Land use patterns exert significant impacts on river water quality variations. However, controversy has long surrounded the optimal spatial scale and river grade at which land use patterns most effectively reflect water quality variability. Identifying how different land use patterns influence river water quality across spatial scales and river grades is therefore crucial for optimizing land use and implementing effective water quality management strategies. In this study, we employed redundancy analysis, random forest modeling, and nonparametric change-point analysis to quantify the combined effects of spatial scale and river grade on land use-water quality relationships, and to identify the key land use type affecting river nutrients in the large-scale Yellow River Basin. The results indicated that the sub-basin scale land use patterns exerted a greater influence on water quality variations in the Yellow River mainstream (YR) during both dry and wet seasons, while the riparian buffer scale land use patterns had stronger effects in the first-order tributaries (FT) during the wet season and in the second-order tributaries (ST) during the dry season. The Yellow River Basin was primarily affected by nitrogen pollution, with farmland identified as the key land use type influencing total nitrogen (TN) concentration. At the sub-basin scale, maintaining farmland proportions below 27.4 % effectively reduced TN concentrations in the main river, with respective thresholds of 33.3 % and 54.5 % for the FT and ST tributaries. At the riparian buffer scale, thresholds of 34.6 %, 53.5 %, and 70.9 %, and at the circular buffer scale, thresholds of 40.6 %, 63.2 %, and 82.8 % for the mainstream, FT, and ST tributaries respectively, significantly reduced TN concentrations throughout the river system. This study provides new insights into the scale effects of land use on water quality and offers valuable implications for water quality management and land use planning in the Yellow River Basin. • Land use patterns affect water quality with significant scale-dependent and hierarchical differences. • The YRB is mainly affected by TN pollution, with farmland as the key contributing land use type. • Farmland proportion thresholds rise with smaller spatial scales and higher river grades. • Water quality management should consider both spatial scales and river grades.
Zang et al. (Fri,) studied this question.