Satellites Reveal Two Opposing Pollution Seasons in Türkiye’s Farm Belt

In the agricultural heart of Turkish Thrace, the rivers do not behave the same way all year round — and a new seven-year satellite study has now shown just how differently they behave. Research published in Environmental Science and Pollution Research has mapped the seasonal rhythms of water quality pressure across the Meriç-Ergene Basin, revealing that organic pollution and sediment pollution follow strikingly opposite calendars. While organic pressures peak in spring and summer, inorganic-sediment pressures surge during autumn and winter, a contrast that could reshape how authorities monitor and protect one of Türkiye’s most intensively farmed river systems.

The study, conducted by Enes Özgenç of Trakya University in Edirne, is notable less for any single measurement than for the scale and consistency of its approach. Rather than relying on scattered field samples, the research built a monthly monitoring framework spanning 2019 to 2025, combining data from the Sentinel-2 and Sentinel-1 satellites with rainfall records from CHIRPS, land surface information from ERA5-Land, land use and land cover maps, terrain and drainage data, and long-term indicators of surface water extent. All of this was processed through an integrated Google Earth Engine, Python and ArcMap workflow, producing a continuous picture of the basin that would be nearly impossible to assemble from boat trips and laboratory bottles alone.

The choice of study area was deliberate. Cropland covered 70.49 percent of the Meriç-Ergene Basin as of 2025, making it a textbook example of an agriculturally dominated watershed. At the same time, the basin’s surface water is limited in extent, its hydroclimate swings sharply with the seasons, and land use pressures are intense — conditions that make conventional water quality assessment difficult and that reward approaches capable of tracking pressures over time rather than snapping isolated optical portraits of the water.

What the framework produced were not direct chemical concentrations. Instead, the study generated relative pressure scores: an organic pressure score, an inorganic-sediment pressure score, and a total water quality pressure score. These scores act as screening indicators, ranking when and where the basin faces the greatest stress from different pollution pathways. The author is explicit that they should not be read as estimates of actual concentrations of nutrients, sediments or other substances — a distinction that matters for how the results are used in practice.

The headline finding is the seasonal split. Organic pressure showed the strongest seasonal contrast of any component, with a seasonal mean range of 0.181 on the study’s relative scale, and reached its highest values during spring and summer conditions. That pattern fits the biology of intensively farmed landscapes: warmer months bring biological activity, algal growth potential and the processing of organic matter in slow-moving, nutrient-rich waters. Inorganic-sediment pressure, by contrast, exhibited the mirror-image pattern, with higher values during autumn and winter — the seasons when rainfall strips exposed soils from fields and carries them into the drainage network.

Sediment pressure also proved to be the most spatially uneven of the three components, displaying the largest spatial range among the pressure scores at 0.410. In other words, the difference between the cleanest and most sediment-stressed parts of the basin was far greater for inorganic-sediment pressure than for organic pressure or the combined total. This spatial patchiness suggests that sediment problems in the Meriç-Ergene Basin are concentrated in specific zones rather than spread evenly across the landscape, which has direct implications for where erosion control and field-edge interventions would deliver the most benefit.

Perhaps the most striking technical result concerns how episodic severe sediment events actually are. High inorganic-sediment pressure score records accounted for only 0.49 percent of all valid monthly observations across the seven-year record, and the maximum persistence of a sediment hotspot — the degree to which a high-pressure location stayed high over time — reached just 0.036. Severe sediment pressure, in this basin, is not a chronic condition but a series of short-lived episodes, most likely tied to intense runoff events that mobilize soil and then subside. A monitoring program that samples monthly or less could easily miss these windows entirely; a satellite-based screening system, by contrast, can flag them as they happen.

Against these sharp seasonal swings, the year-to-year picture was remarkably flat. Interannual fitted changes were small for all three pressure scores, indicating limited basin-scale directional change during 2019 to 2025. The basin’s water quality pressures, in other words, are dominated by the annual cycle rather than by any steady deterioration or improvement over the study period. That finding is a double-edged sword: it suggests no alarming long-term trend in the observed window, but it also means that seasonal management — timing agricultural activities, buffer maintenance and monitoring campaigns to the right months — is where the leverage lies.

Total water quality pressure, the combined measure, was more compressed seasonally than either component alone, with a seasonal mean range of just 0.062. This compression is a natural consequence of averaging two opposing signals: when organic pressure rises in spring and summer, sediment pressure tends to fall, and vice versa in the wetter months. The result is a total pressure curve that looks deceptively stable even though the underlying drivers are swinging in opposite directions. It is a cautionary lesson for anyone relying on a single aggregate index — the aggregate can mask exactly the seasonal dynamics that matter most for management.

The practical promise of the framework lies in its reproducibility and its data economy. Because it runs on freely available satellite products and reanalysis datasets through Google Earth Engine, the same workflow can be applied to other data-limited basins where ground-based monitoring networks are sparse or absent. The author positions the pressure scores as a screening tool for identifying seasonal pressure windows and prioritizing field monitoring — telling water managers when and where to send sampling teams, rather than replacing laboratory measurements altogether. In basins like the Meriç-Ergene, where cropland dominates and surface water is scarce and seasonal, that kind of targeted intelligence could mean the difference between catching a pollution episode and missing it entirely. As satellite archives continue to grow and processing platforms become more accessible, studies of this kind point toward a future in which the seasonal pulse of an entire river basin can be read from orbit, one month at a time.

Subject of Research: Seasonal pathways of organic and inorganic-sediment water quality pressures in the agriculturally dominated Meriç-Ergene Basin, Türkiye, assessed through monthly multisource satellite observations from 2019 to 2025

Article Title: Contrasting seasonal pathways of organic and inorganic-sediment pressures in the Meriç-Ergene Basin, Türkiye: evidence from monthly multisource observations, 2019–2025

Article References: Özgenç, E. (2026). Contrasting seasonal pathways of organic and inorganic-sediment pressures in the Meriç-Ergene Basin, Türkiye: evidence from monthly multisource observations, 2019–2025. Environmental Science and Pollution Research, 33(28), 14546-14573. https://doi.org/10.1007/s11356-026-38212-x

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38212-x

Keywords: water quality, remote sensing, Meriç-Ergene Basin, sediment pressure, organic pollution, Sentinel-2, Google Earth Engine, seasonal variability, agricultural basins, Türkiye, monitoring, land use