DAILY AND MONTHLY WIND-SOLAR COMPLEMENTARITY ACROSS NIGER'S SAHARA-TO-SAHEL GRADIENT: HOW MUCH OF THE RAW SIGNAL SURVIVES DESEASONALIZATION?

Negative wind-solar correlations are often cited as evidence that hybrid systems can deliver steadier supply, yet a long daily series contains fewer independent observations than its nominal length suggests, because both resources share an annual cycle and successive days are serially dependent. This study quantifies how much daily complementarity survives once these effects are controlled, using nine years (2015-2023) of synoptic wind and sunshine-duration records from Bilma, Agadez Aéro and N'Guigmi, three Nigerien stations spanning the Sahara-to-Sahel transition. Daily wind-energy proxies were built from a zero-inflated wind-speed mixture and a generic turbine power curve, and daily global irradiance was estimated with the Ångström-Prescott model. Spearman correlations were computed on raw series, deseasonalized anomalies and harmonic day-of-year residuals, with 95% confidence intervals from a month-year block bootstrap, and a stability coefficient was computed for monthly hybrid mixes. Raw correlations were weakly negative and significant at all stations (r = −0.094 to −0.152, p < 0.001). After deseasonalization, the signal was no longer robust at Agadez (r = −0.036, interval including zero), became weak and borderline at Bilma and remained robust only at N'Guigmi (r = −0.12 to −0.13, interval excluding zero). The variance-minimising monthly mix was solar-dominated, with wind weights of 14-40% and stability coefficients of 0.83-0.95; this stability arose mainly from the low month-to-month variability of solar resource rather than from anti-phase behaviour. Daily wind-solar complementarity in this region is real but modest and station-specific, and pooled daily correlations uncorrected for seasonality and serial dependence overstate it.