The_preferential_flow_of_soil_A_widespread_phenomenon_in_pedological_perspectives1.docx

ISSN 1064-2293, Eurasian Soil Science, 2016, Vol. 49, No. 6, pp. 661672. © Pleiades Publishing, Ltd., 2016. SOIL PHYSICS The Preferential Flow of Soil: A Widespread Phenomenon in Pedological Perspectives1 Yinghu Zhanga, Mingxiang Zhanga, Jianzhi Niub, and Haijin Zhengc aSchool of Nature Conservation, Beijing Forestry University, Beijing, 100083, China bSchool of Soil and Water Conservation, Beijing Forestry University, Beijing, 100083, China cJiangxi Provincial Institute of Soil and Water Conservation, Nanchang, 330029, China e-mail: zhangfan2012oo163.com Received April 28, 2015 AbstractThe article provides an overview of studies about the preferential f low phenomenon. This phenom- enon is one of the types of the transportation of water solution through the soil profile by preferential channels (pathways) with a relatively high speed and with a slight change in the chemical composition of the solution. Interest in this phenomenon has risen sharply in the last two decades due to the observed fast transportation of contaminants from soil surface into groundwater level. On the basis of the literature data, the authors give the definition of this phenomenon, consider its types, degree, features, mechanisms, methods and models and research perspectives, in particular the interaction between preferential f low and soil matrix f low. The article considers the aspects of the movement of soil water carrying heavy metals and pesticides; hence, it concerns the protection of environment and peoples health. It provides the thorough review of the studies on the preferential f low, and describes the research directions and their development. Keywords: preferential f low, soil matrix, soil water movement, solute transportation, water infiltration, mac- ropore f low, runoff DOI: 10.1134/S1064229316060120 INTRODUCTION In the 1980s, residents lived in the eastern part of Long Island, United States of America, unexpectedly found their water wells were polluted by pesticide called Aldicarb and could not understand the reason why the contaminant transported so fast from soil upper layers to groundwater level. Soil water move- ment and solute transportation evaluated by preferen- tial f low pathways are hot spots both in biological and ecological aspects from pedological perspectives 14. Solution including heavy metal, radionuclides, pesti- cides, and contaminants could transport from soil sur- face to subsurface zone even groundwater level by preferential f low pathways 46. This phenomenon is important for understanding and estimating of bio- geocenotic and global ecological functions of soil, such as the transformation of substances and energy entering the soil, sorption of substances and water by soil, formation of chemical composition of groundwa- ter, and the regulation of water balance of landscapes. In general, the preferential f low of soil research enlightened by Schumacher in 1864 originated from macropores phenomenon during water infiltration. Some studies found that soil water and solute trans- 1 The article is published in the original. ported through soil layers with high f low rate bypass- ing soil matrix. The result was consistent with Lawes et al. 63. Afterwards, no close attention had been paid to preferential f low until people stated that water infiltration was positively correlated with soil cracks, root channels and fissures. Free et al. 37 also reported that macroporosity had a pivotal effect on water infiltration rate. Bouma and Dekker 13 stated that dye stain coverage could be used to examine the number, size and shape of conducting pores to charac- terize water movement characteristics of soils. Studies on preferential f low were not of great interest gradually until Beven and Germann 9 proposed a kinematic wave model. Helling and Gish 47 characterized the role of root channels, cracks, fissures in soil water movement and solute transportation, and they regarded those continuous pores as the preferential f low pathways. Beven and Germann 9, Bundt et al. 14, Hagedorn and Bundt 45 showed that the pref- erential f low pathways in forest soils persisted for decades. Franklin et al. 36 demonstrated that the mechanism of the preferential f low pathways was ambiguous though lots of studies had been done for macropore f low. Etana et al. 31 concluded that per- sistence of subsoil compaction might enhance prefer- ential f low and that width of f lowpath was more related to dye coverage, and also stated that smaller 661 662 YINGHU ZHANG et al. EURASIAN SOIL SCIENCE Vol. 49 No. 6 2016 numbers and width of f lowpath in the upper subsoil (3040 cm) caused smaller dye coverage because of water movement and solute transportation through small pores space of the compaction treatment. While, larger numbers and width of f lowpath beneath subsoil (50 cm) were pronounced for compaction treatment. In general, homogeneous and heterogeneous f lows are two different f low patterns as water infiltration and rainfall events occur, and heterogeneous f low is also regarded as preferential f low. Petersen et al. 76 stated that preferential f low was a prevailing phenomenon in soils, and earthworm channels were the dominating preferential f low pathways in the subsoil below 50 cm. Banaszuk et al. 8 confirmed the impacts of preferen- tial f low pathways during snowmelt period on trans- portation of contaminants in agricultural catchments. To date, a large number of studies have reported the relationship between contaminants or heavy metal and preferential f low 60. Though there have been an increasing number of studies on the mechanisms and calculating methods of preferential f low 26, 81, 72, promising methods and models that could character- ize the inf luence of preferential f low at the larger catchment scales are extremely rare. The article deals with the phenomenon of preferen- tial flow and water solution transportation in soils from a pedological perspective. A number of studies demonstrate preferential flow can potentially pose seri- ous problems to the quality of groundwater and surface water bodies. Other studies are included to explain the mechanisms leading to preferential flow in soils, and experimental methods are listed and some models are mentioned. At the end, the authors provide an overview of the research perspectives that have been identified from the collected literature. Topics dealing with prefer- ential flow and water quality are of general importance because there are still many unresolved problems. It may indeed be a valuable idea to provide a special review of the progress in the soil-related preferential flow research for the applied water science community, and to explain phenomena, observations, processes, models, and parameters, and to identify what is still missing or unresolved to improve predictions of unin- tended groundwater contamination in various soil land- scapes, for land use systems, and depending on chang- ing climatic conditions. PREFERENTIAL FLOW OF SOIL: DEFINITION, CONTROLLING FACTORS, TYPES, FEATURES, DEGREE, MECHANISMS, SCALES, METHODOLOGY AND MODELS Definition. Preferential f low could be recognized as encompassing phenomena where water movement and solute transportation by preferential f low path- ways, these phenomena occur at a wide range of tem- poral and spatial scales, while quickly bypassing the soil matrix and reaching groundwater level. For long, people treat water as a media. Water containing pollut- ing substances is not in contact with the upper layers of soil. The preferential f low of soil is considered as a combination of phenomena in which water partici- pates as the carrier of polluting substances. Therefore, preferential f low of soil is a dominant parameter in water infiltration, contaminant transportation, as well as ecohydrology, because it typically generates high speed, high volume f low with minimal exposure to solid earth materials. Publications on the preferential f low of soil increase in the last two decades. On the topic of preferential f low now really indicates growing number of published works (Fig. 1). As is shown from Fig. 1, preferential f low studies are of interest increas- ingly. In particular, citations of soil preferential f low increase with high speed. Controlling factors. Many controlling factors play pivotal roles in the preferential f low, such as soil bulk density 78; soil moisture content 82; soil water repellency 53; specificity of soil as a nature body 84; scale changes of soil functions 84; plant roots systems 90; rock fragments 51, 55; soil cracks 100; shrink-swell phenomena 84; solutes hydro- philicity/hydrophobicity 84; freezing/thawing, wet/dry seasonal recycling; cultivation patterns; irri- gation and rainfall events; vegetation types and aspect 16, 17; climate changes 18, 19; fire 11, 12, 20, 42, 49, 66; biota 25; land use 22, 65, 69, 99. Gjetter- mann et al. 41 reported the effects of irrigation on the preferential f low and demonstrated additional con- trolling factors containing soil tillage and earthworm channels. Wang et al. 97 confirmed the impacts of air entrapment, surface saturation, soil layering and water repellency on the preferential f low. Clothier et al. 26 stated the effects of soil surface phenomena, soil pro- file heterogeneities, plant roots systems on the prefer- ential f low generation. Kramers et al. 61 investigated the effects of soil type, irrigation and antecedent soil moisture content on the preferential f low by dye trac- ing experiments in three grassland soils and found that antecedent soil moisture content had inf luences on water movement between macropores and soil matrix. Among those controlling factors above, a large number of studies pay special attentions to soil mois- ture content, soil structure, plant roots systems, rock fragments, biota, fire and water repellency, land use and climate changes. Soil moisture content. Antecedent soil moisture content had a profound effect on the occurrence of preferential f low 9. Antecedent soil moisture content reduced lateral f low from macropores into the soil matrix and increased the size and number of channels. Compared with wet zone, dry zone may result in higher water infiltration and faster rates during water movement and solute transportation process. Flury et al. 34 reported the effect of antecedent soil moisture content on preferential f low patterns and demon- strated that antecedent soil moisture content had a less inf luence on staining dye coverage but the dye moved THE PREFERENTIAL FLOW OF SOIL 663 EURASIAN SOIL SCIENCE Vol. 49 No. 6 2016 Numbersofpublications ofpreferentialow Numbersofcitations ofpreferentialow 600 20000 550 18000 500 450 400 350 1 300 250 200 150 2 100 50 0 16000 14000 12000 10000 8000 6000 4000 2000 0 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 Years Fig. 1. Papers published on the preferential f low of soil (source from web of science): 1publications of papers topic on soil pref- erential f low; 2citations of soil preferential f low each year. slightly deeper into wet than dry soils. However, researchers are in disagreement on the controversy which antecedent soil moisture content could affect preferential flow. In macroporous soils, higher anteced- ent soil moisture content increases the depth of macro- pore flow penetration because higher antecedent soil moisture content reduces lateral flow into the soil matrix 9, 52. Granovsky et al. 43, Shipitalo and Edwards 87 reported that only preferential flow path- ways contributed more to the flow in dry soils, while soil matrix flow had fewer interactions with preferential flow pathways. Merdun et al. 71 also reported that preferential flow was more evident when soil was ini- tially dry. Soil structures. Soil structures are inf luenced by the presence of soil cracks. Soil cracks are recognized as preferential f low pathways even as they are close to the upper soil layers. The role of soil cracks in the prefer- ential f low might depend on the depth of soil cracks. Meanwhile, the geometry of soil cracks determines the preferential f low patterns. Reports on the relation of soil cracks to preferential f low are of interest gradually. Zhang et al. 103 stated that soil cracks increased soil water infiltration in young paddy fields. Zhang et al. 102 concluded the structure of soil cracks was com- plex and that many irregular branches of soil cracks led to water to downward into deeper soils. Soil pore space. The heterogeneity of soil pore space affects preferential f low of water, heat and solutes 84. Rapid transportation of substances through soil pore space could be characterized as the high input rates of water (exceeding rates of water absorption) to the upper soil surface 83. However, it is unclear that how we should take into account the geometry of soil pore space, because we still do not know which of these soil pores affects water movement and solute transporta- tion 83. Soil pore size and pore continuity also may be vital parameters. It is still well unknown about the disputes caused by soil pore size: which pore size is large enough to facilitate water movement and solute transportation. Gregory 44 confirmed that soil pore size larger than 10 m in diameter could prompt more liquid movement. However, not all macropores could contribute to solute transportation because of macro- pore connectivity and tortuosity. Research shows that only 30% of macropore networks could form macrop- ore f low and preferential f low pathways to accelerate water movement and solute transportation 81. Spatial heterogeneity and anisotropy of soil proper- ties. Soil profile is a heterogeneous body with uneven soil properties, in particular soil bulk density, anteced- ent soil moisture content, soil pore space, soil water retention capacity 84. This phenomenon is cor- related with the specificity of soil structures from tem- poral and spatial scales 84. In general, spatial hetero- geneity and anisotropy of soil properties leads to the variations of soil preferential f low patterns at the soil profiles 60. Water pressure or flow conditions at the upper bound- ary of soils. More attentions have been paid to charac- terize the impacts of water pressure characteristics on soil preferential f low increasingly. For example, water infiltration tends to be stable in the upper soil layers without the inducement of preferential f low if rainfall intensity is low. While if soil has lower water pressure and is so dry that fissures form at the soil profiles, meanwhile rainfall intensity is high, water movement will be characterized by preferential f low because the input rates of water into the upper soil layers exceed the rates of water imbibition capacity from soils 84. Soil granulometric and aggregate composition. Field studies sh