Representing the macropore flow processes in a watershed hydrological model and evaluating the effect of macropore flow on the flood peaks

Macropore flow in soils, also known as preferential flow in Soils, is an important hydrological process that generally enhances the soil infiltration capacity and velocity of subsurface water. The evolution of macropore is impacted by both of natural processes (such as soil fauna and plant roots) and human activities (such as agricultural planting and landfills).

Up till now, macropore flow is mostly simulated with high-resolution models. One possible drawback of this modeling approach is the difficulty to effectively represent the overall typology and connectivity of the macropore networks. We hypothesize that modeling macropore flow directly at the catchment scale may be complementary to the existing modeling strategy and offer some new insights. This study represents the macropore flow processes in a semi-distributed watershed hydrological model and evaluates the effect of macropore flow on the floods. Tsinghua Representative Elementary Watershed model (THREW model) is a semi-distributed hydrology model, where the fundamental building blocks are representative elementary watersheds (REW) linked by the river channel network. In the framework of THREW model, the methods to describe the macropore infiltration, water exchange with unsaturated soil, and macropore flow generation are proposed, and the constitutive relationships of macropore flow processes are established as part of THREW. THREW model including macropore flow processes is applied to simulate the hourly discharge of 11 years in two river basins of about 2000 km² in USA, which are Blue River and Illinois River in Oklahoma State. The simulated discharges are evaluated by Nash-Sutcliffe efficiency coefficient and Coefficient of determination, and the simulated regime curve of Blue River are raised from October to April by the macropore flow. The flood peaks are enhanced by 26.8% in Blue River and 17.6% in Illinois River. The macropore flow processes improve the high flow part of flow duration curve and enhance the flood peaks comparing with the baseline hydrological model. The simulation results reveal the role of macropore flow on the flood at the river basin scale.