Assessing sediment yield and streamflow with SWAT model in a small sub-basin of the Cantareira System

ABSTRACT Hydro-sedimentological models might be useful tools for investigating the effectiveness of soil and water conservation practices. However, evaluating the usefulness of such models requires that predictions are tested against observational data and that uncertainty from model parameterization is addressed. Here we aimed to evaluate the capacity of the SWAT model to simulate monthly streamflow and sediment load in the Posses creek catchment (12 km2), Southeast Brazil. The SUFI-2 algorithm from SWAT-CUP was applied for calibration, testing, uncertainty, and sensitivity analysis. The model was calibrated and initially tested using discharge and sediment load data, which were measured at the catchment outlet. Additionally, we used soil loss measurements from erosion plots within the catchment as independent data for model evaluation. Average monthly streamflow simulations obtained satisfactory results, with Nash-Sutcliffe coefficient (NSE) values of 0.75 and 0.51 for the calibration and testing periods, respectively. Sediment load simulations also displayed satisfactory results for calibration (NSE = 0.65) and testing (NSE = 0.52). However, the comparison with independent plot data revealed that SWAT severely overestimated hillslope erosion rates and compensated it with high sediment channel deposition. Moreover, the model was not sensitive to the parameters used for calculating hillslope sediment yields. Therefore, it should be used with caution for evaluating the interactions between land use, soil erosion, and sediment delivery. We found that the commonly used outlet-based approach for model calibration and testing can lead to internal misrepresentations, and models can reproduce the right answer for the wrong reasons.

摘要 水文泥沙模型（Hydro-sedimentological models）或许是探究水土保持措施有效性的实用工具。然而，评估此类模型的应用价值，需将模型预测结果与观测数据进行验证比对，并处理模型参数化过程中产生的不确定性。本研究旨在评估SWAT模型（Soil and Water Assessment Tool）在巴西东南部波塞斯溪流域（面积12 km²）中模拟月径流量与输沙量的能力。研究采用SWAT-CUP工具中的SUFI-2算法开展率定、验证、不确定性及敏感性分析。模型以流域出口处实测的径流量与输沙量数据完成率定与初步验证。此外，本研究还使用流域内部侵蚀小区测得的土壤流失量作为独立数据，用于模型的额外评估。月径流量模拟结果表现良好，率定期与验证期的纳什-舒特克利夫系数（Nash-Sutcliffe coefficient, NSE）分别为0.75与0.51。输沙量模拟在率定期（NSE=0.65）与验证期（NSE=0.52）同样取得了令人满意的结果。但与独立侵蚀小区数据对比后发现，SWAT模型严重高估了坡面侵蚀速率，并通过高估河道泥沙沉积量抵消了该偏差。此外，该模型对用于计算坡面产沙量的参数并不敏感。因此，在评估土地利用、土壤侵蚀与泥沙输移之间的相互作用时，需谨慎使用该模型。本研究表明，常规采用的基于流域出口的模型率定与验证方法，可能会导致模型内部过程失真，甚至出现模型因错误机理却得到正确预测结果的情况。