Abstract. Local peoples from Niakhar in the Senegalese peanut basin highlight a dramatic increase of water access problems due to marked rainfall deficits and salinization of surface and ground water resources. The chemical quality of groundwaters is often critical because of the salinization process, whereas water surfaces, which should be used in such situations, are up early. More and more, lowlands and rivers beds are pervaded by salt crusts. Then the salinization of wells is increasing, leading to the extension of tans (salty of acidified soils). To study the impacts of climatic pejoration on the agroecosystems and on the living conditions of the populations, we carried out the analysis of the time series of the precipitations with daily and annual time steps from 1950 to 2015 on 6 meteorological stations, in situ measurements on 78 wells for an area of 311 km2, as well as local population interviews and field observation. The results confirm an important climatic break in the region in 1970. The long dry period, from 1970 to 2009, has increased the annual rain variability, decreased the number of rainy days per year. We confirm a real and large extension of well salinization, and salt crusting in the lowlands and the riverbeds. From the local people, it seems the process of degradation of the aquifers continues to progress from a large tidal event in 1984. The rainfall increase noted in the last decade does not seem to be enough to reverse the trend and to ensure both the rise of the piezometric level of the aquifers and the desalinization of surface and ground waters.

Dans le bassin arachidier (en zone soudano-sahélienne) et particulièrement dans le bassin de Thysse Kaymore, le déficit pluviométrique continu et prononcé depuis les années 1970 et la pression démographique croissant ont conduit à la dégradation des ressources en eau, rendant d e p l u s e n p l u s difficile l 'accès à l'eau. Les eaux souterraines sont souvent très profondes, alors que les eaux de surface qui devraient servir de recours dans de pareilles situations connaissent un tarissement précoce. Dans ce petit bassin de 162km 2 , le déboisement massif, la mise en culture de zones marginales et la disparition de la jachère conjugués au contexte climatique défavorable et à la pression démographique ont entrainé la dégradation du réseau hydrographiques et modification du fonctionnement hydrologique. Pour étudier les impacts de la péjoration climatique et de la pression anthropique sur les ressources en eau du bassin de Thysse Kaymore et sur les conditions de vie des populations, nous avons effectué l'analyse des séries chronologiques des précipitations de 1918 à 2011, des m esur e s pi éz om ét r i qu es a i n si qu e d es d é bi t s. Pour l'analyse diachronique de l'évolution des espaces ruraux des images Landsat de 1986, 1996, 2007 et 2016 sont utilisés. Les déficits pluviométriques annuels varient entre 54 et 60%. On constate un abaissement du niveau des nappes phréatiques, le tarissement précoce des puits, l'assèchement et l'ensablement des bas-fonds, une profondeur importante du niveau de l'eau sous le sol (58 à 116m pour le Continental Terminal et 300m pour le Maestrichtien) dans cette zone rurale ou les eaux souterraines sont les principales sources d'approvisionnement en eau des populations. A cette péjoration pluviométrique, il faut ajouter la forte pression démographique avec l'augmentation de la population qui a doublé en 30 ans (de 9400 habitants en 1976 elle est passée à 22524 en 2013) entrainant la multiplication des besoins en eau. Nonobstant ces problèmes accrus d'approvisionnement en eau potable des populations, cette zone constitue un bassin agricole, dont 62% des 90,76% de sa superficie cultivable sont exploités. Dans le même temps, on note le développement du maraîchage, une activité fortement consommatrice d'eau. Les besoins futurs en eau seront donc en croissance. Il

This work is part of a study for the conservation of fish products through solar drying. The grey seabream is dried after a pre-treatment of 16 hours in a salt brine under two conditions: on a rack in open air and in a solar dryer. The tests were carried out under average irradiance conditions of 592.76 W/m², an average ambient temperature of 30°C and an average air temperature in the dryer of 50°C. The results obtained, allowed us to reach a final humidity of 32% from an initial humidity of 65% in a wet basis. The drying was processed in 15 hours, (i.e. two days in the dryer) and 24 hours (i.e. three days in the open air). The modeling of the drying kinetics of the grey sea bream is carried out from empirical or semi-empirical models taken from the previous works. Several criteria were defined for the choice of the two-term model as being the one that can describe in the best way, the drying of the fish in both conditions. The effective diffusivity was determined using Ficks diffusive model whose solution is given by Crank, so that the logarithm of the reduced moisture allows us to find effective diffusivities for fish of 9.88823 10-7 (m²/s) and 1.72534 10-6 (m²/s) for open-air and in dryer drying respectively.

<p>In Senegal, the groundnut basin is the main agricultural region under a semi-arid climate, heavily cultivated in an agrarian system combining agricultural rotation and agroforestry dominated by <em>Faidherbia albida</em> trees. The soils of the groundnut basin, essentially sandy, have a low water retention capacity. In this area, water is a limiting factor, and the climate variability represents an additional constraint on an already precarious agricultural production system. It is therefore essential to improve knowledge on water saving practices and soil humidity dynamics. The management of water resources in agricultural fields requires reliable information about soil hydraulic properties, which control the partition of rainfall into infiltration and runoff, and their spatio-temporal variability.</p> <p>To investigate the variability of soil hydraulic parameters we have carried out infiltration measurement in open space without tree and below tree canopies. A total of 24 infiltration measurements were carried out using an automatic single-ring infiltrometer in the nearby of each plot (4 measurements × 6 plots), and after removing the first 10 cm of uncompacted sand. The infiltration tests were carried out in June, October and December, respectively before, during and after the crop season. We used the Beerkan Estimation of Soil Transfer Parameters (BEST) method to retrieve the soil hydraulic parameters from infiltrometer data and field measurements of soil porosity, initial and saturated soil water contents and soil bulk density.</p> <p>The statistical analysis of the data showed a high variability during the cultivating period, both in time and space, especially of the saturated soil hydraulic conductivity Ks. However, the Ks seems higher under tree cover, around 0.186 mm/s, for 0.167mm/s without any tree canopy influence.  Despite the expected homogeneity of the investigated sandy soil, the presence of the perennials triggered a patchy distribution of soil hydraulic conditions. These preliminary results evidenced the importance of taking into account parameters variability and landscape structure when simulating soil water dynamics in the Senegalese groundnut basin.</p>

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<p>In the Sahel region, agroforestry is a land-use system widely adopted as a more sustainable agricultural production system. In this type of system, woody perennials that are grown in association with agricultural crops and pastures, constitute spatially disconnected zones where microclimate and soil’s infiltrability, physical, chemical, and biological conditions are assumed locally improved. Particularly the stemflow concentrates a part of the intercepted rainfall from the canopies to the stems. Hence stemflow can induce preferential infiltration around the stem base and promote groundwater recharge.</p> <p>In the West African Sahel, <em>Faidherbia albida</em> (Delile) A.Chev. is commonly adopted as multi-purpose woody perennial in agroforestry systems. It is a deciduous tree with an inverse phenology as it loses the leaves during the rainy season. Although, the absence of leaves during the rainy season is expected to decrease the interception and to consequently decrease stemflow, evidence of stemflow at the base of <em>F. albida</em> trees were reported in the literature when the stems were partially covered with green leaves (Chinen, 2007).</p> <p>In this study, we carried out timelapse ground penetrating radar (GPR) surveys in conjunction with a simulated stemflow event to investigate stemflow-induced infiltration by an <em>F. albida</em> tree trunk and root system. We established a survey grid (2.1 m × 2.1 m) around an <em>F. albida</em>, consisting of twelve horizontal and ten vertical parallel survey lines with 0.3 m intervals between them. Two stemflow pulses, each of 20 L, were poured on the tree trunk using a PVC pipe with a 1-mm-diameter hole every 50 mm. The pipe was connected to a plastic funnel and positioned around the tree trunk at 0.4 m from the soil surface. One grid GPR survey was carried out before the stemflow simulation experiment. A total of 40 L of water was used during the experiment. A second survey was carried out after the injection of the first 20 L, while the last survey was carried out after the second stemflow pulse. We collected a total of 66 (3 GPR surveys × 22 survey lines) radargrams using a GSSI (Geophysical Survey System Inc., Salem, NH) SIR 3000 system with a 900-MHz antenna. We therefore obtained for each survey line a pre-wetting and two post-wetting radargrams. Next, we created other forty-four matrixes based on absolute differences between pre- and post-wetting amplitude values. Higher differenced values occurred because of amplitude changes and time shifts related to wave propagation.</p> <p>The analysis of the differentiated radargrams provided evidence of deep infiltration along the tap roots. The wetted zone extended mainly in-depth providing evidence of the potential role played by the <em>F. albida</em> trees in groundwater recharge processes due to their deep rooting, preferably reaching the groundwater table. Put all together, this study shows a first signal of the importance of accounting for stemflow infiltration in the water balance of agroforestry systems with <em>F. albida</em> trees.</p> <p><strong>References</strong></p> <p>Chinen, T., 2007. An observation of surface runoff and erosion caused by acacia albida stemflow in dry savanna, in the south-western republic of Niger 10.</p>

<p>Vegetation strongly affects the water cycle, and the interactions between vegetation and soil moisture are fundamental for ecological processes in semiarid regions. Therefore, characterizing the variation in soil moisture is important to understand the ecological sustainability of cropping systems towards food security. The present study aims at exploring factors and mechanisms influencing soil moisture variability in the Faidherbia albida (FA) parkland at Sob basin located in the center of Senegal [1]. Volumetric soil moisture content at multiple depths was monitored at 15 locations distributed along a transect (upper slope, mid-slope and lower slope) and different FA tree position (under, at the limit and outside canopy) from August to October 2020. A portable TRIME Time Domain Reflectometry (TDR) Tube Probe (IMKO, Germany) was used to determine soil volumetric moisture content while being placed at specific depth intervals inside a PVC access tube set up at each location. Soil moisture was monitored at 10 cm interval from 20 to 420 cm during the rainy season from July to October 2020. Results of soil moisture profiles along the transects exhibit two main zones based on the standard deviation (SD) and the inflection of the coefficient of variation (CV): shallow soil moisture (SSM) and deep soil moisture (DSM). For SSM observed at 20-60 cm of the soil layer, both mean soil moisture and SD increase with depth, the lowest mean value (8%) being observed at the top surface. This soil layer is influenced by rainfall infiltration and daily evaporation. For DSM observed at 70-420 cm, the moisture pattern can be further divided into 4 soil sublayers taking the mean soil moisture vertical distribution as reference: (i) a rainfall infiltration layer (70-160 cm) which appears mainly influenced by cumulative rainfall infiltration in addition to transpiration of grassland and crops (shallow root system); (ii) a rainfall-transpiration layer (170-250 cm) which is still an infiltration layer but more influenced by crops transpiration; (iii) a transpiration layer (260-350 cm) which can be recharged by rainfall infiltration during heavy rainfall and supply deep root system; and (iv) deep transpiration layer (360-400 cm) which has DSM that can be influenced by extremely deep root vegetation such as FA. The factors influencing the soil water content varied with the topography. The soil water content SWC (mean and median value of 27.2 and 29.6% respectively) in the lower slope was significantly higher than that at middle (mean and median value of 14.4 and 13.2 % respectively) and upper slope (mean and median value of 16.8 and 18.4 % respectively). At last, soil water content was positively correlated with the distance from the FA, regardless the slope. The higher water content for both SSM and DSM was observed outside the FA canopy. This result refutes the initial hypothesis of higher SWC under trees and support a more detailed analysis of the infiltration capacity in relationship with the FA position.</p><div> <div> <p>[1] Faidherbia-Flux : https://lped.info/wikiObsSN/?Faidherbia-Flux</p> </div> </div>

<p>Agro-silvo-pastoralism is a highly representative Land Use in Africa, often presented as a strategical option for ecological intensification of cropping systems towards food security and sovereignty.</p><p>We set up a new long-term observatory (“Faidherbia-Flux”) to monitor and model microclimate, energy and C balance in Niakhar (central Senegal, rainfall ~ 500 mm), dominated by the multipurpose tree Faidherbia albida (12.5 m high; 7 tree ha<sup>-1</sup>; 5% canopy cover). Faidherbia is an attractive agroforestry tree species in order to partition fluxes, given that it is on leaf during the dry season (October-June) and defoliated during the wet season, just when crops take over. Pearl-millet and groundnut crops were conducted during the wet season, following annual rotation in a complex mixed mosaic of ca. 1 ha fields.</p><p>Early 2018, we installed an eddy-covariance (EC) tower above the whole mosaic (EC1: 20 m high). A second EC system was displayed above the crop (EC2: 4.5 m if pearl-millet, 2.5 m if groundnut) in order to partition ecosystem EC fluxes between tree layer and crop+soil layers. Sap-flow was monitored from April 2019 onwards in 5 faidherbia trees (37 sensors).</p><p>The ecosystem displayed moderate but significant daily CO<sub>2</sub> and H<sub>2</sub>O fluxes during the dry season, when faidherbia (low canopy cover) was in leaf and the soil was evaporating. At the onset of the rainy season, the soil bursted a large amount of CO<sub>2</sub>. Just after the growth of pearl-millet in 2018, CO<sub>2</sub> uptake by photosynthesis increased dramatically. However, this was largely compensated by high ecosystem respiration. Surprisingly in 2019, although the crop was turned to groundnut, the fluxes behaved pretty much the same as with pearl millet in 2018: comparing annual balances between 2018 and 2019 we obtained [454, 513] for rainfall (P: mm yr<sup>-1</sup>), [3500, 3486] for potential evapotranspiration (ETo: mm yr<sup>-1</sup>), [0.13, 0.15] for P/ETo, [470, 497] for actual evapotranspiration (E: mm yr<sup>-1</sup>), [2809, 2785] for net radiation (R<sub>n</sub>: MJ m<sup>-2</sup> yr<sup>-1</sup>), [1686, 1645] for sensible heat flux (H: MJ m<sup>-2</sup> yr<sup>-1</sup>),  [-3.2, -2.8] for net ecosystem exchange of C (NEE: tC ha<sup>-1</sup> yr<sup>-1</sup>), [-11.8, -11.1] for gross primary productivity (GPP: tC ha<sup>-1</sup> yr<sup>-1</sup>) and [8.6, 8.3] for ecosystem respiration (R<sub>e</sub>: tC ha<sup>-1</sup> yr<sup>-1</sup>). The energy balance (R<sub>n</sub>-H-LE) was nearly nil indicating that the EC system behaved reasonably. E was very close to P, indicating that little or no water would recharge the deep soil layers.</p><p>Now comparing the dry (2/3 of the year) and wet (1/3) seasons: surprisingly, NEE was more effective during the dry season [-3.9, -1.7]. This was the result of R<sub>e</sub> being much lower on a daily basis as well as cumulated over the entire seasons [57, 84], whereas GPP was similar [-10.8, -12.1].</p><p>We found a good match between E measured above the whole ecosystem (EC1), and the sum of tree transpiration (T, measured by sapflow) + E measured just above crops + soil (EC2) throughout the wet and dry seasons.</p><p>The “Faidherbia-Flux” observatory is registered in FLUXNET as SN-Nkr and is widely open for collaboration.</p>