水文响应单元划分对SWAT模型总氮模拟效果的影响

Effect of hydrologic response unit delineation on the SWAT model results of total nitrogen simulation

为分析水文响应单元(HRU)划定要素的阈值选取对SWAT模型模拟的影响,以东苕溪上游区域为研究对象,对土地利用、土壤类型和坡度三要素设置5%~30%阈值范围,以5%为梯度形成18种方案,以纳什系数(NSE)和均方根差与标准偏差的比值(RSR)为主要评判指标,探究三要素划分阈值对SWAT模型总氮负荷模拟效果的影响。研究结果显示:较为粗放的坡度分级有利于缩小总氮模拟值与实测值的偏差,而土地利用和土壤类型在阈值较小的情况下,总氮模拟的效果较好;各种方案对总氮模拟的结果表明土地利用阈值变化对其影响最大,而耕地面积占比是主要的影响因素,土壤类型阈值的影响相对较小。综合18种方案的总氮模拟结果,土地利用阈值5%、土壤类型阈值5%、坡度分级阈值20%是相对最合适的HRU划分方案。

In the Soil & Water Assessment Tool(SWAT)model, hydrologic response unit(HRU)delineation largely depends on the parameters selected, such as slope, land use, and soil type. Different HRU delineation schemes directly impact the results of the SWAT model simulation. In this study, to achieve a suitable HRU delineation scheme for simulating total nitrogen(TN)export from the East Tiaoxi watershed, 18 HRU schemes in 3 groups with different thresholds of parameters were investigated. Each group included one particular parameter of slope, land use, or soil type that ranged in the threshold from 5% to 30% with the 5% gradient, with the other two parameters remaining at determined thresholds. The Nash–Sutcliffe efficiency(NSE)and the ratio of the root mean square error to the standard deviation(RSR)were used to assess the effect of 18 HRU schemes on SWAT model simulation. Results showed that extensive slope classification was beneficial for reducing the deviation between simulated TN values and observed TN values, whereas the lower thresholds of land use and soil type helped obtain better TN simulation results. The land use threshold had the greatest impact on TN simulation. The optimal HRU scheme was found in the group with 5% land use, 5% soil type, and 20% slope.