Spatial and temporal controls on post-fire hydrologic recovery in Southern California watersheds

The current study investigates the spatial and temporal dynamics of post-fire vegetation and the subsequent influence on seasonal and annual hydrologic responses in chaparral-dominated watersheds. Post-fire climatology, burn severity, slope aspect, and vegetation behavior are evaluated for two basins burned during the 2003 Old Fire in the San Bernardino Mountains in Southern California. Climate and discharge data are used to evaluate seasonal and annual variability of post-fire hydrologic fluxes. Data obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) are used to estimate Enhanced Vegetation Index (EVI) and differenced Normalized Burn Ratio (dNBR). A Savitzky–Golay filtering technique and an integrated EVI annual fraction are utilized to assess vegetation recovery under a range of potential controls. Vegetation recovery is highly variable in both watersheds and is related to slope aspect (solar and water availability), initial biomass levels, and burn severity. South and west facing slopes show higher pre-fire EVI (biomass) and significant loss of vegetation cover after fire. Vegetation in both watersheds responds to an extreme wet season during the second post-fire year, however recovery rates are not sustained. North and east aspects show the quickest biomass gain relative to pre-fire conditions by the end of the study period (WY 2010), while the west and south slopes show lower biomass recovery. High burn severity areas show the slowest recovery across all slope aspects, with these regions just approaching 90% of pre-fire biomass by the end of the seven-year post-fire period. The variable rate of vegetation recovery across the watersheds results in significant changes in annual and seasonal discharge throughout the post-fire period. Runoff ratios remain elevated in both systems and there is increased dry season flow for much of the study period, indicating that plant water consumption and flowpaths are not back to pre-fire behavior by the end of WY 2010.     

A comparison of fire severity measures: An Australian example and implications for predicting major areas of soil erosion

Modelled results of two popular fire severity measures NBR_diff and NDVI_diff were compared with field data for two small wildfires that occurred in eucalypt-dominated forest near Sydney, Australia during the spring of 2006. NBR_diff and NDVI_diff are computed by differencing specific spectral reflectance properties of satellite images captured before and after the fire event. In this case indices were derived from the Landsat 5 Thematic Mapper platform. It is demonstrated that the spatial distribution of severity classes computed using NBR_diff were not statistically different to severity classes computed for NDVI_diff in the Tallowa fire, which was a ‘hot’ burn. However, in the ‘cooler’ Thirlmere Lakes fire, their respective classifications differed significantly. NBR_diff was clearly superior to NDVI_diff in discriminating lower fire severities, which predominantly burn understorey vegetation whilst leaving the canopy un-scorched. It is also demonstrated using post-fire soil profiles from 50 sample sites that charring depth, which is related to the hydrophobic properties of the soil landscape, is significantly correlated to fire severity. This has important implications for the probability of developing a soil erosion risk model based on remotely sensed data, because in this environment, charred soil landscapes will have lost some of their naturally highly water repellent behaviour during burning, which in turn alters their susceptibility to overland flow development and surface wash erosion.     

Effect of vegetation cover on soil erosion in a mountainous watershed

We applied the Revised Soil Loss Equation (RUSLE) to assess levels of soil loss in a Geographic Information System (GIS). In this study, we used the k-NN technique to estimate vegetation cover by integrating Landsat ETM+ scenes and field data with optimal parameters. We evaluated the root mean square errors and significance of biases at the pixel level in order to determine the optimal parameters. The accuracy of vegetation cover estimation by the k-NN technique was compared to that predicted by a regression function using Landsat ETM+ bands and field measurements as well as to that predicted by the Normalized Difference Vegetation Index (NDVI). We used a regression equation to calculate the cover management (C) factor of the RUSLE from vegetation cover data. On the basis of the quantitative model of soil erosion, we explored the relationship between soil loss and its influencing factors, and identified areas at high erosion risk. The results showed that the k-NN method can predict vegetation cover more accurately for image pixels at the landscape level than can the other two methods examined in this study. Of those factors, the C-factor is one of the most important affecting soil erosion in the region. Scenarios with different vegetation cover on high-risk areas showed that greater vegetation cover can considerably reduce the loss of soil erosion. The k-NN technique provides a new method to estimate the C-factor for RUSLE erosion mapping. The quantitative model of different vegetation cover scenarios provides information on how vegetation restoration could reduce erosion.     

Stratified vegetation cover index: A new way to assess vegetation impact on soil erosion

Vegetation cover is an important parameter used in assessing the relationship between vegetation and soil erosion. However, the intensity of soil erosion actually changes not only with vegetation cover but also with differences in vegetation type and structure. How to integrate the cumulative effect of the different growth forms making up a vegetation community into one index for inclusion in soil loss predictive equations is an open research question. This paper proposes a method to separately measure the cover of different vegetation strata, estimate their contribution to reducing soil loss, and then to integrate this into a single vegetation index called the stratified vegetation cover index (C_s). The results show that C_s is more effective than projected vegetation cover for the assessment of soil erosion and also can overcome the disadvantages of vegetation indices such as NDVI. This means that C_s is a good substitute for vegetation cover or cover-related vegetation indices in studies on the relationship between vegetation and soil erosion. The concept of Cs may help the local governors or forest department understand the importance of vegetation structure and make right management decisions.     

基于多时相遥感植被指数的柑橘果园识别

柑橘普遍种植于中国南方地区,受天气多云多雨、种植类型复杂等因素影响,利用光谱信息直接识别柑橘果园信息存在一定困难。该研究根据柑橘特有的物候特征,提出了"柑橘在其果实生长膨大过程中柑橘果园的植被信息可能减弱"的假设。根据此特征提出柑橘果园信息识别方法,确定关键时间窗口的阈值,并以广西壮族自治区南宁市武鸣区为研究区,开展柑橘果园信息遥感识别实证研究。首先,获取2018年研究区多时相Sentinel-2遥感影像,构建归一化植被指数（Normalized Difference Vegetation Index,NDVI）、绿色归一化植被指数（Green Normalized Difference Vegetation Index,GNDVI）、差值植被指数（Difference Vegetation Index,DVI）和红边波段指数（Sentinel-derived Red-edge Spectral Indices,RESI）等多个植被光谱指数;其次,根据地面样本点信息,对比不同植被类型在不同时期的遥感植被信息差异,进而确定柑橘果园识别的最优特征。研究结果表明,柑橘果园与研究区其他主要作物类型（如甘蔗、香蕉、玉米、水稻等）没有明显的光谱特征差异,但研究区多时相遥感植被指数显示10月的柑橘果园NDVI相比11月出现明显低值0.47,且明显低于其他作物类型;10月的柑橘果园GNDVI也出现了低值0.43,但与其他月份相比差异不明显;而柑橘果园DVI的离散程度低,分离性不强。根据作物物候历,9-10月为柑橘果实迅速膨大期,这验证了该研究提出的科学假设,即该时期柑橘果园的植被信息会减弱。柑橘果实膨大期不同植被指数的离散程度差异明显,NDVI离散程度最高,差异性最强。根据10月柑橘果园NDVI的物候特征,进一步构建归一化指数,通过阈值法识别研究区柑橘果园空间分布,该识别方法的总体精度达到82.75%,优于其他植被指数的识别结果,该研究结果可为柑橘果园信息遥感识别研究提供较好的理论与实践支撑。     

The vegetation cover dynamics (1982–2006) in different erosion regions of the Yellow River Basin,China

Vegetation significantly influences human health in the Yellow River basin and the plant cover is vulnerable to people. Typical types of erosion in the Yellow River basin include that caused by water, wind and freeze–thaw. In this paper, vegetation cover change from 1982 to 2006 was studied for a number of different erosion regions. The Global Inventory Monitoring and Modeling Studies Normalized Difference Vegetation Index (GIMMS NDVI) data were employed, while climatic data were also used for analysis of other influencing factors. It was shown that: (1) generally the vegetation cover in different erosion regions displayed similar increasing trends; (2) spatially the vegetation cover was highest in the water erosion region, the second highest was in the freeze–thaw region and the lowest in the wind erosion region; and (3) vegetation cover in the Yellow River basin is influenced by climate factors, especially by temperature. In water erosion regions, the temporal change of vegetation cover seemed complicated by comprehensive climatic and human influences. In wind erosion regions, the vegetation cover had close relations to precipitation. In freeze–thaw erosion regions, the vegetation cover was primarily altered by temperature. In all the three erosion regions, significant change of the vegetation cover occurred from 2000 just after the ‘Grain for Green’ (GFG) programme was implemented throughout China. Copyright © 2010 John Wiley & Sons, Ltd.     

基于遥感温度植被干旱指数的小蠹虫害预警

针对小蠹虫对森林的危害隐蔽强,症状滞后性明显,在其早期发生时进行遥感识别非常困难。该文基于干旱和虫害存在一定的时滞相关性的假设,提出基于温度植被干旱指数(temperature vegetation dryness index,TVDI)预报小蠹危害的方法。以遭受大面积连续干旱和小蠹危害的云南省中部的石林县为案例区,利用Landsat数据,建立归一化植被指数(normalized difference vegetation index,NDVI)-地表温度(land surface temperature,Ts)特征空间,估算逐像元TVDI。基于地面小班调查的虫害等级数据(健康、轻度、中度和重度4个等级),比较不同虫害等级斑块TVDI差异。同时,以持续干旱2011年轻度受害区为例,结合受害前后云南松林NDVI差值(difference of NDVI before and after the forest attacked by bark beetles,d NDVI)表征实际受害程度的方法,建立TVDI与d NDVI的关系,对2012年进行预测。结果表明,2010-2015年,受害区整体呈下降趋势,TVDI由西向东逐渐变大。健康云南松林TVDI显著高于虫害云南松林(P0.05),且虫害越严重,TVDI越小;2011年,TVDI与d NDVI呈显著负相关(P0.05),可以用线性模型进行拟合,拟合决定系数R2为0.322。采用模型对2012年实际发生情况进行预测,得到预测与实测d NDVI均方根误差RMSE为0.237。在整体干旱的环境下,相对湿润的地方小蠹虫害更严重。因此,可以根据TVDI空间分布特征,找出TVDI相对较小的区域,作为虫害可能发生的重点关注区域,该研究对及时发布虫情监测信息有建设性的意义。     

Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia

This paper examines post-fire erosion response in a sub-alpine environment in south-eastern Australia for a period of 2.2 years. Few studies have examined fire impacts on sediment transfer in this environment. Erosion pins were used in grids located at upper, mid and lower slope positions on adjacent burnt and unburnt hillslopes to assess fire effects on the extent of surface level change. The results indicated that there was a significant difference between the surface level change regimes on the burnt and unburnt hillslopes. Estimated erosion rates for the burnt slope over the study period ranged from 2.7 to 94.3 t ha^− 1, which could be considered low given the high slope angles, high precipitation and moderate fire severity. Slope position was critical in modifying post-fire erosion response, as it controlled slope angle and the rate of surface cover regrowth. Analysis of lower slope sites, for which more detailed data was available, indicated a second delayed erosion peak after the initial elevated post-fire response during the following spring snowmelt period. Surface recovery on the lower burnt site was slow, with vegetation cover still comparatively low 2.3 years after the fire. Evidence of post-fire sediment supply limitation was found on this site, with a declining rate of increase in the magnitude of total surface level change, despite limited regrowth and an increasing number of precipitation events > 20 mm for measurement intervals since the fire. Modification of the hillslope surface by fire leads to changing hillslope erosion process dominance in this environment. The post-fire hillslope undergoes erosion by direct rain-drop impact and overland flow, whereas the unburnt slope rarely experiences overland flow due to the thick ground cover. As a result surface level change on the unburnt slope was largely influenced by wetting–drying effects rather than sediment transfer by surface flow. Downslope biotransfer appears to be the dominant sediment movement process in the unburnt sub-alpine forest environment.     

Relationships among dust outbreaks, vegetation cover, and surface soil water content on the Loess Plateau of China, 1999–2000

We analyzed relationships among dust outbreaks, Normalized Difference Vegetation Indices (NDVI), and surface soil water content (0 to 2 cm depth) on the Loess Plateau, a significant dust source area of East Asia. World Surface Data for wind speed and current weather, coarse-resolution data for NDVI, and a three-layer soil model for surface soil water content were used. The threshold NDVI for preventing dust outbreaks was about 0.2 when the wind speed ranged from 7 to 8 m s^− 1. This threshold NDVI corresponds to a vegetation cover of 18%. The threshold ratio of surface soil water content to the field capacity (θ_r) was about 0.2. Conditions facilitating dust outbreaks on the Loess Plateau are when NDVI is less than 0.2 with wind speed  7 m s^− 1 and θ_r < 0.2, and when NDVI is greater than 0.2 with wind speed  9 m s^− 1 and θ_r < 0.2.     

长江经济带降雨侵蚀力与NDVI耦合协调关系时空变异分析

为探究长江经济带降雨侵蚀力和植被覆盖关系之间的耦合作用及其与土壤侵蚀风险的关系,该研究基于逐日降水数据和逐月NDVI数据,使用日尺度的降雨侵蚀力计算模型和Mann-Kendall趋势检验方法分析了长江经济带月尺度的降雨侵蚀力和NDVI时空演变特征,并通过耦合协调度理论和Pettitt突变点检验识别了降雨侵蚀力与NDVI耦合协调关系的时空变异特征,最后讨论两者之间耦合协调关系变化的原因。结果表明：1)1965—2019年,7—10月长江经济带多年平均降雨侵蚀力在全区空间上分布比较均匀,其他月份多年平均降雨侵蚀力均呈现从东南向西北减少的空间分布格局。东部地区7月和9月的降雨侵蚀力呈现显著增加趋势（P<0.05）;2）不同月份NDVI在1999—2019年间显著增加的总面积比例均超过长江经济带总面积30%,NDVI显著减少面积主要在东部地区,该地区土壤侵蚀风险较大;3）降雨侵蚀力与NDVI耦合协调度的多年平均值呈现东高西低的空间分布格局,耦合协调程度整体较低,耦合协调度显著减少的区域主要位于中部和西南部,显著增加的区域主要位于东部;4）长江经济带东部地区面积占比较小的常绿针叶林、常绿阔...     

Fire and Tillage as Degrading Factors of Soil Structure in Northern Zagros Oak Forest,West Iran

The effects of fire and the conversion to vineyard on soil organic carbon (SOC), and soil aggregate size distribution and stability were studied in a forest of Iran. For this purpose, topsoil was sampled in an unburned area, a portion of the forest burned three years earlier, and a vineyard, all three contiguous and showing similar topographic features. In the burned forest, soil was sampled in areas undergone high, moderate, or low severity. Air‐dried soil samples were sieved to obtain four aggregate size classes, which were subsequently wet sieved. Soil aggregate distribution index, mean weight diameter, geometric mean diameter, and aggregate stability index were determined on both dry and wet specimens. No significant differences in SOC between burned and unburned forest were found, most probably because of the supply of charred biomass to soil, while in the vineyard thirty years of cultivation had removed half of initial SOC. Both severe fire and cultivation had decreased the stability of aggregates and the relative amount of the biggest ones (8 to 2‐cm diameter). However, aggregate stability was significantly lower in the vineyard than in the burned forest, which points out to a stronger impact of prolonged cultivation than a single fire, although severe. Cultivation and severe fire had decreased the proportion of C in macroaggregates, to the advantage of meso (1 to 0.25 mm) and micro (<0.25 mm) aggregates. A hierarchical cluster analysis of all investigated properties and indices demonstrated that cultivation and highly severe fire both were causes of soil degradation. Copyright © 2016 John Wiley & Sons, Ltd.     

基于归一化植被指数的流域植被覆盖分形维数研究

量化和表征植被覆盖空间分布的复杂性是研究植被与地表物质迁移关系的重要问题。为研究植被覆盖分形维数的表征特点及其在不同尺度上的变化特征,该文基于地理信息系统平台,利用分形布朗运动理论,结合像元归一化植被指数(normalized difference vegetation index,NDVI)的空间分布,提出并计算了植被覆盖分形布朗运动(fractional brownian motion,FBM)分形维数。结果显示,流域植被覆盖的空间分布具有统计自相似性,可用分形维数表征,其值在2.5~3之间,越接近2.5,表示植被覆盖空间分布越复杂。植被覆盖FBM分形维数与流域内均值化NDVI值和NDVI值的变异系数无直接关系,与单位面积不同NDVI值的像元数呈极显著负相关(R=0.66,P0.01)。植被覆盖FBM分形维数具有尺度效应,随流域面积增大而增大,到一定尺度后趋于平稳。流域植被覆盖FBM分形维数能够克服NDVI像元点奇异值等对植被空间分布量化表征计算的干扰,相对传统表征植被覆盖的指数,其在水文、土壤侵蚀等模型中具有更广泛的应用意义。     

Topographic variation in burning-induced loss of carbon from organic soils in Tasmanian moorlands

Although it is known that cool fire can result in carbon loss in organic soils, data are lacking on the effects of such fire on the distinctive lowland organic soils of the southern hemisphere and on the relationship of fire-induced carbon loss to topography. We established an experiment to determine the effects of a low severity burn on organic and total mass of soils, the position of the water table and vegetation cover. We recorded soil losses directly after the fire, after the first rain and after 4 years. We also recorded losses as a result of the first rain and at 4 years after a wildfire at another locality with the same vegetation and topography. We compared soil surface temperatures over summer between burned and unburned moorland. The planned fire resulted in substantial soil and carbon losses, which, up to 4 years after the wildfire, occurred mostly as a result of the fire itself and the first rains. The topographic wetness index was strongly related to soil and organic matter loss for the pooled data for both sites, while fire severity, slope and vegetation cover were less predictive. Burning increased dissolved organic material in streams; the depth of the water table; and, soil temperatures. The continuing soil loss on slopes 18 years after fire contrasts with faster recovery on the shelves and in the basins, reflecting a strong topographic component in variation. Fire is a major influence on the depth of the organic soils of the southern hemisphere, as in the northern hemisphere. The influence of fire on soils varies markedly between topographic positions, with the time interval between fires that would maintain organic soils in basins being markedly less than that necessary to maintain organic soils on slopes. The topographic effect appears to be a consequence of relative drying rather than relative exposure to water or aeolian erosion or differences in fire severity.     

Spatial and temporal patterns of vegetation recovery following sequences of forest fires in a Mediterranean landscape,Mt. Carmel Israel

The Mediterranean ecosystem of Mt. Carmel is subjected to increasing number of forest fires at various extents and severities due to increasing human activities. Accordingly, we tested whether in areas exposed to different fire histories vegetation regeneration is different in north versus south facing slopes, and the potential impact on erosion processes. Using remote sensing techniques we evaluated the Enhanced Vegetation Index (EVI) to monitor vegetation recovery following a single fire and three successive fires, using a series of Landsat images taken between 1985–2002. Following a single fire, vegetation cover reached pre-disturbance values within less than 5 years. Repeated fires caused further reduction of EVI values, especially at south facing slopes (SFS). The effects of three successive fires within 10 years, followed by a three year recovery period, however, are negligible when considering vegetation cover values. This was deduced as north facing slope EVI values returned to pre-disturbance conditions at the end of the 3 years and SFS EVI values to 80% of the pre-disturbance conditions. Our results indicate that Mediterranean eco-geomorphic systems are quite resilient, showing quick response, at least in terms of return to pre-disturbance states of vegetation cover, and hence of soil erosion rates. This is true not only in response to a disturbance caused by a single fire, but also for repetitive fire incidents.     

Effects of fire and torrential rainfall on erosion in a Mediterranean gorse community

Wildland fires and torrential rainfall are important factors affecting Mediterranean nature and human society. In this paper, we investigate their combined impact on a fire‐prone vegetation community at the east coast of Spain. Our results show that shrub cover provides an effective control of erosion even under simulated extreme rainfall events. After fire, soil erosion increased noticeably; however, the amount of sediment produced varied greatly, which can be explained by variations in fire severity. The quantity of litter debris after the fire plays a decisive role in controlling the erosion that follows. Copyright © 2003 John Wiley & Sons, Ltd.     

Contribution of phytoecological data to spatialize soil erosion: Application of the RUSLE model in the Algerian atlas

Among the models used to assess water erosion, the RUSLE model is commonly used. Policy makers can act on cover (C-factor) and conservation practice (P-factor) to reduce erosion, with less costly action on soil surface characteristics. However, the widespread use of vegetation indices such as NDVI does not allow for a proper assessment of the C-factor in drylands where stones, crusted surfaces and litter strongly influence soil protection. Two sub-factors of C, canopy cover (CC) and soil cover (SC), can be assessed from phytoecological measurements that include gravel-pebbles cover, physical mulch, annual and perennial vegetation. This paper introduces a method to calculate the C-factor from phytoecological data and, in combination with remote sensing and a geographic information system (GIS), to map it over large areas. A supervised classification, based on field phytoecological data, is applied to radiometric data from Landsat-8/OLI satellite images. Then, a C-factor value, whose SC and CC subfactors are directly derived from the phytoecological measurements, is assigned to each land cover unit. This method and RUSLE are implemented on a pilot region of 3828 km² of the Saharan Atlas, composed of rangelands and steppe formations, and intended to become an observatory. The protective effect against erosion by gravel-pebbles (50%) is more than twice that of vegetation (23%). The C-factor derived from NDVI (0.67) is higher and more evenly distributed than that combining these two contributions (0.37 on average). Finally, priorities are proposed to decision-makers by crossing the synthetic map of erosion sensitivity and a decision matrix of management priorities.     

大巴山地区植被覆盖变化及其对气候变化的响应

利用1998—2009年的SPOT VGT-NDVI逐旬数据集,对大巴山地区植被覆盖变化及其对气候的响应进行了研究。结果表明:(1)1998—2009年大巴山地区植被NDVI值变化整体呈显著上升趋势,年平均NDVI值在0.54～0.64之间;大巴山弧形褶皱带植被NDVI值较高,河流附近NDVI值较低;大巴山地区植被覆盖呈显著性增加趋势,在县城城区附近有植被退化迹象,占总面积的0.24%。(2)1998—2009年,提取6种植被类型的NDVI值在四季的变化总体呈上升趋势,尤其是春季NDVI变化显著;不同类型的植被NDVI值大小顺序为:阔叶林>针叶林(冬季为针叶林>阔叶林)>灌丛>草甸>草丛>栽培植被,其中,草甸在四季的变幅最大。(3)在时间尺度上,NDVI与同期、前一个月、前两个月的气温呈极显著相关性,与降水量的相关性不显著。空间尺度上,6月月均NDVI与气温呈负相关的像元数占67%,表明该时段温度升高可能导致蒸发量增大,从而抑制了植物生长。     

群体小麦条锈病发病动态无人机遥感监测方法

针对当前育种群体小麦条锈病表型分析手段单一、效率低下等问题,该研究提出了一种基于无人机低空遥感和多光谱成像技术的群体小麦田间条锈病高通量表型动态分析方法。该方法利用无人机采集自然发病的育种群体小麦（共600个样本,516个基因型）冠层多时相的光谱图像,并提取22个植被指数作为后续分析的表型,同时按照发病后的时间顺序与传统条锈病人工鉴定标准记录条锈病发病阶段和发病严重度数据;使用随机森林算法建立22个光谱植被指数同条锈病发病阶段与病害严重度的分类模型,并筛选出对上述两个分类问题敏感的植被指数;同时,使用随机蛙跳算法对特征进行筛选以降低仅使用随机森林算法对特征筛选的偶然性,并将随机蛙跳算法给出被选择概率排名在约前1/3的特征作为SVM算法的输入,构建发病阶段与病害严重度模型以验证随机蛙跳算法对特征筛选的有效性;综合两次特征选择的结果,分别筛选出对发病阶段和病害严重度敏感的3个植被指数,并基于这些指数响应的时间序列分析了群体中6个参考品种发病动态的差异。对条锈病发病阶段的分类模型构建中,随机森林和SVM模型测试集的F1分数分别为0.970和0.985;对条锈病严重度等级分类中,二者的F1分数为0.740和0.780,表明通过所建立的模型可以实现对群体小麦发病阶段和病害严重度等级的分类,且随机森林算法和随机蛙跳算法都能够筛选出对条锈病发病阶段和病害严重度敏感的特征。筛选出的差分植被红边指数（Difference Vegetation Index - Rededge,DVIRE）的响应对病害胁迫较为敏感,可用于同时描述田间条锈病发病阶段和严重度。该研究提出的高通量表型分析方法,基于无人机成像光谱提取的植被指数对群体小麦田间条锈病进行时间序列动态分析,能够精准量化群体小麦受条锈病胁迫状态,并可为其他作物抗病育种的表型分析提供一定的参考。     

从黄土高原植被指数数据中获取森林覆盖率信息

 由于存在既包含农作物又包含森林植被的混合像元,以年平均归一化植被指数(NDVI)作为反映森林覆盖率的指标有很大的局限性。在黄土高原,森林植被的季节变化与农作物的季节变化有很大差异,基于这种差异,以不同月份NDVI的不同组合和考虑某种权系数后计算出来的加权平均NDVI与森林覆盖率相联系,筛选对于森林覆盖率有较好反映的NDVI指标。结果表明:6、10月2月森林覆盖率与NDVI的决定系数最高,故以6、10月平均NDVI(INDVI,6+10)作为反映森林覆盖率的指标;全年森林覆盖率与INDVI,6+10之间的决定系数为0.5384,比全年森林覆盖率与全年平均NDVI之间的决定系数0.4432提高21.48%;以各月的决定系数作为权系数求得的NDVI加权平均值(INDVI,w)与全年森林覆盖率之间的决定系数为0.5144,比森林覆盖率与年平均NDVI(INDVI,m)之间的决定系数0.4432提高16.07%,因此,推荐全年各月加权平均NDVI(INDVI,w)或6、10月平均NDVI(INDVI,6+10)作为反映黄土高原森林覆盖率的遥感植被指标。     

黄河中游多沙粗沙区侵蚀产沙与植被相互作用的临界现象

黄河中游多沙粗沙区是黄土高原境内侵蚀最为强烈的地区,也是下游河道强烈淤积的主要物源区。该区的土壤侵蚀受到多重因素的复合作用,植被作为其最显著的影响因子之一而广受关注。以统计资料为依据,论述了研究区侵蚀模数、林草覆盖度和林木覆盖度的空间分布特征,发现一些相对较大的侵蚀模数与植被覆盖度之间具有良好的非线性关系。将这些相对较大的侵蚀模数定义为极端侵蚀模数——相似植被覆盖度条件下的最大侵蚀模数,并拟合出它们与林草和林木覆盖度之间的定量函数关系,得出极端侵蚀模数由增加到减小的临界林草覆盖度为24.2%,临界林木覆盖度为12%。这就是说,在其它复合因素不发生明显改变的情况下,当研究区林草覆盖度小于24.2%或林木覆盖度小于12%时,极端侵蚀模数随着林草或林木覆盖度的增大而增大;当覆盖度大于上述临界值后,极端侵蚀模数则随之减小。该研究成果对于更有效地管理黄河中游流域具有一定的指导意义。