Foliage temperature extraction from thermal imagery for crop water stress determination

Crop water stress determination methods from canopy temperatures, derived from the surface energy balance equations, treat the canopy temperature under the assumption that the canopy behaves as a virtual “big-leaf”, covering the ground surface. Introduction of very high-resolution thermal imagery, 0.01–0.3-m pixel size, acquired from low altitude platforms, enabled finely detailed observation of the whole canopy, raising the question how to select the relevant canopy temperatures. One approach is to select the sunlit leaves confirming to the “big leaf” energy balance paradigm. However, thermal imagery alone is incomplete and needs additional marking or synchronized visible imagery for interpretation, which makes the process complicated and expensive. The other approach, used in reference surface based water stress evaluation, is to use full frame pixel statistics without pattern recognition by selecting the mean temperature of the cold fraction from the pixel histogram. That greatly simplifies processing for large-scale aerial thermography. Here are presented the results of experiments conducted in cotton and vine grapes, where both approaches were evaluated simultaneously. Ground referenced thermal and visible images were overlapped, and sunlit, shaded and whole canopy leaves were selected for crop temperature evaluation. The pixel histograms of the same images were analyzed in a two-step method, after discarding soil pixels where their temperature was 7 °C higher than air temperature at step one, and calculation of the mean temperatures of the lowest 33 and 100 % of the remaining pixels for step two. Several crop water stress indices were compared with leaf and stem water potentials and stomatal conductance. Good agreement was found between both image segmentation and histogram analysis methods, demonstrating the suitability of both methods in canopy temperature evaluation for crop water stress evaluation.     

Crop water requirements revisited: The human dimensions of irrigation science and crop water management with special reference to the FAO approach

Halfway through the 20thcentury, a curious shift took place in theconcept and definition of the agronomic termcrop water requirements. Where these cropneeds were originally seen as the amount ofwater required for obtaining a certain yieldlevel, in the second half of the 20thcentury, the term came to mean the water neededto reach the potential or maximum yield in acertain season and locality. Some of themultiple academic, economic, social, andgeopolitical aspects of this conceptual shiftare addressed here. The crucial role of theproduction ecologist Cees de Wit in formulatingthis paradigmatic shift in the 1950s isdiscussed. It is seen how the incipient concernfor an expected global scarcity of waterresources has contributed to a trend back tothe conservative view of crop water control ofde Wit. The development over the years ofengineering and agricultural science conceptsconcerning irrigation and crop water control ispresented as an evolution from practicalhusbandry to specialized applied science;from an empirical, ecological approach to amainly physical/mathematical discipline. Inthe section ``The scientific heritage of Occamand Bacon,' it is argued that this developmentregarding irrigation is part of a general trendin agricultural (and other) sciences andtechnologies over the last 150 years, althoughtendencies to return to a more holisticapproach have, at times, occurred. The current mainstream concepts and methodsin the art and science of crop water control,far from being objective and value-free, oftenact as ``a siren song' for decision-makersresponsible for daily irrigation practice andregional or global water resource management.The seductive ``tune' of maximum yields,concurrently the highest crop water use, drownsout the more modest aim of making anefficient use of the available waterresources. The latter's allure might,however, become the morecompelling as a greater scarcity ofphysical water resources becomes moreimportant than scarcity of land and labor.     

Soil water status mapping and two variable-rate irrigation scenarios

Irrigation is the major user of allocated global freshwaters, and scarcity of freshwater threatens to limit global food supply and ecosystem function—hence the need for decision tools to optimize use of irrigation water. This research shows that variable alluvial soil ideally requires variable placement of water to make the best use of irrigation water during crop growth. Further savings can be made by withholding irrigation during certain growth stages. The spatial variation of soil water supplied to (1) pasture and (2) a maize crop was modelled and mapped by relating high resolution apparent electrical conductivity maps to soil available water holding capacity (AWC) at two contrasting field sites. One field site, a 156-ha pastoral farm, has soil with wide ranging AWCs (116–230 mm m⁻¹); the second field site, a 53-ha maize field, has soil with similar AWCs (161–164 mm m⁻¹). The derived AWC maps were adjusted on a daily basis using a soil water balance prediction model. In addition, real-time hourly logging of soil moisture in the maize field showed a zone where poorly drained soil remained wetter than predicted. Variable-rate irrigation (VRI) scenarios are presented and compared with uniform-rate irrigation scenarios for 3 years of climate data at these two sites. The results show that implementation of VRI would enable significant potential mean annual water saving (21.8% at Site 1; 26.3% at Site 2). Daily soil water status mapping could be used to control a variable rate irrigator.     

Crop water stress index derived from multi-year ground and aerial thermal images as an indicator of potato water status

Potato yield and quality are highly dependent on an adequate supply of water. In this study, 3 years of information from thermal and RGB images were collected to evaluate water status in potato fields. Irrigation experiments were conducted in commercial potato fields (Desiree; drippers). Two water-deficit scenarios were tested: a short-term water deficit (by suppressing irrigation for a number of days before image acquisition), and a long-term cumulative water deficit. Ground and aerial images were acquired in various phenological stages along the potato growing season. Effects of irrigation treatments were recorded by thermal indices and biophysical measurements of stomatal conductance (SC), leaf water potential, leaf osmotic potential and gravimetric water potential in soil. Canopy temperature was delineated from the thermal images with and without fused information from the RGB image. Crop water stress index (CWSI) was calculated, using three forms of minimum baseline temperature: empirical, theoretical and statistical. An empirical evaluation of maximum baseline temperature of Tair + 7 °C was used in all CWSI forms examined. Statistical tests and comparison of CWSI with biophysical measurements were performed to evaluate the responses to irrigation treatments. The results indicated a high correlation of CWSI with SC from tuber initiation to maturity based on ground and aerial data (0.64 ≤ R² ≤ 0.99). Similar trends of increasing CWSI from well to deficit-irrigated treatments were found in all three growing seasons. The results also showed that CWSI may be calculated based merely on thermal imagery data.     

Hyperspectral species mapping for automatic weed control in tomato under thermal environmental stress

This work studied the impacts of variations in environmental temperature on hyperspectral imaging features in the visible and near infrared regions for robust species identification for weed mapping in tomato production. Six major Californian processing tomato cultivars, black nightshade (Solanum nigrum L.) and redroot pigweed (Amaranthus retroflexus L.) were grown under a variety of diurnal temperature ranges simulating conditions common in the Californian springtime planting period and one additional treatment simulating greenhouse growing conditions. The principal change in canopy reflectance with varying temperature occurred in the 480-670 and 720-810 nm regions. The overall classification rate ranged from 62.5% to 91.6% when classifiers trained under single temperatures were applied to plants grown at different temperatures. Eliminating the 480-670 nm region from the classifier’s feature set mitigated the temperature effect by stabilizing the total crop vs. weed classification rate at 86.4% over the temperature ranges. A site-specific recalibration method was also successful in alleviating the bias created by calibrating the models on the extreme temperatures and increased the classification accuracy to 90.3%. A global calibration method, incorporating all four temperature conditions in the classifier feature space, provided the best average total classification accuracy of 92.2% out of the methods studied, and was fairly robust to the varying diurnal temperature conditions.     

基于虚拟仪器杨树苗木水胁迫监测与灌溉系统研究

在分析适时性节水灌溉传统监测方法并指出其不足之处的基础上,研究了采用杨树苗木茎部声发射信号作为植株需水信息,通过AE传感器采集杨树苗木的茎部声发射信号,经相应的板卡和软件系统的分析、处理,并运用目前在测控领域国际流行的虚拟仪器技术,建立了实时、快速的微机监测和灌溉控制系统,并提出排水沟干湿交替喷/滴灌溉的新型的节水方法。试验证明:该系统实现自动化灌溉并且达到了显著的节水效果。     

Airborne multi-spectral imagery for mapping cruciferous weeds in cereal and legume crops

Cruciferous weeds are competitive broad-leaved species that cause losses in winter crops. In the present study, research on remote sensing was conducted on seven naturally infested fields located in Córdoba and Seville, southern Spain. Multi-spectral aerial images (four bands, including blue (B), green (G), red (R) and near-infrared bands) taken in April 2007 were used to evaluate the feasibility of mapping cruciferous patches (Diplotaxis spp. and Sinapis spp.) in winter crops (wheat, broad bean and pea) and compare the accuracy of different supervised classification methods (vegetation indices, maximum likelihood and spectral angle mapper). The best classification method was selected to develop site-specific cruciferous treatment maps. Cruciferous patches were efficiently discriminated with red/blue (R/B) and blue/green (B/G) vegetation indices and the maximum likelihood classifier. At all of the locations, the accuracy of the results obtained from the spectral angler mapper was relatively low. The cruciferous weed-classified imagery of each location were created according to the method that provided the best discrimination results and were used to obtain site-specific treatment maps for in-season post-emergence control measures or herbicide applications for subsequent years. By applying the site-specific treatment maps, herbicide savings from 71.7 to 95.4% for the no-treatment areas and from 4.3 to 12% for the low-dose herbicide were obtained.     

Evaluation of different approaches for estimating and mapping crop water status in cotton with thermal imaging

Canopy temperature has long been recognized as an indicator of plant water status, therefore, a high-resolution thermal imaging system was used to map crop water status. Potential approaches for estimating crop water status from digital infrared images of the canopy were evaluated. The effect of time of day on leaf temperature measurements was studied: midday was found to be the optimal time for thermal image acquisition. Comparison between theoretical and empirical approaches for estimating leaf water potential showed that empirical temperature baselines were better than those obtained from energy balance equations. Finally, the effects of angle of view and spatial resolution of the thermal images were evaluated: water status was mapped by using angular thermal images. In spite of the different viewing angles and spatial resolution, the map provided a good representation of the measured leaf water potential.     

株25S冷灌繁殖技术研究

介绍了株25S优质高产冷灌繁殖的基地选择、繁殖季节选择、冷灌技术、栽培技术及保纯技术,繁殖产量能稳定在3 500kg/hm2以上,种子纯度99.9%以上。     

Airborne hyperspectral imagery and linear spectral unmixing for mapping variation in crop yield

Spectral unmixing techniques can be used to quantify crop canopy cover within each pixel of an image and have the potential for mapping the variation in crop yield. This study applied linear spectral unmixing to airborne hyperspectral imagery to estimate the variation in grain sorghum yield. Airborne hyperspectral imagery and yield monitor data recorded from two sorghum fields were used for this study. Both unconstrained and constrained linear spectral unmixing models were applied to the hyperspectral imagery with sorghum plants and bare soil as two endmembers. A pair of plant and soil spectra derived from each image and another pair of ground-measured plant and soil spectra were used as endmember spectra to generate unconstrained and constrained soil and plant cover fractions. Yield was positively related to the plant fraction and negatively related to the soil fraction. The effects of variation in endmember spectra on estimates of cover fractions and their correlations with yield were also examined. The unconstrained plant fraction had essentially the same correlations (r) with yield among all pairs of endmember spectra examined, whereas the unconstrained soil fraction and constrained plant and soil fractions had r-values that were sensitive to the spectra used. For comparison, all 5151 possible narrow-band normalized difference vegetation indices (NDVIs) were calculated from the 102-band images and related to yield. Results showed that the best plant and soil fractions provided better correlations than 96.3 and 99.9% of all the NDVIs for fields 1 and 2, respectively. Since the unconstrained plant fraction could represent yield variation better than most narrow-band NDVIs, it can be used as a relative yield map especially when yield data are not available. These results indicate that spectral unmixing applied to hyperspectral imagery can be a useful tool for mapping the variation in crop yield.     

土壤水分胁迫对小麦产量的影响及节水灌溉的可行性

综述了土壤水分胁迫对小麦产量形成的影响及小麦节水灌溉的可行性.近年我国北方时常出现的土壤水分胁迫,既影响小麦的物质生产,也影响籽粒形成的物质来源.由于土壤胁迫影响器官生长,使叶面积减小,叶绿素含量降低,群体和单叶光合性能都有所变劣.单叶光合性能的研究多集中在旗叶,干旱使旗叶光合速率降低,光合功能期缩短,而这些表现又与气孔导度的变化有关.土壤水分胁迫也使开花后的光合产物减少,灌浆物质不足,因此原贮存在营养器官中的物质向籽粒的运转速度加快,贮存物质在粒重中的比例提高,这可以在一定程度上弥补粒重和产量的降低.在小麦生产中的很多例证表明只要采用适宜的栽培技术,适当减少灌溉次数和灌水量并不严重影响产量,且能提高水分利用效率.通过试验研究,这些现象近年在理论上也得到证明.从理论和实践上说明了小麦节水灌溉的可行性.     

基于无人机影像的作物土壤水分亏缺反演方法

【目的】基于无人机影像进行作物土壤水分亏缺反演研究有利于提高农田集约化管理效率,提升农田精准灌溉水肥空间分布信息高效获取技术水平。【方法】文章以冬小麦为例,采用大型喷灌机变量灌溉的方式,设置3个灌溉水平和6个不同施肥处理;利用无人机携带热红外、可见光、多光谱相机,规划航线飞行采集影像数据,结合田间布点取样校准,反演冬小麦长势;根据获取的无人机遥感影像,经图像拼接、数据提取等处理,获得田间不同试验小区光谱数据,计算植被指数,反演水肥指标,构建土壤水分反演模型。【结果】水分亏缺情境下,灌水的多少直接影响冬小麦的生理生长指标,灌水量越多,冬小麦株高和叶面积发育越好;3个灌溉水平处理间冠层温度差异在2～5℃;植被指数与灌溉处理一致性较好,而不同的施肥处理反演效果不理想。【结论】热红外影像反演作物冠层温度计算得到的作物水分亏缺指数可以展现冬小麦作物水分亏缺空间分布,作物水分亏缺指数与土壤水分平均含水率有很好的相关性。无人机遥感在田间水肥管理的应用有效提升了大田时空数据的获取能力,下一步应开展水肥时空变异性研究,探讨水肥精准施用技术,从理论和技术两方面着手提升精准管理水平。     

不同灌水定额对欧李抗逆性状、耗水特性及水分利用效率的影响

通过研究不同灌水定额下欧李抗逆性状及耗水特性,本试验探索了欧李在宁夏中北部半干旱地区栽培的可行性和最优灌水定额。结果表明：成年欧李树在灌水定额为7 mm时仍能完成生育全过程,表明其具有较强的抗旱性;随着灌水定额的增加,丙二醛含量和细胞膜透性均呈先降低后升高的变化趋势,游离脯氨酸和叶绿素a含量均呈先升高后降低的变化趋势。综合分析欧李抗逆性、耗水特性及水分利用效率可知,灌水定额为20 mm处理是兼顾产量和节水效益的最佳处理,与灌水定额为7 mm处理相比,虽然水分利用效率不高,但产量增加84.08%。总体来看,在该地区将欧李作为经济林兼作生态林的栽培树种,具有良好的经济效益和生态效益。     

株1S冷灌繁殖技术

介绍了株1S冷灌繁殖的基地选择，季节选择，主要栽培措施，冷水串灌的水温、水深、流量，灌溉时期和每天冷灌的时间，以及防杂保纯措施等操作要点，冷灌繁殖产量稳定在2500kg/hm^2以上，最高单产达5180kg/hm^2。     

干旱区棉花水分胁迫指数对滴灌均匀系数和灌水量的响应

为了修订和完善滴灌均匀系数的设计与评价标准,在新疆干旱区研究了滴灌均匀系数和灌水量对作物水分胁迫指数（CWSI）的影响。供试作物为棉花,试验中滴灌均匀系数（C_u）设置0.65（C1）、0.78（C2）和0.94（C3）三个水平,灌水量设置充分灌水量的50%、75%和100%三个水平。结果表明：棉花冠层温度和CWSI表现出随灌水量增加而降低的趋势;冠层温度和CWSI均匀系数的变化范围分别为0.91～0.98和0.65～0.91,均随滴灌均匀系数增加而增大;灌水量对冠层温度和CWSI均值的影响达到极显著水平（α=0.01）,滴灌均匀系数对冠层温度和CWSI均匀系数的影响达到显著水平（α=0.05）或极显著水平。CWSI与皮棉产量呈显著或极显著的负相关关系;滴灌均匀系数越低,水分亏缺引起的减产幅度越小。     

“水肥一体化”技术在马铃薯栽培中的应用

广东马铃薯种植主要在秋冬季节,传统的马铃薯栽培是通过“大水大肥”来实现增产,但由于广东降雨分布不均匀,秋冬季节雨水较少,干旱时有发生,马铃薯灌溉问题突出,加上封行后人工施肥较为困难,因此马铃薯生产极易受到影响,产量很不稳定.通过滴灌施肥的“水肥一体化”技术,可很好地解决马铃薯生长各个时期的水、肥供应问题,达到节水、节肥、省工、增产的效果.在多年实践的基础上,总结了“水肥一体化”在马铃薯栽培中应用的原理、优势、方法和效果,以期为今后“水肥一体化”技术在马铃薯栽培中的广泛应用提供依据.     

Mapping water status based on aerial thermal imagery: comparison of methodologies for upscaling from a single leaf to commercial fields

Aerial thermal remote sensing can provide a means for collecting spatial plant water status data. Many studies have shown their potential in irrigation management but the adaptation of this technology is not straight forward. In this paper, knowledge accumulated in recent years on thermal imagery analysis methodology for water status mapping is summarized aiming at indicating alternatives to calculate the Crop Water Stress Index (CWSI) for commercial scale water status mapping. Based on literature overview, four forms of wet-baselines to calculate CWSI were selected, namely: artificial wet reference surface, two theoretical calculations and a statistical bio-indicator. These baselines were used to calculate CWSI based on multi-temporal aerial thermal images of cotton fields. CWSI based on a statistical bio-indicator and one of the theoretical wet-baselines provided the best correlations. It is argued though that the statistical one is preferable since it includes the plant characteristics and it is farmer-friendly. Based on bio-indicators, leaf water potential maps of three commercial fields were produced on several dates through the season. Water status spatial patterns were not static and the effect of static factors like sandy soil patches also changed through the season. The maps show the importance of in-season variability mapping for rational irrigation management. To improve current variable-rate irrigation, the concept of in-season irrigation management zones (IMZ) based on thermal-images should be considered and integrated with the delineation of static irrigation IMZ.     

油葵对沼液灌溉引起的盐碱胁迫响应

为解决养殖业废弃物沼液长期灌溉引起农田盐渍化风险问题,本研究选择油葵作为沼液消纳作物研究其耐盐碱性并进行大田验证.通过室内模拟沼液中盐分主要的胁迫类型,配制不同浓度盐、碱离子进行油葵种子发芽胁迫实验,明确油葵发芽期对沼液中盐、碱胁迫的耐受阈值,同时大田实验设置不同沼液灌溉量灌溉油葵,观测生长状况和农艺性状,综合分析油葵对盐、碱胁迫的响应机制.结果 表明,油葵发芽对中性盐耐受阈值在180～240 mmol?L-1之间,对碱性盐离子耐受阈值在60～120 mmol?L-1之间.发芽过程中,碱性盐离子浓度为60 mmol?L-1时,细胞膜相对透性为24％,与中性盐(22％)相近,但相对盐害率为32.36％,显著高于中性盐处理(1.45％),K+/Na+下降91.11％～92.81％,中性盐处理下K+/Na+下降79.90％～87.33％.大田试验利用Na+浓度约为35 mmol?L-1沼液长期灌溉油葵,在低灌溉量(150 m3?hm-2)下,油葵各农艺性状变化不大,且K+/Na+差异不显著,但在高灌溉量(600 m3?hm-2)下,油葵生长各农艺指标受到抑制,各组织K+/Na+下降57％～88％.研究表明,碱性盐产生的高pH危害高于盐离子渗透胁迫,且碱性盐会更大程度破坏油葵离子稳态,影响油葵发芽与生长.利用养殖业废弃物中沼液作为再生水资源灌溉农田,应控制其HCO-3、CO2-3的含量,调控废水pH,以降低对农作物的伤害.     

镉污染灌溉水入田前快速净化材料和装置研究

为开发灌溉水入田前低成本、快速净化技术,创制了一套农田Cd污染灌溉水的快速净化装置,筛选出四类低成本、环保、高效的Cd净化材料以及材料配比方案。研究表明,不同材料Cd静态和动态吸附效果为赤泥粒石灰石沸石油菜秸秆。模拟灌溉时长3 h,灌水Cd浓度60μg·L~(-1),过水流量53.3 m3·h~(-1)(内径40 cm圆形过水断面),保障净化后水中Cd浓度低于农田灌溉水安全标准(10μg·L~(-1)),Cd污染水净化率≥83.3%,上述四类材料对Cd吸附总量变化范围为344.4~358.2 g·m~(-3),各材料间Cd吸附量无显著差异。净化装置主要工艺参数:进水口径10 cm,壳体内径40 cm,壳体高度65 cm,最大处理水量64.8 m3·h-1,滤层高度45 cm,材料承载体积56.5 dm~3,滤层分为三层,每层由10 cm厚单一材料构成,材料粒径5~8 mm。灌溉水推进速度1.4 m·s~(-1),组合材料处理Cd污染水(10~60μg·L~(-1))净化率85%,材料一次装填可净化Cd 15.2~20.1 g,预计可处理水507~671 m~3,满足0.13~0.18 hm~2单季稻田灌水净化需求(灌水Cd浓度和灌水定额分别以40μg·L~(-1)和3750 m3·hm~(-2)计)。