Concurrent use of active and passive microwave remote sensing data for monitoring of rice crop

Estimation of crop area, growth and phenological information is very important for monitoring of agricultural crops. However, judicious combination of spatial and temporal data from different spectral regions is necessary to meet the requirement. This study highlights the use of active microwave QuikSCAT Ku-band scatterometer and Special Sensor Microwave/Imager (SSM/I) passive microwave radiometer data to derive information on important phenological phases of rice crop. The wetness index, a weekly composite product derived using brightness temperatures from 19, 37 and 85 GHz channels of SSM/I, was used to identify the puddling period. Ku-band scatterometer data provided the signal of transplanted rice seedlings since they acts as scatterers and increases the backscattering. Dual peak nature of temporal backscatter curve around the heading stage of rice crop was observed in Ku-band. The decrease of backscatter after first peak was associated with the threshold value of 60% crop canopy cover. The symmetric (Gaussian) and asymmetric (lognormal) curve fits were attempted to derive the date of initiation of the heading phase. The temporal signature from each of these sensors was found to complement each other in crop growth monitoring. Image showing pixel-wise timings of heading stage revealed the differences exists in various parts of the study area.     

Can vegetation productivity be derived from greenness in a semi-arid environment? Evidence from ground-based measurements

Trends of biomass production and land processes in the Sahel have been widely studied since the droughts of 1970s. Satellite data have been an important source of information because of limited in situ data. Previous studies relied on the assumed existence of a relationship between vegetation productivity and the NDVI, in particular the annually integrated NDVI (iNDVI). This study examines this assumption and its limitations, based on in situ time series measurements of biomass, species composition, NDVI and soil moisture at the Dahra test site in northern Senegal. It is shown that, there are large differences between the NDVI – vegetation productivity relationships, and these differences can be linked to species composition. There is moderate correlation between NDVI and above-ground net primary productivity (ANPP) at the peak season (r² = 0.39). In particular, the species Zornia glochidiata is characterized by high peak NDVI and low ANPP, compared to other common species such as Cenchrus biflorus and Aristida adscensionis. It is concluded that spatial and temporal variations in species dominance is likely to add noise to the relationship between NDVI and biomass. However, the seasonal cyclic fraction of the NDVI – “small seasonal integral” – reduces such noise.     

Unexpected climate impacts on the Tibetan Plateau: Local and scientific knowledge in findings of delayed summer

Knowledge of climate change and its impacts can facilitate adaptation efforts. However, complex system dynamics, data scarcity, and heterogeneity often generate both contradictory findings and unexpected climate change impacts. Here we present local ecological knowledge of climate and ecological change in central Tibet to support the finding of delayed summer on the Tibetan Plateau, a finding that has been subject to vigorous, ongoing debate based on Western scientific data. Tibetans who actively herd on a daily basis and are located at higher elevations were most likely to notice changes in seasonality, reported as later start of summer and green-up, and as delayed and shortened livestock milking season. Local meteorological data, indigenous observations of higher snowlines and long-term animal number data suggest that a regional warming trend, rather than land use change, may underlie the delayed phenology trends. We demonstrate that local ecological knowledge can reveal counter-intuitive outcomes and help resolve apparent contradictions through its strengths in situations of high variability, ability to integrate over a range of variables and time scales, and operation outside of Western scientific logic. This suggests local knowledge does not exist to be confirmed or disproved by Western science, but rather can also advance Western science and help contribute to its debates. It is precisely points of apparent contradiction within and between knowledge systems that are most productive for more extensive inquiry. Future research on climate change, and climate adaptation policy-making, will benefit from careful, contextual dialog with local observations, focusing on observable biophysical phenomena that are affected by temperature and precipitation and that are important to livelihoods.     

Differences between cropland and rangeland MODIS phenology (start-of-season) in Mali

Start-of-season data are more and more used to qualify the land surface phenology trends in relation with climate variability and, more rarely, with human land management. In this paper, we compared the phenology of rangeland vs cropped land in the Sahel belt of Africa, using the only currently available global phenology product (MODIS MCD12Q2 – Land Cover Dynamics Yearly), and an enhanced crop mask of Mali. The differences in terms of start-of-season (SOS) are spatially (north south gradient), and temporally (10 years, 2001–2009) analyzed in bioclimatic terms. Our results show that globally the MODIS MCD12Q2 SOS dates of croplands and rangelands differ, and that these differences depend on the bioclimatic zone. In Sahelian and Guinean regions, cropland vegetation begins to grow earlier than rangeland vegetation (8-day and 4-day advance, respectively). Between, in the Sudanian and Sudano-Sahelian parts of Mali, rangeland vegetation greens about one week earlier than croplands. These results are discussed in the context of the land surface heterogeneity at MODIS scale, and in the context of the natural vegetation ecology. These results could help interpreting phenological trends in climate change analysis.     

近20年气候变暖对北京时令旅游的影响--以北京市植物园桃花节为例

北京植物园桃花节是赏花专题时令旅游的典型代表,以近20年植物园桃花节的起讫时间、持续时间及相关气温、物候期等数据为依据,探讨了气候变暖对时令旅游的影响。结果表明：1994年前后桃花节开始日期存在着显著的均值差异,后一时段比前一时段约提前6天,植物园桃花节的开始日期在总体上响应了以山桃始花为代表的北京春季物候提前的变化趋势。同时,桃花节开始日期与上年山桃始花日期、上年年均温度存在显著相关性,反映人们对桃花节的决策是参照上一年的物候现象做出的,相对于气候年际波动存在1年的滞后。桃花节结束日期受人为因素影响较大,但总的来说气候变暖有利于桃花节持续日数的延长。此外,桃花节响应气候变化的滞后性,使得气候波动容易引起时令旅游产品的不稳定,不利于时令旅游的经济效益和社会效益。     

Global analysis of time-lag and -accumulation effects of climate on vegetation growth

Climate dominantly controls vegetation over most regions at most times, and vegetation responses to climate change are often asymmetric with temporal effects. However, systematic analysis of the time-lag and time-accumulation effects of climate on vegetation growth, has rarely been conducted, in particular for different vegetation growing phases. Thus, this study aimed to leverage normalized difference vegetation index (NDVI) to determine the spatiotemporal patterns of climatic effects on global vegetation growth considering various scenarios of time-lag and/or accumulation effects. The results showed that (i) climatic factors have time-lag and -accumulation effects as well as their combined effects on global vegetation growth for the whole growing season and its subphases (i.e., the growing and senescent phases). However, these effects vary with climatic factors, vegetation types, and regions. Compared with those of temperature, both precipitation and solar radiation display more significant time-accumulation effects in the whole growing season worldwide, but behave differently in the growing and senescent phases in the middle-high latitudes of the Northern Hemisphere; (ii) compared to the scenario without time effects, considering time-lag and -accumulation effects as well as their combined effects increased by 17 %, 15 %, and 19 % the overall explanatory power of vegetation growth by climate change for the whole growing season, the growing phase, and senescent phase, respectively; (iii) considering the time-lag and -accumulation effects as well as their combined effects, climate change controls 70 % of areas with a significant NDVI variation from 1982 to 2015, and the primary driving factor was temperature, followed by solar radiation and precipitation. This study highlights the significant time-lag and -accumulation effects of climatic factors on global vegetation growth. We suggest that these effects need to be incorporated into dynamic vegetation models to better understand vegetation growth under accelerating climate change.     

物候对全球变暖响应的研究综述

近100年来，尤其是在最近20多年，全球平均表面温度出现了显著上升，全球变暖已成为全球关注的重要问题。物候现象与气候等环境因素息息相关，物候对全球变暖的响应研究正在成为物候研究的一个新的热点领域，NDVI正日益成为植被对气候响应研究的重要手段。概述了当前物候对全球变暖响应研究的主要进展。基于实地动植物等物候观测和遥感监测的大量研究表明，近期动植物等物候正发生着显著变化：北半球中高纬度地区植被生长季延长、植物提早开花、昆虫提早出现、鸟类提早产蛋以及冰川退缩、永冻土带融化、江河湖泊结冰推迟而融化提早等，与气候变暖有密切关系，是对全球变暖的明显响应。目前国内的许多研究者在物候对气候变暖响应方面做了一些工作，但与国际研究进展相比，还有许多研究工作有待于进一步开展。     

Mapping crop phenology using NDVI time-series derived from HJ-1 A/B data

With the availability of high frequent satellite data, crop phenology could be accurately mapped using time-series remote sensing data. Vegetation index time-series data derived from AVHRR, MODIS, and SPOT-VEGETATION images usually have coarse spatial resolution. Mapping crop phenology parameters using higher spatial resolution images (e.g., Landsat TM-like) is unprecedented. Recently launched HJ-1 A/B CCD sensors boarded on China Environment Satellite provided a feasible and ideal data source for the construction of high spatio-temporal resolution vegetation index time-series. This paper presented a comprehensive method to construct NDVI time-series dataset derived from HJ-1 A/B CCD and demonstrated its application in cropland areas. The procedures of time-series data construction included image preprocessing, signal filtering, and interpolation for daily NDVI images then the NDVI time-series could present a smooth and complete phenological cycle. To demonstrate its application, TIMESAT program was employed to extract phenology parameters of crop lands located in Guanzhong Plain, China. The small-scale test showed that the crop season start/end derived from HJ-1 A/B NDVI time-series was comparable with local agro-metrological observation. The methodology for reconstructing time-series remote sensing data had been proved feasible, though forgoing researches will improve this a lot in mapping crop phenology. Last but not least, further studies should be focused on field-data collection, smoothing method and phenology definitions using time-series remote sensing data.     

Spatially detailed retrievals of spring phenology from single-season high-resolution image time series

Vegetation indices derived from satellite image time series have been extensively used to estimate the timing of phenological events like season onset. Medium spatial resolution (≥250 m) satellite sensors with daily revisit capability are typically employed for this purpose. In recent years, phenology is being retrieved at higher resolution (≤30 m) in response to increasing availability of high-resolution satellite data. To overcome the reduced acquisition frequency of such data, previous attempts involved fusion between high- and medium-resolution data, or combinations of multi-year acquisitions in a single phenological reconstruction. The objectives of this study are to demonstrate that phenological parameters can now be retrieved from single-season high-resolution time series, and to compare these retrievals against those derived from multi-year high-resolution and single-season medium-resolution satellite data. The study focuses on the island of Schiermonnikoog, the Netherlands, which comprises a highly-dynamic saltmarsh, dune vegetation, and agricultural land. Combining NDVI series derived from atmospherically-corrected images from RapidEye (5 m-resolution) and the SPOT5 Take5 experiment (10m-resolution) acquired between March and August 2015, phenological parameters were estimated using a function fitting approach. We then compared results with phenology retrieved from four years of 30 m Landsat 8 OLI data, and single-year 100 m Proba-V and 250 m MODIS temporal composites of the same period. Retrieved phenological parameters from combined RapidEye/SPOT5 displayed spatially consistent results and a large spatial variability, providing complementary information to existing vegetation community maps. Retrievals that combined four years of Landsat observations into a single synthetic year were affected by the inclusion of years with warmer spring temperatures, whereas adjustment of the average phenology to 2015 observations was only feasible for a few pixels due to cloud cover around phenological transition dates. The Proba-V and MODIS phenology retrievals scaled poorly relative to their high-resolution equivalents, indicating that medium-resolution phenology retrievals need to be interpreted with care, particularly in landscapes with fine-scale land cover variability.     

An overview of Calanus helgolandicus ecology in European waters

We review current knowledge and understanding of the biology and ecology of the calanoid copepod Calanus helgolandicus in European waters, as well as provide a collaborative synthesis of data from 18 laboratories and 26 sampling stations in areas distributed from the northern North Sea to the Aegean and Levantine Seas. This network of zooplankton time-series stations has enabled us to collect and synthesise seasonal and multi-annual data on abundance, body size, fecundity, hatching success and vertical distribution of C. helgolandicus. An aim was to enable comparison with its congener Calanus finmarchicus, which has been studied intensively as a key component of European and north east Atlantic marine ecosystems. C. finmarchicus is known to over-winter at depth, whereas the life-cycle of C. helgolandicus is less well understood. Overwintering populations of C. helgolandicus have been observed off the Atlantic coast between 400 and 800 m, while in the Mediterranean there is evidence of significant deep-water populations at depths as great as 4200 m. The biogeographical distribution of C. helgolandicus in European coastal waters covers a wide range of habitats, from open ocean to coastal environments, and its contribution to mesozooplankton biomass ranges from 6% to 93%. Highest abundances were recorded in the Adriatic and off the west coast of Spain. C. helgolandicus is generally found in 9-20 °C water, with maximum abundances from 13-17 °C. In contrast, C. finmarchicus is found in cooler water between 0 and 15 °C, with peak abundances from 0 to 9 °C. As water has warmed in the North Atlantic over recent decades, the range of C. helgolandicus and its abundance on the fringes of its expanding range have increased. This review will facilitate development of population models of C. helgolandicus. This will not only help answer remaining questions but will improve our ability to forecast future changes, in response to a warming climate, in the abundance and distribution of this important species.