2015年厄尔尼诺年东南亚主要国家活跃火发生类型与影响分析

李文君, 肖池伟, 封志明, 李鹏, 祁月基

自然资源学报 ›› 2020, Vol. 35 ›› Issue (10) : 2539-2552.

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自然资源学报 ›› 2020, Vol. 35 ›› Issue (10) : 2539-2552. DOI: 10.31497/zrzyxb.20201018
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2015年厄尔尼诺年东南亚主要国家活跃火发生类型与影响分析

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Occurrence types and impact analysis of active fires in the major countries of Southeast Asia during the 2015 strong El Nino

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摘要

2015年强厄尔尼诺引起东南亚干旱少雨与活跃火加剧,但目前有关活跃火时空特征、发生类型与强度及其对人口—社会经济影响仍分析不足。利用美国国家航空航天局火灾信息资源管理系统(FIRMS)VIIRS V1活跃火位置矢量产品,通过月际、地形、土地覆被类型等GIS分析确定东南亚主要国家活跃火主要类型,并评价各国活跃火对人口分布的影响范围与国别差异。结果表明:(1)东南亚2015年活跃火发生频次达81.40×105次,中南半岛与马来群岛各占69.60%与30.40%,分别集中发生在2-4月与8-10月,各国活跃火频次与发生时间差异很大。(2)中南半岛五国活跃火地形差异明显,缅甸和越南活跃火集中分布于25 m以下;老挝(85~105 m、140~200 m)、泰国(5~15 m、70~110 m)和柬埔寨(5~15 m、70~110 m)活跃火随海拔呈双峰特征;马来群岛国家(印度尼西亚、马来西亚与菲律宾)活跃火集中分布于60 m以下的平原地带;且东南亚35%以上的活跃火均集中分布5~15°的斜坡。(3)东南亚森林、农田活跃火发生率为76%,其中森林活跃火发生率由柬埔寨的52.00%到老挝的74.27%不等,农田活跃火由老挝的13.18%到泰国的42.68%不等。(4)综合活跃火发生月份、海拔、坡度与覆被特征,可从山区刀耕火种农业与平原秸秆焚烧界定东南亚主要国家活跃火发生类型。(5)东南亚活跃火随人口密度增加呈先增后减至平稳变化的趋势,且多集中于人口稀少的乡村和原始森林,其中缅甸、泰国、越南三国有10%以上的活跃火发生在人口密度为72~91人/km2的区域,而马来西亚、老挝、柬埔寨、印度尼西亚30%以上的活跃火集中发生于人口密度在20人/km2以下的区域。

Abstract

The strong El Nino in 2015 caused droughts, little rainfall and intensified active fires in Southeast Asia (SEA). To our knowledge, the spatio-temporal characteristics, occurrence types and intensity of active fires as well as their impacts on population and socio-economy have not been fully analyzed. In this paper, we used the active fire data (point in Shapefile format) derived from Visible Infrared Imaging Radiometer Suite (VIIRS) Version 1 (V1) provided by the NASA's Fire Information for Resource Management System (FIRMS), the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM, 30 m, V2), Global Land Cover products (30 m, 2015), and Gridded Population of the World (GPW, 1 km, V4, 2015). On this basis, GIS-based monthly, topographical, and land-cover category analyses of VIIRS active fires were comprehensively applied to determine the major occurrence types of active fires at national scale, followed by impact evaluation of active fires on national population density in the major counties of Southeast Asia (excluding Brunei, East Timor, and Singapore). The results showed that: (1) The active fire frequency in SEA reached 81.40×105 in 2015, with Mainland Southeast Asia (MSEA) and Insular Southeast Asia (ISEA) accounting for 69.60% and 30.40%, respectively. Active fires in MSEA and ISEA mainly occurred in the periods of February to April and August to October, respectively. There were huge differences in the occurrence frequency and timing of active fires. (2) The topographical features of active fires were quite different in MSEA. Specifically, active fires in Myanmar and Vietnam were distributed at the elevations below 25 m, while those in Laos (85-105 m and 140-200 m), Thailand (5-15 m and 70-110 m) and Cambodia (5-15 m and 70-110 m) showed consistently a bimodal pattern. The ISEA's active fires were primarily distributed in the plains below 60 m. Additionally, over 35% of the occurrence frequency of active fires was mainly observed in the slope range of 5-15. (3) The frequency proportion of active fires in forests and farmland in SEA was estimated to be 76%. Among them, the figures in forests increased from 52.00% in Cambodia to 74.27% in Laos, while those in farmland increased from 13.18% in Laos to 42.68% in Thailand. (4) Combining the comprehensive features of active fires in occurrence timing, elevation, slope and land cover types, national occurrence types of active fires could be discriminated in two aspects, namely upland swidden agriculture and open burning of crop residues in the plain area. (5) With the increment of population density, the occurrence frequency of active fires first increased and then decreased to a stable level in SEA, and active fires mostly concentrated in the sparsely populated countryside and natural forests. Specifically, over 10% of the occurrence frequency of active fires were located in the areas with the population density ranging from 72 to 91 p/km2 in Myanmar, Thailand, and Vietnam, while more than 30% of the active fire counts were found in Malaysia, Laos, Cambodia, and Indonesia, where the population density was below 20 p/km2.

关键词

厄尔尼诺 / 活跃火 / 地形 / 土地覆被 / 发生类型 / 人口密度

Key words

El Nino / active fires / topography / land cover / occurrence types / population density

引用本文

导出引用
李文君, 肖池伟, 封志明, 李鹏, 祁月基. 2015年厄尔尼诺年东南亚主要国家活跃火发生类型与影响分析[J]. 自然资源学报, 2020, 35(10): 2539-2552 https://doi.org/10.31497/zrzyxb.20201018
LI Wen-jun, XIAO Chi-wei, FENG Zhi-ming, LI Peng, QI Yue-ji. Occurrence types and impact analysis of active fires in the major countries of Southeast Asia during the 2015 strong El Nino[J]. JOURNAL OF NATURAL RESOURCES, 2020, 35(10): 2539-2552 https://doi.org/10.31497/zrzyxb.20201018
厄尔尼诺是全球大气—陆地生态系统的重要影响因素[1,2,3],通常引起温度升高与持续干旱[1,2,4-6],导致区域降水减少和陆地蓄水能力增加[7,8,9],进而加剧森林与农田活跃火(Active fires)现象发生[10,11,12]。在厄尔尼诺发生周期内,尤其是8-10月,易引发亚洲热带区域活跃火现象,即东南亚活跃火首先达到顶峰[13]。2000年以来,东南亚屡遭厄尔尼诺[2,14-17],如2001年厄尔尼诺使区域(印度尼西亚等)活跃火发生剧烈[18];2014—2016年的极强厄尔诺现象已对区域气候(干旱少雨)[5]、土地利用与碳储存[15,19]、空气污染与生物多样性[16]经济社会发展造成了不同类型、不同程度的影响[20]。一般而言,厄尔尼诺的影响主要表现为不同土地利用/覆被发生的高强度活跃火。东南亚地处热带,且农业为区内主要国家最主要的经济活动,对国民经济贡献较大。与此同时,东南亚又是全球活跃火高发区[12,21],土地开发(如刀耕火种农业)与农业生产在很大程度上依赖粗放又经济的野火焚烧。在类型上,主要包括山区刀耕火种农业引起的植被焚烧[22,23]、平原土地开发与农田(含泥炭地)秸秆焚烧[12,24]与各种自然原因(雷电等)引发的野火等类型。
遥感为活跃火信息识别、提取与动态监测提供了技术支持与数据基础[21,25,26]。当前,搭载于热带降雨测量任务(TRMM)卫星上的可见光红外成像辐射仪(VIIRS)[27]提供了较高空间分辨率、且全球覆盖的改进型活跃火位置矢量产品(VIIRS V1,375 m),较MODIS公里格网活跃火数据产品精度更高,尤其对小规模植被火情监测更为有效[24,25]。自2012年VIIRS V1活跃火产品发布以来,已广泛应用于火烧信息提取动态监测[28]、厄尔尼诺影响评估[29]、以及森林覆被变化[30]等领域。相比之下,针对VIIRS与MODIS活跃火的发生类型、成因及其对人口分布影响缺乏深入研究[31,32]。此外,当前对VIIRS及其同类活跃火产品(MODIS C6)的信息挖掘仍显不足。
为有效界定东南亚主要国家活跃火发生类型,本文利用美国国家航空航天局火灾信息资源管理系统(FIRMS)发布的VIIRS V1活跃火位置矢量产品,以及先进星载热发射和反射辐射仪全球数字高程模型(ASTER GDEM,30 m)与全球土地覆被类型产品(30 m,2015),基于GIS从活跃火发生月份、地形(海拔、坡度)与土地覆被等角度界定东南亚主要国家(柬埔寨、老挝、缅甸、泰国、越南、印度尼西亚、马来西亚与菲律宾)活跃火的发生类型,并基于全球公里格网人口密度数据,分国家研究活跃火对人口密度的影响范围。

1 研究区概况与数据来源

1.1 研究区概况

东南亚陆地面积为4.54×106 km2,中南半岛与马来群岛各占42.72%和57.28%。中南半岛包括柬埔寨、老挝、缅甸、泰国与越南,马来群岛包括印度尼西亚、马来西亚、菲律宾、文莱、新加坡和东帝汶,共11国(图1)。中南半岛地势北高南低,北部山地为横断山脉向南延伸部分,南北向山系发育了湄公河、伊诺瓦底江、萨尔温江、湄南河及红河等河流,并在河口形成一系列三角洲冲积平原(如湄公河平原)。而马来群岛地形起伏较大,仅在沿海处有小块平原。该区域处于板块交接地带,地壳运动活跃,火山地震活动频繁。在国家层面,东南亚地形国别差异明显。其中,柬埔寨84.49%的面积平均海拔在200 m以下,老挝61.93%的面积平均海拔在500 m以上,缅甸54.05%的面积平均海拔在500 m以上,其他东南亚国家平均海拔介于200~500 m,泰国、菲律宾、印度尼西亚、马来西亚平均海拔在300 m之下的面积均占62%以上。东南亚8国坡度最大值均在72°以上,其中老挝、缅甸、越南、菲律宾、马来西亚五国的坡度均值在10°以上,柬埔寨、泰国、印度尼西亚三国坡度均值在10°以下。就土地覆被类型而言,东南亚8国土地覆被类型以森林和农田为主,面积占各国八成以上,其中,森林面积占比由泰国的41.64%到马来西亚的86.10%不等,农田面积占比由马来西亚的7.27%到泰国的50.72%不等(表1)。
图1 东南亚研究区概况
注:本图基于自然资源部标准地图服务系统下载的标准地图制作,底图无修改。

Fig. 1 Maps showing the topography, land cover types and population density of Southeast Asia

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表1 东南亚8个主要国家地形、土地覆被人口密度

Table 1 Topography, land cover types, and population density of the eight major Southeast Asia (SEA) countries

国家 值/类型 老挝 缅甸 柬埔寨 泰国 越南 菲律宾 印度尼西亚 马来西亚
海拔/m 均值 671.10 590.17 123.56 283.85 395.9 330.68 346.08 281.18
坡度/(°) 最大值 78.75 89.15 72.26 86.27 89.36 89.52 89.54 89.43
均值 14.91 12.97 5.60 8.23 12.16 11.85 9.64 11.38
人口密度/
(人/km2)
最小值 0.18 0.06 0.38 0.03 5.89 0.23 0.85 0.01
最大值 31118 39864 61774 22384 55175 126446 22277 11275
均值 30.88 74.77 91.84 137.03 286.83 346.27 138.1 93.23
土地覆被
类型/%
农田 9.43 25.86 33.42 50.72 38.58 22.22 13.68 7.27
森林 80.88 64.29 55.93 41.64 49.41 67.49 78.09 86.10
草地 8.38 5.74 3.14 4.29 6.29 6.49 3.84 1.89
灌木 0.17 1.33 0 0 1.59 0.03 0 0
湿地 0.17 0.64 4.17 0.62 0.49 0.89 1.85 1.84
水体 0.85 1.20 2.66 1.67 1.66 1.53 1.34 0.91
据联合国人口估计和预测(http://worldpopulationreview.com/),2018年东南亚人口约为6.56×108人,占全球人口的8.59%,是世界上人口较为稠密的地区之一,集中分布于平原和河口三角洲地区。东南亚平均人口密度为145人/km2,但国别人口密度差异明显,从老挝的30.88人/km2到菲律宾的346.27人/km2不等,除新加坡(人口密度约8000人/km2)外。近三十年来,东南亚人口持续增长,而山区土地资源有限,导致传统耕作形式的刀耕火种农业休耕期缩短,即刀耕火种的烧伐阶段更加频繁,加剧了植被焚烧与森林野火发生。

1.2 数据来源及处理

1.2.1 FIRMS 2015年VIIRS V1活跃火产品
美国国家航空航天局(NASA)应用科学计划和联合国粮食及农业组织(FAO)联合开发的火灾信息资源管理系统(FIRMS)发布了可见红外成像辐射仪(VIIRS)V1活跃火数据集。该数据提供了过去24小时、48小时和7天的(近)实时活跃火产品,数据格式包括矢量(Shapefile)、Google Earth(KML)、WMS(Web Map Service)或文本文件格式。本文从FIRMS网站(https://firms.modaps.eosdis.nasa.gov/download/)分国家下载了东南亚11国2012—2017年VIIRS V1活跃火矢量数据产品。每个国家的Shapefile数据均包含相应栅格中心点的经纬度、火烧时间(年/月/日/时/分/秒)、火烧亮度(强度)、采集方位与传感器信息等。VIIRS VI数据在赤道附近重访周期时为12小时,活跃火“点”代表375 m×375 m的像元中心。
表3 东南亚8个主要国家坡度的活跃火发生频率

Table 3 Distribution characteristics of active fire frequency on different slopes in the eight major Southeast Asia (SEA) countries (%)

坡度 东南亚 老挝 缅甸 泰国 越南 柬埔寨 菲律宾 印度尼西亚 马来西亚
平原 19.25 4.34 8.34 12.45 10.78 15.60 6.73 30.31 21.67
缓斜坡 26.61 15.58 17.79 27.26 16.87 42.35 16.77 29.29 29.84
斜坡 35.51 39.70 39.40 35.01 35.41 39.72 42.02 31.77 37.28
陡坡 12.33 26.74 21.71 17.09 23.43 2.05 22.35 6.05 8.87
急坡 4.92 11.19 10.04 6.75 10.61 0.26 9.26 1.80 1.88
陡坡 1.35 2.44 2.70 1.43 2.86 0.02 2.85 0.73 0.44
垂直坡 0.03 0.02 0.02 0.01 0.04 0.00 0.02 0.05 0.02
2012—2017年VIIRS VI活跃火数据统计分析表明,东南亚活跃火频次呈先增后减趋势,由2012年的12.03×105次逐步增长至2014年的16.52×105次,并于2015年(19.55×105次)达峰值,2016年后东南亚活跃火频次持续减少至2017年的8.05×105次,相较其他年份厄尔尼诺年活跃火发生更为剧烈[33]。基于ArcGIS空间分析,本文以活跃火像元点图层提取对应位置的高程坡度土地覆被人口密度信息,统计每一属性分级下的活跃火频次,以揭示并解释活跃火的地理空间分布特征与发生类型及对人口分布的影响。由于活跃火现象集中发生于除文莱、东帝汶和新加坡的东南亚8国,加之这三国国土面积狭小,故本文主要选取东南亚8国活跃火数据开展其发生类型与影响研究。
1.2.2 地形数据、土地覆被数据及人口密度数据
先进星载热发射和反射辐射仪全球数字高程模型(ASTER GDEM)为全球(83°N~83°S)30 m分辨率数字高程产品数据集,按1°×1°分片。它采用全自动化方法对NASA对地观测卫星Terra采集的150万景ASTER存档数据制作而成,已完成去云与异常值等处理,以像元平均值代表GDEM对象区域的最后像素值,其垂直精度为20 m,水平精度为30 m,采用UTM/WGS 84投影。该数据集包括两个文件:DEM和质量评估(QA)数据。2009年美国NASA与日本产经省共同发布GDEM V1(2011年更新到V2),该数据集已在地球物理、水平建模、以及遥感图像配准等方面得到了广泛应用。基于GDEM数据,本文运用ArcGIS 10.5软件提取东南亚海拔与坡度信息。
2015年土地覆被数据,来源于我国国家科技基础条件平台—国家地球系统科学数据共享服务平台http://www.geodata.cn),分辨率为30 m,采用UTM/WGS 84投影,数据格式为栅格(TIF 格式)。土地覆被类型包括全球9类一级分类(农田、森林、草地、灌从、湿地水体苔原、不透水层、裸地等)和25类二级分类(包括水田、阔叶林、牧草地与湖泊等)的地表覆盖类型。该数据集可为研究不同分辨率下全球不同土地覆盖类型提供重要数据基础。该数据集生产所使用的原始数据包括:Landsat 5/7/8、中国高分卫星资源卫星、环境卫星SRTM地形数据等。基于36630个验证样本对全球土地覆盖数据集进行精度验证,一级类整体精度70.2%,可满足大尺度上的精度需求。
2015年人口密度数据,来源于美国NASA社会经济数据与应用中心(Socioeconomic Data and Applications Center)发布的世界栅格人口集(Gridded Population of the World,GPW,http://sedac.ciesin.columbia.edu/data/collection/gpw-v4)。自1995年第一版发布以来,哥伦比亚大学国际地球科学信息网络中心已持续更新至第四版(GPW V4)。该数据集主要基于全球不同空间尺度2010年人口与住房普查结果和相应的地理界线进行模拟预测,并提供了2000—2020年每隔5年的人口统计预测格网化数据产品(包括人口数量与人口密度),空间分辨率为30弧秒(赤道处约1 km)。该数据集可与社会、经济、地球科学和遥感的数据集兼容使用,并在研究与决策中得到普遍使用。

2 结果分析

2.1 东南亚活跃火时间动态分析

利用VIIRS VI数据包含的时间信息对东南亚中南半岛与马来群岛和分国家的活跃火发生时间进行逐月统计(图2)。结果表明,东南亚各国活跃火频次差异明显,缅甸最多达21.26×105次,马来西亚最少(1.30×105次)。此外,中南半岛与马来群岛活跃火逐月分布差异明显,中南半岛活跃火集中发生于旱季的2月(20.20%)、3月(40.50%)和4月(22.70%);而马来群岛活跃火则集中发生于8月(13.48%)、9月(24.51%)和10月(22.93%)。
图2 东南亚8个主要国家活跃火频次时间(逐月)分布特征

Fig. 2 Monthly differences in the number of active fire events in the eight major Southeast Asia (SEA) countries

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东南亚8国活跃火逐月分布地域性差异明显,受厄尔尼诺影响,柬埔寨活跃火最早于2月(发生率达36.41%)出现最大值,其次为越南(33.29%)、泰国(41.66%)和缅甸(51.27%)均于3月达最大值,最后为内陆国家老挝(40.81%)于4月达最大值。马来群岛3国活跃火逐月分布差异较大,马来西亚(19.43%)和菲律宾(31.24%)分别于3月和4月达最大值,印度尼西亚(27.63%)则于9月达最大值。

2.2 东南亚2015年活跃火的地形分布特征

为研究主要地貌单元的活跃火频次分布特征,将其分为<200 m的平原、200~500 m的丘陵、500~1000 m的低山及>1000 m的高山/高原。2015年东南亚活跃火频次随着海拔的升高而减少(图3),但是国别差异非常突出。其中,53.84%的活跃火集中分布在平原,尤以马来群岛较为突出(表2),如印度尼西亚和马来西亚平原活跃火发生率分别达89.18%与88.85%,而菲律宾活跃火则集中于平原(45.14%)和丘陵(29.38%)。此外,柬埔寨(91.99%)和泰国(40.30%)活跃火高度集中在平原,中南半岛另外三国活跃火发生的地形特征表现出较强的波动性。缅甸和越南活跃火较为均匀地分布在平原、丘陵和低山地带,分别占各国活跃火总频次的25%~30%左右;而老挝活跃火则集中于低山(40.19%)和丘陵(29.33%)。相比之下,中南半岛国家活跃火频次随海拔变化特征差异性明显。其中,缅甸和越南呈明显的单峰特征,分别有7.13%、16.71%的活跃火集中分布于0~25 m海拔范围内,可推断5~25 m发生的活跃火主要由秸秆焚烧引起;此外,老挝(100~125 m、150~220 m)、泰国(5~35 m、120~190 m)和柬埔寨(5~15 m、70~110 m)活跃火随海拔分布呈明显的双峰特征,发生率分别达15.28%、26.97%、34.19%。
图3 东南亚8个主要国家活跃火频次海拔分布特征

Fig. 3 Elevational characteristics of active fire frequency in the eight major Southeast Asia (SEA) countries

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表2 东南亚8个主要国家地形类型的活跃火发生频率

Table 2 Topographical characteristics of active fire frequency in the eight major Southeast Asia (SEA) countries (%)

地形 东南亚 老挝 缅甸 泰国 越南 柬埔寨 菲律宾 印度尼西亚 马来西亚
平原 53.84 15.82 27.10 40.30 32.36 91.99 45.14 89.18 88.85
丘陵 19.79 29.33 24.35 27.76 27.30 6.57 29.38 5.26 8.34
低山 19.00 40.19 29.94 27.01 29.46 1.42 18.64 2.80 2.56
高山/高原 7.37 14.66 18.61 4.93 10.88 0.02 6.84 2.76 0.25
类似地,分别统计8国活跃火在不同坡度分布特征,包括平地(<2º)、缓斜坡(2~5º)、斜坡(5~15º)、陡坡(15~25º)、急坡(25~35º)和急陡坡(35~55º)、垂直坡(>55º)7级。表3表明东南亚活跃火频次随坡度加大而呈现先增后减的特征,但是国别差异较为突出。具体地,东南亚8国有35.51%的活跃火分布于斜坡,其次为缓斜坡(26.61%)和平地(19.25%),即超过八成的活跃火分布在20º以下的斜坡,在急坡以上分布极少。中南半岛国家活跃火坡度分布呈较强的规律性,35%以上的活跃火发生于斜坡。其中,老挝、缅甸、泰国、越南以及菲律宾5国的活跃火主要分布在斜坡、陡坡与缓斜坡。而印度尼西亚、马来西亚以及归属于中南半岛的柬埔寨3国的活跃火集中分布在斜坡、缓斜坡与平地(表3)。

2.3 东南亚2015年活跃火的土地覆被类型特征

东南亚8国2015年活跃火集中分布于森林、农田,占比在76%以上(图4)。具体而言,森林活跃火发生率由柬埔寨的52.00%到老挝的74.27%不等;其次,8国活跃火集中在农田发生,由老挝的13.18%到泰国的42.68%不等。针对森林和农田活跃火,进一步统计其二级地类对应的活跃火频次。
图4 东南亚8个主要国家一级土地覆被类型对应活跃火频次分布特征

Fig. 4 Distribution characteristics of active fire frequency in different land cover types (Level-1) in the eight major Southeast Asia (SEA) countries

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表4显示8国活跃火普遍发生于常绿阔叶林,活跃火频率由泰国的33.87%到印度尼西亚的69.24%不等;而落叶阔叶林与常绿针叶林发生的活跃火均较少,各国活跃火发生频率都在4%以下。此外,老挝、缅甸、泰国与越南于混交林活跃火发生率也仅在3%左右,其他4国相应活跃火更少。相应地,农田活跃火集中于其他农地(即新开荒地、休闲地和轮歇地等),如泰国、柬埔寨、菲律宾、缅甸与越南相应占比都在10%以上。
表4 东南亚8个主要国家二级土地覆被类型的活跃火发生频率

Table 4 Distribution characteristics of active fire frequency of varied land cover types (Level-2) in the eight major Southeast Asia (SEA) countries (%)

土地覆被类型 老挝 缅甸 泰国 越南 柬埔寨 菲律宾 印度尼西亚 马来西亚
其他农地 7.28 12.03 24.19 10.88 15.32 13.18 2.54 3.46
常绿阔叶林 66.15 41.04 33.87 50.03 48.77 53.92 69.24 57.83
落叶阔叶林 0.51 3.19 3.46 0.65 0.57 0.26 0.11 0
针阔混交林 2.15 3.66 1.05 3.17 0.86 1.45 0.54 0.32
混交林 5.45 9.83 3.07 4.96 1.77 1.12 0.22 0.17

2.4 东南亚2015年活跃火发生类型国别分析

综合活跃火发生的时空特征,各国活跃火类型界定如下。就时间特征而言,中南半岛国家于3-5月发生的活跃火事件由刀耕火种或森林野火引发,此时正值旱季,适宜砍伐和焚烧。以印度尼西亚为代表的马来群岛国家活跃火现象集中分布在8-10月,结合其地形与土地覆被特征,其活跃火现象主要由秸秆焚烧引发。雨季即将结束,农作物逐渐成熟并进入收割阶段,秸秆焚烧现象非常普遍。
就其空间特征而言,老挝近2/3的活跃火集中发生在常绿阔叶林,且其多分布在5~25°坡及低山丘陵地区(200~1000 m),由此可推断主要由山区刀耕火种引发,另有较少量由平原农田秸秆焚烧引起。泰国、柬埔寨活跃火现象集中发生于5º以下及5~15 m的平原地带,结合其土地覆被特征,推断其活跃火主要由秸秆焚烧引起;另在70~110 m斜坡上发生的活跃火由刀耕火种引起。缅甸、越南超2/5的活跃火发生于5º及200 m以上的丘陵、低山地带,即由刀耕火种引发;另在平原地带由秸秆焚烧引发较多的活跃火。就马来群岛国家而言,因其地势平坦,其活跃火类型以平原活跃火为主。结合土地覆被特征,菲律宾5~100 m平原中有50%的活跃火由秸秆焚烧引起(占其全部活跃火比例13.18%),而大部分在平原、丘陵发生的活跃火由刀耕火种引起。印度尼西亚和马来西亚与菲律宾情况相似,近3/5的活跃火现象由刀耕火种引起,少部分平原活跃火由秸秆焚烧引起。此外,秸秆焚烧和刀耕火种的焚烧过程都有可能造成森林野火,加之自然引起的森林火灾,以上界定的刀耕火种活跃火中还包含森林野火。
中南半岛国家由坡度、海拔界定的活跃火成因,与土地覆被类型界定的成因较为一致。而马来群岛国家由坡度、海拔界定的活跃火类型4/5以上为平原活跃火,无法判断其具体类型;但结合土地覆被类型后,可界定平原内发生的活跃火也由刀耕火种引起。

2.5 东南亚2015年活跃火对人口分布影响的空间差异

结合2015年人口密度数据,统计8国人口密度间隔为1人/km2的活跃火频次。总体而言,东南亚国家活跃火频次随人口密度增加呈先增后减的趋势(图5),活跃火集中于人口密度<50人/km2范围内,但国别波动变化区间和幅度差异较大。以印度尼西亚为首,在人口密度<50人/km2范围内,活跃火频次最高约为7×104次;柬埔寨和缅甸的波动幅度较大,达1.5×104次以上;其次为老挝、泰国、越南和马来西亚,活跃火频次波动幅度达0.4×104次以上;最后为菲律宾,活跃火频次—人口密度变化幅度在0.1×104次以下。
图5 东南亚8国活跃火频次与人口密度相关性分析

Fig. 5 Correlation analysis between the occurrence frequency of active fires and population density in the eight major Southeast Asia (SEA) countries

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具体而言,老挝和柬埔寨34%以上的活跃火集中于人口密度10人/km2以下范围内,但老挝活跃火随人口密度的变化更为平稳,而柬埔寨活跃火影响至150人/km2人口密度范围。印度尼西亚和马来西亚的活跃火集中发生在16人/km2以下范围内,发生率分别为27.46%、36.44%;该区间内两国活跃火频次的波动幅度较大,人口密度>50/km2后频次骤减。缅甸、泰国、越南活跃火影响的人口密度区间变大,10%左右的活跃火发生在72~91人/km2人口密度区间内;此外,缅甸、泰国和越南3国的活跃火频次波动幅度大且频繁,在人口密度>150人/km2活跃火频次仍有中等幅度的波动变化。以人口密度>100人/km2的区域可视为人口密集区(即城镇),据此可推断活跃火多集中于人口稀少的乡村和原始森林,其中缅甸、泰国和越南的活跃火对人口的影响范围较大,在>100人/km2的城区仍有发生。

3 结论与讨论

本文以2015年强厄尔尼诺年VIIRS VI活跃火为基础,结合海拔、坡度土地覆被及全球人口密度栅格产品,基于GIS揭示东南亚主要国家活跃火频次的发生时间、地形及土地覆被分布特征,据此在国家层面界定了活跃火发生类型及其对人口分布的影响,主要结论包括:
(1)2015年东南亚主要国家活跃火发生频次差异明显。其中中南半岛活跃火频次占到69.60%,尤以缅甸居多,而马来群岛以印度尼西亚最多。且活跃火逐月分布差异明显,中南半岛活跃火集中发生于旱季的2-4月,马来群岛活跃火则集中发生于8-10月。
(2)缅甸和越南活跃火随海拔变化呈单峰特征,集中分布在25 m以下;而老挝、泰国、柬埔寨活跃火随海拔分布呈显著双峰特征;马来群岛国家活跃火发生频次随海拔分布呈单峰特征,集中分布在60 m以下的平原地带。东南亚活跃火均集中分布5~15º的斜坡,分别占各国活跃火的35.01%以上,且活跃火随坡度增加呈先增后减趋势。
(3)东南亚国家活跃火集中于森林、农田,其中以森林活跃火为首,集中发生率由柬埔寨的52.00%到老挝的74.27%不等,尤其集中于常绿阔叶林,活跃火频率由泰国的33.87%到印度尼西亚的69.24%不等。其次,农田活跃火发生率由老挝的13.18%到泰国的42.68%不等。
(4)东南亚国家活跃火随人口密度增加呈先增后减至平稳变化的趋势。老挝、柬埔寨、和菲律宾的活跃火集中发生于10人/km2以下;印度尼西亚27.46%的活跃火集中发生在6~16人/km2范围内,而马来西亚36.44%的活跃火集中于1~15人/km2范围内;缅甸、泰国、越南则集中在72~91人/km2;东南亚于城镇中发生的活跃火甚少,多集中于人口稀少的乡村和原始森林,此外,缅甸、泰国和越南在>100人/km2的城区仍有发生。
(5)东南亚2015年活跃火主要发生于坡度在15º以下的平原地带,且集中于常绿阔叶林和其他农地类型。2015年东南亚活跃火频次骤增源于刀耕火种的焚烧过程,及由此(或自然因素)引发的森林野火。除印度尼西亚外,活跃火影响范围在50人/km2以下的乡村和原始森林,对人口分布的影响较小;而缅甸、泰国和越南的活跃火对人口的影响范围较大。
基于发生月份、地形分布与覆被类型的活跃火发生类型评价体系,能客观界定东南亚8个主要国家的活跃火发生类型,即由秸秆焚烧和刀耕火种所引起,但目前未能有效识别由森林野火引发的活跃火。实际上,就处在热带地区的东南亚而言,农林业在其8个主要国家中占有重要地位[34,35,36],森林野火现象并不突出,而丘陵及中低山的刀耕火种农业以及平原地区的秸秆焚烧则是其活跃火主要成因。活跃火发生类型分析结果可为后续开展中高分辨率影像动态监测提供科学支持。目前,本文仅在发生类型与影响两个层面进行了初步分析,对于活跃火焚烧强度与发生强度(概率)等未有涉及,未来可结合活跃火数据中的火烧强度信息以及特定格网大小下的发生概率进行深入分析。

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基金

中国科学院地理科学与资源研究所“秉维”优秀青年人才计划(2018RC201)
国家自然科学基金项目(41971242)
鄱阳湖湿地与流域研究教育部重点实验室(江西师范大学)开放基金资助项目(PK2019005)
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