Macropore flow in PEARL

Within the framework of the APECOP project, a PEARL version for pesticide and water transport in cracking clay soil has been developed. This version was developed and tested by Rob Hendriks, Aaldrik Tiktak, Jos van Dam, Jos Boesten and Mechteld ter Horst.

The PEARL model describes the fate and transport of pesticides in soil and uses the SWAP model as the submodel for soil water flow. Preferential water flow is the driving force of preferential flow of solutes. The concepts for preferential water flow are described first, and thereafter the concepts for preferential solute flow.

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Preferential soil water flow

SWAP is based on Richard's equation and has been modified to account for macropore flow by introducing an adapted version of the FLOCR model (Hendriks et al., 1999). Two classes of macropore are distinguished with respect to pore continuity. One domain continuous throughout the profile (i.e. the main bypass domain), and one domain represents macropores ending at different depths in the profile, resulting in 'internal catchment' (i.e. the internal catchment domain). The figure shows a conceptual visualisation of these two classes of macropores. As shown in this figure, the volume of macropores in the main bypass domain consists of a network of interconnected macropores (e.g. structural and shrinkage cracks). It is constant with depth up to the depth where the internal catchment domain stops; thereafter the volume of pores in the main bypass domain decreases linearly with depth. The volume of the internal catchment consists of macropores that are not interconnected and that end at different depths. The decline of the number of internal catchment macropores is described by a power law function. Additionally, two types of macropore are included in the model to describe the dynamics of the macropore volume resulting from swelling and shrinking: a permanent static macropore volume independent of the soil moisture status and dynamic shrinkage cracks whose volume depends on the shrinkage characteristic and the current soil moisture content. SWAP simulates the swelling and shrinking dynamics via a simplified procedure: the soil level remains fixed and swelling and shrinking influences only the pore volumes. The figure visualises the permanent/static and the dynamic macropore volumes. Water enters the macropores at the soil surface either as rain falling directly into the macropores or as 'runoff' if the rainfall rate exceeds the infiltration capacity of the matrix. Water flowing into the macropores accumulates at the bottom, while uptake into the matrix takes place only in the saturated part of the macropore.

Details Figure 1 Conceptual model of the soil pore system used in the SWAP module describing preferential water flow in cracking clay soils. The left part is the main bypass domain and the right part is the internal catchment domain. The blue colour represents water stored in the two domains.
Details Figure 2: Dynamic and static macropores in SWAP. The dark area shows the volume of the soil matrix in unswollen status; the light brown area shows the increase of the volume of the soil matrix due to swelling (and thus the volume of dynamic macropores); the white area is the volume of permanent/static macropores

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Preferential pesticide transport

In FOCUS PEARL version 1.1.1 solute flow is based on the convection-dispersion equation. This is also the case in the new model, but transport in the macropores is added. Surface applied pesticides are introduced into the macropores using the mixing-cell concept developed by Steenhuis and Parlange (1980). This concept has also been used in MACRO (Jarvis, 1994). It is assumed that the two classes of macropore (main bypass and internal catchment) each have a uniform pesticide concentration (assuming perfect mixing and ignoring adsorption and transformation). Exchange of pesticide between macropores and the matrix is calculated as the product of the water uptake rate and the pesticide concentration in the corresponding domain (i.e. convective transport only). Thus the solute behaviour in the macropores is described in a strongly simplified way (the only parameter being the thickness of the mixing-cell layer).

Details Figure 3: Conceptual model of the soil pore system used in the PEARL model. The figure shows that one uniform pesticide concentration is used for each domain.
Details Figure 4: Mathematical formulation of the processes in the mixing layer.
Details Figure 5: Mathematical formulation of the processes in the macropores.

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Model testing

The MacroPore version of PEARL was tested against the Andelst dataset. Only bentazone leaching was considered. All parameters (except those describing preferential flow) were taken from the calibrated runs of version 1.1.1. The thickness of the mixing-cell layer and the saturated hydraulic conductivity were calibrated using the measured concentrations in drain-water.

Calculations with initial estimates of the calibration parameters showed a zero concentration of bentazone for the first and most important drain-flow event after about 20 d (see figure 6). To obtain the calculated results shown in this figure, a mixing-cell thickness of 1 mm was used and the saturated hydraulic conductivity of the soil matrix had to be lowered to about 1 cm/d (thus forcing more water to flow into the macropores).

Details Figure 6: Measured and simulated bentazone concentrations in drainwater at the Andelst experimental field. Only calibrated calculations are shown for PEARL v 1.1.1 and the new PEARL version including preferential flow.

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Conclusions

These initial results show that the new version has a large potential for simulating pesticide transport in cracking clay soils. It should be noticed, however, that the results are preliminary. Currently, the model is further tested and improved. It is expected that a version including macropore flow will be made available to the general public in 2009.

This study has further shown that drainflow concentrations cannot be properly simulated without considering preferential flow. The new model version is therefore considered a prerequisite for building Dutch surface water scenarios.


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