The study, conducted by Stefanos P. Stefanidis of the Forest Research Institute of the Hellenic Agricultural Organization DIMITRA and Nikolaos D. Proutsos of the Institute of Mediterranean Forest Ecosystems, mined a remarkable archive: 20,817 scheduled observation intervals recorded at the Pertouli station between 2001 and 2020, with 2006 treated as a complete data gap. Rainfall and fresh snowfall were accumulated over intervals ending at 08:00, 14:00 and 20:00 local time, while 2-meter air temperature and relative humidity were read at each endpoint. This is precisely the kind of ground-truth data that Mediterranean mountains have long lacked, even though the phase of precipitation there determines whether water arrives as immediate liquid input to streams and soils, or as temporary snow storage that shapes the timing of spring runoff.

The headline number is a 50% snow-occurrence temperature of 0.34 °C, with a 95% confidence interval running from 0.10 to 0.60 °C. In other words, at Pertouli, precipitation is equally likely to fall as rain or snow when the air is just a third of a degree above freezing — slightly warmer than the 0 °C assumption embedded in many hydrological models. More revealing is the width of the transition itself: the zone in which the probability of snow slides from 90% down to 10% stretches from −1.10 °C to 1.78 °C. That nearly three-degree window is where forecasting becomes genuinely difficult, and where the study’s most striking diagnostic results emerge.

To test that difficulty quantitatively, the researchers set aside intervals in which rain and fresh snow occurred simultaneously — 112 of them — and refused to use them when fitting the occurrence models. Instead, these mixed-phase events were held out as an independent diagnostic of the transition zone. The result was emphatic: 97 of the 112 co-occurring intervals, or 86.6%, fell inside the transition zone defined by held-out years, compared with only 15.1% of single-phase intervals. The odds ratio of 36.3, with a confidence interval from 25.4 to 59.1, means that mixed precipitation was more than thirty-six times as likely to occur within the transition band as outside it — a powerful confirmation that the fitted probability function captures where the atmosphere genuinely straddles both phases.

Methodologically, the study is notable for its validation discipline. Logistic occurrence models and nominal precipitation-amount models were evaluated with leave-one-year-out cross-validation, in which each calendar year was excluded in turn and the model was tested against it, or with year-block bootstrap resampling. This approach respects the strong temporal structure of climate data, avoiding the optimistic bias that arises when models are validated on randomly shuffled observations from the same seasons they were trained on. The researchers also confronted a stubborn practical problem: converting fresh snow depth to snow-water equivalent requires an assumed density ratio, and the archive’s convention of 10:1 (ten centimeters of snow to one centimeter of water) is itself a modeling choice. Under that baseline, the amount-weighted 50% threshold came out at 0.17 °C; across fixed ratios from 5:1 to 20:1, it ranged from 0.55 down to −0.22 °C. Crucially, occurrence-based estimates were unchanged by these assumptions, which is why the authors treat the occurrence threshold as the more robust quantity.

Perhaps the most practically useful finding is the performance of the simplest possible rule. A fixed 1 °C air-temperature threshold — the kind of rule a field technician or a coarse model might apply — achieved a record-wide balanced accuracy of 0.951 across the entire twenty-year dataset, a figure that rivals far more elaborate schemes. But that headline skill conceals a weakness: accuracy deteriorated sharply inside the transition zone, where the atmosphere itself refuses to commit to a single phase. Outside that band, the 1 °C rule is nearly perfect; inside it, even the best near-surface meteorology struggles.

That struggle motivated the study’s second question: can humidity information rescue the ambiguous cases? On a response-qualified subset of relative humidity data, adding RH improved logistic temperature classification within the transition zone by 0.081 in balanced accuracy, with a confidence interval of 0.038 to 0.119 — a real but modest gain. The researchers also tested wet-bulb temperature estimated with the Stull formulation, a physically motivated alternative that combines temperature and humidity into a single variable often favored in snow-modeling literature. Neither humidity-enhanced approach outperformed the plain 1 °C temperature benchmark across the full record. The finding aligns with recent machine-learning work suggesting there is a fundamental limit to how accurately precipitation phase can be partitioned using near-surface meteorology alone, because the phase of a falling hydrometeor reflects conditions aloft as much as conditions at the gauge.

The hydrological stakes are quantified directly: about 25.1% of nominal November-to-April precipitation at Pertouli fell inside the observed transition zone under the 10:1 baseline. One quarter of the cold-season water budget, in other words, arrives in conditions where a one-degree shift in temperature — or a slightly different snow-density assumption — could flip it from snowpack to streamflow. For a Mediterranean mountain forest, where snow acts as a delayed reservoir feeding soils and streams well into spring, that fraction represents a substantial exposure to phase uncertainty. As regional warming pushes more winter precipitation events toward the warm edge of the transition zone, the practical difference between a 0.34 °C threshold and a 0 °C assumption becomes a matter of water-supply planning, not academic refinement.

The authors frame their results as a locally validated probability function and a quantified hydrological exposure for snow and forest-hydrology models, and the deliverables are concrete: a fitted logistic curve with confidence bounds, a documented sensitivity of amount-based thresholds to snow-density conventions, and a benchmark against which humidity-based schemes can be judged. The underlying meteorological data from the Pertouli station, operated by the Aristotle University Forest Administration and Management Fund, are freely available, allowing other researchers to replicate the analysis or extend it to neighboring sites. In a region where direct phase observations remain scarce and snow hydrology is chronically under-constrained, a single well-observed mountain station has now produced both a precise threshold and an honest measure of the uncertainty that surrounds it — a combination that modelers of Mediterranean water resources will find hard to ignore.

Subject of Research: Rain–snow partitioning thresholds and transition-zone uncertainty at a Mediterranean mountain forest station

Article Title: Observed rain–snow partitioning at a Mediterranean mountain forest station: temperature thresholds, humidity sensitivity, and transition-zone uncertainty

Article References: Stefanidis, S. P., & Proutsos, N. D. (2026). Observed rain–snow partitioning at a Mediterranean mountain forest station: temperature thresholds, humidity sensitivity, and transition-zone uncertainty. Theoretical and Applied Climatology, 157(11), Article 711. https://doi.org/10.1007/s00704-026-06658-1

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06658-1

Keywords: rain-snow partitioning, precipitation phase, temperature threshold, Mediterranean mountains, snow hydrology, Pertouli, Greece, logistic model, transition zone, relative humidity, wet-bulb temperature, forest hydrology

Freshwater ecosystems rank among the most biodiverse and most threatened environments on Earth, and headwater streams sit at the top of the conservation agenda because of their outsized influence on the biodiversity, functioning, and connectivity of entire river networks. In Mediterranean mountains, where irrigated agriculture and climate change have extensively reshaped flow regimes, these small streams face mounting pressure. Sierra Nevada, a designated Biosphere Reserve and a recognized Mediterranean biodiversity super hotspot, offers a striking case study. There, careo ditches have been documented since the eighth to tenth centuries, diverting snowmelt from headwater reaches and distributing it across upper slopes to artificially recharge aquifers in weathered fractured rock, extending water availability for lowland farms and towns through the dry season.

A research team led by scientists from the University of Almeria, the Andalusian Centre for Global Change, and the Geological and Mining Institute of Spain examined five first- and second-order headwater streams on the southern face of the park, in the Alpujarra region, at altitudes between 1800 and 2000 meters. In each stream, a small, rudimentary weir built of loose rocks and gravel diverts water into a careo ditch. The researchers compared upstream reference reaches with downstream flow-impacted reaches, sampling benthic macroinvertebrates in spring during snowmelt and in autumn after the prolonged dry period, while continuously monitoring discharge and water temperature from November 2022 onward.

The magnitude of diversion was severe. Proportional flow reduction downstream of the weirs most frequently ranged from 76 to 98 percent of upstream discharge, with mean reductions exceeding 0.85 for much of the year in the Cáñar, Mecina, and Bérchules streams. In absolute terms, downstream flows often fell below 10 liters per second. Yet despite these extreme withdrawals, the impacted reaches never dried. They remained perennial throughout the year, maintaining the longitudinal hydrological connectivity that ecologists consider critical for stream life. The team attributes this persistence to two concurrent processes: seepage returning from the leaky weirs and unlined or poorly sealed ditches, and groundwater effluents from the region’s weathered hard-rock aquifers.

The groundwater signal was unmistakable in the temperature records. Downstream reaches showed significantly higher winter temperatures despite their lower discharge, along with lower summer maxima and reduced spring daily fluctuations, a thermal fingerprint of groundwater-fed baseflow. Water chemistry, by contrast, remained remarkably stable across reach types, with only slightly lower oxygen and higher salinity downstream, differences that were not statistically significant. Total dissolved nitrogen was actually higher upstream, suggesting stronger surface runoff influence at reference sites. This chemical stability matters, because it means that any ecological differences between reaches could be traced primarily to physical habitat alteration, such as reduced flow diversity and increased sedimentation, rather than to water quality degradation.

The biological consequences were nuanced. Macroinvertebrate density was substantially reduced downstream, particularly in spring, when it was roughly 50 percent lower than at upstream sites, a pattern consistent across all functional feeding groups and supported by medium effect sizes. The prolonged duration of the reduction, which persisted for at least a full year, contracted the wetted channel width by 63 to 85 percent, slashing total benthic abundance per reach length to about one-tenth of reference values. The authors warn that this decline in benthic production, together with reduced insect drift and emergence, likely creates severe food shortages for trout and riparian insectivorous predators that depend on aquatic insects crossing the land-water interface.

Surprisingly, local alpha diversity and interlocal beta diversity, measured as taxa richness, Shannon diversity, and community turnover, did not differ significantly between upstream and downstream reaches, and community composition showed no detectable separation in ordination analyses. The researchers propose that the maintenance of hydrological connectivity prevented the impacted reaches from degrading into isolated pools, a fate that typically traps invertebrates, intensifies predation, and triggers prey population collapse. Groundwater subsidies may also have mitigated local diversity losses, since moderate upwelling is known to enhance macroinvertebrate abundance and richness in alpine streams by moderating the harsh abiotic stress imposed by meltwater.

However, the buffering was incomplete, and the damage emerged at larger scales. Regional gamma diversity, estimated from rarefaction curves, was significantly lower in the sets of downstream reaches in both seasons, indicating that even small, non-significant losses of local richness can accumulate into substantial regional declines. Sensitive components of the community also eroded: the taxonomic richness of collector-gatherers dropped significantly in spring, and several biotic indices based on the pollution- and disturbance-sensitive insect orders Ephemeroptera, Plecoptera, and Trichoptera, the EPT taxa, declined significantly at impacted sites. Notably, the IBMWP index, the regulatory standard for Iberian river biomonitoring, classified both reach types as being of good quality and failed to distinguish them, suggesting that current monitoring frameworks may be blind to the early functional erosion caused by flow diversion unless they incorporate EPT-driven metrics.

The study’s most consequential implication concerns the global push toward irrigation efficiency. Modern water policy has favored concrete-lined canals and sealed infrastructure that minimize seepage, yet the Sierra Nevada findings suggest that these very inefficiencies, the leaky weirs and porous ditches of the traditional system, are essential to sustaining downstream flow and biodiversity, particularly in reaches that lack groundwater inflow. The authors invoke the well-documented irrigation efficiency paradox, in which saving water at the infrastructure scale can paradoxically intensify overall water consumption and ecological harm, and they argue that reversing the efficiency-first trend is urgent to halt river degradation in biodiversity-rich mountain regions.

Ultimately, the researchers argue that preserving as many stream reaches as possible under natural or ecologically compatible flow regimes is mandatory in biodiversity hotspots, because moderate local losses compound into regional extirpation. As climate change intensifies aridity across the Mediterranean and agricultural water demands climb, the centuries-old careo systems, increasingly framed as nature-based solutions for aquifer recharge, offer a rare example of human water infrastructure coexisting with, and even underwriting, freshwater conservation. The lesson from Sierra Nevada is that sometimes the most valuable feature of an irrigation system is not how efficiently it moves water, but how much it lets slip away.

Subject of Research: Ecological effects of traditional irrigation water diversion on benthic macroinvertebrate biodiversity in Sierra Nevada headwater streams

Article Title: Leakage from Traditional Irrigation Systems and Groundwater Upwelling Buffer the Impact of Heavy Streamflow Diversion on Benthic Biodiversity

Article References: Casas, J. J., Fenoy, E., Rubio-Ríos, J., Villegas, J., Salinas-Bonillo, M. J., Zakaluk, T., Martos-Rosillo, S., & Cabello, J. (2026). Leakage from Traditional Irrigation Systems and Groundwater Upwelling Buffer the Impact of Heavy Streamflow Diversion on Benthic Biodiversity. Water Resources Management, 40(11), Article 517. https://doi.org/10.1007/s11269-026-04861-3

Image Credits: AI Generated

DOI: 10.1007/s11269-026-04861-3

Keywords: benthic macroinvertebrates, headwater streams, irrigation ditches, groundwater upwelling, hydrological connectivity, water abstraction, Sierra Nevada, EPT taxa, biotic indices, environmental flows, Mediterranean mountains, biodiversity conservation