The research rests on a methodological choice that matters as much as the findings: rather than computing drought indices once at the national scale, the study disaggregated national-scale calculations to the basin level. This allowed the analysis to capture the spatial continuity of hydro-climatic change and the real differences between neighboring basins, something a single countrywide index would smooth away. Türkiye spans Mediterranean, continental, and semi-arid climate zones, and its twenty-five hydrological basins respond to atmospheric forcing in very different ways. Averaging across them risks diluting exactly the signals that water managers need to see.

Four indices formed the backbone of the analysis. The Standardized Precipitation Index, or SPI, and the Percent of Normal Index, or PNI, are purely precipitation-based measures that compare rainfall against long-term climatological norms. The Standardized Precipitation Evapotranspiration Index, or SPEI, and the Reconnaissance Drought Index, or RDI, add a second ingredient: the atmospheric demand for water, driven by temperature. This distinction proved decisive. Precipitation-only indices and evaporation-sensitive indices told strikingly different stories about the same half-century of Turkish climate, and the divergence between them is the study’s central finding.

Each index was computed at timescales of 1, 3, 6, 12, and 24 months, and trends in the resulting time series were evaluated with three complementary statistical approaches: the Mann-Kendall test, Sen’s slope estimator, and Innovative Trend Analysis, or ITA, a method developed to detect hidden trends that classical tests can miss. Because hydro-climatic series are serially correlated, meaning this month’s conditions influence the next, the study applied the Hamed-Rao autocorrelation-corrected version of the Mann-Kendall test. Results were then visualized through heat maps that make the spatial patterns across the basin network immediately legible, revealing where trends cluster and where they fade.

The short-timescale picture is one of statistical silence. At the 1- to 3-month scales, no statistically significant trends appeared in the SPI or PNI in most basins. In other words, month-to-month and season-to-season rainfall across Türkiye has not shifted in any consistent direction detectable by these tests. This is a genuinely important negative result. It means that drought monitoring systems keyed to short-term precipitation anomalies would conclude that little has changed, even as deeper, slower changes accumulate in the water balance.

Stretch the window to 6 to 24 months, however, and trends begin to emerge and, crucially, to gain spatial continuity. Neighboring basins start to agree with one another, which is the signature of a coherent regional climate shift rather than local noise. The strongest and most alarming signals appear in the SPEI and RDI at the 12- and 24-month scales, where widespread, statistically significant negative trends were detected. The drying is most pronounced in the Mediterranean, Aegean, Central Anatolia, and Southeastern Anatolia basins, regions that anchor much of Türkiye’s agriculture and water supply.

The interpretation follows directly from the index comparison. Because precipitation-based indices remain relatively stable while indices incorporating the evaporation component show persistent moisture deficit, the study concludes that rising temperatures and increased evaporation demand are the primary drivers of hydro-climatic stress. This mechanism, sometimes called the evaporative intensification of drought, is exactly what climate science predicts for a warming world: warmer air holds and demands more moisture, drawing water from soils, reservoirs, and vegetation even when rainfall totals hold steady. Türkiye’s basins are experiencing this demand-side drought in full force.

The study also took unusual care to make sure the drying signal is not a statistical artifact. The negative trends, while reduced in extent after autocorrelation correction, remained detectable, and they were independently corroborated by permutation-based trend testing, a nonparametric approach that reshuffles the data to build a null distribution from scratch. Structural breaks in the series were located with the Pettitt change-point test, and these breaks turned out to occur at basin-specific times rather than at a single common year, underscoring that each basin has followed its own trajectory into deficit rather than responding to one synchronized climatic event.

The practical implications are pointed. The findings suggest that drought assessment and water management strategies should be grounded in basin-scale analyses using medium- and long-term hydro-climatic indices rather than short-term indicators. Reservoir operation, irrigation planning, and agricultural policy calibrated to 1- or 3-month precipitation indices would systematically underestimate the stress that water systems face. Indices like the SPEI and RDI at 12- to 24-month scales capture the slow accumulation of deficit that determines whether aquifers recover, whether reservoirs refill, and whether rain-fed agriculture remains viable.

For a country positioned at the intersection of a warming Mediterranean and a drying Middle East, the message of this analysis is that the most consequential drought is not the one announced by a dry month, but the one written into the multi-year water balance. The precipitation record may look reassuring; the evaporative ledger does not. By disaggregating national indices to the basin scale and triangulating across multiple statistical tests, the study provides a template for how other nations with complex regional climates can detect drying signals that would otherwise hide in plain sight, and it makes a strong case that the timescale of analysis is not a technical detail but the difference between seeing the trend and missing it entirely.

Subject of Research: Basin-scale trends in hydro-climatic drought indices across Türkiye

Article Title: Basin-scale trends in hydro-climatic drought indices across Türkiye

Article References: Basin-scale trends in hydro-climatic drought indices across Türkiye. (n.d.). https://doi.org/10.1007/s10113-026-02690-z

Image Credits: AI Generated

DOI: 10.1007/s10113-026-02690-z

Keywords: drought indices, SPI, SPEI, RDI, trend analysis, Mann-Kendall test, Innovative Trend Analysis, evapotranspiration, Türkiye, river basins, climate change, water management