ACTA UNIVERSITATIS AGRICULTURAE SUECIAE AGRARIA 164 Quantifying Macropore Flow Effects on Nitrate and Pesticide Leaching in a Structured Clay Soil Field experiments and modelling with the MACRO and SOILN models Martin Larsson SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES Quantifying Macropore Flow Effects on Nitrate and Pesticide Leaching in a Structured Clay Soil Field experiments and modelling with the MACRO and SOILN models Martin Larsson Department of Soil Sciences Uppsala Doctoral thesis Swedish University of Agricultural Sciences Uppsala 1999 Acta Universitatis Agriculturae Sueciae Agraria 164 ISSN 1401-6249 ISBN 91-576-5489-1 © 1999 Martin Larsson, Uppsala Tryck: SLU Service/Repro, Uppsala 1999 Abstract Larsson, M. 1999. Quantifying macropore flow effects on nitrate and pesticide leaching in a structured clay soil. Field experiments and modelling with the MACRO and SOILN models. Doctoral thesis. ISSN 1401-6249, ISBN 91-576-5489-1 Rapid non-equilibrium flow of water in soil macropores is one significant, yet poorly understood, process controlling solute transport in soils. To protect surface waters and groundwater from pollution of agro-chemicals, it is important to improve our understanding of the effects of macropore flow on solute leaching. In this study, the effect of macropore flow was quantified for nitrate and pesticides on a structured clay soil in south-west Sweden, using two models, the dual-porosity, dual-permeability model MACRO and the nitrogen turnover model SOILN. This was done by first calibrating the MACRO model against extensive field measurements of tile drain flow, soil moisture - contents, and Br tracer (drainage concentrations and amounts in the soil). Model simulations with MACRO and the coupled models MACRO-SOILN were then performed with and without macropore flow and compared with measurements of solute content in the soil profile and solute concentrations in drain discharge of NO3-N and a weakly- sorbed herbicide, bentazone. Ten-year simulations with 60 hypothetical compounds were also conducted to evaluate the effect of macropore flow on pesticides with widely different sorption and degradation properties. The results showed that macropore flow reduced total leaching of bentazone to tile drains by as much as 50 % for the one-year experimental period. This is because much of the bentazone was stored in the micropores, moving at a ‘reduced’ velocity, ‘protected’ against bypass of water in the macropores. However, the scenario simulations indicated that macropore flow considerably increases leaching for less mobile pesticides, since this is the dominant leaching mechanism. The largest increase, up to five orders of magnitude, was found for moderately to strongly sorbed pesticides with half-lives <10 days. For most pesticides, macropore flow also significantly reduced the influence of compound properties on leaching. For nitrate, the simulated effect of macropore flow was a reduction in leaching by 28 % from June 1990 to July 1995, but due to the influence of climatic conditions, large variations were found between years (from 3 to 45 %). Without taking macropore flow into consideration, the model could not depict the observed leaching pattern of bromide and bentazone. It was also impossible to match all the measured components of the nitrogen mass balance without accounting for macropore flow. Even though there are many uncertainties in the modelling approach, it is concluded that macropore flow models could be useful in the evaluation of alternative soil and crop management practices to minimize adverse impacts of non-point source pollution on water quality. Key words: leaching, pesticide, nitrate, field-scale, macropore, model, MACRO, SOILN. Author’s address: Martin Larsson, Department of Soil Sciences, P.O. Box 7072, SLU, SE-75007 Uppsala, Sweden. E-mail: [email protected] Preface This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I. Larsson, M.H. & Jarvis, N.J. 1999. Evaluation of a dual-porosity model to predict field-scale solute transport in a macroporous soil. Journal of Hydrology, 215:153-171. II. Larsson, M.H. & Jarvis, N.J. 2000. Quantifying interactions between compound properties and macropore flow effects on pesticide leaching. Pest Management Science, 56:133-141. III. Larsson, M.H. & Jarvis, N.J. 1999. A dual-porosity model to quantify macropore flow effects on nitrate leaching. Journal of Environmental Quality, 28:1298-1307. Contents Introduction, 7 Literature review, 7 Macropores, macropore flow and transport, 7 Experimental evidence of macropore flow and transport, 8 Extent of macropore flow, 9 Factors influencing macropore flow effects on leaching, 10 Quantifying macropore flow effects, 13 Field evaluation of macropore flow models, 13 Aims and objectives, 15 Materials and methods, 15 The models, 15 Description of the field experiments, 19 Modelling strategy and parameterization, 21 Results and discussion, 21 Effects of macropore flow on hydrology, 21 Effects of macropore flow on bromide leaching, 22 Effects of macropore flow on pesticide leaching, 22 Effects of macropore flow on nitrate leaching, 23 Uncertainties and limitations in the modelling approach, 24 Conclusions and recommendations, 27 References, 29 Acknowledgements, 34 Introduction Macropore flow is the process by which infiltrating water rapidly moves downwards in structural pore spaces such as shrinkage cracks, worm channels, and root holes and thereby bypasses a part of the soil profile. Macropore flow may dramatically influence the transport of solutes (Thomas & Phillips, 1979) since the buffering capacity of the chemically and biologically reactive topsoil is bypassed. Consequently, the risk of leaching of surface-applied agrochemicals (e.g. pesticides and nitrogen) to surface waters and groundwater may increase. It is therefore of great importance to fully understand the effects of macropore flow on leaching to protect valuable water resources against non-point source pollution by agrochemicals. Only when transport through macropores can be accurately quantified and predicted, can the most efficient countermeasures be taken to minimize leaching from structured soils. Many models have been developed in recent years to quantify and predict non- point source leaching of nitrate and/or pesticides to surface waters and groundwater, and some of these models now include a description of macropore flow (Jarvis, 1998). However, few macropore flow models have been thoroughly tested under natural field-scale conditions. The purpose of this thesis is to (i.) evaluate one widely-used macropore flow model MACRO (Jarvis, 1994; Jarvis & Larsson, 1998), and (ii.) quantify the effects of macropore flow on leaching of pesticides and nitrate by applying the model to measurements made in two comprehensive field-scale experiments in a structured clay soil in south-west Sweden. Literature review The aim of this brief literature review is to (i.) discuss definitions and concepts related to macropores and macropore flow (ii.) highlight some important factors governing solute leaching in macroporous soil, and (iii.) discuss methods to quantify macropore flow effects on solute leaching. Macropores, macropore flow and transport Luxmoore (1981) proposed a division of the soil pore system into three size classes: micro-, meso-, and macropores, based on fixed values of soil water pressure head (or equivalent pore size). Such an approach may be useful for some pragmatic purposes, such as in numerical modelling of non-equilibrium flow and transport processes, but the use of well-defined static terms representing fixed 7 physical limits may be unnecessarily restrictive. Beven (1981), Bouma (1981), and Skopp (1981) all argued that the definition of macropores should instead be based on measurements of transport characteristics or the hydraulic conductivity function rather than fixed physical limits. Based on these ideas, Othmer et al. (1991), Durner (1992), Wilson (1992), and Jarvis (1999) all demonstrated the existence of dual or multiple pore systems in soil by curve-fitting bimodal or multi-modal hydraulic functions to measurements of water retention and/or soil hydraulic conductivity. Many dual- and multi-domain flow and transport models have also been developed in recent years based on these concepts (Jarvis, 1998). Skopp (1981) emphasised the transport processes occurring in the soil and introduced a functional definition of the soil pore system on this basis. He divided the pore space into macropores and matrix pores, where macropores are pores that provide “preferential paths of flow so that mixing and transfer between such pores and remaining pores is limited”, while matrix pores “transmit water and solute at a rate slow enough to result in extensive mixing.. ..of molecules between different pores”. As pointed out by Wilson et al. (1998) and Jarvis (1998), it is important in this respect to distinguish between macropore flow and macropore transport. Macropore flow can be defined in terms of non-equilibrium in hydraulic pressure between pore regions, while macropore transport can be described as non- equilibrium in solute concentrations between pore regions. Consequently, equilibrium water flow (e.g. in saturated soil) can cause non-equilibrium transport. Experimental evidence of macropore flow and transport The first experimental observations of macropore flow were made more than 100 years ago by Schumacher (1864) and Lawes et al. (1882). Nevertheless, as a whole, the research