OTRAS FORMAS DE REPRESENTACIÓN

model builds on the important influence which soil properties have on the translation of rainfall into runoff and, hence, the transfer of pesticides out of the soil profile. The principal hydrological pathways from the soil profile, identified as entry routes for pesticides to water bodies, are: field drains, surface runoff, lateral throughflow and percolation into the unsaturated zone (Foster, 1998; Heppell et al., 2004a; Holvoet et al., 2007; Brown and van Beinum, 2009). The importance of these pathways will vary according to several key drivers

including: climate, soil type, antecedent soil water content, topography, geology, land management, application date and rate, and pesticide physico-chemical properties (Brown et al., 1995; Blanchard and Lerch, 2000; Carter, 2000a; Leu et al., 2004a; Bloomfield et al., 2006b). These drivers are often inter-related and can vary spatially and temporally (Capel et al., 2001; Leu et al., 2004b).

The explicit pathway of pesticide transport through the soil zone during a rainfall event (i.e. via macropores or through the bulk matrix) is not explicitly

represented because it is a complex process and hence difficult to represent using a few readily available parameters at a daily time-step (a requirement of the model developed in this thesis). There is, however, an implicit assumption of the pesticide mass flux bypassing the soil matrix when a rainfall event occurs which is greater than a certain magnitude. Pesticide transport via macropores, where present in the soil, can promote preferential flow to deeper layers or connect to field drains potentially rapidly transporting pesticides to surface water (Harris et al., 1994; Jones et al., 2000; Peterson et al., 2002; Brown and van Beinum, 2009; Tediosi et al., 2012; Tediosi et al., 2013).

Several field studies (e.g. Johnson et al., 1995; Uusitalo et al., 2001; Shipitalo et al., 2004; Tediosi et al., 2012) have suggested that a large proportion of water in field drains originates from the topsoil. In drained clay soils a good

thereby providing a route for rapid pesticide transport from the soil surface layers to field drains (Harris and Catt, 1999; Shipitalo et al., 2004; Tediosi et al., 2012). In a study on an alluvial aquifer in Switzerland, Mermoud and Meiwirth, (2004) found that the first significant rainfall event would quickly transport pesticides through the soil and unsaturated zone to groundwater. The

transportation of pesticides through the unsaturated zone is covered in detail in Section 5.2.

Any displaced pesticide is assumed to be transported, during the current time- step, directly to the surface water catchment outlet and/or to the top of the unsaturated zone for transportation to the water table (Figure 3.11) depending on the water flux boundary conditions at the interface between the soil and the unsaturated zone (Section 3.3). It should be noted that the pathways taken by water and solutes out of the soil profile are not explicit in the model. It is therefore assumed implicitly that pesticide will be transported by an active hydrological pathway: field drains, lateral throughflow, surface runoff or percolation to the top of the unsaturated zone (further explanation of the hydrological pathways mentioned above is provided in Sections 3.2.3 – 3.2.7). During transportation to the catchment outlet or to the top of the water table it is assumed there will be relatively little transformation or sorption. Of course, in reality, pesticide travelling down macropores and in overland flow could be exchanged with the soil matrix and may be subject to degradation en route (as described in more physically-explicit models such as MACRO: Larsbo and Jarvis, 2003). However, the fact that a relatively simple description of displacement, similar to the model described here, can represent observed pesticide concentrations in field drains (Tediosi et al., 2012) suggests that the limiting step in the leaching processes is the process description of

displacement itself (i.e. sorption and degradation along transport pathways are limited), at least where field drains dominate the hydrological response.

Pesticides can be transported in either the dissolved phase or sorbed to eroded soil particles or soil colloids (Liess et al., 1999; Carter, 2000a; Holvoet et al.,

pesticides classified as strongly sorbing, KOC > 1,000 L kg-1 transport of eroding

soil particles is potentially a significant mechanism (Wu et al., 2004), who found that for pesticides with a very high KOC, almost 50 % of the pesticides in the

water samples were particle bound. Transport pathways for pesticide sorbed to soil particles include surface runoff (Syversen and Bechmann, 2004) and field drains (Williams et al., 1996).

In the model any displaced pesticide is assumed to be transported directly to the surface water catchment outlet and/or to the top of the unsaturated zone for transportation to the water table (Figure 3.11) depending on the water flux boundary conditions. The pesticide mass flux to surface water (Jsw; µg m-2 day-1)

is calculated as:

(3.33)

where qquick is the water flux from the hydrologically active pathways that will

reach surface water resources (this is the sum of surface runoff, lateral

throughflow and drain flow; mm day-1) and qtot is the total flux of water displaced

from the soil (this includes the qquick and the water percolating to the top of the

unsaturated zone; mm day-1).

The pesticide mass flux entering the top of the unsaturated zone (Juz; µg m-2

day-1) is calculated as:

(3.34)

Between subsequent rainfall events there is assumed to be no transport of pesticides out of the soil profile and the remaining mass of pesticide is assumed to undergo further sorption, degradation and internal redistribution as described in Sections 3.3.2, 3.3.3 and 3.3.4.