<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 &#215; 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. &#160;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>
<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 (&#8220;Faidherbia-Flux&#8221;) 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>), &#160;[-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 &#8220;Faidherbia-Flux&#8221; observatory is registered in FLUXNET as SN-Nkr and is widely open for collaboration.</p>
<p>The soil hydraulic properties controlling infiltration are dynamic depending on interrelated factors such as soil texture and structure, climate (rainfall intensity), land use, vegetation cover and plant root systems. These physical and biological factors directly influence the size and geometry of the conductive pores, and therefore the bulk density, soil structure and finally water infiltration at surface. In the Sahelian zone, the slightest modification of the physical properties of the soil has severe consequences on the soil properties and thus on hydrological processes. It is therefore essential to improve knowledge on the spatial distribution of the hydraulic behavior of soils for optimization of agricultural uses.</p><p>We used the BEST method (Beerkan Estimation of Soil Transfer parameters) on a toposequence of the Senegalese groundnut basin (Fatick region) in the Faidherbia-Flux observatory[1] where the average rainfall is 590 mm/yr. The studied toposequence (400 m long) is representative of a common agroforestry zone with annual cultivation of millet and peanuts and a sparse density of Faidherbia albida. The slope is low (1%) with small lowland areas made up of sandy soil with more clay (clay soil), while the glacis is represented by more or less compacted sand. The infiltrometry measurements were made with the automatic single-ring infiltrometer developed by Di Prima et al. (2016), used here for the first time in West Africa. The explicative variables tested are the type of soils, including: clay soils under tree (CLUT) and outside tree (CLOT), sandy soils under tree (SSUT) and outside trees (SSOT), and cattle trampled soils outside trees (TSOT) particularly compacted and largely present in the study area. BEST algorithms were applied to the experimental data to determine the hydraulic properties of the soils of the different variables and to draw water retention and hydraulic conductivity curves.</p><p>There are significant differences in infiltration rates between the sampled zones and in relation with the studied factors. The highest infiltration rate is found on sandy soils under tree (SSUT) with an average infiltration rate of 14.0 mm/min, followed by SSOT with 11.6 mm/min. Then the clay soils CLUT and CLOT are characterized by similar lower hydraulic responses with average infiltration rates of 6.9 mm/min and 6.2 mm/min, respectively. The average infiltration rate is the lowest on the compacted sandy soils TSOT, with only 5.4 mm/min. The study of the variability of the infiltration rates measured by class of variable shows a large variability for CLOT, CLUT and SSUT (decreasing order of variability). These results are in agreement with the measured values of dry soil bulk density. The high infiltration rates in the clay soils outside and under trees can be explained by the higher content of organic matter observed on the sampling, and probably by the existence of preferential flow activated by the macropores particularly present on clay soils (CLOT and CLUT) and on sandy soils under tree (SSUT).</p><p>Di Prima, S., et al., 2016. Testing a new automated single ring infiltrometer for Beerkan infiltration experiments. Geoderma, 262, 20&#8211;34. doi:10.1016/j.geoderma.2015.08.006</p><div> <div> <p>[1] Faidherbia-Flux&#160;: https://lped.info/wikiObsSN/?Faidherbia-Flux</p> </div> </div><p>&#160;</p>
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