Hydrological evidence to guide climate resilient agricultural policies in Senegal

Many agricultural and land conservation programmes could use the latest hydrological evidence and methodology at early stages of programme design to improve long-term climate resilience and sustainability in Sahel region. This report presents stakeholder consultation, awareness (e.g., World Water Forum) and interdisciplinary research conducted in 2022 in Senegal. We evaluate the impact of bench terracing on water recharge and vegetation resilience to drought using satellite NDVI data and AMMA Catch long-term hydroclimate observatory datasets across 34 land conservation sites, covering 6600ha (period 2018-2021). Results show positive impacts from bench terracing but important variability. The causes of variability need to be determined to guide climate adaptation and land restoration

Observed long-term land cover vs climate impacts on the West African hydrological cycle: lessons for the future ? [P-3330-65]

West Africa has experienced a long lasting, severe drought as from 1970, which seems to be attenuating since 2000. It has induced major changes in living conditions and resources over the region. In the same period, marked changes of land use and land cover have been observed: land clearing for agriculture, driven by high demographic growth rates, and ecosystem evolutions driven by the rainfall deficit. Depending on the region, the combined effects of these climate and environmental changes have induced contrasted impacts on the hydrological cycle. In the Sahel, runoff and river discharges have increased despite the rainfall reduction (“less rain, more water”, the so-called "Sahelian paradox "). Soil crusting and erosion have increased the runoff capacity of the watersheds so that it outperformed the rainfall deficit. Conversely, in the more humid Guinean and Sudanian regions to the South, the opposite (and expected) “less rain, less water” behavior is observed, but the signature of land cover changes can hardly be detected in the hydrological records. These observations over the past 50 years suggest that the hydrological response to climate change can not be analyzed irrespective of other concurrent changes, and primarily ecosystem dynamics and land cover changes. There is no consensus on future rainfall trend over West Africa in IPCC projections, although a higher occurrence of extreme events (rainstorms, dry spells) is expected. An increase in the need for arable land and water resources is expected as well, driven by economic development and demographic growth. Based on past long-term observations on the AMMA-CATCH observatory, we explore in this work various future combinations of climate vs environmental drivers, and we infer the expected resulting trends on water resources, along the west African eco-climatic gradient. (Texte intégral

Analyse des essais d'infiltration conduits sur le bassin de la Claduègne en Mai-Juin 2012. Contribution au WP3.4 du projet ANR FloodScale 2012-2015

In this report, we present the infiltration field campaign conducted in May-June 2015 in the 48 km2 Claduègne catchment (Ardèche, France). First we present the different measurement protocols using either simple ring infiltrometers (Beerkan method) or suction disk infiltrometers. Then the sampling strategy is presented. The theoretical aspects of infiltration equations and different methods used in infiltration tests analysis are described in details. In particular, the BEST (Beerkan Estimation of Soil Transfer) method is used in the analysis of single ring infiltration data and mini-disk infiltrometer with a -20 mm suction. This method is compared to the differential linear (DL) method, showing reasonable agreement, although systematic biases are evidenced. Multiple suction disk infiltrometers are analyzed using permanent regime equations. The single ring infiltration tests provide estimates of the retention curve and hydraulic conductivity curves. The infiltrometers provide information about the hydraulic conductivity close to saturation. We present the statistical analysis of the results, which show that the soils have generally high hydraulic conductivity, especially in forested soils. Dry bulk density is also much lower in natural soils than in cultivated soils. In situ measurements are compared with three representative pedo-transfer functions showing poor capability of the latter in reproducing observations. On the other hand, land use is found more discriminant in determining soil hydraulic properties. The statistical analysis on the samples allows defining two classes for dry bulk density with significantly different values gathering respectively natural and cultivated soils. In terms of saturated hydraulic conductivity, two land uses classes gathering forested fields and moors on the one hand, and pasture and cultivated land on the other hand can be distinguished. This allows proposing a first strategy of soil hydraulic properties spatialization in the Claduègne catchment, by combining the dry bulk density and hydraulic conductivity classes in three main families of soil hydraulic properties, depending on land use

Interactions surface continentale/atmosphère : l'expérience HAPEX-Sahel

Durant un épisode pluvieux, le partage entre eaux d'infiltration et de ruissellement dans un sol encroûté dépend des propriétés hydrodynamiques du système sol-croûte. Une méthode fondée sur l'utilisation simultanée d'infiltromètres à pression contrôlée et de microtensiomètres est proposée pour la détermination in situ de la conductivité hydraulique et de la sorptivité capillaire proches de la saturation. Cette méthode repose sur la mesure du flux transitoire d'infiltration à la surface du sol, pour des potentiels imposés variant entre -110 et -10 mm de colonne d'eau. Le microtensiomètre, placé horizontalement dans le sol à l'interface croûte-sol, est utilisé pour limiter l'analyse de l'infiltration à la croûte seule. Des résultats sont présentés pour des sols caractéristiques des différentes parcelles du site Central-Est de l'expérience Hapex-Sahel : WABs mil et jachère et brousse tigrée. Sur les sols de jachère, la méthode transitoire s'est avérée consistante avec les résultats de l'infiltrométrie classique fondée sur l'analyse du régime permanent. Dans le cas de la brousse tigrée, le contraste de conductivité hydraulique à saturation entre croûte et sol sous-jacent varie de 2 à 6. Les résultats obtenus pourront servir de base à une modélisation des transferts en vue de la prévision du temps d'apparition du ruissellement et de la quantification des lames ruisselées. (Résumé d'auteur

Réponse hydrologique à la variabilité des caractéristiques du sol dans un contexte Méditerranéen d'épisodes extrêmes

International audienceThe poster presents the sensitivity of the hydrological response with respect to rainfall variability and soil characteristics for the extreme event of september 2002 in the Gard

Une méthode pour cartographier la conductivité hydraulique à saturation du sol de surface dans la région cévenole à l'aide d'essais d'infiltration réalisés avec différentes techniques

International audienceTopsoil field-saturated hydraulic conductivity, Kfs, is a parameter that controls the partition of rainfall between infiltration and runoff. It is a key parameter in most distributed hydrological models. However, there is a mismatch between the scale of local in situ measurements and the scale at which the parameter is required in models. Therefore it is necessary to design methods to regionally map this parameter at the model scale. The paper propose a method for mapping Kfs in the Cévennes-Vivarais region, south-east France, using more easily available GIS data: geology and land cover. The mapping is based on a data set gathering infiltration tests performed in the area or close to it for more than ten years. The data set is composed of infiltration tests performed using various techniques: Guelph permeameter, double ring and single ring infiltration tests, infiltrometers with multiple suctions. The different methods lead to different orders of magnitude for Kfs rendering the pooling of all the data challenging. Therefore, a method is first proposed to pool the data from the different infiltration methods, leading to a homogenized set of Kfs, based on an equivalent double ring/tension disk infiltration value. Statistical tests showed significant differences in distributions among different geologies and land covers. Thus those variables were retained as proxy for mapping Kfs at the regional scale. This map was compared to a map based on the Rawls and Brakensiek (RB) pedo-transfer function (Manus et al., 2009, Vannier et al., 2016), showing very different patterns between both maps. In addition, RB values did not fit observed values at the plot scale, highlighting that soil texture only is not a good predictor of Kfs