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.

https://ddd.uab.cat/pub/artpub/2014/184441/remsen_a2014m3v6n3p2408-2434.pdf

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

In order to understand the impact of climatic and environmental changes as well as socio-economic drivers on human migration, it remains a challenging task to find a method to analyse the knowledge from different scientific disciplines in an integrated way. The Sahel region with its high ecological dynamic has a long history of migratory movements. Within this work, we integrate and analyse socio- and natural-scientific data from two Sahelian study areas in Mali and Senegal using Bayesian belief networks. The core of the network’s structure is formed by four main motives to migrate which are education, family, visit and curiosity and sustenance and employment. It is assumed that these motives determine the spatial and temporal patterns of migration. On the basis of submodels for each migration motive, we identify the decisive factors that constitute the socio-economic and ecological conditions with a combination of sensitivity analyses and train-and-test validation method. In combining these factors, the model avoids implying monocausal dependencies and allows an analytical view on the likely consequences of different settings of social-ecological conditions on migration. Furthermore, we use the model to estimate the consequences of alternative future developments in contrasting scenarios. The results show that changing environmental conditions lead to changing patterns of migration, regarding its duration and destination. These patterns can be very specific for different motives and their underlying factors. One principal result of the analysis is that uncertainty in the main income sources correlates with an increase of short-term migrations in order to increase the households' possibilities for income generation. Nevertheless, socio-economic conditions show a greater impact on the people's decision to migrate than environmental conditions.

Using Bayesian belief networks to analyse social-ecological conditions for migration in the Sahel

Climatic stress and anthropogenic disturbances have caused significant environmental changes in the Sahel. In this context, the importance of soil is often underrepresented. Thus, we analyze and discuss the interdependency of soil and vegetation by classifying soil types and its woody cover for a region in the Senegalese Ferlo. Clustering of 28 soil parameters led to four soil types which correspond with local Wolof denotations: Dek, Bowel, Dior and Bardial. The soil types were confirmed by a Non-metric Multidimensional-Scaling (NMDS) ordination and extrapolated via a Random Forest classifier using six significant variables derived from Landsat imagery and a digital elevation model (out-of-bag error rate: 7.3%). In addition, canopy cover was modeled using Landsat and a Reduced-Major-Axis (RMA) regression (R2 = 0.81). A woody vegetation survey showed that every soil type has its own species composition. However, 29% of Bowel regions are deforested (i.e., degraded) and interviews revealed extensive environmental changes and a strong decline and local extinction of woody species. The differences between the soil types are significant, showing that vegetation changes (i.e., degradation and greening), resilience to climatic stress and human activities largely depend on soil properties. We highlight that spatial heterogeneity is an important aspect when dealing with environmental changes in the Sahel, and local knowledge can be well used to classify spatial units by means of public Earth observation data.

https://www.mdpi.com/2073-445X/3/3/770/pdf

Modeling Soil and Woody Vegetation in the Senegalese Sahel in the Context of Environmental Change

Climate variability, expressed by random short time variation in rainfall and temperature, has severe effects on agriculture in the Sahel. This region depends on a short and uncertain rainy season with many uncertainties in short-term climate variations that affect farm decisions in regards to. sowing period, crop variety, water management, etc. The most conspicuous climate variability issue in West Africa is the high intra-seasonal and inter-annual variability associated with dry (or too wet) spells, and in most cases, season length modifications. We derived selected climate descriptors from 35-year daily climate data to assess inter-annual rainfall anomalies, seasonal variation and temperature trends in southern Burkina Faso. Results show the evolution in the number of rainy days and rainfall sum from the drier years in the late 1970s and early 1980s, which were followed by positive but variable rainfall in the 1990s and 2000s. We further observed widespread occurrence of unsuccessful season ons...

Unstacking high temporal resolution meteorological data for multidimensional analysis of climate variability in southern Burkina Faso

Introduction: In the 20th century, the West African Sahel has been a hot-spot of climatic changes. After severe drought-events in the 1970s and 1980s which were followed by a significant drop in annual precipitation, rainfall seems to increase again during the past years. This study breaks the spatial and temporal scale down to the weather-station of Linguère and analyses the benefits of daily rainfall data compared to annual sums.

Aida Diongue-Niang

Papers

Analysis of daily rainfall of the Sahelian weather-station Linguère (Senegal) - Trends and its impacts on the local population