Soil Macropore Size Distribution from Water Breakthrough Curves

A new method for characterizing macropore size distribution is presented for a well‐aggregated Oxic Dystropept at La Selva, Costa Rica, known to exhibit macropore flow. Samples were taken as undisturbed cores from a secondary forest and an abandoned pasture. The method is based on hydraulic principles including the Poiseulle equation, and on water breakthrough curves obtained by timing and measuring water flow through the cores from a drained state (simulating field capacity) to a saturated steady‐state flow condition. Application of the method to the La Selva soil (0‐ to 20‐cm depth) yielded macropore radii between 50 and 200 µm, with a peak between 80 and 140 µm. The macropore size distributions thus obtained were coupled to micropore size distributions calculated from moisture release curves to yield a set of complete pore size distributions. The method is simple, and the results are based on dynamic measurements, making them more applicable to flow models.