Prosopis spp. are a fast growing invasive tree originating from the American dry zones, introduced to Kenya in the 1970s for the restoration of degraded pastoral lands after prolonged droughts and overgrazing. Its deep rooting system is capable of tapping into the ground water table reducing its dependency on rain water and increasing its drought tolerance. It is believed that the Prosopis invasion was eased by a hybridization process, described as the Prosopis Juliflora – Prosopis Pallida complex, suggesting that introduced Prosopis spp. evolved into a hybrid, specifically adapted to the environmental conditions, rendering it a superior and aggressive competitor to endemic species. In many dry lands in Kenya Prosopis has expanded rapidly and has become challenging to control. On the other hand, in some cases, an economic use seems possible. In both cases, detailed and accurate maps are necessary to support stakeholders and design management strategies. The aim of this study is to map the distribution of Prosopis spp. in a selected area in north-west Turkana (Kenya), covering a section of the Tarach water basin. The study is funded by the European Union through the National Drought Management Authority (NDMA) in Kenya, and the main purpose is to assess the potential production of Prosopis pods, which can be used to manufacture emergency livestock feeds to support animals during drought events. The classification was performed using novel Sentinel-2 data through a non-parametric Random Forest classifier. A selection of reference sites was visited in the field and used to train the classifier. Very high classification accuracies were obtained.
Prosopis juliflora is a fast growing tree species originating from South and Central America with a high invasion potential in semi-arid areas around the globe. It was introduced to East Africa for the stabilization of dune systems and for providing fuel wood after prolonged droughts and deforestation in the 1970s and 1980s. In many dry lands in East Africa the species has expanded rapidly and has become challenging to control. The species generally starts its colonization on deep soils with high water availability while in later stages or on poorer soils, its thorny thickets expand into drier grasslands and rangelands. Abandoned or low input farmland is also highly susceptible for invasion as P. juliflora has competitive advantages to native species and is extremely drought tolerant.
In this work we describe a rapid approach to detect and map P. juliflora invasion at country level for the whole of Somaliland. Field observations were used to delineate training sites for a supervised classification of Landsat 8 imagery collected during the driest period of the year (i.e., from late February to early April). The choice of such a period allowed to maximise the spectral differences between P. juliflora and other species present in the area, as P. juliflora tends to maintain a higher vigour and canopy water content than native vegetation, when exposed to water stress.
The results of our classification map the current status of invasion of Prosopis in Somaliland showing where the plant is invading natural vegetation or agricultural areas. These results have been verified for two spatial subsets of the whole study area with very high resolution (VHR) imagery, proving that Landsat 8 imagery is highly adequate to map P. juliflora. The produced map represents a baseline for understanding spatial distribution of P. juliflora across Somaliland but also for change detection and monitoring of long term dynamics in support to P. juliflora management and control activities.
The aim of this paper is to present a freely available data service platform (http://ivfl-info.boku.ac.at/) for executing preprocessing operations (such as data smoothing, spatial and temporal sub-setting, mosaicking and reprojection) of time series of Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices (NDVI and EVI) on request. The web-application is based on the integration of various software and hardware components: a web-interface and a MySQL database are used to collect and store user’s requests. A server-side application schedules the user’s requests and delivers the results. The core of the processing system is based on the “MODIS” package developed in R, which provides MODIS data collection and pre-processing capabilities. Smoothed and gap-filled data sets are derived using the state-ofthe- art Whittaker filter implemented in Matlab. After the processing, data are delivered directly via ftp access. An analysis of the performance of the web-application, along with processing capacity is presented. Results are discussed, in particular in view of an operative platform for real time filtering, phenology and land cover mapping.
Evaluation is an essential step of model development. However, there is a missing definition of appropriate validation strategies, needed to guarantee reproducibility and generalizability of modeling results. Also, there is a lack of a generally agreed set of 'optimal' statistical measure(s) to assess model accuracy. The objective of the present study is to provide for remote sensing practitioners (i.e., non-statisticians) guidance for model validation strategies and to propose an optimal set of statistical measures for the quantitative assessment of model performance in the context of vegetation biophysical variable retrieval from Earth observation (EO) data. For these purposes, main terms and concepts were reviewed. Then, validation strategies were tested on a polynomial regression model and discussed. Moreover, a literature review was carried out, summarizing the statistical measures used to evaluate model performances. Supported by some exemplary datasets, these measures were calculated and their meanings discussed in view of several model validation criteria. From the results, we recommend to further exploit cross-validation and bootstrapping strategies to guarantee the development/validation of reliable models. An 'optimal' statistic set is suggested, including root mean square error (RMSE), coefficient of determination (R 2 ), slope and intercept of Theil-Sen regression, relative RMSE, and Nash-Sutcliffe efficiency index. A wide acceptance and use of these statistics should enable a better intercomparison of scientific results, urgently needed in times of increasing model development activities that are carried out with respect to upcoming EO missions.
The capability of models to predict vegetation biophysical variables is usually evaluated by means of one or several
goodness-of-fit measures, ranging from absolute error indices (e.g. the root mean square error, RMSE) over correlation
based measures (e.g. coefficient of determination, R2) to a group of dimensionless evaluation indices (e.g. relative
RMSE). Hence, the greatest difficulty for the readers is the lack of comparability between the different models'
accuracies. Therefore, the objective of our study was to provide an overview about the quantitative assessment of
biophysical variable retrieval performance. Furthermore, we aimed to suggest an optimal set of statistical measures. This
optimum set of statistics should be insensitive to the magnitude of values, range and outliers. For this purpose, a
literature review was carried out, summarizing the statistical measures that have been used to evaluate model
performances. Followed by this literature review and supported by some exemplary datasets, a range of statistical
measures was calculated and their interrelationships analyzed. From the results of the literature review and the test
analyses, we recommend an optimum statistic set, including RMSE, R², the normalized RMSE and some other
indicators. Using at least the recommended statistics, comparability of model prediction accuracies is guaranteed. If
applied, this will enable a better intercomparison of scientific results urgently needed in times of increasing data
availability for current and upcoming EO missions.
Vegetation indices (VI) combine mathematically a few selected spectral bands to minimize undesired effects of soil background, illumination conditions and atmospheric perturbations. In this way, the relation to vegetation biophysical variables is enhanced. Albeit numerous experiments found close relationships between vegetation indices and several important vegetation biophysical variables, well known shortcomings and drawbacks remain. Important limitations of VIs are illustrated and discussed in this paper. As most of the limitations can be overcome using physically-based radiative transfer models (RTM), advantages and limits of RTM are also presented.
In the context of defining a procedure for near real time land use/land cover (LULC) mapping with seasonal updated
products, this research examines the use of time-series and phenological indicators from MODIS NDVI. 16-day NDVI
composites from MODIS (MOD13Q1) covering the period from 2001 to the present were acquired for three test sites
located in different parts of Europe. The newly proposed Whittaker smoother was used for filtering purposes. Metrics of
vegetation dynamics (such as minimum, maximum and amplitude, etc.) were extracted from the filtered time-series.
Subsequently, the capability of three data sets (raw, filtered data and phenological indicators) was evaluated to separate
between different LULC classes by calculating the overall classification accuracy for the years 2002 and 2009. Ground
truth data for model calibration and testing set was derived combining existing land cover products (GLC2000 and
GlobCover 2009). Based on these results, the benefits of using phenological indicators and cleaned data for land cover
classification are discussed.
The robust and accurate retrieval of vegetation biophysical variables using radiative transfer models (RTM) is seriously
hampered by the ill-posedness of the inverse problem. With this research we further develop our previously published
(object-based) inversion approach [Atzberger (2004)]. The object-based RTM inversion takes advantage of the
geostatistical fact that the biophysical characteristics of nearby pixel are generally more similar than those at a larger
distance. A two-step inversion based on PROSPECT+SAIL generated look-up-tables is presented that can be easily
implemented and adapted to other radiative transfer models. The approach takes into account the spectral signatures of
neighboring pixel and optimizes a common value of the average leaf angle (ALA) for all pixel of a given image object,
such as an agricultural field. Using a large set of leaf area index (LAI) measurements (n = 58) acquired over six different
crops of the Barrax test site (Spain), we demonstrate that the proposed geostatistical regularization yields in most cases
more accurate and spatially consistent results compared to the traditional (pixel-based) inversion. Pros and cons of the
approach are discussed and possible future extensions presented.
The current work aimed at testing a methodology which can be applied to low spatial resolution satellite data to assess interannual
crop area variations on a regional scale. The methodology is based on the assumption that within mixed pixels such
variations are reflected by changes in the related multitemporal Normalised Difference Vegetation Index (NDVI) profiles.
This implies that low resolution NDVI images with high temporal frequency can be used to update land cover estimates
derived from higher resolution cartography. More particularly, changes in the shape of annual NDVI profiles can be detected
by a Neural Network trained by using high resolution images for a subset of the study years. By taking into account the
respective proportions of the remaining land covers within a given low resolution pixel, the accuracy of the net can be further
increased. The proposed methodology was applied in a study region in central Italy to estimate area changes of winter crops
from low resolution NDVI profiles. The accuracy of such estimates was assessed by comparison to official agricultural
statistics using a bootstrap approach. The method showed promise for estimating crop area variation on a regional scale and
proved to have a significantly higher forecast capability than other methods used previously for the same study area.
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