis of the characteristic flow in terms of tip leakage flow, blade load, rotor wake feature, and blockage indicates that the self-recirculating injection can postpone the occurrence of stalling in the proposed compressor through a coupling influencing mechanism. One mechanism is to weaken the self-induced unsteadiness of tip leakage flow and to delay the forward movement of the interface between the tip leakage flow and the main flow. The other mechanism is to unload the blade tip and to recover the rotor wake. All these responses can lead to improved stall margin in the self-recirculating injection. This study may be helpful to guide the design of self-recirculating injection in actual application.

The Dynamics of Prestall Process in an Axial Low-Speed Compressor With Single Circumferential Casing Groove

The fact that the location of single circumferential casing groove can have a large impact on the stall margin of axial compressors has been actively investigated in recent years. However, it remains a tough challenge to numerically predict the groove performance and clarify its underlying mechanism on the difference of stall margin improvement (SMI) for different groove locations. In this paper, a single rotor, which had been proven to be a tip sensitive rotor with spike type stall inception, is tested and numerically simulated with an unsteady Reynolds averaged Navier-Stokes (URANS) solver. The test results show that the rear grooves perform better than the front grooves in this rotor. A multi-passage numerical scheme is used to capture the prestall process involving the unsteady cross-passage flow interaction. Although the calculation did not fully capture the measured trend of stall margin improvement, the numerical result did show that the front groove, which is the closest to the leading edge, generates the worst stall margin extension, and the rear groove, which is located right behind the front groove, gives the best stall margin improvement.The prestall dynamics for smooth casing and the two typical grooves are chosen for a comparative study to clarify the underlying mechanism. Three different prestall processes are found. For smooth casing, a rotating disturbance evolves into spike after the interface between tip leakage flow (TLF) and incoming main flow (MF) spills in front of the leading edge. For the front groove, the interface is prevented by the groove to move forward during the throttling process. A modal wave is captured before stall. When the rear groove is applied, the interface location as a function of flow coefficient behaves much similar to the case of smooth casing. However, there is no any rotating disturbance, neither the modes nor the spike, with this groove. The flow is symmetric until all the passages break down at the same time.Copyright

Numerical Study on Effects of Axial-Slot Casing Treatment on Peak Efficiency of Axial Compressors

Axial compressors can obtain substantial improvement on stall margin by using axial-slot casing treatments. However, this type of casing treatment usually yields large peak efficiency penalty due to the interaction between the slots and rotor tip region where the tip leakage flow plays an important role. Therefore, as a main factor that influences the peak efficiency, the tip leakage loss was examined in this paper with a variety of slot geometries. Unsteady numerical simulations were performed on both low speed and transonic compressors with axial skewed slot casing treatments with different geometric parameters. In addition, an equation which can be applied to evaluate the tip leakage loss under casing treatment cases was derived from Denton’s leakage mixing model. The leakage loss can be expressed in terms of the cube of the tip leakage flow rate. Combined with the simulation results, the effects of the number, depth and width of the slots on both the leakage loss and peak efficiency deficit were investigated. For the transonic compressor, the impacts of shock wave and its interaction with the tip leakage flow /vortex were assessed as well. Lastly, two axial-slot casing treatments with an isosceles-trapezoid shaped opening were designed to reduce the loss in the rotor tip region. It was shown that the newly designed axial-slot casing treatments were capable of improving the peak efficiency of both compressors.Copyright

The Analysis of Axial Momentum of the Rotor Tip Flows for Axial Compressors With Circumferential Grooves

A new control volume analysis is developed in this paper aiming at assessing the circumferential grooves effectiveness on stability improvement. The underlying mechanism for this approach is based on the hypothesis that the spike stall precursors can be triggered by the forward spillage of the rotor tip leakage flow and the onset condition of such a spillage is determined by the axial momentum balance within the rotor tip region. Control volumes are defined to quantify the axial momentum balance of the whole region where the grooves influence the flow at the rotor tip. The distribution curve of the cumulative axial momentum along the axial chord indicates that the grooves change the rotor tip loading and increase the stability, which is useful to assess the different grooved casings. As an example, multiple-groove configurations for a transonic rotor are analyzed. The results verified the cumulative axial momentum distribution for different grooved casings are in accordance with the stall margin extension variations. Another example is to prescreen the best double-groove configurations for a low speed compressor. By using this current approach, a double-groove configuration was selected and validated by experiments. These examples demonstrate the current approach has great potential in helping pre-screen circumferential grooves. As an important issue, peak efficiency changed by the grooves is discussed in the last section. Entropy production is quantitatively compared with and without grooves.Copyright

Seamless Upscaling of the Field-Measured Grassland Aboveground Biomass Based on Gaussian Process Regression and Gap-Filled Landsat 8 OLI Reflectance

The spatially explicit aboveground biomass (AGB) generated through upscaling field measurements is critical for carbon cycle simulation and optimized management of grasslands. However, the spatial gaps that exist in the optical remote sensing data, underutilization of the multispectral data cube and unavailability of uncertainty information hinder the generation of seamless and accurate AGB maps. This study proposes a novel framework to address the above challenges. The proposed framework filled the spatial gaps in the remote sensing data via the consistent adjustment of the climatology to actual observations (CACAO) method. Gaussian process regression (GPR) was used to fully exploit the multispectral data cube and generated the pixelwise uncertainty concurrent with the AGB estimation. A case study in a 100 km × 100 km area located in the Zoige Plateau, China was used to evaluate this framework. The results show that the CACAO method can fill almost all of the gaps, accounting for 93.1% of the study area, with satisfactory accuracy. The generated AGB map from the GPR was characterized by a relatively high accuracy (R2 = 0.64, RMSE = 48.13 g/m2) compared to vegetation index-derived ones, and was accompanied by a corresponding uncertainty map that provides a new source of information on the credibility of each pixel. This study demonstrates the potential of the joint use of gap-filling and machine-learning methods to generate spatially explicit AGB.

Spatio-Temporal Patterns of the Net Primary Productivity in Southern Himalayas During 2001–2015

Vegetation in southern Himalayas is susceptible to both extrinsic (e.g., climate change) and intrinsic (e.g., earthquakes) factors. Analysis of spatio-temporal patterns and variability of net primary productivity (NPP) help to understand ecological functioning in this area. Based on MODIS net primary productivity data (MOD17A3H), we investigated the spatial distribution of NPP values and its spatio-temporal variation in southern Himalayas during 2001–2015 with the methods of gross statistics, correlation analysis and spatial statistics. The impacts of the Nepal Ms8.1 earthquake in April 2015 were also analyzed. The results indicate that: (1) in the past 15 years, NPP in southern Himalayas maintains a growth trend in general, with an average amount of 1.60 g Cm−2 a−1, while the tendency appears negative at lower altitudes below 1000 m, and positive in the mid-altitude areas and above; (2) NPP in southern Himalayas is characterized by vertical altitudinal belts, among which, NPP reaches to a maximum at the altitude of 1200–2700 m with an average of about 800 g Cm−2 a−1; (3) The average NPP values of evergreen broad-leaved forest, deciduous forest, coniferous forest, grassland, sparse vegetation in southern Himalayas were 835.0, 711.6, 623.9, 144.8, 25.5 g Cm−2 a−1, respectively, and their average growth rates are 5.08, 2.17, 1.135, 0.7086, 0.22 g Cm−2 a−1; (4) Along the major axis of high seismic intensive region, anomalous decreases of NPP could be found in a period of five years before earthquake (from 2010 to 2014). There is a positive correlation between these anomalous areas and average temperatures, but negative correlation with rainfall, which suggests that anomalous decrease of NPP may correlate with tectonic activity before the earthquake occurs.

Postearthquake Landslides Mapping From Landsat-8 Data for the 2015 Nepal Earthquake Using a Pixel-Based Change Detection Method

The 2015 Nepal earthquake and its aftershocks not only caused huge damage with severe loss of life and property, also induced many geohazards with the major type of landslide which should bring continuous threats to the affected region. To gain a better understanding of the landslides induced by this earthquake, we proposed a pixel-based change detection method for postearthquake landslide mapping by using bitemporal Landsat-8 remote sensing data [May 29, 2014 (pre-earthquake) and June 1, 2015 (postearthquake)]. Two river valleys (Trishuli river valley and Sun Koshi river valley) that contain important economic arteries linking Nepal and China were selected as the study areas. Validation of the mapping results with postearthquake high-resolution images from Google Earth shows that the pixel-based landslide mapping method is able to identify landslides with relatively high accuracy, and it also approves the applicability of Landsat-8 satellite for landslide mapping with its multispectral information. The spatial distribution analysis indicates that both river valleys are substantially affected by landslides, and the situation is even more serious in the high mountain areas. Landslides are generally found in areas of high elevation and large surface slopes, with mean values above 1600 m and 30°, respectively. These findings suggest that these areas suffer greatly from these geohazards, and the threat will continue for the next few years.

The Evaluation of Water Resources Sustainable Utilization in Kosi Basin Based on DPSIR Model

Kosi basin located between the north Nepal and south Tibet of China has abundant water resources. Its recharge sources are glacier meltwater and precipitation in this area. However due to the topographical effects, the distribution of rainfall is uneven in space and time. In order to guarantee the harmonious development of social economy, it’s necessary to evaluate the sustainable utilization of water resources. This paper adopts GIS spatial analysis methods to establish evaluation indexes of water resources sustainable based on DPSIR model, and determine the weights of each evaluation index by utilizing analytic hierarchy process. The results show that: (1) the proposed methods can reflect the development level of regional water resources system and its harmonious state with society and economy. (2) The comprehensive evaluation value changes are between 4.0 and 6.9. (3) The sustainable utilization level gradually increases from northwest to southeast. (4) The presented methods can quantitatively evaluate the sustainability of regional water resources.