Partitioning of Infiltration into Macropore and Soil-Matrix Flow: Predictive Model Based on Mesoscale Heterogeneity of Infiltrability

Friday, 19 December 2014: 5:00 PM
John R Nimmo, USGS National Research Program, Menlo Park, CA, United States
A condition that initiates macropore flow at the land surface is the application of water faster than it can infiltrate into the soil matrix material. Sometimes this is taken to require ponding, but accumulated evidence shows preferential flow to be commonplace when wetness is less than saturation and when macropores are not completely filled. Examples include water flowing into shrinkage cracks or funneled into macropores by hydrophobic surface material. A more inclusive criterion is that macropore flow is generated when the water application rate exceeds the infiltrability of a small area associated with a macropore.

A new model based on this criterion considers the representative elementary area (REA), as would be appropriate for measurement of field-scale infiltrability, to be divided into a mosaic of functional sub-areas (FSA). A single value of matrix infiltrability characterizes each FSA. The REA as a mosaic of FSAs is hydraulically represented by a characteristic distribution of infiltrabilities. During rainfall or irrigation, each FSA absorbs water into its soil matrix material up to the rate of its matrix infiltrability. Water applied in excess of this infiltrability is assumed to flow into a macropore within or adjacent to the FSA, becoming preferential flow. Especially if crusted or hydrophobic, an FSA can generate preferential flow even during low-intensity rainfall when other FSAs are absorbing all incident water into the matrix. The total flux of preferential flow at given depth is the sum of contributions from all FSAs. In this way the characteristic distribution of FSA infiltrabilities controls the field-scale partitioning of matrix and macropore flow as an emergent phenomenon.

Illustrative case studies use field-measured data concerning water application rate and preferential flux. Results show this model can quantitatively represent observations of preferential flow occurring in relatively dry soils or at modest rainfall intensities.