Predicting Macropore Flow Generation over Wide Ranges of Rain Intensity and Soil Wetness.

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 in soils when wetness is less than saturation and when macropores are not completely filled. 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 model based on this concept 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). When water from rain, irrigation, or other source reaches the land surface, each FSA absorbs water into its soil matrix material up to the rate of its own infiltrability. (Overland flow and subsurface macropore-matrix interaction are assumed negligible in this simple version of the model.) Water applied in excess of an FSA’s infiltrability is assumed to flow into a nearby macropore and become preferential flow. Especially if crusted or hydrophobic, an FSA can generate significant preferential flow even during low-intensity rainfall when most 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. Two field-scale properties, the effective matrix infiltrability of the REA and the quantitative partitioning of matrix and macropore flow, both emerge from the characteristic distribution of mesoscale (FSA) infiltrabilities. Tests with field-measured data, including water application rate, soil water conditions, and preferential flow evaluated from tile drainage or water table fluctuations, show that this distribution can quantitatively represent observations of preferential flow occurring in relatively dry soils or at modest rainfall intensities.