The AMMA-CATCH experiment in the cultivated Sahelian area of south-west Niger – Investigating water cycle response to a fluctuating climate and changing environment

Summary

Among the three sites distributed along the West African latitudinal gradient in the AMMA-CATCH observation system, the experimental setup in the Niamey area of south-west Niger samples the cultivated Sahel environment, for hydrological, vegetation and land surface processes. The objective is to investigate relationships between climate, land cover, and the water cycle, in a rapidly changing semiarid environment. This paper first presents the main characteristics of the area, where previous research, including the EPSAT and HAPEX-Sahel experiments, had evidenced a widespread decadal increase in water resources, concurrently with severe drought conditions. The specifics of AMMA-CATCH research and data acquisition at this site, over the long-term (∼2001–2010) and enhanced (∼2005–2008) observation periods, are introduced. Objectives and observation strategy are explained, and the main characteristics of instrument deployment are detailed. A very large number of parameters – covering rainfall, vegetation ecophysiology, phenology and production, surface fluxes of energy, water vapour and CO₂, runoff and sediment, pond water, soil moisture, and groundwater – were monitored at local to meso scales in a nested structure of sites. The current state of knowledge is summarized, connecting processes and patterns of variation for rainfall, vegetation/land cover, and the terrestrial hydrologic cycle. The central role of land use and of its spectacular change in recent decades is highlighted. This paper provides substantial background information that sets the context for papers relating to the south-west Niger site in this AMMA-CATCH special issue.

Introduction

In the semiarid Sahelian belt, the generalised rainfall deficit of the 1970s and 1980s reached 25–50%, and continued almost unabated until the end of the 20th century (Lebel and Ali, 2009). This drought had especially severe consequences in the cultivated Sahel (roughly the 400-800 mm yr⁻¹ rainfall band stretching east-west across Africa, acknowledging that mean isohyet locations fluctuate over time and that different criteria could be used), which is one of the most densely populated rural areas in West Africa (see Fig. 3 in Lebel et al., 2009). Relying mainly on traditional techniques, local rain-fed agriculture is very sensitive to highly variable precipitation patterns. The AMMA-CATCH¹ Niger mesoscale site (ACN), located at latitude 13–14°N in SW Niger (see Fig. 8 in Lebel et al., 2009, for a location map of the three AMMA-CATCH mesoscale sites), is typical of a large fraction of the cultivated Sahel. It is thus a highly appropriate area for documenting interannual variability and decadal-scale trends in climate, land use, ecosystem, and hydrological cycle, and for improving our understanding of the mechanisms of interaction between these factors in one of the most challenging regions in the world for sustainable development.

In contrast to the northern Sahel, where the AMMA-CATCH Gourma site (15–17.5°N) is located and where rangeland is the dominant land use (Mougin et al., 2009), the two major activities in the cultivated Sahel are mixed crop-livestock agriculture and wood harvesting for cooking fuel, which are rapidly modifying the land cover. As a consequence, the response of the terrestrial biophysical system to the variability of the monsoon system at different scales in space and time (decadal, interannual, seasonal and sub-seasonal), even though strong, is blurred at the long timescales by man-induced environmental changes. Separating the role of climate from that of local human activity in the drastic changes observed in the hydrological cycle over the past decades is thus a major issue for the cultivated Sahel.

While the main climate-induced difference between the ACN and Gourma sites is land use, the major such difference between the ACN area and the southern AMMA-CATCH site of Ouémé in Benin (Sudanian climate, 9.5–10.5°N) is the hydrological environment. The Ouémé catchment, like most of the Sudanian or Guinean catchments located south of 10°N, or like the very large rivers that combine hydro-climatic regimes (Guinean, Sudanian, Sahelian), such as the Rivers Senegal or Niger, had their discharges reduced by the drought in a higher proportion than rainfall (Olivry, 2002, Descroix et al., 2009). By contrast, in SW Niger, both surface and ground water resources have been increasing for several decades, despite the duration and intensity of the drought (Leblanc et al., 2008). While degradation of soils and land cover provides a logical qualitative explanation for this phenomenon (Favreau et al., 2009), there is still no consistent quantitative scheme able to account for this spectacular change at any significant scale. The long-term monitoring investment in this region has thus specific goals in terms of understanding and modelling the complex relationships between climate, environment and the water cycle in the cultivated Sahel.

The ACN benefits from a rich history of hydrological observations that began with the EPSAT-Niger (Lebel et al., 1992) and HAPEX-Sahel (Goutorbe et al., 1997) experiments in the early 1990s. Substantial background data and knowledge exist on rainfall distribution (Ali et al., 2003), runoff processes (Estèves and Lapetite, 2003, Cappelaere et al., 2003), aquifer replenishment (Favreau et al., 2002a), or bioclimatology and land–atmosphere exchanges (Dolman et al., 1997). In the ACN area, the River Niger roughly separates the regional domains of endorheic and exorheic hydrology, to the north-east and south-west, respectively (Fig. 1). Endorheic hydrology, with small-scale catchments mixing both highly runoff-prone and infiltration-prone surfaces, is typical of many arid to semiarid areas and raises challenging questions and difficulties for field observation as well as for modelling, thus making the ACN of particular interest to hydrologists. Beyond hydrology and associated bio-physical processes, a large corpus of data and knowledge exists in the ACN area from a great variety of past or current environmental studies, on such important topics as wind erosion (Rajot et al., 2008), soil fertility (de Rouw, 2004, Renard et al., 1997), ecosytems (e.g., Mahamane et al., 2007), as well as on agro-economic and social issues (e.g., La Rovere et al., 2005).

The overall objectives and strategy of the AMMA-CATCH observation programme are described by Lebel et al. (2009), in the wider context of the AMMA studies (Redelsperger et al., 2006). The aim of the present paper is to provide a specific overview of the ACN study area (“Study area”), of the programme and rationale for recent data acquisition at this site (“ACN data acquisition and monitoring programme”), and of the current state of knowledge (“A short review of current knowledge”). Many results obtained on the area’s vegetation and hydrology are detailed in several other papers of this special issue.