CHARACTERISTICS OF MACROPORE TRANSPORT STUDIED WITH THE ARS ROOT-ZONEWATER-QUALITY MODEL

The ARS Root Zone Water Quality Model components dealing with preferen tial water and chemical transport are presented and used to study macr opore flow and transport in a silty clay loam soil. Macroporosity of t he soil was assumed to be 0.05% by volume, half of which was continuou s and the rest discontinuous. Two rainfall sequences with two initial soil water contents, evaporation versus transpiration, macropore radiu s ranging from 1.0 to 0.125 mm, and three different chemicals were eva luated Over a five-week period, weekly rainfall of 25.4 mm in one hour , with soil water redistribution and evaporation or transpiration occu rring between storms, generated no macropore flow when the soil was in itially dry (-1500 kPa). A slight amount of macropore flow was generat ed under the same rainfall when the soil was initially wet (-33 kPa). Doubling the weekly rainfall amount and intensity generated macropore flow varying between 30 to 50% of rainfall depending on initial and bo undary conditions. Chemicals transported with this flow were 0.05 to 8 % of the surface-applied amount, depending on conditions and type of c hemical. A moderately adsorbed chemical (Atrazine) was the most suscep tible to macropore transport, followed in order by a strongly adsorbed chemical (Prometryn), and a mobile chemical (Nitrate). The flow enter ing the macropores was partially absorbed by soil at progressively dee per depths; it. increased the water content of the root zone, and crea ted a tail of low concentrations in the soil chemical content distribu tions. The macropore size had very little effect on macropore flow and transport, but the smallest size pores retarded the downward chemical movement by wall adsorption a little more than the largest size pores . Surface evaporation decreased macropore flow, soil water contents, a nd downward chemical movement, but increased chemical content of the m acropore flow. Transpiration, on the other hand, decreased both macrop ore flow and its chemical content. Thus, this modeling study gives ver y useful insights into the macropore flow behavior that are very diffi cult to obtain experimentally, and which will be useful in characteriz ing macropore flow in the field.