ABSENCE OF PREFERENTIAL FLOW IN THE PERCOLATING WATERS OF A CONIFEROUS FOREST SOIL

Evidence for the functioning of macropores and the presence of prefere ntial flow in forest soils is equivocal. This is partly because many w orkers use only one diagnostic technique to indicate whether or not ma cropore flow occurs. In this paper three lines of evidence are used to suggest that preferential flow does not occur in the percolating wate rs of a coniferous forest soil under the range of hydrological conditi ons that prevail in the field. To simulate field conditions, realistic rainfall intensities were used in conservative solute transport exper iments on four undisturbed soil columns. A method is described in whic h breakthrough data can be used to calculate the percentage of anteced ent water displaced from a soil column during frontal-type breakthroug h experiments. Calculations based on this method using the experimenta l data show that as little as five percent of the antecedent water was immobile, The simple form of the functional advection-dispersion equa tion, based on a single value for linear velocity and the dispersion c oefficient was fitted to two of the breakthrough curves with reasonabl e accuracy, further suggesting that preferential flow did not occur in the experiments. Finally, soil moisture characteristic curves were de termined for replicate soil samples from the forest soil. The operatio nal water contents of the columns during the breakthrough experiments were compared with the soil moisture characteristics and it was found that pores exerting pressure heads greater than -0.5 kPa did not appea r to contribute to flow through the columns, again suggesting an absen ce of preferential flow. (C) 1997 by John Wiley & Sons, Ltd.