SPATIAL-DISTRIBUTION OF WATER AND CHLORIDE MACROPORE FLOW IN A WELL-STRUCTURED SOIL

Macropore how is recognized as an important component of water and sol ute transport in many soils. Our understanding of its magnitude and si gnificance is not well developed. The objective of this experiment was to measure the effect of application rate on the magnitude and spatia l distribution of macropore how in a Maury silt loam (fine, mixed, mes ic Typic Paleudalf). An undisturbed block of soil (32.5 by 32.5 by 32. 5 cm) was obtained and placed on a specially designed apparatus that a llowed effluent to be collected under - 2.0 kPa vacuum and separated i nto 100 cells. Water tagged with Cl- was applied by 100 needles, 0.25- mm i.d., which were positioned directly above the 100 collection cells . Approximately 7.5 cm of solution was applied at four application rat es: 3.14, 1.97, 1.23, and 0.5 cm h(-1). A large fraction of the water and solute moved through a small percentage of the total soil for all application rates: 50% of the water and Cl- were collected in <20 and 10%, respectively, of the cells. Displacement of antecedent water by a pplied water was much higher for the 0.5 cm h(-1) application rate tha n for the higher three rates. The position and spatial pattern of macr opore flow remained fairly constant for the three highest how rates. A reas of high soil water flow and high Cl- concentrations were clustere d, not randomly distributed across the column. We believe these findin gs support our hypotheses that how takes place in the same network of pores for similar rainfall events and that interpedal voids between te rtiary structural units are important pathways of