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Rainfall consistently enhanced around the Gezira Scheme in East Africa due to irrigation

Abstract

Land-use and land-cover changes have significantly modified regional climate patterns around the world1,2. In particular, the rapid development of large-scale cropland irrigation over the past century has been investigated in relation to possible modification of regional rainfall3,4,5,6,7,8,9,10,11,12,13,14. In regional climate simulations of the West African Sahel, hypothetical large-scale irrigation schemes inhibit rainfall over irrigated areas but enhance rainfall remotely13,14. However, the simulated influence of large-scale irrigation schemes on precipitation patterns cannot be substantiated without direct comparison to observations15. Here we present two complementary analyses: numerical simulations using a regional climate model over an actual, large-scale irrigation scheme in the East African Sahel—the Gezira Scheme—and observational analyses over the same area. The simulations suggest that irrigation inhibits rainfall over the Gezira Scheme and enhances rainfall to the east. Observational analyses of rainfall, temperature and streamflow in the same region support the simulated results. The findings are consistent with a mechanistic framework in which irrigation decreases surface air temperature, causing atmospheric subsidence over the irrigated area and clockwise wind anomalies (in background southwesterly winds) that increase upward vertical motion to the east. We conclude that irrigation development can consistently modify rainfall patterns in and around irrigated areas, warranting further examination of potential agricultural, hydrologic and economic implications.

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Figure 1: Background information regarding irrigation in the Gezira Scheme.
Figure 2: Comparison of simulated and observed changes in rainfall.
Figure 3: Temporal and spatial changes in rainfall from station observations.
Figure 4: Spatial changes in horizontal wind and temporal changes in surface air temperature.

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Acknowledgements

Funding for this research was provided by the Cooperative Agreement between the Masdar Institute of Science and Technology (Masdar Institute) and the Massachusetts Institute of Technology (MIT), and by the Singapore–Massachusetts Institute of Technology Alliance for Research and Technology (SMART). We are grateful to M. Siam and the other members of the Eltahir Research Group for their support and valuable feedback.

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E.A.B.E. conceived and supervised the study. E.-S.I. performed the numerical modelling experiments and analysis. R.E.A. performed the observational analyses. R.E.A. wrote the paper, with input from E.-S.I. and E.A.B.E. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Ross E. Alter or Eun-Soon Im.

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The authors declare no competing financial interests.

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Alter, R., Im, ES. & Eltahir, E. Rainfall consistently enhanced around the Gezira Scheme in East Africa due to irrigation. Nature Geosci 8, 763–767 (2015). https://doi.org/10.1038/ngeo2514

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