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In the early dry season, between the end of the rains and the end of December, the loss rates of vegetation cover are high at 35.95% on average in the four rangeland sites with a variation coefficient of 12.34%. Dry-season vegetation mass and cover fraction from SWIR1.6 and SWIR2.1 band ratio: Ground-radiometer and MODIS data in the Sahel. Located near village and permanent water points, the rangeland site of Tara shows the lowest herbaceous cover due to intense grazing and trampling, <2% vegetation cover at the end of the dry season. [Extracted from the article]

Summary: Twenty-five rangeland sites were monitored over two decades (1984–2006) first to assess the impact of the 1983–1984 droughts on fodder resources, then to better understand ecosystem functioning and dynamics. Sites are sampled along the south–north bioclimatic gradient in Gourma (Mali), within three main edaphic situations: sandy, loamy-clay and shallow soils. In addition, three levels of grazing pressure where systematically sampled within sandy soils. Located at the northern edge of the area reached by the West African monsoon, the Gourma gradient has recorded extremes in inter-annual variations of rainfall and resulting variations in vegetation growth. Following rainfall variability, inter-annual variability of herbaceous yield increases as climate gets dryer with latitudes at least on the sandy soils sites. Local redistribution of rainfall explains the high patchiness of herbaceous vegetation, especially on shallow soils. Yet spatial heterogeneity of the vegetation does not buffer between year yield variability that increases with spatial heterogeneity. At short term, livestock grazing during the wet season affects plant growth and thus yield in direction and proportions that vary with the timing and intensity of grazing. In the longer term, grazing also impinges upon species composition in many ways. Hence, long histories of heavy grazing promote either long cycle annuals refused by livestock or else short cycle good quality feed species. Primary production is maintained or even increased in the case of refusal such as Sida cordifolia, and is lessened in the case of short cycle species such as Zornia glochidiata. These behaviours explain that the yield anomalies calculated for the rangelands on sandy soils relative to the yield of site less grazed under similar climate tend to be negative in northern Sahel where the scenario of short cycle species dominates, while yield anomalies are close to nil in centre Sahel and slightly positive in South Sahel where the refusal scenario is more frequent. Because grazing promotes short cycle species, grazed rangelands respond faster to droughts. Year to year changes in species composition are abrupt as expected from the transient soil seed stock. However, some decadal trends in species composition are identified, with a wave of pioneer species following the 1983–1984 droughts, and a more progressive diversification and return to typical Sahel flora from 1992 onwards. [Copyright &y& Elsevier]

This paper presents an analysis of ENS!VAT Advanced Synthetic Aperture Radar data acquired over a Sahehan region located in Mali, West Africa. The considered period is 2004–2005 and includes two rainy seasons. Emphasis is put on two ScanSAR modes, namely, the Global Monitoring (GM) and the Wide Swath (WS) modes characterized by spatial resolutions of about 1 km and 150 m, respectively. Results show that the WS mode offers better performance in terms of radiometric resolution, radiometric stability, and speckle reduction than the GM mode. The latter is more appropriate for studies at large scale (> 10 × 10 km). In both modes, pronounced angular and temporal signatures are observed for most soil surfaces, and azimuthal effects are observed on markedly orientated rocky surfaces. In contrast, polarization differences (VV/HH) are small during the dry season except on flat loamy soil surfaces. Finally, a relationship is observed between the normalized WS backscattering signal at HH polarization and the surface soil moisture of sandy soils. [ABSTRACT FROM AUTHOR]

Summary: A series of numerical experiments has been designed to investigate how effective satellite estimates of radiometric surface temperatures and soil surface moisture are for calibrating a Soil–Vegetation–Atmosphere Transfer (SVAT) model. Multi–objective calibration based on error minimization of temperature and soil moisture model outputs is performed in a semi–arid environment. Model accuracy when calibrated using in situ versus satellite objectives is explored in detail. Observational meteorological datasets from the African Monsoon Multidisciplinary Analysis (AMMA) were used to force a column model during a growing season in Mali. Fourier Amplitude Sensitivity Test (FAST) revealed the most sensitive parameters to model outputs. Parameters found sensitive were subsequently optimized in a series of model calibrations to reveal trade-offs between model objectives. Our main findings are (1) the SVAT model performs well in the semi–arid environment, but underestimates peak growing season evapotranspiration and overestimates soil moisture, (2) most of the parameters important for flux estimates can be constrained using surface temperature and soil surface moisture with the three exceptions: root depth, the extinction coefficient and unstressed stomatal resistance, (3) flux simulations are improved when the model is calibrated using in situ surface temperature and soil surface moisture versus satellite estimates. [ABSTRACT FROM AUTHOR]

Summary: The Sahel is characterized by low and highly variable rainfall, which strongly affects the hydrology and the climate of the region and creates severe constraints for agriculture and water management. This study provides the first characterization of the rainfall regime for the Gourma region located in Mali, Central Sahel (14.5–17.5°N and 2–1°S). The rainfall regime is described using two datasets: the daily long term raingauge records covering the period 1950–2007, and the high frequency raingauge records collected under the African Monsoon Multidisciplinary Analysis (AMMA) project between 2005 and 2008. The first rainfall dataset was used to analyse the interannual variability and the spatial distribution of the precipitation. The second dataset is used to analyse the diurnal cycle of precipitation and the nature of the rainfall. This study is complementary to previous analyses conducted in Sahelian areas located further south, where the influence of the continental Sahara heat low is expected to be less pronounced in summer. Rainfall regimes in the Gourma region present a succession of wet (1950–1969) and dry decades (1970–2007). The decrease of summer cumulative rainfall is explained by a reduction in the number of the rainy days in southern Gourma, and a decrease in both the number of rainy days and the daily rainfall in northern and central Gourma. This meridional difference may be related to the relative distances of the zones from the intertropical discontinuity, which is closer to the northern stations. The length of the rainy season has varied since the 1950s with two episodes of shorter rainy seasons: during the drought of the 1980s and also since 2000. However, this second episode is characterized by an increase in the daily rainfall, which suggests an intensification of rainfall events in the more recent years. High-frequency data reveal that a large fraction of the rainfall is produced by intense rain events mostly occurring in late evenings and early mornings during the core of the rainy season (July–September). Conversely, rainfall amounts are less around noon, and this mid-day damping is more pronounced in northern Gourma. All these characteristics have strong implications for agriculture and water resources management. [Copyright &y& Elsevier]

Summary: The patterns of the changes in woody plant population densities, size and species composition is documented and discussed for 24 rangeland sites monitored from 1984 to 2006 in Gourma (Mali). The sites are sampled along the North–South bioclimatic gradient on each of the main soils and levels of grazing intensity. Site woody plant populations range from extremely sparse on shallow soils, to scattered on sandy soils, to open forest in temporarily flooded clayed soils, and to narrow thickets on hard pans. Three different methods contributed to assess and monitor woody plant density and canopy cover. In the short term woody populations were struck by the 1983–1984 droughts irrespective of their edaphic situation and location along the bioclimatic gradient. Drought induced mortality was not more severe under drier climate within the Sahel gradient but occurred sooner after drought in shallow soils, and with a lag of a year or two on flooded clay soils. No evidences were found of higher mortality rates in stands with history of intense grazing. Although rainfall remained below average for a decade after the drought, active recruitment of woody plants occurred in all sites starting as soon as 1985. Recruitment proceeded by successive cohorts, often with short-living perennial undershrubs and pioneer shrubs settling first. Acacia species were among the first to settle or re-establish, especially on the sites most intensively grazed. The release of competition due to drought induced mortality and to the reduction of herbaceous cover contributed to the success of the recruitment. The species composition change that resulted could first be interpreted as a shift toward a more arid tolerant flora, then some diversification occurred since the mid 1990s that could indicate a possible return to previous composition in the long term, confirming the resilience Sahel vegetation. [Copyright &y& Elsevier]

The West African Sahel experienced a long drought from the 1970s to the 1990s during which runoff has paradoxically increased, as a response to human and climate-induced changes in surface conditions. Despite the vegetation recovery (re-greening) observed at regional scale over the past 30 years, surface runoff is still increasing, suggesting that the Sahelian eco-hydrological system passed a tipping point and is now trapped in a « high runoff » state. To study this hypothesis, we developed a system dynamics model incorporating vegetation-hydrology interactions at annual time scale. The model successfully reproduced the vegetation collapse and the increase of runoff-prone bare soil areas monitored over 65 years on a pilot site in Northern Mali. Our results confirmed the existence of a tipping point between alternative high/low runoff states at the small catchment scale. According to the model, a reverse shift to the pre-drought low runoff state is possible, but the conditions in which this shift would occur remain uncertain. The system trajectory presents a strong sensitivity to annual rainfall variability (amplitude and temporal structure). This study suggests that the increasing runoff in a re-greening environment is caused by the tipping of some areas to a high runoff/low vegetation state, illustrating how a regime shift in sub-systems can result in eco-hydrological changes at larger scale. The associated large-scale changes of the rainfall partitioning may alter evapo-transpiration and thus the surface-atmosphere feed-back. Those changes also bear strong environmental and socio-economic consequences, either adverse (increase of degraded areas to the detriment of agriculture, and increased flood risk) or beneficial (increased water resource in ponds and water tables). [ABSTRACT FROM AUTHOR]

Following 25 years of below average annual rainfall in the Sahel between 1970 and 1995, the return to more humid conditions has led to rapid postdrought recovery of the woody cover. However, the increase in the woody cover is not spatially homogeneous raising questions about the resilience of some woody vegetation types. Based on the analysis of field and remote sensing data collected on the tiger bush systems in the northern Sahel in Mali, this study noted the current and persistent degradation of these systems in the Sahel since the 1970s despite the recent improvement in rainfall since the mid-1990s and the general Sahel re-greening. Profound changes in the woody population pattern, tree density and cover, and floristic composition took place regardless of the site location along the south-north rainfall gradient. Associated with definite structural changes of the woody population, surface hydrology shifted from a sheet to concentrated run-off accelerating the collapse of the patterned woody population. Currently, there is no evidence in favour of reversing the current degradation process, at least at a decadal scale, although very sparse recolonization by pioneer woody vegetation has been observed in the driest sites along recently formed gullies. These observations support the hypothesis of an ecosystem shift, with long-term implications for the structure and functioning of the patterned vegetation, as well as the whole watershed landscape through increased run-off leading to stronger water flows in enlarged wadis, increased soil erosion upstream and sediment deposition downstream, enhanced water storage in ponds, and greater recharge of aquifers, which is an illustration of the "Sahelian paradox"., (© 2018 John Wiley & Sons Ltd.)