Integrating meteorological data in biomass prediction models

Diouf, A.A.; Hiernaux, P.; Brandt, M.; Faye, G.; Djaby, B.; Diop, M.B.; Ndione, J.A.; Tychon, B. Do Agrometeorological Data Improve Optical Satellite-Based Estimations of the Herbaceous Yield in Sahelian Semi-Arid Ecosystems? Remote Sens. 2016, 8, 668.

Quantitative estimates of forage availability at the end of the growing season in rangelands are helpful for pastoral livestock managers and for local, national and regional stakeholders in natural resource management. For this reason, remote sensing data such as the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) have been widely used to assess Sahelian plant productivity for about 40 years.

This study combines traditional FAPAR-based assessments with agrometeorological variables computed by the geospatial water balance program, GeoWRSI, using rainfall and potential evapotranspiration satellite gridded data to estimate the annual herbaceous yield in the semi-arid areas of Senegal.

It showed that a machine-learning model combining FAPAR seasonal metrics with various agrometeorological data provided better estimations of the in situ annual herbaceous yield (R2 = 0.69; RMSE = 483 kg·DM/ha) than models based exclusively on FAPAR metrics (R2 = 0.63; RMSE = 550 kg·DM/ha) or agrometeorological variables (R2 = 0.55; RMSE = 585 kg·DM/ha). All the models provided reasonable outputs and showed a decrease in the mean annual yield with increasing latitude, together with an increase in relative inter-annual variation. In particular, the additional use of agrometeorological information mitigated the saturation effects that characterize the plant indices of areas with high plant productivity.

The date of the onset of the growing season derived from smoothed FAPAR seasonal dynamics showed no significant relationship (0.05 p-level) with the annual herbaceous yield across the whole studied area. The date of the onset of rainfall was significantly related to the herbaceous yield and its inclusion in fodder biomass models could constitute a significant improvement in forecasting risks of a mass herbaceous deficit at an early stage of the year.

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Recent woody vegetation trends in Sahel

Our new paper looks at recent dynamics in woody vegetation in Sahel and finds some interesting patterns which are mainly controlled by human population density.

Martin Brandt, Pierre Hiernaux, Kjeld Rasmussen, Cheikh Mbow, Laurent Kergoat, Torbern Tagesson, Yahaya Ibrahim, Abdoulaye Wele, Compton J. Tucker, Rasmus Fensholt. Assessing woody vegetation trends in Sahelian drylands using MODIS based seasonal metrics. Remote Sensing of Environment, 2016, 183, 215-225.

  • Woody cover trends are estimated for Sahel based on MODIS dry season metrics.
  • Interannual fluctuations in foliage density are attenuated to monitor woody plant trends.
  • Increases (decreases) are seen in areas of low (high) human population.
  • Recent decreases only partially offset a general post-drought increase in Sahelian woody cover.

Woody plants play a major role for the resilience of drylands and in peoples’ livelihoods. However, due to their scattered distribution, quantifying and monitoring woody cover over space and time is challenging. We develop a phenology driven model and train/validate MODIS (MCD43A4, 500 m) derived metrics with 178 ground observations from Niger, Senegal and Mali to estimate woody cover trends from 2000 to 2014 over the entire Sahel at 500 m scale.

Over the 15 year period we observed an average increase of 1.7 (± 5.0) woody cover (%) with large spatial differences: No clear change can be observed in densely populated areas (0.2 ± 4.2), whereas a positive change is seen in sparsely populated areas (2.1 ± 5.2). Woody cover is generally stable in cropland areas (0.9 ± 4.6), reflecting the protective management of parkland trees by the farmers. Positive changes are observed in savannas (2.5 ± 5.4) and woodland areas (3.9 ± 7.3).

The major pattern of woody cover change reveals strong increases in the sparsely populated Sahel zones of eastern Senegal, western Mali and central Chad, but a decreasing trend is observed in the densely populated western parts of Senegal, northern Nigeria, Sudan and southwestern Niger. This decrease is often local and limited to woodlands, being an indication of ongoing expansion of cultivated areas and selective logging.

We show that an overall positive trend is found in areas of low anthropogenic pressure demonstrating the potential of these ecosystems to provide services such as carbon storage, if not over-utilized. Taken together, our results provide an unprecedented synthesis of woody cover dynamics in the Sahel, and point to land use and human population density as important drivers, however only partially and locally offsetting a general post-drought increase.

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Remote sensing of vegetation in drylands: Evaluating vegetation optical depth (VOD) using NDVI and in situ data over Sahel

Tian, F.; Brandt, M.; Liu, Y. Y.; Verger, A.; Tagesson, T.; Diouf, A. A.; Rasmussen, K.; Mbow, C.; Wang, Y.; Fensholt, R. Remote sensing of vegetation dynamics in drylands: Evaluating vegetation optical depth (VOD) using AVHRR NDVI and in situ green biomass data over West African Sahel. Remote Sensing of Environment 2016, 177, 265–276.

 

  •  A Long-term VOD dataset is evaluated against NDVI and in situ biomass observations.
  • Both VOD and NDVI reflect the spatio-temporal patterns of biomass in West Sahel.
  • VOD captures variations of woody plant foliage biomass better than NDVI.
  • VOD and NDVI seasonal metrics differ for optimal long-term monitoring of biomass.

Monitoring long-term biomass dynamics in drylands is of great importance for many environmental applications including land degradation and global carbon cycle modeling. Biomass has extensively been estimated based on the normalized difference vegetation index (NDVI) as a measure of the vegetation greenness. The vegetation optical depth (VOD) derived from satellite passive microwave observations is mainly sensitive to the water content in total aboveground vegetation layer. VOD therefore provides a complementary data source to NDVI for monitoring biomass dynamics in drylands, yet further evaluations based on ground measurements are needed for an improved understanding of the potential advantages.

In this study, we assess the capability of a long-term VOD dataset (1992–2011) to capture the temporal and spatial variability of in situ measured green biomass (herbaceous mass and woody plant foliage mass) in the semi-arid Senegalese Sahel.

Results show that the magnitude and peak time of VOD are sensitive to the woody plant foliage whereas NDVI seasonality is primarily governed by the green herbaceous vegetation stratum in the study area. Moreover, VOD is found to be more robust against typical NDVI drawbacks of saturation effect and dependence on plant structure (herbaceous and woody compositions) across the study area when used as a proxy for vegetation productivity. Finally, both VOD and NDVI well reflect the spatial and inter-annual dynamics of the in situ green biomass data; however, the seasonal metrics showing the highest degree of explained variance differ between the two data sources. While the observations in October (period of in situ data collection) perform best for VOD (r2 = 0.88), the small growing season integral (sensitive to recurrent vegetation) have the highest correlations for NDVI (r2 = 0.90).

Overall, in spite of the coarse resolution of 25 km, the study shows that VOD is an efficient proxy for estimating biomass of the entire vegetation stratum in the semi-arid Sahel and likely also in other dryland areas.

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Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series

From: Diouf, A.A., Brandt, M., Verger, A., Jarroudi, M.E., Djaby, B., Fensholt, R., Ndione, J.A., Tychon, B., 2015. Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series. Remote Sensing 7, 9122–9148. doi:10.3390/rs70709122

Livestock farming constitutes the most widespread human activity and the dominant land use in rangeland ecosystems. At a global scale, it contributes 40% of the agricultural gross domestic product, and provides income for more than 1.3 billion people and nourishment for at least 800 million food-insecure people. In particular for the West African Sahel, livestock constitutes the first renewable resource and is mainly characterized by an extensive use of pastures in rangelands.

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Since 1987 the Centre de Suivi Ecologique (CSE) operationally estimates the total annual biomass in Senegal in order to monitor the fodder availability of the national pastoral rangelands. Field data is collected along 1 km transects at 24 sites at the end of the wet season. Here, herbaceous and woody leaf biomass is measured and summed to the total available fodder biomass. This is done each year since 1987.

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Using a linear regression with satellite images, the field data is projected to whole Senegal and gives stakeholders an estimation on the quantity and distribution of fodder biomass. Between 1987 and 1999, this method was implemented using the seasonal integrated NDVI (i.e., seasonal weighted average) from the Advanced Very High Resolution Radiometer (AVHRR) of the National Oceanic and Atmospheric Administration (NOAA) satellites acquired in Local Area Coverage (LAC) format at the CSE receiving station in Dakar. Since 2000, the 1-km SPOT-VEGETATION NDVI have been used.

In this context, we developed a new operational system for monitoring total fodder biomass, including both herbaceous and woody leaf biomass. The proposed method is based on multiple linear regression models using phenological variables derived from the seasonal dynamics of the FAPAR SPOT-VEGETATION time series and ground measurements of total biomass production collected in different Sahelian ecosystems in Senegal over 15 years.

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A model with three variables – large seasonal integral (LINTG), length of growing season and end of season decreasing rate – performed best (MAE = 605 kg DM/ha; R² = 0.68) across Sahelian ecosystems in Senegal (data for the period 1999-2013). A model with annual maximum (PEAK) and start date of season showed similar performances (MAE = 625 kg DM/ha; R² = 0.64), allowing a timely estimation of forage availability. The subdivision of the study area using metrics related to ecosystem properties increased overall accuracy (MAE = 489.21 kg DM/ha; R² = 0.77). LINTG was the main explanatory variable for woody rangelands, whereas for areas dominated by herbaceous vegetation it was the PEAK metric. The proposed approach outperformed the established single-predictor model (MAE = 818 kg DM/ha and R² = 0.51) and should improve the operational monitoring of forage resources in Sahelian rangelands.

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In the future, such early warning models should enable stakeholders to take decisions as early as September (current year as biomass shortage) with regard to livestock by triggering protocols designed for livestock management (e.g., Opération de Sauvegarde du Bétail ) in Senegal.

see the full document here: MDPI

Text and Figures: A.A. Diouf; Fotos: M. Brandt

Climate, Environment and Vegetation in the West African Sahel

Here’s a press release summarizing parts of my PhD. It was released by the University of Bayreuth (http://www.uni-bayreuth.de/pressemitteilungen-html/121-Sahelzone/index.html) and IDW (http://idwf.de/-CREwAA) in German (thanks to Pierre Gosselin for translating). As I got many emails on this, we want to make clear at this point that we do not aim to question climate change with this article! It shall give background information on the greening of the Sahel/desertification debates, and not on climatic changes/variability.

New research works show: Not global climate change alone, but rather foremost the local actions of people impact the face of their environment

Are the earth’s deserts continuously expanding? Or is green vegetation now spreading into regions that were once barren deserts? The West African section of the Sahel zone located at the southernmost edge of the Sahara, which extends from the Atlantic to the Red Sea, has been the source of reason for a wide variety of prognoses over the recent years. Extreme periods of drought during the 1970s and 1980s were considered as indices of growing desert regions across the globe. “Desertification” was the buzzword. However, over the last two decades a rise in precipitation has been observed across the West African Sahel. As a result there has been talk about the blanket perception that “the desert is greening”.

With this controversy as the backdrop, an international research team led by geographer Martin Brandt of the University of Bayreuth examined the vegetation development in the West African Sahel more closely. High and coarse resolution satellite data as well as wide range of measurement results from the last decades enabled conclusions to be drawn on climate and vegetation trends and field research brought regional and local particularities to light. Here some determinations were made: There is no uniform development in the West African Sahel. Not only the climate but also especially various forms of land-use – farming, forestry management or village development – are mostly responsible for the way the landscape there appears, and which resources it offers the people.

In the journal “remote sensing” researchers from Bayreuth (Germany), France, Spain and the Senegal report on their results. “The activity of man on location, for example the sustainable cultivation of selected green plants or the reforestation of forests, can impact the face of the landscape considerably,” says Martin Brandt. “Such initiatives and measures by the local population are far less dependent on large-scale climatic trends than what was earlier assumed. For this reason environmental and climate research should not be one-sidedly guided by blanket buzzwords such as ‘desertification’ or ‘greening Sahel’.”

Regional differences due to land and forest management – case studies in Mali and in the Senegal

Thanks to  satellite time series analyses , the scientists were able to determine that the vegetation density in the West African Sahel increased from 1982 to 2010. This development is especially pronounced in the Senegal and in western Mali. Here there are clear regional differences with respect to plants that have multiplied over time: Not only does one observe the wild growth of trees, bushes and grass, but also foremost the expansion of crops and plants due to farm and forest management measures. In total one notices that in the West African countries, with the exception of Gambia and the Ivory Coast, the forest levels have decreased markedly even though the vegetation density has increased as a whole.

Bayreuth_2The field research work by Martin Brandt (left) concentrated on two regions: the Senegal and Mali: The region surrounding the city of Bandiagara in southern Mali has seen a complete transformation of its vegetation over the last 50 years: Many tree and bush types that were still common in the 1960s have disappeared. Periods of drought did not alone damage the plants through a lack of water, but also it was because income from agriculture fell due to poor harvests, and so the people tried to compensate by felling trees and selling lumber. However in the meantime, a vegetation-rich landscape has since appeared – and not only because the precipitation amounts have been increasing for two decades and extended periods of droughts have failed to occur. “A targeted reforestation and planting of trees on agricultural land have changed the landscape considerably,” reports Brandt, and adds: “Without a sound botanical and ecological knowledge by the local population, this development would not have been possible.”

The transformation to an agricultural landscape  was also found by the scientists from Bayreuth at another region – one located in the Senegal, north of the city of Linguère. This region is mainly settled by nomads belonging to the Fulbe ethnic group who practice intensive pasture farming. In order to feed their livestock with leaves during dry periods, they cut or fell trees and bushes during dry periods. Nevertheless, state-sponsored reforestation and protective measures have led to a considerable increase in vegetation over the last two decades and it has become more adaptable to climate fluctuations. Today three especially robust tree types make up more than 90% of the vegetation found in the region surrounding Linguère. “Alone in the immediate proximity of the city there is a fenced-in area of at least 5000 hectares on which a special species of acacia has been placed,” says Martin Brandt. However he also points to the unmistakable damage in some places arising from the overuse of the tree stock. This completely bare ground is very difficult to regenerate – an example of how intervention into the vegetation by man can be destructive when it is not approached with ecological farsightedness.

Intervention by man stimulates a differentiated agricultural landscape – Plea for research without blanket buzzwords

The newly published scientific results refute the claims that the West Sahel is being hit by a growing desert that is a consequence of a global climate change. However they also refute the suggestion that the “greening of the desert” will take off by itself due to the increasing annual precipitation. The moderate trend reversal after a severe period of drought indeed does entail an increase in vegetation density. But it neither means a return to the conditions that existed before these extreme climatic events, nor does it automatically mean a widespread growth of green vegetation. Moreover, anthropogenic factors – and conversely their absence – have had a decisive impact on landscape and vegetation. Targeted farm and forest management measures that are oriented on scientific knowledge can significantly foster a differentiated man-made landscape.

Martin Brandt, who will soon receive his doctorate at the University of Bayreuth, also here sees reason for hope: “Should the climate prognoses of the UN IPCC do come true, the living conditions in some arid and semi-arid regions of West Africa – foremost in the region of the Sahel zone – will worsen. Appropriate concepts for land and forest management and for environmental protection however do offer the opportunity to adapt quickly enough to such climate developments in order to soften the impacts on poeple..”

International support project

The research works in Mali and in the Senegal were part of an international project: “Climate Change, Environmental Changes and Migration: Social-Ecological Conditions of Population Movements with the Example of the Sahel Countries Mali and Senegal (micle)”. The “micle” research project was funded from 2010 to April 2014 by the German Ministry for Education and Science (BMBF) and coordinated by the Institute for Social Ecological Research (ISOE) in Frankfurt. The Geographical Institute of the University of Bayreuth – together with the Institute for Geography and Regional Research of the University of Vienna – was involved as an associated partner. The leadership of the sub-project “Physical Geographical Perspectives ” was done by Prof. Dr. Cyrus Samimi, who today leads the research group for climatology at the University of Bayreuth. Prof. Dr. Martin Doevenspeck , Professor Regional-Related Conflict Research at the University of Bayreuth, was responsible for the sub-project “Social-Geographic Perspectives”.

Publication:

Martin Brandt, Aleixandre Verger, Abdoul Aziz Diouf, Frederic Baret and Cyrus Samimi, Local Vegetation Trends in the Sahel of Mali and Senegal Using Long Time Series FAPAR Satellite Products and Field Measurement (1982–2010), in: Remote Sensing 2014, 6, pp. 2408-2434 DOI:10.3390/rs6032408.

Photos: University of Bayreuth

 source: http://www.uni-bayreuth.de/pressemitteilungen-html/121-Sahelzone/index.html

Local Vegetation Trends in the Sahel of Mali and Senegal Using Long Time Series FAPAR Satellite Products and Field Measurement (1982–2010)

We finally published an article dealing with local vegetation trends in the Sahel and data quality of long term time series (GEOV1 and GIMMS3g). It is published in the open access journal “Remote Sensing” and can be downloaded for free:

http://www.mdpi.com/2072-4292/6/3/2408

Brandt, Martin; Verger, Aleixandre; Diouf, Abdoul A.; Baret, Frédéric; Samimi, Cyrus. 2014. “Local Vegetation Trends in the Sahel of Mali and Senegal Using Long Time Series FAPAR Satellite Products and Field Measurement (1982–2010).” Remote Sens. 6, no. 3: 2408-2434.

Abstract: Local vegetation trends in the Sahel of Mali and Senegal from Geoland Version 1 (GEOV1) (5 km) and the third generation Global Inventory Modeling and Mapping Studies (GIMMS3g) (8 km) Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) time series are studied over 29 years. For validation and interpretation of observed greenness trends, two methods are applied: (1) a qualitative approach using in-depth knowledge of the study areas and (2) a quantitative approach by time series of biomass observations and rainfall data. Significant greening trends from 1982 to 2010 are consistently observed in both GEOV1 and GIMMS3g FAPAR datasets. Annual rainfall increased significantly during the observed time period, explaining large parts of FAPAR variations at a regional scale. Locally, GEOV1 data reveals a heterogeneous pattern of vegetation change, which is confirmed by long-term ground data and site visits. The spatial variability in the observed vegetation trends in the Sahel area are mainly caused by varying tree- and land-cover, which are controlled by human impact, soil and drought resilience. A large proportion of the positive trends are caused by the increment in leaf biomass of woody species that has almost doubled since the 1980s due to a tree cover regeneration after a dry-period. This confirms the re-greening of the Sahel, however, degradation is also present and sometimes obscured by greening. GEOV1 as compared to GIMMS3g made it possible to better characterize the spatial pattern of trends and identify the degraded areas in the study region.

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