Impacts of Climate Change on Environmental Flows in West

Impacts of climate change on environmental ﬂows in West Africa’s Upper Niger Basin and the Inner Niger Delta

Julian R. Thompson a,*, Cédric L. R. Laizé b, Michael C. Acreman b, Andrew Crawleya and Daniel G. Kingston c a Wetland Research Unit, UCL Department of Geography, University College London, Gower Street, London WC1E 6BT, UK b UK Centre for Ecology & Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford OX10 8BB, UK c School of Geography, University of Otago, P.O. Box 56, Dunedin, New Zealand *Corresponding author. E-mail: [email protected]

JRT, 0000-0002-8927-6462; CLRL, 0000-0002-7560-7769; MCA, 0000-0002-8946-739X; DGK, 0000-0003-4205-4181

ABSTRACT

Modified water regimes due to climate change are likely to be a major cause of freshwater ecosystem alteration. General Circulation Model (GCM)-related uncertainty in environmental flows at 12 gauging stations in the Upper Niger Basin and flooding within the Inner Niger Delta is assessed using the Ecological Risk due to the Flow Alteration method and a hydrological model forced with projections from 12 GCM groups for RCP 4.5 in the 2050s and 2080s. Risk varies between GCM groups and stations. It increases into the future and is larger for changes in low flows compared to high flows. For the ensemble mean, a small minority of GCM groups projects no risk for high flows in the 2050s (low risk otherwise). This reverses for the 2080s. For low flows, no risk is limited to three stations in the 2050s and one station in the 2080s, the other experience either low or medium risk. There is greater consistency in the risk of change in flood extent, especially in the dry season (medium risk for all groups and the ensemble mean). Some (low or medium) risk of change in peak annual inundation is projected for most groups. Changing flood patterns have implications for wetland ecology and ecosystem services.

Key words: climate change, environmental ﬂows, Inner Niger Delta

HIGHLIGHTS

• Projected river discharge in the Upper Niger for 12 GCM groups is combined with the Ecological Risk due to Flow Alteration (ERFA) environmental flow method. • ERFA is also applied to projected flood extent within the Inner Niger Delta. • Risk of river flow change varies between GCM groups and stations. It increases into the future and is larger for low flows than high flows. • Risk for flood extent is more consistent especially in the dry season (medium risk).

INTRODUCTION Hydrological conditions within rivers exert critical controls upon aquatic ecosystems. This influence is implicit within the natural flow paradigm (Poff et al. 1997) that recognises that a river’s regime, characterised by variability, magnitude, fre- quency, duration, timing and rate of change of discharge, is central to sustaining ecosystem integrity. All elements of the flow regime influence some aspect of riverine ecosystems. Discharge variability, for example, drives fish communities both directly (e.g. influencing migration, spawning and recruitment) and indirectly (e.g. impacting availability and diversity of habi- tat; e.g. Nestler et al. 2012). Indirect influences include the important role of high flows in controlling connections between rivers, floodplains and riparian wetlands. In turn, the resulting wetland hydrological conditions, particularly water-level/inundation regimes, are a dominant influence upon vegetation, animals and biochemical processes that support numerous ecosystem services (e.g. Baker et al. 2009). For example, Africa’s floodplains are sustained by seasonal floods that underpin agriculture, grazing and fisheries, provide water for direct abstraction or groundwater recharge and sustain habitats contribut- ing to internationally important biodiversity (e.g. Adams 1992). Modifications to river regimes and floodplain inu