Ulansuhai, the largest shallow lake of the Yellow River of China, is an important component of the Hetao region irrigation system. Many concerns have concentrated on its water quality, which affects the local water security and sustainable economic development. In this study, the water pollution index (WPI), an effective water quality evaluation method, was used to compare the pollution levels among pollution indicators and to determine the major pollution indicators. The regime shift index (RSI) approach was employed to identify the water quality trends. Cluster analysis and Daniel trend test methods were employed to analyse the inner-annual and inter-annual spatio-temporal trends of the typical water quality indicators (e.g., total nitrogen (TN), total phosphorus (TP), dissolved oxygen (DO), and chemical oxygen demand (COD)) in Lake Ulansuhai. The results show that the water quality of Ulansuhai improved from 1998 to 2017; spatial variations in the WPI_TN, WPI_TP, and WPI_DO followed the order of inlet > centre and outlet, while spatial variations in the WPI_COD showed the order of outlet > inlet > centre. TN was the critical pollution indicator throughout the year. In 2017, the dry season and wet season were determined using cluster analysis. The WPI_COD was higher than the WPI_TN, WPI_TP, and WPI_DO in the dry season, while the WPI_TN, WPI_TP, and WPI_DO were higher than the WPI_COD in the wet season. WPI was grouped into three clusters: highly polluted regions, moderately polluted regions, and less polluted regions, However, there is a discrepancy between the three polluted regions that were divided into the dry season and the wet season. The WPI_COD was highest among all pollution indicators in 2017. Major sources of pollution that contribute to the deterioration of water quality include inner-annual or inter-annual pollution, agricultural non-point pollution, point source pollution, and internal pollution. This study provides useful information for authorities to effectively manage water quality and control water pollution.

Lake Ulansuhai, one of the main water sources for semi-arid areas of China, has a local deposit caused by soil erosion during past decades. However, a lack of monitor stations prevents better estimation of soil erosion levels. Therefore, we try to estimate soil erosion in the Huangtuyaozi (HTYZ) watershed, an ungauged small watershed of the lake’s eastern watershed, by using the revised universal soil loss equation (RUSLE) model and multi-source remote sensing data, and analyze its key drivers and effect on the lake siltation. The result showed that the soil erosion rate in the HTYZ watershed ranged from 0 to 129.893 t ha⁻¹ yr⁻¹ with an average of 6.45 t ha⁻¹ yr⁻¹ during 1986–2015. In particular, 80.06% of the area was less than 10 t ha⁻¹ yr⁻¹, and just 0.06% was over 50 t ha⁻¹ yr⁻¹, mainly in the mountain area, the southern part of the HTYZ watershed. Moreover, rainfall erosivity factor is the key factor, and rainfall during flood season plays a key role in soil erosion. Due to the soil erosion of HTYZ, siltation in Lake Ulansuhai reached 223.83 ha, with the annual siltation area increasing at a rate of 7.46 ha/yr. The results could provide a reference for estimating soil erosion of ungauged small watershed in semi-arid areas.