Türkiye’s Rainfall Is Shifting in Opposite Directions at Sea Level and in the Mountains

Türkiye is a country where climate refuses to sit still. Squeezed between the Black Sea, the Aegean, the Mediterranean and the arid interior of Anatolia, and carved by mountain ranges that rise from sea level to well over 3,000 meters, it hosts everything from lush temperate rainforest along the northeastern coast to semi-arid steppe in the heart of the peninsula. A new study published in Theoretical and Applied Climatology has now mapped how monthly precipitation across this patchwork has changed over half a century, and the results reveal a landscape divided: rainfall is creeping upward along the coasts while quietly draining away from the interior and the east, with altitude emerging as a surprisingly powerful arbiter of which way the trend bends.

The research, led by Murat Şan of Gümüşhane University together with Sinan Nacar of Tokat Gaziosmanpaşa University and Murat Kankal of Bursa Uludağ University, examined long-term monthly total precipitation records spanning 1969 to 2020. Rather than lumping the year into the familiar four seasons, the team treated each calendar month as its own season, a choice that matters enormously in a country where a single region can receive the bulk of its annual water in winter while another depends on spring and autumn storms. This monthly resolution allowed the researchers to detect shifts that coarser seasonal averages would smooth into invisibility.

At the heart of the study are two statistical engines that approach the same question from very different angles. The first is the seasonal Mann–Kendall test, a workhorse of hydrology that checks whether a time series is drifting upward or downward while explicitly accounting for the natural rhythm of the seasons. Because it is non-parametric, it makes no assumption that the data follow a tidy bell curve, which is essential for precipitation, a variable notorious for wild swings, occasional zeros and stubborn skew. The second tool is newer and more visual: the innovative polygon trend analysis framework, paired here with a Wilcoxon significance test, which plots data against themselves in a polygon and lets researchers see not just whether a trend exists but how low, medium and high values within each month behave differently.

The contrast between the two methods is itself one of the study’s most instructive findings. Across the country, the seasonal Mann–Kendall test flagged significant increases at roughly 7 percent of station-month combinations and significant decreases at about 6 percent, a near balance that captures the genuine tug-of-war between wetting and drying influences. The innovative polygon approach, however, proved more sensitive, identifying significant or weak trend signals at a larger number of stations, particularly when those signals were tied to altitude. For water managers, this matters: a method that stays silent about subtle changes may lull decision-makers into planning for a climate that no longer exists.

Geographically, the study draws a clean line between the margins and the core of the country. Along the Aegean and Black Sea coasts, the rate of precipitation increase outpaces the rate of decrease, suggesting that these maritime corridors are, on balance, getting wetter. The Black Sea coast, fed by moisture-laden air masses riding in from the north, and the Aegean, influenced by Mediterranean storm tracks, both show this coastal intensification. In the inland regions and the east, the picture reverses: decreases dominate, a pattern that will resonate uneasily in a part of the world where snowpack on the eastern mountains feeds the headwaters of the Tigris and Euphrates.

The innovative polygon method added nuance that the classical test missed, pointing to more pronounced increasing trends in the Marmara and Mediterranean regions than the seasonal Mann–Kendall analysis alone revealed. The Marmara region, home to Istanbul and a dense concentration of industry and population, and the Mediterranean coast, a hub of agriculture and tourism, would both face very different management challenges if their rainfall regimes are indeed strengthening. The discrepancy between the two methods is not a failure of either; it is a reminder that trend detection is partly a matter of perspective, and that graphical approaches can surface structure that rank-based tests compress into a single statistic.

Perhaps the most striking thread running through the results is the role of elevation. The researchers stratified their analysis by altitude and slope, alongside geographical region and the Köppen climate classification, and found that while analogous trends appeared at lower slopes and altitudes, the trends were more strongly related to altitude than to any other factor. The rate of precipitation increase, they report, diminishes as altitude rises. In other words, the lowlands are absorbing the wetting signal while the high country is not keeping pace, a pattern that echoes a growing international literature on elevation-dependent climate change in mountain environments.

The implications of that elevation gradient ripple far beyond academic curiosity. Mountain precipitation in eastern Anatolia accumulates as snow through winter and melts through spring, timing the release of water to downstream reservoirs, irrigated fields and hydropower plants across Türkiye and its neighbors. If increases in precipitation are concentrated at low elevations while higher terrain sees weaker gains or outright declines, the seasonal storage function of the mountains may erode even as some lowland gauges record wetter years. Farmers who depend on irrigation schedules calibrated to historical runoff, and dam operators who time releases around snowmelt, would both feel the consequences of such a shift.

The study’s framing of each month as a separate season also carries practical weight for agriculture. Conventional trend analyses that overlook periodicity can blur the distinction between, say, a drying October and a wetting February, yet these two signals demand opposite responses from anyone deciding when to plant, when to irrigate and when to conserve. By resolving trends month by month and testing them with complementary statistical machinery, the researchers have produced a picture granular enough to inform water resource planning and irrigated agriculture at the scale where decisions are actually made, from the cotton fields of the Aegean lowlands to the orchards of the Mediterranean coast.

Türkiye sits within the Mediterranean climate change hotspot, one of the regions projected to experience some of the sharpest drying and most volatile hydrology as the century progresses, and the new findings add observed, station-based evidence to that global picture. The near-even split between stations showing increases and decreases nationwide, the coastal-versus-inland divide, and the altitude dependence of the trends together sketch a country whose water future will not be decided by a single national number but by a mosaic of local trajectories. For a nation whose rivers cross borders and whose agriculture anchors rural economies, understanding that mosaic, the authors suggest, is no longer optional; it is the foundation on which the next half-century of water policy will have to be built.

Subject of Research: Seasonal trend analysis of monthly precipitation across Türkiye's topographically complex and climatically diverse regions from 1969 to 2020.

Article Title: Monthly precipitation trends in Türkiye under complex topography and varying climate: a seasonal Mann–Kendall and innovative polygon trend framework

Article References: Şan, M., Nacar, S., & Kankal, M. (2026). Monthly precipitation trends in Türkiye under complex topography and varying climate: a seasonal Mann–Kendall and innovative polygon trend framework. Theoretical and Applied Climatology, 157(10), Article 619. https://doi.org/10.1007/s00704-026-06538-8

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06538-8

Keywords: precipitation trends, Türkiye, climate change, seasonal Mann-Kendall test, innovative polygon trend analysis, Köppen climate classification, altitude effects, water resources, irrigated agriculture, Mediterranean climate, Black Sea coast, hydrology