نوع مقاله : پژوهشی
نویسندگان
1 عضوء هیات علمی مرکز تحقیقات و آموزش کشاورزی و منابع طبیعی استان زنجان
2 2 دانشیار بخش تحقیقات حفاظت خاک و آبخیزداری، مرکز تحقیقات و آموزش کشاورزی و منابع طبیعی استان آذربایجان شرقی، سازمان تحقیقات، آموزش
3 3 استادیار بخش تحقیقات حفاظت خاک و آبخیزداری، مرکز تحقیقات و آموزش کشاورزی و منابع طبیعی استان زنجان، سازمان تحقیقات، آموزش و ترویج
4 محقق بخش تحقیقات حفاظت خاک و آبخیزداری، مرکز تحقیقات و آموزش کشاورزی و منابع طبیعی استان زنجان، سازمان تحقیقات، آموزش و ترویج کشاورزی،
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Introduction and Goal
Climate change, as one of the most critical environmental challenges of the present century, exerts wide-ranging impacts on hydrological and geomorphological processes, including soil erosion and sediment production. According to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2021), the global mean surface temperature rose by approximately 0.99°C during the period 2001–2020 relative to the baseline period of 1850–1900. This global warming is profoundly altering precipitation patterns and hydrological regimes of watersheds, particularly in semi-arid regions such as Iran. Soil erosion and sedimentation are among the most direct consequences of these changes, posing serious threats to ecosystem stability, water resources, and agricultural lands. The severity of these threats underscores the necessity of employing accurate modeling approaches in small watersheds to forecast future impacts. In this context, the Chesb watershed in Zanjan Province was selected as a representative catchment within Iran's cold semi-arid zone to evaluate the effects of climate change on erosion and sediment dynamics.
Materials and Methods
Study Area: The Chesb watershed, covering an area of 12,645 hectares, is located in Ijrud County, Zanjan Province, at geographic coordinates between 36°13′ to 36°27′N longitude and 48°1′ to 48°41′E latitude. The watershed has a cold semi-arid climate, with elevations ranging from 1,449 to 2,357 meters above sea level and a mean annual precipitation of 403 mm. The dominant land uses are cropland (52%) and rangeland (43%). The study was structured around three principal and complementary components: Climate Change Detection: In the first phase, climatic variables — including precipitation, minimum temperature, and maximum temperature — were simulated using the CanESM5 global climate model and the Statistical Downscaling Model (SDSM) for three future periods (2020–2049, 2050–2079, and 2080–2100), under two emission scenarios: SSP1-2.6 (optimistic) and SSP5-8.5 (pessimistic), as defined in the IPCC Sixth Assessment Report. Erosion and Sediment Simulation: In the second phase, the process-based physical model WEPP was implemented within the GeoWEPP environment (integrated with ArcGIS) for the baseline period (1979–2014) and validated against observed hydrometric data from the Chesb gauging station. Input data layers included a 30×30 m Digital Elevation Model (DEM), a land use map derived from Sentinel-2 imagery, and soil data from the Zanjan Province Natural Resources Authority. Model accuracy was assessed using the combined USBR-FAO method. Future Projections: In the third phase, the GeoWEPP model was re-executed using downscaled future climate data to project changes in erosion and sediment yield relative to the baseline period across all future time horizons.
Results and Discussion
Baseline Period: Simulation results for the baseline period indicated a mean annual sediment discharge of 2,589.2 tonnes per year at the watershed outlet, with a specific sediment yield of 0.2 t/ha/year — equivalent to 18.64 t/km²/year when accounting for a 15% bedload contribution. The Sediment Delivery Ratio (SDR) was estimated at 0.249. These results reflected an overestimation of 8.98% relative to observed data, attributed primarily to the model's limitations in representing dynamic vegetation cover. Projected Climate Changes: Analysis of future climate data projected a reduction in mean annual precipitation of 10–20% under SSP5-8.5 and up to 20% during the mid-century period under SSP1-2.6. This decline is concentrated predominantly in the spring months (March through June). Additionally, maximum and minimum temperatures are projected to increase by up to 7°C under the SSP5-8.5 scenario. Future Erosion and Sediment Trends under SSP5-8.5: Under this pessimistic scenario, mean annual sediment discharge is projected to decline from 2,589.2 t/year in the baseline to 1,845.8 t/year during 2020–2049 (a 15% reduction), 648.1 t/year during 2050–2079 (a 20% reduction), and 18.3 t/year during 2080–2100 (a 25% reduction). Future Erosion and Sediment Trends under SSP1-2.6: Under the optimistic scenario, sediment discharge is projected to initially increase to 3,811 t/year in the first future period (2020–2049), before declining markedly to 48 t/year and 35.5 t/year in the subsequent periods — reflecting an overall reduction of 10–15%. The anomalous increase in the first period is likely associated with a relative rise in winter precipitation projected under this scenario.
Conclusion and Suggestions
The findings of this study demonstrate that sediment reduction across both scenarios is driven primarily by declining precipitation. Nevertheless, rising temperatures introduce complex long-term dynamics: intensified evapotranspiration, reduced soil moisture retention, and progressive degradation of vegetation cover — all of which can increase soil susceptibility to erosion. Consequently, a quantitative decline in sediment yield does not necessarily indicate an improvement in ecological conditions. Comparison with regional and global studies — including Eekhout and De Vente (2020) in Spain and Borrelli et al. (2022) at the global scale — confirms that the trend of declining sediment in semi-arid regions under SSP scenarios is consistent with broader findings in the literature. The estimated SDR of 0.249 is also in agreement with values reported by Zhang et al. (2019) in China (0.2–0.3), further affirming the reliability of the WEPP model as a physically-based simulation tool. Overall, future watershed management demands an integrated approach that addresses not only the reduction in sediment production, but also the collateral risks associated with diminishing water resources, vegetation degradation, and increasing soil aridity. It is recommended that watershed conservation policies be aligned with national greenhouse gas emission reduction strategies and broader climate change adaptation frameworks.
کلیدواژهها [English]
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