Effects of Macropores on Infiltration and Runoff Generation in Tropical Saprolitic Soils at the Small Catchment Scale

Abstract:

Soil water flow in macropores is subject to different process controls than water flow in the soil matrix. Depending on macropore areal concentration, size distribution and tortuosity, they can lead to an abrupt increase in infiltration. In the saprolitic soils of Central Panama, macropores are ubiquitous and originate from the processes of soil shrinkage upon drying, growth and decay of roots and burrowing animals. Our experiment data show that macropores have great impact on infiltration and runoff production. Existing models based on soil physics take into account preferential flow using either dual continuum, dual porosity or dual permeability assumptions, but are still rooted firmly in the Richards equation. Our research is taking a different numerical approach by extending the improved Talbot-Ogden (T-O) 1-D finite water-content infiltration method, which discretizes the soil not in space but in the water-content domain and provides the opportunity to mathematically describe non-Darcian flows through high Bond number flow paths. The entire watershed is discretized into cells and the water flow processes in each cell are simulated using a modified quasi 1-D extended T-O approach. Provided that the density of the macropores is high enough that they are on-average well in contact with the soil matrix, the macropore in each cell can be treated as a moisture bin embedded in the matrix, dominated by gravity and pressure-driven flow physics. Interactions between flows in macropores and the soil matrix are governed by the conductivity of the macropore wall and the corresponding wetting contact angle. This presentation discusses simulation of macropore flow using non-Darcian flow physics, and the effects of macropore geometry and wall parameters on bulk infiltration and small catchment response. The simulation results are compared against measured discharge and tracer breakthrough.