Thomas Theis Nielsen

Savanna fires in east-central Senegal: distribution patterns, resource management and perceptions.

The temporal and spatial distribution of fires for an area in east-central Senegal was determined on the basis of multi-temporal NOAA AVHRR satellite images. Three years of data (1990–1992) were analyzed defining the boundary between two different fire regimes: very few and scattered fires to the north with the majority of fires south of the boundary. This boundary was stable for the three dry seasons examined and was identical to the northernmost extension of fires as determined by visual inspection of a hard copy Landsat image mosaic. Fire frequencies were analyzed in relation to dominant vegetation types and yearly precipitation, and the findings compared to results of a field survey of the local populations perceptions of the causes and implications of fires. Survey results clearly showed that the use of fire in the study area is closely linked to the utilization of the environment for livestock grazing and crop production. We conclude that the local population has a high degree of awareness about the application of fire, that different fire use practices concerning can be identified respectively in the grasslands of the northern and the savanna of the southern parts of the study area, and that these practices reflect a well adapted production strategy. Finally, we recommend policy decisions be more flexible in the light of local understanding of fire use.

Evaluation of AVHRR PAL and GIMMS 10‐day composite NDVI time series products using SPOT‐4 vegetation data for the African continent

Global 8 km resolution AVHRR (advanced very high resolution radiometer) NDVI (normalized difference vegetation index) 10‐day composite data sets have been used for numerous local to global scale vegetation time series studies during recent years. AVHRR Pathfinder (PAL) NDVI was available from 1981 until 2001, and the new AVHRR GIMMS NDVI was available from 1981 to the present time. A number of aspects potentially introduce noise in the NDVI data set due to the AVHRR sensor design and data processing. NDVI from SPOT‐4 VGT data is considered an improvement over AVHRR, and for this reason it is important to examine how and if the differences in sensor design and processing influence continental scale NDVI composite products. In this study, the quality of these AVHRR NDVI time series are evaluated by the continental scale 1 km resolution SPOT‐4 vegetation (VGT) 10‐day composite (S10) NDVI data. Three years of AVHRR PAL (1998–2000) and seven years of GIMMS (1998–2004) have been compared to 8 km resampled SPOT‐4 VGT (1998–2004) data. The dynamic range of SPOT‐4 VGT NDVI tends to be higher than the AVHRR PAL NDVI, whereas there is an exact match between AVHRR GIMMS NDVI and SPOT‐4 VGT NDVI. Ortho‐regression analysis on annually integrated values of AVHRR PAL/GIMMS and SPOT‐4 VGT on a continental scale reveals high correlations amongst the AVHRR and the SPOT data set, with lowest RMSE (root mean square error) on the GIMMS/SPOT‐4 VGT compared to the PAL/SPOT‐4 VGT. Analyses on decade data likewise show that a linear relation exists between Spot‐4 VGT NDVI and the two AVHRR composite products; GIMMS explaining most of the Spot‐4 VGT NDVI variance compared to PAL. These results show that the AVHRR GIMMS NDVI is more consistent with Spot‐4 VGT NDVI compared to AVHRR PAL versus Spot‐4 VGT NDVI (in terms of RMSE and dynamic range) and can therefore be considered the more accurate long time AVHRR data record. Analyses performed on monthly maximum composites and decade composite data, however, reveal intra‐annual variations in the correlation between SPOT‐4 VGT and the two AVHRR data sets, which are attributed to different cloud masking algorithms. The SPOT‐4 VGT cloud‐screening algorithm is insufficient, thereby suppressing the rainy season NDVI.

Going Where the Grass is Greener: On the Study of Pastoral Mobility in Ferlo, Senegal

Based on a case study from Ferlo in Senegal, this paper discusses how pastoral mobility can be studied and understood with special emphasis on the use of GPS data. It has a dual objective: first, to investigate the methodological potential of using GPS data; second, to discuss the analytical use of GPS data for understanding mobility. The methodological potential for using GPS data is related to quantifying mobility and characterizing mobility patterns in space and time. Analytically, GPS data can be used in combination with qualitative information to make method triangulation. The GPS data can be used both prior to qualitative interviews to make informed questions about mobility and they can be used after qualitative investigations to illustrate points made or to reveal inconsistencies. The study shows that cattle walk about 5000 km per year (excluding night grazing) and different mobility patterns occur depending on the season. Issues such as “the cattle complex” and the notion of the independent, nomadic pastoralist are discussed in relation to pastoral mobility. Although cattle are of major importance to the Fulani, it is not important for them to walk with their animals, which are left to roam freely or supervised by paid herders. It is necessary to take into account all these issues if we want to go beyond the simple understanding of mobility as a means to find pasture and water.

The Distribution in Time and Space of Savanna Fires in Burkina Faso as Determined from NOAA AVHRR Data

Abstract Geografisk Tidsskrift, Danish Journal of Geography 97: 86–97, 1997. Fires have significant effects on both biological diversity, nutrient cycling and emission of greenhouse gasses in the savanna zones of Africa. The meaning and characterization of the fire regimes of such areas are briefly discussed, and it is demonstrated that time series of NOAA AVHRR satellite images may be used for identifying active fires in Burkina Faso, and thus contribute to the characterization of the fire regime. Most fire activity in Burkina Faso is shown to take place in a brief period after the beginning of the dry season, defined as the date of a sharp increase in surface temperature. Based on an analysis of a small area over a short period the possibility of identifying fire scars, rather than active fires, is demonstrated. Furthermore it is shown that the number of pixels detected as active fire pixels may only be a small fraction of all pixels affected by fire. This has important implications for the use of fire ...

Utilization of NOAA AVHRR for assessing the determinants of savanna fire distribution in Burkina Faso

Temporal and spatial patterns of active fires, detected using NOAA AVHRR LAC data, in Burkina Faso are identified and related to vegetation, tree cover and land use classes. Initially, fires are classified into early and late dry season fires (EDSF and LDSF). Early dry season fires are defined as fires occurring earlier than 45 days after the start of the dry season, marked by a levelling out in the post-rainy season temperature increase, determined on the basis of surface temperature data derived from NOAA AVHRR. The date of the start of the dry season, defined in this way, is shown to be a linear function of the latitude. The distribution of fire occurrence are shown to display distinctively different patterns. These distribution patterns are related to information on vegetation class, woody biomass and land use intensity as well as net primary productivity, estimated from NOAA AVHRR rainy season data. It is shown that overall fire frequency and the ratio of early to late dry season fire activity depend strongly on net primary productivity, land use intensity and vegetation class. Late fires tend to occur mainly in agricultural areas, whereas early fires are much more frequent in areas of low land use intensity in the wooded savannas of southern Burkina Faso.

Advances in monitoring vegetation and land use dynamics in the Sahel

Vegetation dynamics of the West African Sahel has attracted great scientific interest over the last 40 years because of the dramatic inter-decadal variability observed in the resource base of the region directly impacting on the livelihoods of the West African population. From farmers to pastoralists, agro-pastoralists and forest-users, all depend on the availability of vegetation resources and are affected by fluctuations in the available vegetation resource. Vegetation dynamics are controlled by both natural and human factors, including climate change and variability, increased concentration of CO2 in the atmosphere, grazing pressure, bush fires and agricultural expansion or contraction. The use of satellite data in combination with field data played a major role in the monitoring of vegetation dynamics and land use in the Sahel, since the mega drought of the 1970s and the 1980s. This paper briefly reviews the advance of satellite-based monitoring of vegetation dynamics over these 40 years. We discuss the promises of current and likely future data sources and analysis tools, as well as the need to strengthen in situ data collection to support and validate satellite-based vegetation and land use monitoring and modelling.

The Role of Methodology and Spatiotemporal Scale in Understanding Environmental Change in Peri-Urban Ouagadougou, Burkina Faso

In recent decades, investigations of NPP (net primary production) or proxies here of (normalized difference vegetation index, NDVI) and land degradation in Sahelian West Africa have yielded inconsistent and sometimes contradicting results. Large-scale, long-term investigations using remote sensing have shown greening and an increase in NPP in locations and periods where specific, small scale field studies have documented environmental degradation. Our purpose is to cast some light on the reasons for this phenomenon. This investigation focuses on the south of Ouagadougou, Burkina Faso, a city undergoing rapid growth and urban sprawl. We combine long-term MODIS (moderate resolution imaging spectroradiometer) image analysis of NDVI between 2002 and 2009, and by using high resolution satellite images for the same area and a field study, we compare trends of NDVI to trends of change in different categories of land cover for a selected number of MODIS pixels. Our results indicate a strong, positive association between changes in tree cover vegetation and trends of NDVI and moderate association between man-made constructions and trends of NDVI. The observed changes are discussed in relation to the unique processes of urban sprawl characterizing Ouagadougou and relative to their spatiotemporal scale.

The Push for Plantations: Drivers, Rationales and Social Vulnerability in Quang Nam Province, Vietnam

This chapter compares the impact of land use changes in two municipalities. We look at the long-term changes regarding the expansion of acacia production and the effects of tropical storms. Due to households experiencing significant economic gains after only a few years, both primary forest and agricultural land are being replaced with acacia tree plantations. The downside to this is an increasing social inequality, which follows in the wake of both government decree support given to monoculture promotion, and the influences of market forces. We observe signs of social differentiation, where poor households end up serving as casual labour for the richer families on their acacia plantations. In addition, the poor can be rendered even more vulnerable after becoming labourers for the richer families, because they may no longer have an alternative source of income, yet they still face the risk of an increasing frequency of typhoon exposure.

The fire regime of Senegal and its determinants

Abstract Biomass burning in Senegal has a range of different causes and affects many different environments. Each cause has a unique spatial and temporal distribution with its own consequences for the fire regime in Senegal. To date, this regime has been only poorly understood due to numerous methodological problems with monitoring fires and mapping their extent. Satellite remote sensing techniques provide the only realistic means of obtaining fire information at the national scale, yet even their use involves considerable problems. This paper uses NOAA AVHRR data from 1989, 90. 91 and 92, supplemented with Landsat and SPOT data, to describe the Senegalese fire regime based on the ‘active fire detection approach’. Possible explanations for the observed patterns will be discussed. The results show that the northern limit of intense fire activity in savannah woodlands can not be adequately explained by variations in rainfall, herbaceous biomass distribution or land cover type. Senegal may be divided into regions characterized by a more or less well-defined pattern of fire occurrences. High frequency of fires, especially early in the dry season, is typical of the savannah woodlands in the east-central part of Senegal. While the central, predominantly agricultural areas experience relatively few fires, areas in the margin of the central ‘Peanut Basin’ have an abundance of fires late in the dry season. In the northern rangelands of the Ferlo few fires occured in the years studied.

Documentation and evaluation of the CSE NOAA AVHRR data set

Abstract A NOAA AVHRR data set covering Senegal and parts of the surrounding countries during the period from 1987 to present is under construction and improvement in an ongoing collaboration between Centre de Suivi Écologique (CSE). Dakar. Senegal, and Institute of Geography, University of Copenhagen (IGUC). This paper details the entire processing chain from raw images to a properly calibrated, geometrically rectified and cloud-masked time-series tracking a number of well-defined variables. Two vital aspects of this time series, the cloud masking procedure and the geometrical rectification, are evaluated in detail. A two-step, multi-criteria cloud masking procedure requiring a manual input does not consistently improve the quality of the data set compared to the simpler, automated procedure. With respect to geometrical rectification an accuracy on the order of I km is obtained. Finally, suggestions for further improving the data set are forwarded