Simulation and evaluation of water quality purification performance in irrigation district drainage ditch wetlands under the influence of regional external water inputs

Abstract

This study investigates the impact of external water inputs on the water quality purification capacity of drainage ditches in large and medium-sized irrigation districts. Taking the drainage ditch wetlands in the Lubotan saline-alkali land improvement area as a case study, the research constructs and validates the QUAL-2K water quality model based on water quality monitoring data. It examines the longitudinal variation of water quality indicators under different gradients of external water input. The study calculates and assesses the comprehensive influence of upstream water on the purification effects of the drainage ditches. Results indicate that the QUAL-2K model exhibits good consistency and correlation with actual observed data in terms of standardized standard deviation, root mean square error, and correlation coefficient for various water quality indices, making it suitable for water quality management of drainage ditches. The external water volume and concentration significantly affect the water quality and reduction rates in the ditches, with increased water volume or decreased concentration positively enhancing purification. Conversely, the purification effect is reduced. Under current conditions and with increased external water volumes or decreased concentrations, the water quality in the drainage ditches can achieve compliance with discharge standards. However, a reduction in water volume by more than 20 % or an increase in concentration by more than 10 % results in failure to meet these discharge criteria.