| 1. |
- Bouras, El houssaine, et al.
(författare)
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Wheat Yield Estimation at High Spatial Resolution through the Assimilation of Sentinel-2 Data into a Crop Growth Model
- 2023
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Ingår i: Remote Sensing. - 2072-4292. ; 15:18
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Tidskriftsartikel (refereegranskat)abstract
- Monitoring crop growth and estimating crop yield are essential for managing agricultural production, ensuring food security, and maintaining sustainable agricultural development. Combining the mechanistic framework of a crop growth model with remote sensing observations can provide a means of generating realistic and spatially detailed crop growth information that can facilitate accurate crop yield estimates at different scales. The main objective of this study was to develop a robust estimation methodology of within-field winter wheat yield at a high spatial resolution (20 m × 20 m) by combining a light use efficiency-based model and Sentinel-2 data. For this purpose, Sentinel-2 derived leaf area index (LAI) time series were assimilated into the Simple Algorithm for Yield Estimation (SAFY) model using an ensemble Kalman filter (EnKF). The study was conducted on rainfed winter wheat fields in southern Sweden. LAI was estimated using vegetation indices (VIs) derived from Sentinel-2 data with semi-empirical models. The enhanced two-band vegetation index (EVI2) was found to be a useful VI for LAI estimation, with a coefficient of determination (R2) and a root mean square error (RMSE) of 0.80 and 0.65 m2/m2, respectively. Our findings demonstrate that the assimilation of LAI derived from Sentinel-2 into the SAFY model using EnKF enhances the estimation of within-field spatial variability of winter wheat yield by 70% compared to the baseline simulation without the assimilation of remotely sensed data. Additionally, the assimilation of LAI improves the accuracy of winter wheat yield estimation by decreasing the RMSE by 53%. This study demonstrates an approach towards practical applications of freely accessible Sentinel-2 data and a crop growth model through data assimilation for fine-scale mapping of crop yield. Such information is critical for quantifying the yield gap at the field scale, and to aid the optimization of management practices to increase crop production.
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| 2. |
- Domeij, Rickard, et al.
(författare)
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Different ways of evaluating a Swedish grammar checker
- 2002
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Ingår i: Proceedings of the third international conference on language resources and evaluation (LREC). ; , s. 262-267
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Three different ways of evaluating a Swedish grammar checker are presented and discussed in this article. The first evaluationconcerns measuring the program's detection capacity on five text genres. The measures (precision and recall) are often used inevaluating grammar checkers. However, in order to test and improve the usability of grammar checking software, they need to becomplemented with user-oriented methods. Consequently, the second and the third evaluations presented in the article both involveusers. The second evaluation focuses on user reactions to grammar error presentations, especially with regard to false alarms anderroneous error identification. The third and last evaluation focuses on problems in supporting users' cognitive revision processes. Italso examines user motives behind choosing to correct or not to correct problems highlighted by the program. Advantages anddisadvantages of the different evaluation methods are discussed.
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| 3. |
- Jin, Hongxiao, et al.
(författare)
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Disentangling remotely-sensed plant phenology and snow seasonality at northern Europe using MODIS and the plant phenology index
- 2017
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Ingår i: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 198, s. 203-212
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Tidskriftsartikel (refereegranskat)abstract
- Land surface phenology is frequently derived from remotely sensed data. However, over regions with seasonal snow cover, remotely-sensed land surface phenology may be dominated by snow seasonality, rather than showing true plant phenology. Overlooking snow influences may lead to inaccurate plant phenology estimation, and consequently to misinterpretation of climate-vegetation interactions. To address the problem we apply the recently developed plant phenology index (PPI) to Moderate Resolution Imaging Spectroradiometer (MODIS) data for estimating plant phenology metrics over northern Europe. We compare PPI-derived start and end of the growing season with ground observations by professionals (6 sites) and nonprofessional citizens (378 sites), with phenology metrics derived from gross primary productivity (GPP, 18 sites), and with data on the timing of snow cover. These data are also compared with land surface phenology metrics derived from the normalized difference vegetation index (NDVI) using the same MODIS data. We find that the PPI-retrieved plant phenology agrees with ground observations and GPP-derived phenology, and that the NDVI-derived phenology to a large extent agrees with the end-of-snowmelt for the start-of-season and the start-of-snowing for the end-of-season. PPI is thereby useful for more accurate estimation of plant phenology from remotely sensed data over northern Europe and other regions with seasonal snow cover.
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| 4. |
- Kelly, Julia, et al.
(författare)
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Can thermal imagery improve ecosystem respiration modelling and UAV-based upscaling?
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Temperature is a key constraint on ecosystem respiration but is typically measured at only one or a few points across a study site. Given the exponential relationship between temperature and respiration, small differences in temperature across a heterogeneous ecosystem may result in large differences in respiration fluxes. These differences are not captured in current respiration models which use single measurements of air or soil temperature to represent a whole ecosystem. As a result, uncertainties are introduced when upscaling modelled respiration estimates or partitioning eddy covariance fluxes.Thermal cameras provide a new method for observing the spatial variability of surface temperature across an ecosystem at a high temporal resolution. We used thermal images, air and soil temperature data to model chamber respiration measurements collected over the growing season at a peatland in central Sweden. The measurements covered the two main vegetation communities (bryophyte-dominated hollows and graminoid-dominated hummocks) present at the site. We tested whether surface temperature data from the tower-based thermal camera predicted ecosystem respiration more accurately than air or soil temperature. We also assessed the impact of vegetation heterogeneity on the modelled fluxes by comparing model accuracy when including either one or bothvegetation communities in a single model. Finally, we upscaled our modelled predictions to the whole site using a vegetation classification and thermal data collected with an Umanned Aerial Vehicle (UAV). We could thus quantify the uncertainties in modelled fluxes when using single versus spatially-distributed temperature estimates at the ecosystem scale.
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| 5. |
- Kelly, Julia, et al.
(författare)
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Challenges and Best Practices for Deriving Temperature Data from an Uncalibrated UAV Thermal Infrared Camera
- 2019
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 11:5
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Tidskriftsartikel (refereegranskat)abstract
- Miniaturized thermal infrared (TIR) cameras that measure surface temperature are increasingly available for use with unmanned aerial vehicles (UAVs). However, deriving accurate temperature data from these cameras is non-trivialsince they are highly sensitive to changes in their internal temperature and low-cost models are often not radiometrically calibrated. We present the results of laboratory and field experiments that tested the extent of the temperature-dependency of a non-radiometric FLIR Vue Pro 640. We found that a simple empirical line calibration using at least three ground calibration points was sufficient to convert camera digital numbers to temperature values for images captured during UAV flight. Although the camera performed well under stable laboratory conditions (accuracy +/- 0.5 degrees C), the accuracy declined to +/- 5 degrees C under the changing ambient conditions experienced during UAV flight. The poor performance resulted from the non-linear relationship between camera output and sensor temperature, which was affected by wind and temperature-drift during flight. The camera's automated non-uniformity correction (NUC) could not sufficiently correct for these effects. Prominent vignetting was also visible in images captured under both stable and changing ambient conditions. The inconsistencies in camera output over time and across the sensor will affect camera applications based on relative temperature differences as well as user-generated radiometric calibration. Based on our findings, we present a set of best practices for UAV TIR camera sampling to minimize the impacts of the temperature dependency of these systems.
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| 6. |
- Kelly, Julia, et al.
(författare)
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Modelling and upscaling ecosystem respiration using thermal cameras and UAVs: Application to a peatland during and after a hot drought
- 2021
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 300
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Tidskriftsartikel (refereegranskat)abstract
- Field-based thermal infrared cameras provide surface temperature information at very high spatial and temporal resolution and could complement existing phenological camera and spectral sensor networks. Since temperature is one of the main drivers of ecosystem respiration (ER), field-based thermal cameras offer a new opportunity to model and upscale ER in unprecedented detail. We present such an approach based on manual chamber CO2 flux measurements and thermal imagery from a tower-based camera and from Unmanned Aerial Vehicle (UAV) flights. Data were collected over two growing seasons, including the hot drought of 2018, for the two main vegetation microforms (hummock and hollow) of a hemi-boreal peatland in Sweden. Thermal imagery proved suitable for modelling ER in this ecosystem: ER model accuracies were similar when air, soil or surface temperature measurements were used as input. Our findings allowed us to upscale ER using UAV-derived thermal images and we present maps of ER at sub-decimeter resolution (<7 cm). The significantly different ER measured for each microform highlighted the importance of modelling their ER separately. Not accounting for these differences and the microforms' spatial distribution across the peatland led to a bias in upscaled ER of up to 18%. As a result of the severity and timing of the hot drought in 2018, we observed reductions in the ER of both microforms, but more so for hummocks (-48%) than for hollows (-15%), and modelled ER leveled off at high temperatures. These findings indicate that peatland carbon loss during hot droughts may be lower than expected and strongly relates to vegetation composition. The presented upscaling approach offers a new method to analyse how ER varies across a peatland or within a flux-tower footprint, and to interpret biases that occur when using coarse resolution satellite data to upscale chamber or tower-based flux measurements.
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| 7. |
- Knutsson, Ola, et al.
(författare)
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Designing and developing a language environment for second language writers
- 2007
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Ingår i: Computers and education. - : Elsevier BV. - 0360-1315 .- 1873-782X. ; 49:4, s. 1122-1146
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a field study carried out with learners who used a grammar checker in real writing tasks in an advanced course at a Swedish university. The objective of the study was to investigate how students made use of the grammar checker in their writing while learning Swedish as a second language. Sixteen students with different linguistic and cultural backgrounds participated in the study. A judgment procedure was conducted by the learners on the alarms from the grammar checker. The students’ texts were also collected in two versions; a version written before the session with the grammar checker, and a version after the session. This procedure made it possible to study to what extent the students followed the advice from the grammar checker, and how this was related to their judgments of its behavior.The results obtained demonstrated that although most of the alarms from the grammar checker were accurate, some alarms were very hard for the students to judge correctly. The results also showed that providing the student with feedback on different aspects of their target language use; not only on their errors, and facilitating the processes of language exploration and reflection are important processes to be supported in second-language learning environments.Based on these results, design principles were identified and integrated in the development of Grim, an interactive language-learning program for Swedish. We present the design of Grim, which is grounded in visualization of grammatical categories and examples of language use, providing tools for both focus on linguistic code features and language comprehension.
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| 8. |
- Knutsson, Ola, 1969-
(författare)
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Developing and Evaluating Language Tools for Writers and Learners of Swedish
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Skrivande och skrivet språk är idag en viktig del av många människors liv, i datorns ordbehandlare, i e-postprogram och i chattkanaler på Internet. Skrivet språk har blivit mer eller mindre en förutsättning för människors dagliga kommunikation. Denna utveckling av samhället leder till ökade behov av att på olika sätt hantera text. En teknologi som har stor potential att hjälpa människor med skrivande och skrivet språk är språkteknologi. I denna avhandling ligger fokus på olika språkverktyg vars avsikt är att stödja skribenter och de som lär sig svenska bland annat genom att skriva. Ett språkverktyg som har utvecklats och utvärderats i avhandlingen är språkgranskningsverktyget Granska. I arbetet med Granska har fokus legat på utvecklingen av regelspråk, granskningsregler och generella analysregler samt utvärdering av dessa. Granska kombinerar en statistisk grundanalys av ordens ordklasser med regelbaserade metoder för sökning av grammatiska fel och frasanalys. I utvecklingen av granskningsreglerna är dragkampen mellan felaktiga utpekningar av fel, så kallade falska alarm, och uteblivna utpekningar av fel, det största enskilda problemet. Dragkampen uppstår genom att det är svårt att hitta många fel utan att också göra en del felaktiga utpekningar. Språkverktyg för skrivande kan i stort sett utvärderas på två sätt: med fokus på texten eller på den som skriver. I denna avhandling har båda typerna av utvärdering utförts med såväl modersmålskribenter som skribenter med svenska som andraspråk. I en första textbaserad utvärdering visade det sig att textgenre spelar stor roll för Granskas resultat. Ett vanligt fel i en textgenre förekommer nästan inte alls i en annan. Detta innebär att det blir mycket svårt för programmet att inte avge några falska alarm i de texter där feltypen saknas. I en andra textbaserad utvärdering jämfördes Granska och en kommersiell grammatikkontroll på texter från andraspråksskribenter. Den kommersiella grammatikkontrollen visade sig att ha bättre träffsäkerhet, men upptäckte färre fel än Granska. En första mindre användarstudie utfördes med Granska och fem erfarna skribenter. Syfte med studien var att utveckla Granska i linje med skribenters behov vid revision av text. Resultatet indikerade att användarna inte hade några problem med att välja mellan olika feldiagnoser om ett av ersättningsförslagen var korrekt. Falska alarm verkade vara av varierande svårighetsgrad: falska alarm från stavningskontrollen är mer eller mindre ofarliga, medan falska alarm från granskningen av mer komplicerade feltyper kan försvåra revisionsarbetet för användaren. Granska utvecklades från början för erfarna skribenter med svenska som modersmål, men allteftersom arbetet har fortskridit har även skribenter med svenska som andraspråk blivit en allt viktigare användargrupp. I detta arbete har diskussionen om granskningsmetod blivit mer och mer central. Även om gruppen andraspråksskribenter är mycket heterogen, så innehåller den här gruppens texter generellt sett mer fel, och i många fall fler fel i samma mening. Detta gör granskningsproblemet betydligt svårare. För det första så blir det svårare att avgöra ordens ordklass och frastillhörighet när flera fel finns i samma mening, och därmed har programmet allt mindre att hänga upp den grundläggande språkliga analysen på. För det andra är det svårare att konstruera granskningsregler för fel vars natur är svår att förutsäga på förhand. För att förbättra den grundläggande språkanalysen utvecklades programmet GTA, som gör en frasanalys och satsgränsigenkänning. GTA utvecklades ur de generella analysregler som redan fanns i Granska. GTA designades för att klara av att analysera texter som innehåller vissa avvikelser från språkets norm, t.ex. inkongruens. För att ta reda på hur väl programmet klarade av mindre avvikelser i form av stavfel utvärderades GTA och även två program för ordklassanalys på texter med olika andel stavfel. GTA bygger till mycket stor del på att identifikationen av ordklass fungerar för att fraser och satsgränser skall analyseras korrekt. Detta bekräftas också i utvärderingen, där det visade sig att GTA klarar sig bra så länge som den underliggande ordklassanalysen klarar att hantera avvikelser i texten. En viktig faktor för att klara språkliga avvikelser, i form av stavfel, är en fungerande metod för att hantera ord som är okända för programmet. Nya metoder för språkgranskning har undersökts i samarbete med andra forskare, och där har avhandlingens bidrag varit i form av transformationsregler i den statistiska språkgranskaren ProbGranska. Dessa regler visade sig vid en utvärdering avsevärt förbättra ProbGranskas säkerhet när det gällde att identifiera grammatiska problem. I utvecklingen av språkgranskaren SnålGranska har avhandlingen bidragit med idéer till dess grundläggande algoritm. Denna algoritm bygger på att träna ett maskininlärningsprogram på konstgjorda fel i avsaknad av en korpus med många uppmärkta autentiska fel. För att komma vidare med utvecklingen av språkverktyg för andraspråksskribenter genomfördes en längre fältstudie vid ett svenskt universitet. Syftet var att studera användningen av Granska i autentiska skrivuppgifter som studenterna genomförde i en avancerad kurs i svenska som främmande språk. Sexton studenter med olika språklig och kulturell bakgrund deltog i studien. En viktig del av studien utgjordes av studenternas bedömningar av Granskas alarm. Bedömningarna gjordes på en betygsskala från 1 till 5. Studenternas texter samlades också in i två versioner; en version före och en efter användningen av programmet. Denna metod gjorde det möjligt att studera i vilken grad studenterna följde Granskas råd, och huruvida dåliga eller bra råd från programmet fick höga eller låga betyg. Mest alarmerande var att dåliga råd angående ordföljd alltid fick högsta betyg. Andra ofta lämpliga råd dömdes ut för att beskrivningen av dessa feltyper, t.ex. anmärkningar om saknade tempusböjda verb och uteblivna subjekt, var svåra att förstå samt att de saknade ersättningsförslag. En viktig insikt från fältstudien var att Granska eller liknade verktyg inte är det enda verktyg som andraspråksskribenter behöver när de skriver text. Denna insikt tillsammans med andra resultat från fältstudien mynnade ut i flera designprinciper för program med fokus på andraspråksskribenter. Dessa designprinciper användes sedan i utformningen av språkmiljön Grim. Grim är en ordbehandlingsmiljö med olika interaktiva språkverktyg integrerade: Granska, GTA, den statistiska språkgranskaren ProbGranska, lexikonet Lexin med åtta olika språkpar, konkordansgränssnitt mot stora textmängder från korpusen Parole, och en ordböjningsfunktion. I Grim kan användaren arbeta med egna eller andras texter, och få återkoppling på språkets former från Granska och GTA, undersöka ords användning i autentiska texter samt få en ökad förståelse av målspråket genom integrerade tvåspråkiga lexikon.
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| 9. |
- Lakomiec, Patryk, et al.
(författare)
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Field-scale CH4 emission at a subarctic mire with heterogeneous permafrost thaw status
- 2021
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 18:20, s. 5811-5830
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Tidskriftsartikel (refereegranskat)abstract
- The Arctic is exposed to even faster temperature changes than most other areas on Earth. Constantly increasing temperature will lead to thawing permafrost and changes in the methane (CH4) emissions from wetlands. One of the places exposed to those changes is the Abisko–Stordalen Mire in northern Sweden, where climate and vegetation studies have been conducted since the 1970s.In our study, we analyzed field-scale methane emissions measured by the eddy covariance method at Abisko–Stordalen Mire for 3 years (2014–2016). The site is a subarctic mire mosaic of palsas, thawing palsas, fully thawed fens, and open water bodies. A bimodal wind pattern prevalent at the site provides an ideal opportunity to measure mire patches with different permafrost status with one flux measurement system. The flux footprint for westerly winds was dominated by elevated palsa plateaus, while the footprint was almost equally distributed between palsas and thawing bog-like areas for easterly winds. As these patches are exposed to the same climatic and weather conditions, we analyzed the differences in the responses of their methane emission for environmental parameters.The methane fluxes followed a similar annual cycle over the 3 study years, with a gentle rise during spring and a decrease during autumn, without emission bursts at either end of the ice-free season. The peak emission during the ice-free season differed significantly for the two mire areas with different permafrost status: the palsa mire emitted 19 mg-C m−2 d−1 and the thawing wet sector 40 mg-C m−2 d−1. Factors controlling the methane emission were analyzed using generalized linear models. The main driver for methane fluxes was peat temperature for both wind sectors. Soil water content above the water table emerged as an explanatory variable for the 3 years for western sectors and the year 2016 in the eastern sector. The water table level showed a significant correlation with methane emission for the year 2016 as well. Gross primary production, however, did not show a significant correlation with methane emissions.Annual methane emissions were estimated based on four different gap-filing methods. The different methods generally resulted in very similar annual emissions. The mean annual emission based on all models was 3.1 ± 0.3 g-C m−2 a−1 for the western sector and 5.5 ± 0.5 g-C m−2 a−1 for the eastern sector. The average annual emissions, derived from these data and a footprint climatology, were 2.7 ± 0.5 and 8.2 ± 1.5 g-C m−2 a−1 for the palsa and thawing surfaces, respectively. Winter fluxes were relatively high, contributing 27 %–45 % to the annual emissions.
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| 10. |
- Müller, Mitro, et al.
(författare)
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Features predisposing forest to bark beetle outbreaks and their dynamics during drought
- 2022
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Ingår i: Forest Ecology and Management. - : Elsevier BV. - 1872-7042 .- 0378-1127. ; 523
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Tidskriftsartikel (refereegranskat)abstract
- Climate change is estimated to increase the risk of the bark beetle (Ips typographus L.) mass outbreaks in Norway Spruce (Picea abies (L.) Karst) forests. Habitats that are thermally suitable for bark beetles may expand, and an increase in the frequency and intensity of droughts can promote drought stress on host trees. Drought affects tree vigor and in unison with environmental features it influences the local predisposition risk of forest stands to bark beetle attacks. We aimed to study how various environmental features influence the risk of bark beetle attacks during a drought year and the following years with more normal weather conditions but with higher bark beetle populations. We included features representing local forest stand attributes, topography, soil type and wetness, the proximity of clear-cuts and previous bark beetle attacks, and a machine learning algorithm (random forest) was applied to study the variation of predisposition risk across a 48,600 km2 study area in SE Sweden.Forest stands with increased risk of bark beetle attack were distinguished with high accuracy both during drought and in normal weather conditions. The results show that during both study periods, spruce and mixed coniferous forests had elevated risk of attack, while forests with a mix of deciduous and coniferous trees had a lower risk. Forests with high average canopy height were strongly predisposed to bark beetle attacks. However, during the drought year risk was more similar between stands with lower and higher canopy height, suggesting that during drought periods younger trees can be predisposed to bark beetle attacks. The importance of soil moisture and position within the local landscape were highlighted as important features during the drought year.Identifying areas with increased risk, supported by information on how environmental features control the predisposition risk during drought, could aid adaptation strategies and forest management intervention efforts. We conclude that geospatial data and machine learning have the potential to further support the digitalization of the forest industry, facilitating development of methods capable to quantify importance and dynamics of environmental features controlling the risk in local context. Corresponding methods could help to direct management actions more effectively and offer information for decision-making in changing climate.
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