Spatiotemporal variations in stream chemistry and suspended sediment levels

The present study aimed to analyze to what extent nearby agricultural and semi-natural land use types might affect stream turbidity and chemistry over time. A four-year-long (2021-2024) data on water turbidity and chemistry were measured at different points of the small stream, while soil water content (SWC), and soil temperature were measured at a nearby cropland site with crop rotation.

We analyzed water samples collected daily from the same collection point, and bi-weekly to monthly along the stream from the spring to the outlet (7 to 10 measurement points), whenever water flow was present. We measured stream water turbidity (FNU), total dissolved inorganic nitrogen (as NO₃+NO₂ and NH₄; TDIN) content, water pH, and specific conductance (SPS) using a ProDSS YSI Instrument. Total nitrogen (N_tot) and total phosphorus (P_tot) concentrations were measured using a Nanocolor VIS-II spectrophotometer (Macherey-Nagel) in 2024. Meteorological data was collected from the catchment outlet, while several rain gauges (ECRN-100, Decagon Devices) were also placed at different parts of the small catchment. SWC measurements were collected using 5TM sensors (Decagon Devices) at 10-minute intervals at 15 cm depth.

During the first three years, our results showed a weak correlation between FNU and precipitation (r=0.16, p=0.21), due to high FNU values from low water levels. This mainly occurred during drought conditions. Weak negative connections were observed between SPS and FNU values (r=-0.17, p=0.18), showing that high precipitation lowers water conductivities. Our results showed that the SPC values were inversely proportional to the FNU values.

Based on the 2024 data (n=266) we noted that P_tot levels varied among sites; however, not significantly (p>0.05). N_tot contents were the highest at the site with fresh spring water entering the stream and the lowest at the outlet (p>0.05). We found the strongest correlation to water FNU with orthophosphate (r=0.92; p=0.0003), while a strong correlation between stream discharge and orthophosphate (r=0.65; p=0.005) or total phosphorus concentrations (r=0.63; p=0.006) were also noted. Also, precedent soil moisture content weakly but significantly affected water stream turbidity (r=0.25; p=0.007).

We ran a cluster analysis to determine different levels of rainfall amounts causing significant changes in turbidity values.  Three main clusters were distinguished based on the daily sample data, which divided our dataset into daily precipitation amounts of i) precipitation sums below 4.8 mm, ii) averaging 6.3 mm, and iii) averaging 23.7 mm. The three clusters, especially the extreme events are most significantly separated along precipitation and FNU values.

Acknowledgments: This material is based upon work supported by the Hungarian National Research Fund (OTKA/NKFI) project OTKA FK-131792. The research presented in the article was carried out within the framework of the Széchenyi Plan Plus program with the support of the RRF 2.3.1 21 2022 00008 project.This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 862756 (OPTAIN). The research was funded by the Sustainable Development and Technologies National Programme of the Hungarian Academy of Sciences (FFT NP FTA).