Abstract: Nitrate pollution is one of the major environmental problems facing reservoirs. Small and medium-sized reservoirs, which are sources of drinking water, are more sensitive to seasonal variations in pollution from diffuse pollution. To analyze the changes of nitrate sources in a reservoir located in a hilly watershed dominated by agricultural cultivation under different time periods. The Qiaodian Reservoir basin was selected as the study area, and 16 sites were set up to collect water samples in January (freeze-up period), March (ablation period), June (pre-flood period), August (high water period), and November (low water period) in 2023. These samples were analyzed for water quality indicators, major ion compositions, and nitrogen and oxygen isotopes. Different sources of nitrate contamination in the water and their contributions were identified using various methods, including hydrochemistry analysis, the nitrogen and oxygen isotope tracer technique, and the Bayesian stable isotope mixing model (MixSIAR). Hydrochemical analysis showed that the hydrochemistry in the study area was dominated by the HCO₃·SO₄-Ca type, and the dissolution of rock weathering mainly controlled the ionic composition. The ionic sources were enhanced by water-rock interaction during the abundant water period. Trends in TN and NO₃⁻-N concentrations exhibited relative consistency, with NO₃⁻-N emerging as the primary form of dissolved inorganic nitrogen. Hydrometeorological conditions, land use patterns, and anthropogenic activities primarily influenced fluctuations in nitrate concentrations. The study area was predominantly dry land (35.8% of the watershed area) where various crops were grown, livestock farming existed in the villages, and more animal manure was applied to the farmland. Other major land use types were forest land and grassland, which accounted for 29.6% and 28.5% of the watershed area, respectively. Temporally, nitrate concentrations decline in the order of freeze-up period (3.83 mg/L), high water period (3.57 mg/L), ablation period (3.51 mg/L), low water period (2.54mg/L), pre-flood period (1.90mg/L). At the spatial scale, NO₃⁻-N concentrations were more variable in the upper and middle reaches of the watershed, while downstream NO₃⁻-N concentrations were close to those in the reservoir area. The δ¹⁵N-NO₃⁻ mean values of nitrate were 9.61, 9.11, 8.1, 7.18, and 6.04‰ in the pre-flood, ablation, low water, freeze-up, and high water periods, respectively. The δ¹⁸O-NO₃⁻ mean values of nitrate were 9.52, 4.25, 3.74, 3.46, and 1.96‰ in the pre-flood, low water, high water, freeze-up, and ablation periods, respectively. The range of δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values varied obviously in different periods, indicating that the source of nitrate was not single. Various analyses showed that soil nitrogen and manure and sewage emerged as pivotal contributors to nitrate concentration shifts within the reservoir basin. Leveraging the MixSIAR model facilitates a quantitative assessment of the contribution rates of different nitrate sources. Nitrate was mainly derived from soil nitrogen and manure and sewage during the freeze-up, ablation, and low water periods. The proportions of nitrate sources were more consistent between the freeze-up period and the low water period, in which soil nitrogen contributed the highest proportion of nitrate to the watershed, 37% and 36%, respectively. Nitrate in water during the pre-flood period was most affected by atmospheric deposition, accounting for 13%. Soil nitrogen loss was most severe during the high water period, when nitrate originating from soil nitrogen and chemical fertiliser was the most abundant, contributing 41% and 31%, respectively. This research result can provide a scientific basis for preventing and controlling surface pollution in small and medium-sized reservoir watersheds similarly located in hilly agricultural areas.