A perceptible degradation in water quality complicates safe water supply for drinking and irrigation purposes. Therefore, this study aims at monitoring water quality changes and effective factors in the Dez River Basin, which are required to manage water resources effectively. To this end, the common influence of flow rate changes on water quality was separated by implementing seasonal Mann–Kendall test on residuals resulting from the LOWESS test. The results show that after adjusting the effects of seasonality and streamflow fluctuations, significant positive trends in most water quality parameters are still observed. It emphasizes the role of other factors controlling river water quality in the basin. Comparison of the trends of modified quality parameter time series (residuals) in different subbasins having natural or mad-made conditions, with or without significant groundwater resources, shows almost the same presented trends in water quality. This supports that, overall, minor changes occurred in land use, groundwater table, and environmental and human factors with no important influences on presented trends in water quality. Our analyses show that overall reduction in precipitation as well as positive trends in temperature and evaporation led to intensified streamflow variations, explaining the main changes in the river water quality of the basin.

水质的明显下降使用于饮用水和灌溉目的的安全水供应复杂化。因此，本研究旨在监测Dez流域的水质变化和有效因素，这是有效管理水资源所必需的。为此，通过对LOWESS试验产生的残留物实施季节性Mann-Kendall试验，分离了流速变化对水质的共同影响。结果表明，在调整季节性和水流波动的影响后，大多数水质参数仍观察到明显的正趋势。它强调了控制流域河流水质的其他因素的作用。比较具有自然或疯狂条件的不同子流域中修改后的质量参数时间序列（残差）的趋势，无论有没有重要的地下水资源，其水质趋势都几乎相同。这表明，总体而言，土地使用，地下水位以及环境和人为因素发生了微小变化，而对当前水质趋势没有重大影响。我们的分析表明，降水的总体减少以及温度和蒸发量的正趋势导致加剧的水流变化，解释了流域河流水质的主要变化。