Herbicide leaching as affected by macropore flow and within‐storm rainfall intensity variation: a RZWQM simulation

Abstract

Within-event variability in rainfall intensity may affect pesticide leaching rates in soil, but most laboratory studies of pesticide leaching use a rainfall simulator operating at constant rainfall intensity, or cover the soil with ponded water. This is especially true in experiments where macropores are present--macroporous soils present experimental complexities enough without the added complexity of variable rainfall intensity. One way to get around this difficulty is to use a suitable pesticide transport model, calibrate it to describe accurately a fixed-intensity experiment, and then explore the affects of within-event rainfall intensity variation on pesticide leaching through macropores. We used the Root Zone Water Quality Model (RZWQM) to investigate the effect of variable rainfall intensity on alachlor and atrazine transport through macropores. Data were used from an experiment in which atrazine and alachlor were surface-applied to 30 x 30 x 30 cm undisturbed blocks of two macroporous silt loam soils from glacial till regions. One hour later the blocks were subjected to 30-mm simulated rain with constant intensity for 0.5 h. Percolate was collected and analyzed from 64 square cells at the base of the blocks. RZWQM was calibrated to describe accurately the atrazine and alachlor leaching data, and then a median Mid-west variable-intensity storm, in which the initial intensity was high, was simulated. The variable-intensity storm more than quadrupled alachlor losses and almost doubled atrazine losses in one soil over the constant-intensity storm of the same total depth. Also rainfall intensity may affect percolate-producing macroporosity and consequently pesticide transport through macropores. For example, under variable rainfall intensity RZWQM predicted the alachlor concentration to be 2.7 microg ml(-1) with an effective macroporosity of 2.2 E(-4) cm(3) cm(-3) and 1.4 microg ml(-1) with an effective macroporosity of 4.6 E(-4) cm(3) cm(-3). Percolate-producing macroporosity and herbicide leaching under different rainfall intensity patterns, however, are not well understood. Clearly, further investigation of rainfall intensity variation on pesticide leaching through macropores is needed.

Corn stover harvest increases herbicide movement to subsurface drains – Root Zone Water Quality Model simulations

“…In our simulations when residue was removed, the number of active macropores ( N mac ) decreased and less of the soil matrix was available to contribute to herbicide sorption, resulting in increased transport through macropores. As discussed by Malone et al , more herbicide can leach with a smaller N mac because water and chemicals transported into macropores are distributed between fewer macropores than with a larger N mac . Therefore, less soil was available for chemical partitioning per mass of chemical for the residue removal plots when N mac was reduced.…”

“…Thus, in order more fully to assess the environmental impact of corn stover harvest, the effects on water movement and the transport of herbicides and their degradates to subsurface drains need to be considered in light of its direct effects on soil crusting, earthworm populations and soil macroporosity. A cost‐effective method to accomplish this objective, which is hard to achieve experimentally because field data are often sparse and difficult to collect, is to use a model that incorporates effects of management, weather and soil properties on the movement of pesticides and their breakdown products, considering degradation, sorption and macropore flow …”

Runoff- and erosion-driven transport of cattle slurry: linking molecular tracers to hydrological processes

“…The dynamics of flow pathway partitioning during storm events and the consequential output of agricultural contaminants has been explored (Delpla et al, 2011;Blanchard and Lerch, 2000;Gao et al, 2004;Zhang et al, 1997;Malone et al, 2004), but there is a lack of experimental data quantifying contaminant export via individual flow pathways due to methodological challenges. In a field context it is challenging to monitor multiple flow pathways without destructive sampling, although new research platforms are now making this type of research more possible (see Peukert et al (2014) for an example).…”

“…Experiments run by Gao et al (2004) showed that at the beginning of a rain event raindrop impact was the main mechanism for liberating solute, followed by diffusion at the latter stages of the storm. In addition, research has also shown that storms which have variable rainfall intensities can increase the transport of solute in surface runoff and via preferential subsurface routes (Zhang et al, 1997;Malone et al, 2004).…”

Technical note: Improving the AWAT filter with interpolation schemes for advanced processing of high resolution data

“…However, in other cases the diurnal course of P and ET must be known, e.g. whether root water uptake shall be simulated with a physically based model (Javaux et al, 2008;Couvreur et al, 2012) or macropore flow due to heavy but short precipitation events be simulated under realistic conditions (Malone et al, 2004;McGrath et al, 2008).…”