Understanding the transport of dissolved organic carbon (DOC) and nitrogen (N) as water flows through headwater basins is important for predicting downstream water quality. With increased recognition of climatic impact on nutrient transport, more studies are needed in headwater basins experiencing a Mediterranean-type climate, such as those of the Sierra Nevada, California. We analyzed water samples collected over 5 years from eight low-order and mixed-conifer watersheds to elucidate the temporal variation of water chemistry and evaluate their responses to prolonged drought and low-intensity forest thinning. We observed higher stream DOC concentrations in October compared to other months within water years prior to drought and thinning, suggesting the importance of antecedent moisture conditions on seasonal C export. In unthinned watersheds, stream DOC concentrations were lower (62%) and DOC aromaticity was higher (68 and 92%, depending on the index used) during drought compared to non-drought years. In thinned watersheds during drought years, stream water had higher DOC concentrations (66–94% in three consecutive years following thinning) and dissolved inorganic N (24%, in the third year following thinning) compared to unthinned watersheds during drought. Additionally, lower stream DOC concentrations were found in watersheds with higher elevations and lower drainage densities in the year with near-average precipitation; however, these correlations were not significant in years with greater or extremely low precipitation. Taken together, our results suggest that stream concentrations of DOC and dissolved N in Mediterranean headwater basins are extremely variable over time due to the high temporal climatic variabilities and periodic management practices.

了解水流过上游水源盆地时溶解的有机碳（DOC）和氮（N）的运输对于预测下游水质很重要。随着人们越来越认识到气候对营养物运输的影响，需要对经历地中海型气候的水源流域（例如加利福尼亚内华达山脉的流域）进行更多的研究。我们分析了从5个低阶和针叶树混合流域5年中收集的水样，以阐明水化学的时间变化，并评估其对长期干旱和低强度森林间伐的响应。与干旱和稀疏之前的水年中的其他月份相比，我们观察到10月份的溪流DOC浓度更高，这表明先前的水分条件对季节性C出口很重要。在未变薄的分水岭，与非干旱年份相比，干旱期间的溪流DOC浓度较低（62％），DOC芳香度较高（68和92％，取决于所使用的指数）。与干旱期间未稀疏的集水区相比，在干旱年稀疏的集水区，溪流水的DOC浓度较高（稀疏后连续三年为66–94％）和溶解的无机氮（稀疏后第三年为24％）。此外，在年降水量接近平均的一年中，高海拔地区和流域密度较低的流域中，DOC浓度较低。但是，在降水量较大或极低的年份，这些相关性并不显着。在一起