Coupled simulations of water flow from a field-investigated glacial till slope using a quasi-two-dimensional water and heat model with bypass flow

Abstract

Substantial field investigations of soil physical properties and stratification in a forested slope (10° slope) covered with glacial till revealed that macropores in the form of old root channels as well as coarse structures in the form of well-sorted layers dominate a very permeable upper solid horizon. Rapid response and quick recessions during snowmelt and heavy rains in 1986 were observed in the runoff from the slope. Based on field tracer experiments it was found that such macropores and macrostructures played an important role on these occasions. In order to verify these findings one-dimensional water and heat models were coupled in a serial manner to simulate the formation of runoff from the slope, using a quasi-two-dimensional approach. Both a strict Darcian concept and a Darcian concept with a simple bypass flow concept introduced were tested. The drainage gadient in the model was made equal with the angle of the slope. Coupled slope simulations, with water retention properties and hydraulic conductivities taken from three different levels on the slope, indicated that the conditions in the lower region of the slope were most important in explaining the discharge rate. With a shallow groundwater table in the lower region of the slope and low hydraulic conductivity of the deeper layers, rapid water flows are routed to the uppermost layers where the conductivity is higher. Most of the flow is well described by Richards' equation, although smaller peaks cannot be represented for small rain events, when the measured runoff and receission showed a more rapid response than that simulated, however, the introduction of a simple bypass flow improved the ability of the model to simulate the observations. Much of the simulated surface runoff generated in the Darcian simulation during the winter of 1986, could be diverted through a frost layer in the humus horizon and in the humus-impregnated mineral horizon with a silty-sand character, down to a more conductive and porous layer. Eighty percent of the water flow in the bypass flow simulations was conducted in the upper 40 cm of the soil with peak intensities in the more porous upper 20 cm where the macroporosity was large.