Characterization of macropore transport studied with the ARS root zone water quality model

The ARS Root Zone Water Quality Model components dealing with preferential water and chemical transport are presented and used to study macropore flow and transport in a silty clay loam soil. Macroporosity of the soil was assumed to be 0.05% by volume, half of which was continuous and the rest discontinuous. Two rainfall sequences with two initial soil water contents, evaporation versus transpiration, macropore radius ranging from 1.0 to 0.125 mm, and three different chemicals were evaluated. Over a five-week period, weekly rainfall of 25.4 mm in one hour, with soil water redistribution and evaporation or transpiration occurring between storms, generated no macropore flow when the soil was initially dry (-1500 kPa). A slight amount of macropore flow was generated 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 boundary conditions. Chemicals transported with this flow were 0.05 to 8% of the surface-applied amount, depending on conditions and type of chemical. A moderately adsorbed chemical (Atrazine) was the most susceptible to macropore transport, followed in order by a strongly adsorbed chemical (Prometryn), and a mobile chemical (Nitrate). The flow entering the macropores was partially absorbed by soil at progressively deeper depths; it increased the water content of the root zone, and created a tail of low concentrations in the soil chemical content distributions. 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, and downward chemical movement, but increased chemical content of the macropore flow. Transpiration, on the other hand, decreased both macropore flow and its chemical content. Thus, this modeling study gives very useful insights into the macropore flow behavior that are very difficult to obtain experimentally, and which will be useful in characterizing macropore flow in the field.