SWAT模型融雪模块的改进

SWAT模型是一个具有物理基础的分布式水文模型，利用SCS径流曲线数方法计算地表径流，而采用相对简单的度日因子方法计算融雪径流。因此在湿润半湿润、雨量丰富的平原地区应用SWAT模型进行径流模拟时可以得到较好的模拟结果，但是在干旱半干旱、降水稀少，且春汛期间融雪径流是重要补给来源的高寒山区，模拟的融雪径流明显偏小，不能很好的反映这些地区的融雪过程，导致河道径流模拟精度偏低。FASST模型是具有物理机制的陆面过程模型，其采用能量平衡的方法计算融雪径流，能够较好的模拟复杂地形山区流域的融雪径流。本文以黑河山区流域为研究区，将FASST模型集成到SWAT模型，改善SWAT模型融雪径流的计算方法。通过对比SWAT模型集成前后莺落峡出山口的河道月径流、融雪径流和地表径流对河道的贡献等几个方面，表明了集成FASST融雪模块的SWAT模型能更好的反映黑河山区流域的融雪径流过程，从而提高河道径流的整体模拟精度。

Improvement of snowmelt implementation in the SWAT hydrologic model

SWAT (Soil and Water Assessment Tool) is a physically explicit distributed hydrologic model, which could simulate hydrological processes such as surface runoff, snowmelt runoff and infiltration with geographic information system(GIS).The surface runoff component in SWAT is implemented by using the SCS( USDA Soil Conservation Service) curve number and snowmelt is calculated by a relatively simple, empirical degree-day method. Those approaches work well in humid and semi-humid areas where precipitation dominantly controls runoff. In cold and arid regions, for example, in a case study in the Heihe river basin (HRB) of northwest China, however, it revealed those implementations cannot represent the effects of snowmelt in springs and thus impact surface runoff simulations. At large, snowmelt in such areas is underestimated and more discrepancies are consequently introduced to the overall simulation accuracy. The FASST model is a surface process model with explicit physical base, including a snowmelt runoff component that makes use of mass and energy balance equations. There is a snowmelt implementation in FASST, which takes topography, vegetation, soil type, and snow physical properties into account. Existing applications of FASST show good simulations of snowmelt in terrain-complex mountainous watershed and its applicability is also confirmed by an application to the Tangula site of northwest China located in a similar cold and alpine area. This paper proposes a coupling approach to improve the simulation of snowmelt by integrating FASST snowmelt to SWAT. In this approach, when the snowmelt begins to be calculated in SWAT, it will call FASST snowmelt to calculate, return its value to the SWAT corresponding variable and continue remainder SWAT logics. The technical implementation is presented in detail. An application to the upper mountainous HRB is set up to test its performance. There are abundant snow falls in Upper HRB in winters and snowmelt is the primary water source to river in springs that cannot be simulated well by original SWAT. The improvements in comparison to the original were examined from three aspects, namely, streamflow, snowmelt runoff and surface runoff contribution to streamflow. Both snowmelt and surface runoff estimates with the coupled model were increased so that streamflow estimate was more close to that observed. An improvement of monthly streamflow estimation by 0.11 in Nash-Sutcliffe coefficient (NSE) can be achieved. Examination of surface runoff contribution to streamflow also supports its feasibility in estimating snowmelt by the coupled approach. Moreover, by using optimized parameter sets, the monthly streamflow simulation accuracy in the validation period of 2000-2009 can be up to 0.83 in NSE. The results confirm the applicability of the SWAT-FASST coupled approach in cold and alpine watersheds where snowmelt should be taken into account and suggest its significance in improving the simulations in such areas.