Comparing water quantity and quality in three inland valley watersheds with different levels of agricultural development in central Benin

Achieving sustainable agricultural intensification in inland valleys while limiting the impacts on water quantity and water quality requires a better understanding of the valleys’ hydrological behavior with respect to their contributing watersheds. This study aims at assessing the dynamics of hydrological processes and nitrate loads within inland valleys that are experiencing different land uses. To achieve this goal, an HRU-based interface (ArcSWAT2012) and a grid-based setup (SWATgrid) of the Soil Water Assessment Tool (SWAT) model were applied to three headwater inland valley watersheds located in the commune of Djougou in central Benin that are characterized by different proportions of cultivated area. Satisfactory model performance was obtained from the calibration and validation of daily discharges with the values of R2 and NSE mostly higher than 0.5, but not for nitrate loads. The annual water balance reveals that more than 60% of precipitation water is lost to evapotranspiration at all sites, amounting to 868 mm in Kounga, 741 mm in Tossahou, and 645 mm in Kpandouga. Percolation (302 mm) is important in the Kpandouga watershed which is dominated by natural vegetation at 99.7%, whereas surface runoff (105 mm) and lateral flow (92 mm) are the highest in the Kounga watershed having the highest proportion of agricultural land use (14%). In all the studied watersheds, nitrate loads are very low (not exceeding 4000 KgN per year) due to the low fertilizer application rates, and the water quality is not threatened if a standard threshold of 10 mg/l NO3-N is applied. The results achieved in this study show that SWAT can successfully be used in spatial planning for sustainable agricultural development with limited environmental impact on water resources in inland valley landscapes

Development and analysis of the Soil Water Infiltration Global database

In this paper, we present and analyze a novel global database of soil infiltration measurements, the Soil Water Infiltration Global (SWIG) database. In total, 5023 infiltration curves were collected across all continents in the SWIG database. These data were either provided and quality checked by the scientists who performed the experiments or they were digitized from published articles. Data from 54 different countries were included in the database with major contributions from Iran, China, and the USA. In addition to its extensive geographical coverage, the collected infiltration curves cover research from 1976 to late 2017. Basic information on measurement location and method, soil properties, and land use was gathered along with the infiltration data, making the database valuable for the development of pedotransfer functions (PTFs) for estimating soil hydraulic properties, for the evaluation of infiltration measurement methods, and for developing and validating infiltration models. Soil textural information (clay, silt, and sand content) is available for 3842 out of 5023 infiltration measurements ( ∼ 76%) covering nearly all soil USDA textural classes except for the sandy clay and silt classes. Information on land use is available for 76% of the experimental sites with agricultural land use as the dominant type ( ∼ 40%). We are convinced that the SWIG database will allow for a better parameterization of the infiltration process in land surface models and for testing infiltration models. All collected data and related soil characteristics are provided online in *.xlsx and *.csv formats for reference, and we add a disclaimer that the database is for public domain use only and can be copied freely by referencing it. Supplementary data are available at https://doi.org/10.1594/PANGAEA.885492 (Rahmati et al., 2018). Data quality assessment is strongly advised prior to any use of this database. Finally, we would like to encourage scientists to extend and update the SWIG database by uploading new data to it

Analyse der hydrologischen Prozesse in den sub-humiden Tropen Westafrikas unter besonderer Berücksichtigung der Landnutzung am Beispiel des Aguima-Einzugsgebietes in Benin

Das Ziel der vorliegenden Arbeit stellt die Analyse der hydrologischen Prozesse in einem kleinen Einzugsgebiet der sub-humiden Savannenzone Benins dar. Die Arbeit ist in das IMPETUS-Projekt integriert, das den Einfluss globaler Umweltveränderungen auf den Wasserkreislauf Westafrikas am Beispiel der Einzugsgebiete des Drâa in Marokko und des Ouémé in Benin untersucht. Zur Analyse der lokalen hydrologischen Prozesse wurde in der Untersuchungsregion in Benin ein repräsentatives Einzugsgebiet, das Aguima-Einzugsgebiet (ca. 30 km2), ausgewählt, in dem die lokalen Untersuchungen des Projektes durchgeführt werden. Im Aguima-Gebiet wurden im Rahmen dieser Arbeit mehrere Messstationen eingerichtet, an denen die Bodenwasserdynamik, das Klima sowie die Durchflüsse an den periodisch durchflossenen Gerinnen zeitlich hoch aufgelöst erfasst werden. Zusätzlich wurden zahlreiche bodenphysikalische Messungen durchgeführt und die Infiltrationseigenschaften der Böden untersucht. Um den Einfluss der Landnutzung auf die Abflussbildungsprozesse zu bestimmen, wurden zwei eileinzugsgebiete des Aguimas mit unterschiedlicher Landnutzung (oberer Aguima: vorwiegend natürliche Vegetation, oberer Niaou: vorwiegend landwirtschaftlich genutzt) intensiver untersucht. Die Ergebnisse der Messungen zeigen, dass im Einzugsgebiet laterale Abflussprozesse dominieren. Dies wird v.a. durch geringere Durchlässigkeiten des Unterbodens der terrestrischen Böden (Lixisol/Acrisol und Plinthosol) hervorgerufen. Im Oberboden ist die Permeabilität stark von der Landnutzung abhängig. Auf Feldflächen wurden deutlich geringere Ksat-Werte bei in-situ-Messungen festgestellt, als auf Flächen mit natürlicher Vegetation. Oberflächenabfluss spielt somit nur im Bereich landwirtschaftlicher Nutzflächen eine Rolle. Der Oberflächenabfluss reinfiltriert jedoch meist am Rand der Inland-Valleys, da hier stark sandige Gleysols mit hohen Durchlässigkeiten vorherrschen. Zusätzlich zu den Feldmessungen wurde in der Arbeit auch die Anwendbarkeit verschiedener hydrologischer Modelle für die Prozessanalyse und zur Quantifizierung des Wasserkreislaufes getestet. Die Modelle sind für die im IMPETUS-Projekt geplanten Simulationen von Zukunftsszenarien möglicher Umweltveränderungen im Untersuchungsgebiet von Bedeutung. Da bisher nur wenige Studien über hydrologische Modellierung in den Tropen veröffentlicht wurden, wurden drei verschiedene Modellkonzepte auf ihre Anwendbarkeit in der Region untersucht. Das Modell TOPLATS (Famiglietti et al. 1992), welches lokale und vertikale Prozesse durch eine Kombination lokaler SVATs mit dem TOPMODEL-Ansatz simuliert, konnte im Untersuchungsgebiet nicht zufrieden stellend angewendet werden. Es zeigte sich bei der Abflusssimulation eine starke Überschätzung des Sättigungsoberflächenabflusses, was durch den TOPMODEL-Ansatz hervorgerufen wird. Des Weiteren wurde das physikalisch-basierte Standortmodell SIMULAT (Diekkrüger 1996) in einer modifizierten Hang-Version (SIMLUAT-H) angewendet. Mit diesem Modell konnten zufrieden stellende bis gute Ergebnisse bei der Simulation des Abflusses, der Bodenwasserdynamik und der Abflussanteile erzielt werden. Zusätzlich wurde das konzeptionelle Modell UHP getestet, das bereits bei regionalen Simulationen im oberen Ouémé-Gebiet von Bormann & Diekkrüger (2003b) und (2004) erfolgreich angewendet wurde. Die lokalen Simulationen im Aguima-Gebiet mit UHP waren zufrieden stellend. Die Untersuchungen zeigen, dass durch angepasste Modellkonzepte eine erfolgreiche Simulation des Wasserkreislaufes im tropischen Westafrika auf verschiedenen Raum- und Zeitskalen möglich ist. Die Modellkonzepte SIMULAT-H und UHP können abhängig von der Fragestellung und der Skala für zukünftige Szenarienberechungen des IMPETUS-Projektes eingesetzt werden.Analysis of the hydrological processes in the sub-humid Tropics of West Africa with special regard to land use – the case study of the Aguima catchment The objective of this study is to analyse the hydrological processes in a small catchment in the sub-humid savannah-zone of Benin. The study is integrated in the IMPETUS-Project which investigates the effects of global change on the water cycle in two catchments in West Africa: the Drâa in Morocco and the Ouémé in Benin. The investigations of hydrological processes on a local scale in Benin were carried out in a representative catchment, the Aguima catchment (30 km2). Here, discharge, soil water dynamics and climate were measured by automatic data loggers in a high temporal resolution. In addition soil physical properties and infiltration capacity were determined for all soil types in the investigation area. To analyse the effects of land use on hydrological processes two sub-cachments of the Aguima with different land use types were investigated in detail (upper Niaou: agricultural land use, upper Aguima: natural vegetation). The results of the measurements have shown, that interflow is the dominant process in the investigation area. This is caused by the low permeability of the sub-soil horizons of the dominant soil types (Plinthosol and Lixisol/Acrisol). The permeability of the soil surface depends on the land use. On fields the saturated conductivity is considerably lower than on soils with natural vegetation. Consequently surface runoff represents an important process in the agricultural areas, but not in catchments with natural vegetation. If surface runoff occurs, it mostly reinfiltrates downslope in inland-valleys, which are characterised by sandy Gleysols with high permeability. In addition to the hydrological measurements, different hydrological models were applied to test the suitability of these models for a hydrological process analysis and for the quantification of the water cycle. Hydrological models represent an important tool for the IMPETUS project overall to simulate future scenarios of global change effects on the water cycle and water availability. Only few models have been applied in the tropics up to now. Therefore the applicability of three different model concepts to the investigation area was tested in this study. The model TOPLATS (Famiglietti et al. 1992) combines local SVATs with the TOPMODEL concept in order to simulate vertical and lateral processes. Because of the overestimation of saturated overland flow, this model was not able to reproduce the processes and the water balance of the investigation area, which is due to the representation of the groundwater table by the TOPMODEL concept. In addition, the physically-based 1D-model SIMULAT (Diekkrüger 1996) was applied in a modified hillslope version (SIMULAT-H). With this model good simulation results could be attained in a multi-criteria validation (discharge, soil water dynamics and discharge components). Furthermore the conceptual model UHP (Bormann & Diekkrüger 2003b and 2004), which has already been tested on a regional scale in Benin, was applied on a local scale in the Aguima catchment. The model was able to reproduce the water balance and the discharge of the investigated catchments. The study demonstrates that adapted model concepts are capable to simulate the water cycle in tropical West Africa on different spatial and time scales. Depending on scale and objective, the model concepts SIMULAT-H and UHP can be used in the context of the IMPETUS project to simulate future scenarios of global change effects on the water cycle and the water availability.L'analyse des processus hydrologiques dans la zone tropicale subhumide de l'Afrique de l'Ouest en prenant en considération l'occupation du sol - l'exemple d'étude du bassin versant de l'Aguima L'objectif de cette étude est d'évaluer les processus hydrologiques d´un petit bassin versant en zone sub-humide au Bénin. Le travail est effectué au sein du projet IMPETUS qui analyse les effets du changement global sur le cycle de l'eau dans deux bassin versants en Afrique de l'Ouest : le Drâa au Maroc et l'Ouémé au Bénin. Les études concernant les processus hydrologiques locales au Bénin sont effectuées dans un bassin versant représentatif: l'Aguima (30 km2). Ce bassin versant a été largement équipé d´instruments de mesure tels que des limnigraphes, des stations climatiques, des sondes TDR's et des tensiomètres. De plus, les propriétés physiques du sol et les taux d'infiltration ont été déterminés pour différent sols dans le bassin de l'Aguima. Les effets de l'occupation du sol sur le cycle de l'eau ont été analysés par une comparaison des processus hydrologiques des deux sous-bassins versants présentant des occupations de sol différentes (Niaou supérieure : agriculture et jachère, Aguima supérieure : végétation naturelle (savane et forêt). Les résultats de cette étude ont montrés que l’écoulement latéral domine dans la région à cause des perméabilités réduites des horizons inférieure des sols dominants (Plinthosols et Lixisols/Acrisols). A la surface la perméabilité dépend de l’occupation du sol. Sur les champs la conductivité saturée est fortement réduite par rapport à la surface avec végétation naturelle. Par conséquence les écoulements de surface dominent dans le bassin versant agricultural/cultivé. Mais l’écoulement de surface se réinfiltre souvent au bas du versant dans les bas-fonds, qui sont caractérisés par des sols sableux avec une perméabilité élevée. L’objective secondaire de cette étude est d’examiner l’applicabilité des différents modèles hydrologiques pour une analyse des processus hydrologiques et pour une quantification des composants du cycle de l’eau dans la région d’étude. Dans le cadre du projet IMPETUS les modèles hydrologiques sont un outil indispensable pour simuler des conséquences possibles du changement global sur le cycle de l’eau et la disponibilité de l’eau avec différents scénarios. Comme il existe seulement quelques études concernant la modélisation hydrologique dans les zones tropicales, trois différents modèles ont été testés dans le but d´être appliqué á la région d’étude. Le model TOPLATS (Famiglietti et al. 1992) simule les processus verticaux et lateraux en combinant des SVATs locales avec le concept TOPMODEL. L’application de ce model n’était pas satisfaisante. Causé par le concept TOPMODEL le model simule une grande quantité de ruissellement de surface saturée, qui ne concorde pas avec les processus réels. En plus le 1d-model SIMULAT (Diekkrüger 1996) était appliqué dans une version modifiée pour simuler les processus hydrologiques des versants (SIMULAT-H). Ce model pouvait atteindre des bons résultats pour la simulation du débit, de la dynamique de l’eau dans le sol et des composants du débit. Le troisième model appliqué dans la région d’étude a été un model conceptionel UHP, qui était déjà employé pour la simulation hydrologique en échelle régionale (Bormann & Diekkrüger 2003b et 2004). Les simulations á l´échelle locale dans le bassin versant de l’Aguima se sont révélées satisfaisantes avec ce model. Les études présentées ont montrées que des modèles adaptés sont applicables pour une simulation de cycle de l’eau dans la zone de l’Afrique de l’Ouest tropicale en échelles spatio-temporelles différentes. Les modèles SIMULAT-H et UHP seront des outils pouvant être employés pour simuler des scénarios dans le cadre du projet IMPETUS

Discharge data derived from five water level gauges and discharge measurements in the Aguima and Niaou catchment, Benin, West Africa

This paper analyzes the hydrological processes and the impact of soil properties and land use on these processes in tropical headwater catchment in the sub-humid part of Benin (West-Africa), the Aguima catchment. The presented study is integrated in the GLOWA IMPETUS project, which investigates the effects of global change on the water cycle and water availability on a regional scale in Morocco and Benin. The lack of field investigations concerning soil and surface hydrology in the Benin research area necessitates detailed field measurements including measurements of discharge, soil water dynamics, soil physical properties etc. on the local scale in order to understand the dominant runoff generation processes and its influencing factors. This is a pre-requisite to be able to forecast the effects which global change has on hydrological processes and water availability in the region. The paper gives an overview over the hydrologic measuring concept of the IMPETUS-Benin project focusing on measurements concerning the soil saturated conductivity ksat and discharge behaviour of two different sub-catchment of the Aguima catchment. The results of ksat measurements revealed that interflow is the dominant runoff process on the hillslopes of the investigated catchment. Concerning the impact of land use on the hydrological processes infiltration experiments showed that infiltration rates were reduced on cultivated land compared to natural land cover. This results in significant differences in runoff behaviour and runoff ratios while comparing natural and agricultural used catchments

Rice Intensification in a Changing Environment: Impact on Water Availability in Inland Valley Landscapes in Benin

This study assesses the impact of climate change on hydrological processes under rice intensification in three headwater inland valley watersheds characterized by different land conditions. The Soil and Water Assessment Tool was used to simulate the combined impacts of two land use scenarios defined as converting 25% and 75% of lowland savannah into rice cultivation, and two climate scenarios (A1B and B1) of the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. The simulations were performed based on the traditional and the rainfed-bunded rice cultivation systems and analyzed up to the year 2049 with a special focus on the period of 2030–2049. Compared to land use, climate change impact on hydrological processes was overwhelming at all watersheds. The watersheds with a high portion of cultivated areas are more sensitive to changes in climate resulting in a decrease of water yield of up to 50% (145 mm). Bunded fields cause a rise in surface runoff projected to be up to 28% (18 mm) in their lowlands, while processes were insignificantly affected at the vegetation dominated-watershed. Analyzing three watersheds instead of one as is usually done provides further insight into the natural variability and therefore gives more evidence of possible future processes and management strategies

Development and analysis of the Soil Water Infiltration Global database

© Author(s) 2018. In this paper, we present and analyze a novel global database of soil infiltration measurements, the Soil Water Infiltration Global (SWIG) database. In total, 5023 infiltration curves were collected across all continents in the SWIG database. These data were either provided and quality checked by the scientists who performed the experiments or they were digitized from published articles. Data from 54 different countries were included in the database with major contributions from Iran, China, and the USA. In addition to its extensive geographical coverage, the collected infiltration curves cover research from 1976 to late 2017. Basic information on measurement location and method, soil properties, and land use was gathered along with the infiltration data, making the database valuable for the development of pedotransfer functions (PTFs) for estimating soil hydraulic properties, for the evaluation of infiltration measurement methods, and for developing and validating infiltration models. Soil textural information (clay, silt, and sand content) is available for 3842 out of 5023 infiltration measurements (∼76%) covering nearly all soil USDA textural classes except for the sandy clay and silt classes. Information on land use is available for 76ĝ€% of the experimental sites with agricultural land use as the dominant type (∼40%). We are convinced that the SWIG database will allow for a better parameterization of the infiltration process in land surface models and for testing infiltration models. All collected data and related soil characteristics are provided online in ∗.xlsx and ∗.csv formats for reference, and we add a disclaimer that the database is for public domain use only and can be copied freely by referencing it. Supplementary data are available at https://doi.org/10.1594/PANGAEA.885492 (Rahmati et al., 2018). Data quality assessment is strongly advised prior to any use of this database. Finally, we would like to encourage scientists to extend and update the SWIG database by uploading new data to it.status: publishe