Background The Weining irrigated watershed, located at the main confluence of the Yellow River and Qingshuihe river in Ningxia, boasts abundant water resources and unique irrigation conditions. Despite its importance, few studies have been conducted on water and sediment in this region of China. By elucidating the trends, sudden change years, and cycle patterns of water and sediment in the Weining irrigated watershed, we can provide more robust support for regional decisions on soil and water conservation and subsequent irrigation in the basin.

Methods We collected hydrological station data, including average daily runoff and average daily sediment volume, at the confluence of two major river sections in the Weining irrigated watershed. After normalization and other preprocessing, we analyzed water-sediment changes in the Weining irrigated watershed from 1989 to 2019 using distance level analysis, Mann-Kendall (M-K) test, and wavelet variance analysis. Preliminary conclusions regarding the water-sand trend and overall trend direction can be drawn through distance level analysis. M-K test, when combined with the overall trend direction obtained from distance level analysis, can identify potential mutation points (years) of water-sediment. Furthermore, wavelet variance analysis can elucidate the cyclical nature of water-sediment change.

Results 1) The water-sediment variation pattern in the Weining irrigated watershed showed an oscillating trend prior to 2012. However, after 2012, the water and sediment trends diverged. The runoff volume gradually increased and remained above the average value of total runoff, while the sediment volume gradually decreased and remained below the average value of total sediment. Consequently, the overall trends of water and sediment were opposite, with a weak increase in runoff and a significant decrease in sediment. 2) During the 31-year period from 1989 to 2019, different numbers of mutation points for runoff and sediment volume were observed in the Weining irrigated watershed, corresponding to different numbers of mutation years. The M-K test showed that there should be several mutation points of annual runoff in the study area, but all of them did not reach the significance level, thus the actual mutation point was 0 and the mutation year did not exist, while there was one mutation point of annual sediment volume and it reached the significance level, combined with the trend direction of the distance level analysis, it was identified as the actual mutation point and the corresponding mutation year was 2005. 3) The water and sediment cycle pattern during this period also differed. The annual runoff cycle had a nested structure of "large-medium-small", with "large" corresponding to a 15-year cycle, "medium" to a 9-year cycle, and "small" to a 6-year cycle. It can be inferred that the annual runoff followed a similar general pattern in 15-year units, with the second and third main cycles of 9 and 6 years respectively controlling annual runoff under 15 years. The annual sediment volume demonstrated a single cycle pattern of 6 years, with sediment volume varying in 6-year units between 31 years. The number and magnitude of cycles for annual runoff and sediment volume suggested that the water and sediment variation patterns were similar on a short time scale of 6 years. However, when extended to 9 or even 15 years, the trends direction of water and sediment diverged.

Conclusions There are many factors affecting runoff and sediment of basin, such as land use change, key water-control project and so on. Large human activities like water conservancy facilities will greatly alter underlying surface conditions and thus lead to a greater impact on water and sediment than climate change. In this study, results also show that water and sediment in study basin will decrease first and then increase in the next 6 years due to operation of The Shapotou Control Project.