This study investigates spatiotemporal variations in Terrestrial Water Storage (TWS) over Nigeria during the period 2002-2024 using Gravity Recovery and Climate Experiment (GRACE) satellite data. We employed Google Earth Engine (GEE) and analytical framework including trend analysis, change point detection, anomaly detection, frequency, and rate of change analyses to characterize hydrological dynamics. Results show a statistically significant increasing trend in water storage (Mann-Kendall test: p-value < 0.001, Tau = 0.43), with a higher magnitude after 2018 (from 0.68 cm/year prior to 2018 to 2.87 cm/year post-2018). Change point analysis identified July 2019 as a critical transition date, dividing the time series into two regimes with very distinct characteristics (mean TWS: 2.52 cm vs. 23.13 cm). Frequency analysis revealed multi-scale cyclical behavior, such as annual seasonality (11.1 - 12.3 months), semi-annual, and long-term hydrological variations that may correspond to multi-year climate oscillations. Wavelet analysis demonstrated the non-stationary properties of these cycles, with the annual pattern exhibiting varied intensity over the study interval (strong in 2002-2009, attenuated in 2010-2015, and re-strengthened in 2018-2024) along with the recent development of a 60-month periodicity. Spatial analysis indicated strong regional heterogeneity with stronger positive trends in northern regions than in central and southern regions. The analysis of climate records revealed no corresponding increasing trend, suggesting that water storage changes may be driven by other factors like human activities instead of natural climatic conditions. Such anthropogenic influences may include expanded water infrastructure, improved water management practice, and land use modifications. These findings provide critical information for strategic water management interventions in the creation of water security, as well as the need for adaptive management techniques for both water availability and flood risk in different regions of Nigeria.
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