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<p>In basins where bimodal and unimodal rainfall regimes coexist, the responses of rainfall, temperature, and drought extremes to future warming have not been adequately assessed. This study assesses future rainfall, temperature, and drought extremes of Wami basin in Tanzania using Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensembles. The CMIP6 outputs are statistically downscaled with the Long Ashton Research Station Weather Generator under four Shared Socioeconomic Pathways (SSP126, SSP245, SSP370, SSP585) for near (2041–2060), mid (2061–2080), and far-future (2081–2100). The selected extreme metrics from the Expert Team on Climate Change Detection and Indices (ETCCDI) and multi-timescale Standardized Precipitation-Evapotranspiration Index (SPEI-3/6/12) were analyzed separately for bimodal and unimodal rainfall regimes. The presence or absence of trends was investigated for the 1985–2014 baseline and future periods using the Mann-Kendall test and Sen’s slope estimator. The results indicate that by mid-future under high emissions, bimodal March-May rainfall increases by up to 31% and unimodal rainfall by up to 66%. Maximum temperature increases from + 0.5 to + 1.3&#xa0;°C in the near future and reaches + 1.6 to + 4.2&#xa0;°C by the late future. While, the minimum temperature rises more strongly to about + 3.0 to + 4.6&#xa0;°C under SSP585. Rainfall extremes intensify episodically, with mid-future unimodal very-wet-day rainfall increasing by up to 26&#xa0;mm yr<sup>− 1</sup> and localized strengthening of multi-day extremes. Temperature extremes show coherent historical intensification. However, future responses become increasingly index-specific, with diurnal temperature range emerging as the most consistent indicator, particularly in the unimodal regime. Drought responses are non-linear and regime-dependent. Historical variability shows short-term drying in the bimodal regime, while far-future projections show pronounced long-window drying in the unimodal regime (SPEI-12 ≈ -0.09 yr<sup>− 1</sup>), despite the projected increase in rainfall. Generally, future hydroclimatic impacts in the basin are driven by the interaction between rainfall variability and warming-induced evaporative demand, with the effects emerging at annual drought timescales. These regime-based projections across contrasting rainfall patterns provide decision-relevant evidence for climate-resilient water-resource planning and offer insights transferable to other semi-arid basins.</p> Graphical Abstract <p></p> <p>The graphical abstract synthesizes the workflow, key findings, and implications of assessing future hydroclimatic change in the Wami Basin, Tanzania, a basin characterized by distinct unimodal and bimodal rainfall regimes. The left panel presents the data foundation, combining observed and ERA5 climate records with CMIP6 projections to represent both historical variability and future climate forcing. Bias-corrected statistical downscaling using the LARS-WG weather generator translates large-scale climate model outputs into basin-scale climate information suitable for impact assessment. Multi-model ensemble (MME) is constructed based on model performance. Climate extremes are evaluated through an integrated framework combining ETCCDI indices, Standardized Precipitation-Evapotranspiration Index (SPEI) drought metrics, and non-parametric trend detection using Mann-Kendall and Sen’s slope approaches across four Shared Socioeconomic Pathways (SSP126, SSP245, SSP370, and SSP585). The central panel summarizes the principal scientific findings, showing consistent warming across rainfall regimes alongside intensification of extreme rainfall events and a late-century drying tendency indicated by declining SPEI-12 conditions, particularly in unimodal areas. Spatial patterns and time-series responses highlight regime-dependent hydroclimatic behavior rather than uniform basin responses. The right panel translates these climate signals into system-level consequences, including increased flash-flood occurrence, higher evaporative losses, enhanced soil erosion, and elevated drought risk. The graphical abstract links these findings to key adaptation priorities, including flood-risk management, soil and water conservation, and heat-resilient water planning.</p>

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Future Climate Change and Extremes in the Wami Basin, Tanzania: A CMIP6 Multi-Model Assessment Across Rainfall Regimes

  • 期刊论文

  • 发表日期：

    2026年7月21日

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  • Charles Katikizu Simon,
  • Aqil Tariq,
  • Anna Msigwa,
  • Grite Nelson Mwaijengo,
  • Hans C. Komakech

摘要

In basins where bimodal and unimodal rainfall regimes coexist, the responses of rainfall, temperature, and drought extremes to future warming have not been adequately assessed. This study assesses future rainfall, temperature, and drought extremes of Wami basin in Tanzania using Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensembles. The CMIP6 outputs are statistically downscaled with the Long Ashton Research Station Weather Generator under four Shared Socioeconomic Pathways (SSP126, SSP245, SSP370, SSP585) for near (2041–2060), mid (2061–2080), and far-future (2081–2100). The selected extreme metrics from the Expert Team on Climate Change Detection and Indices (ETCCDI) and multi-timescale Standardized Precipitation-Evapotranspiration Index (SPEI-3/6/12) were analyzed separately for bimodal and unimodal rainfall regimes. The presence or absence of trends was investigated for the 1985–2014 baseline and future periods using the Mann-Kendall test and Sen’s slope estimator. The results indicate that by mid-future under high emissions, bimodal March-May rainfall increases by up to 31% and unimodal rainfall by up to 66%. Maximum temperature increases from + 0.5 to + 1.3 °C in the near future and reaches + 1.6 to + 4.2 °C by the late future. While, the minimum temperature rises more strongly to about + 3.0 to + 4.6 °C under SSP585. Rainfall extremes intensify episodically, with mid-future unimodal very-wet-day rainfall increasing by up to 26 mm yr− 1 and localized strengthening of multi-day extremes. Temperature extremes show coherent historical intensification. However, future responses become increasingly index-specific, with diurnal temperature range emerging as the most consistent indicator, particularly in the unimodal regime. Drought responses are non-linear and regime-dependent. Historical variability shows short-term drying in the bimodal regime, while far-future projections show pronounced long-window drying in the unimodal regime (SPEI-12 ≈ -0.09 yr− 1), despite the projected increase in rainfall. Generally, future hydroclimatic impacts in the basin are driven by the interaction between rainfall variability and warming-induced evaporative demand, with the effects emerging at annual drought timescales. These regime-based projections across contrasting rainfall patterns provide decision-relevant evidence for climate-resilient water-resource planning and offer insights transferable to other semi-arid basins.

Graphical Abstract

The graphical abstract synthesizes the workflow, key findings, and implications of assessing future hydroclimatic change in the Wami Basin, Tanzania, a basin characterized by distinct unimodal and bimodal rainfall regimes. The left panel presents the data foundation, combining observed and ERA5 climate records with CMIP6 projections to represent both historical variability and future climate forcing. Bias-corrected statistical downscaling using the LARS-WG weather generator translates large-scale climate model outputs into basin-scale climate information suitable for impact assessment. Multi-model ensemble (MME) is constructed based on model performance. Climate extremes are evaluated through an integrated framework combining ETCCDI indices, Standardized Precipitation-Evapotranspiration Index (SPEI) drought metrics, and non-parametric trend detection using Mann-Kendall and Sen’s slope approaches across four Shared Socioeconomic Pathways (SSP126, SSP245, SSP370, and SSP585). The central panel summarizes the principal scientific findings, showing consistent warming across rainfall regimes alongside intensification of extreme rainfall events and a late-century drying tendency indicated by declining SPEI-12 conditions, particularly in unimodal areas. Spatial patterns and time-series responses highlight regime-dependent hydroclimatic behavior rather than uniform basin responses. The right panel translates these climate signals into system-level consequences, including increased flash-flood occurrence, higher evaporative losses, enhanced soil erosion, and elevated drought risk. The graphical abstract links these findings to key adaptation priorities, including flood-risk management, soil and water conservation, and heat-resilient water planning.