Reduced Infiltration Rates Research Articles

Effects of forest roads on the hydrological response of a small‐scale mountain watershed in Greece

AbstractThe effects of land use changes on the ecology and hydrology of natural watersheds have long been debated. However, less attention has been given to the hydrological effects of forest roads. Although less studied, several researchers have claimed that streamflow changes related to forest roads can cause a persistent and pervasive effect on hillslope hydrology and the functioning of the channel system. The main potential direct effects of forest roads on natural watersheds hydrologic response are runoff production on roads surfaces due to reduced infiltration rates, interruption of subsurface flow by road cutslopes and rapid transfer of the produced runoff to the stream network through roadside ditches. The aforementioned effects may significantly modify the total volume and timing of the hillslope flow to the stream network. This study uses detailed field data, spatial data, hydro‐meteorological records, as well as numerical simulation to investigate the effects of forest roads on the hydrological response of a small‐scale mountain experimental watershed, which is situated in the east side of Penteli Mountain, Attica, Greece. The results of this study highlight the possible effects of forest roads on the watersheds hydrological response that may significantly influence direct runoff depths and peak flow rates. It is demonstrated that these effects can be very important in permeable watersheds and that more emphasis should be given on the impact of roads on the watersheds hydrological response. Copyright © 2014 John Wiley & Sons, Ltd.

Comparing Two Methods of Determining Infiltration Rates of Permeable Interlocking Concrete Pavers

Adequate infiltration through Permeable Interlocking Concrete Pavements (PICPs) is critical to their hydraulic performance. Detected by monitoring infiltration performance, reduced infiltration rates can indicate that maintenance is required. Measurement of infiltration rates has previously been problematic on PICPs because of a lack of accepted standard methodologies and the practical difficulties in modifying existing testing methodologies. On large sites, standard methodologies necessitate multiple measurements to achieve accuracy. Standard methods also contend with practical issues such as sealing the rings to the surface to prevent lateral water flow. This study examined the performance of two PICP surface infiltration rate measurement methods: a modified double-ring infiltrometer (DRIT), and a specially designed rainfall simulation infiltrometer (RSIT). A positive correlation (R2 = 0.85) of results was found between the two, demonstrating that the RSIT was comparable to the DRIT. The modified DRIT produced surface infiltration results approximately 60% higher than the RSIT results. The RSIT provided lower variation between tests, requiring fewer measurements in large sites whilst still maintaining accuracy, thereby improving testing efficiency. The new RSIT method also eliminates some of the practical difficulties with existing methodologies such as unrealistic pressure heads artificially increasing infiltration rates, and the use of sealant under test measurement infiltration rings.

Measurements of canopy interception and transpiration of openly-grown eastern redcedar in central Okalhoma

Eastern redcedar ( Juniperus virginiana L.) is rapidly encroaching and degrading native prairie and rangeland landscapes in the Great Plains of the U.S. Little is known concerning the impacts of increasing redcedar density and areal coverage on local and regional water budgets through transpiration ( T r ) and canopy interception ( C I ) of precipitation. Limited T r and C I studies have been conducted in dense stands of redcedar but results from these studies may not be applicable to redcedar growing in open environments. Four redcedar trees (two large, two small) were located in central Oklahoma to measure T r . Two limbs (one on the north face and one on the south face) on each of the large trees were instrumented with sapflux sensors to measure T r from August 2010 through mid-July 2012. Limb level T r was scaled to tree level T r using ratios of both leaf and bole areas. Whole tree T r was measured on two small redcedar trees from mid-May 2011 through mid-July 2012. Transpiration of the small redcedars was found to respond quickly to precipitation events, while the large redcedars did not. Redcedar T r was compared to that of native grasses. The large redcedars exhibited higher T r rates than native grasses while the small redcedars transpired at rates closely matching native grasses. Four different redcedars were instrumented to measure C I from October 2009 through mid-July of 2012. Redcedar canopies were found to intercept 100% of precipitation for events ? 2.4 mm. Redcedar canopies reduce annual precipitation received at the surface by about 33%, and as much as 39% in the western portion of the state. Significant canopy interception of precipitation, coupled with T r rates as large as or larger than native grasses and with year-round T r , suggests increases in redcedar density and areal coverage could affect local water resources (e.g. reducing infiltration, runoff, and ground water recharge rates).

Connecting bacterial colonization to physical and biochemical changes in a sand box infiltration experiment

Summary Infiltration through sediments is linked to complex biogeochemical processes occurring at small spatial scales, often leading to a progressive reduction in infiltration rates due to microbial growth and/or mechanical clogging. Unraveling the linkage between microbial dynamics and water infiltration in a heterogeneous medium is of concern in artificial recharge ponds and natural infiltration systems. We present an 84-day laboratory infiltration experiment that aims at studying the temporal variation of selected biogeochemical parameters at different depths along the infiltration path. The experimental setup consists of a 1.2 m high tank packed with a heterogeneous soil and instrumented with arrays of sensors as well as soil and liquid samplers. Results indicate that: (i) microbial processes are responsible for infiltration reduction, enhancing the spatially heterogeneous distribution of infiltration rates with time, (ii) bacteria and extracellular polymeric substances (EPS) are present at all monitored depths, indicating the potential for deep biological clogging, (iii) bacteria functioning and richness exhibit depth zonation after the system reaches a mature state and (iv) the retention curve changes towards highest saturation by the end of the experiment. The increase in water holding capacity is largest at depth, where the presence of EPS is noticeable. The reduction in time of the quantity of water infiltrating along the tank can only be accounted for with a truly interdisciplinary approach involving physical, chemical and biological processes.

Shrub encroachment alters the spatial patterns of infiltration

AbstractEncroachment of open woodlands by shrubs is a global phenomenon associated with marked changes in ecosystem structure and function. We measured sorptivity and steady‐state infiltration at two supply potentials under shrubs and grasses and in their interspaces where shrubs were encroaching into grassland. Steady‐state infiltration (ponded) and sorptivity were greater at the grassland than the shrubland site, and there was substantially greater infiltration under shrubs (48.2 mm h−1) and grasses (50.0 mm h−1) than the corresponding interspaces (17.0 and 32.3 mm h−1for shrubland and grassland, respectively). The difference between grasses and their interspaces was substantially less (1.5 times) than that between shrubs and their interspaces (three times). Shrub encroachment also affected the spatial patterns of infiltration. Although the autocorrelation range for shrublands coincided almost exactly with the average distance between shrub canopies (3.5 m), the range for grasslands was three times greater (1.5 m) than the mean grass canopy, indicating a greater connectivity of infiltration in the grasslands than the shrublands. Our study indicates that encroachment by shrubs does not change infiltration under individual plants. Rather, it reduces the interspace infiltration rates significantly, resulting in lower estimated site‐level infiltration rates in shrublands. Our research suggests therefore that it is the shrubland interspaces that are the likely drivers of reduced infiltration rates when grasslands are encroached, rather than increase in the total cover of shrubsper se. Management strategies that result in greater retention of grass cover and minimize the level of interspace disturbances are likely to result in increased infiltration. Copyright © 2014 John Wiley & Sons, Ltd.

EFEITO DAS ATIVIDADES AGROPASTORIS SOBRE OS ATRIBUTOS FÍSICO-QUÍMICOS DE TRÊS CLASSES DE SOLOS DE UMA BACIA HIDROGRÁFICA NO SEMIÁRIDO BRASILEIRO

The soils of the Brazilian Semiarid region are commonly shallow and stony. Also, due to climatic conditions that alternate between droughts and torrential rains, its development is often incipient. In addition, the use is often at odds with the existing potential. The sum of these factors contributes to the existence of soils with absenceof several attributes that protect it from erosion. The area under consideration here was the Serra Branca river basin, located in western Cariri of Paraíba. The work consisted of an exploratory study of the predominant soil types (Luvisol, Vertisol and Neosoil Regolithic) and the mapping of the area, based on the use of GIS and existing information. The soil characterization aimed the knowledge of some properties related to erosive processes (compaction, particle size, density, infiltration, organic matter and oxygen incorporated). The results showed the absence of clay material, a high percentage of compaction, reduced infiltration rates and low levels of organic matter and organic oxygen incorporated. This shows that not only the climatic conditions therein contribute to land degradation, but also the use and soil management contribute decisively to the occurrence of erosion, affecting the environmental balance of the area.Key words: soil compaction, impacts of use and management on soils, watershed.

Natural soil water repellency in different types of Mediterranean woodlands

Water repellency (WR) is a property of some soils that reduces infiltration rates, enhances runoff generation and increases soil erosion risk. Although wildfires are considered a triggering factor, the characteristics of plant residues and soil properties may contribute to the development of soil WR. Because of its impacts, soil WR must be considered when modeling soil erosion risk and hydrological processes. Although many studies on WR from Mediterranean soils exist, relatively few studies have contributed to the knowledge of the natural baseline of soil WR in wide areas. The objective of this paper is to study the natural background soil WR in Mediterranean soils from south-western Spain under three representative forest types (pines, eucalypts and holm oaks) and its relation with plant cover (trees, shrubs and herbaceous plants) and soil properties. Field sampling was carried out in August 2013 in 15 areas from Huelva (SW Spain) under the studied forest types. Vegetation cover (trees, shrubs and herbaceous plants) was determined using transects at each case. The water drop penetration time test was used for assessing soil WR, and main soil properties were determined (texture, pH, organic C, N, Extractable P, exchangeable base cations – Ca2+, Mg2+, K+ and Na+ – and cation exchange capacity). According to results, soil WR was observed in all areas, increasing according to the sequence soils under holm oaks<eucalypts<pines. The severity of soil WR was always higher under the canopy of trees and usually decreased in bare areas, although bare soils under pine forest showed a proportion of slight to strong water-repellent cases. Severity of WR from soils under eucalypts and holm oaks increased with the presence of shrubs and herbaceous plants, but similar levels were not reached out of the tree-covered areas. Soils under vegetation in pine forests were always water-repellent, and wettability was observed only in some bare areas, suggesting a high potential of pines for induction of WR. Soil pH and the proportion of clay showed negative correlations with soil WR. A negative correlation between WR class and the proportion of exchangeable K+ was found, suggesting that K deficiency for trees and shrubs restricts the input of hydrophobic substances in soil.

A Numerical Study on the Impact of Wind Gust Frequency on Air Exchanges in Buildings with Variable External and Internal Leakages

Wind-driven air infiltration has been recognized among the major reasons for energy loss in buildings, and the impact to energy efficiency under steady conditions has been reported and issued as part of many building codes. The nearly zero-energy building demand makes uncontrolled leakage paths even more undesired and creates the need for further investigation of their behavior under unsteady wind conditions. The present numerical study examines the role of wind gustiness on instantaneous infiltration rates of a low-rise building. For this purpose, two levels of gust frequency Ω have been simulated, expressed as a sinusoidal factor in the wind profile formula. In parallel, a ratio α is employed to represent seven different cases of external leakages distribution, while five scenarios of compartmentalization and internal leakages shows the impact of the latter on the dynamics of building air exchange rates. The results indicate that higher wind gustiness results in higher ACH, marking out gusts as a potential critical factor under unsteady climate conditions. The infiltration rates shown in relation to the leakage distribution ratio α provide arguments for the importance of the detailed detection of external leakages while the comparison of the different internal-volume-scenario highlights the key-role of internal leakages control towards a drastic reduction of infiltration rates.

Bonding implications of heap leach infiltration rates at Nevada mines

One important factor in determining bonding costs for mine reclamation is the steady-state infiltration rate of the heap leach pad(s). The importance of assessing each mine on a case-by-case basis is demonstrated. Long-term infiltration rates were investigated for 19 Nevada heaps: <2% at <1860 m above mean sea level (amsl) and <3% at >1860 m amsl. A common means of reducing steady-state infiltration rate is the application of a vegetative cover. Cover implementation costs can be optimised by mathematical modelling of precipitation flow through the heap. For the proposed Buffalo Valley mine, the post-closure heap equilibrium flux of ∼1 mm/year for a 0.9-, 1.2- or 1.5-m-thick cover was calculated after ∼11, 17, and 23 years, respectively, demonstrating that covers > 0.9 m (adequate for successful revegetation) accrue no incremental reduction in infiltration rate. The infiltration rate for this cover thickness (0.6%) was consistent with heaps at similar elevations.

Mulch application in fruit orchards increases the persistence of soil water repellency during a 15-years period

Application of crop residues to cultivated soils combined with no tillage are management practices used to improve water management, increase soil fertility, crop production and soil erosion control. Conservative practices as mulching and no-tillage increase soil organic matter input in soils and contribute to reduce the soil hydrological response, but also increase soil water repellency. Water repellency is a property of soils that reduces infiltration rates. In this research, we have studied the effect of no-tillage and mulching at different rates (1–4, MR1; 5–8, MR2; and 9–12Mgha−1year−1 wheat straw residues application, MR3) versus conventional tillage with no mulching, during a range of treatment periods (1–15 years) in Southern Spain. Soil water repellency (SWR) and organic matter content were analyzed and rainfall simulations were performed to study the impact of management in the hydrological soil response (time to ponding, Tp; time to runoff, Tr; and runoff rate). Subcritical SWR developed in MR1 soils, and slight SWR was observed in MR2 and MR3 soils after a few years of treatment. Subcritical or slight SWR induced significant changes in Tp and Tr, which increased mainly in MR1 soils, but increased SWR observed in MR2 and MR3 soils reduced the positive impact of organic matter and contributed to accelerate ponding and runoff flow.

1564 Effect of an Intervention on Peripheral Intravenous Catheter Infiltration Rates in the Pediatric Department of King Fahad Armed Forces Hospital

Background and AimsPeripheral intravenous catheterization is the most common invasive procedure among paediatric inpatients. Infiltration and extravasation are common complications, causing pain, discomfort, infection, nerve damage, and tissue necrosis. In...

Soil cultivation for enhanced wastewater infiltration in soil aquifer treatment (SAT)

Summary Soil aquifer treatment is often employed as a tertiary treatment component of reclamation proceeding of wastewater for irrigation in agriculture. Reductions in infiltration rates due to increase in water repellency have been reported to be associated with organic matter (OM) accumulation in the soil (mainly in the top soil layer) as a result of treated wastewater (TWW) infiltration. Our aim was to reduce OM content in soils extensively loaded with TWW. Four model infiltration ponds were built to simulate large infiltration basins: three for TWW infiltration using different application regimes, and the fourth for freshwater (FW) infiltration (control). We examined changes in OM content, hydraulic conductivity (HC) and water repellency in these model ponds as a result of soil plowing. In field experiment, four soil-plowing events were performed. Reduced OM content and water repellency, and increased HC were found in all TWW-applied ponds following each soil plowing. These changes were attributed to OM burial in deeper soil layers elimination of the continuity of the OM based crust, and surface exposure of soil with low OM content. An overall reduction in OM content was found at the end of the experiment in all soil layers as a consequence of plowing. No changes in OM content, water repellency or HC were found in the FW-applied pond as a result of soil plowing.

Pervious concrete with titanium dioxide as a photocatalyst compound for a greener urban road environment

The United States is facing the problem of controlling air pollution from vehicle emissions, especially in growing urban areas. This study investigates the photocatalytic effect of titanium dioxide (TiO2) applied onto pervious concrete pavement to remove some of these pollutants from the air, so that pervious concrete pavement can be installed for two sustainable applications: storm water management and air pollutant removal. The photocatalyst, TiO2, activates with UV radiation to oxidize air pollutants, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). This study compared different methods to apply TiO2 onto the surface of pervious concrete and measured the photocatalytic activity of the concrete, the infiltrating characteristics of the pervious concrete, and its ability to withstand environmental impact. High pollutant reductions were seen with a driveway protector mix, a commercial water-based TiO2 preparation, TiO2 in water, a cement–water slurry with low cement concentration, and the commercial PURETI coating. It was found that nitrogen oxide (NO) was efficiently removed with each of these treatments, while VOCs displayed more variability in removal efficiency. Different coating methods can cause different degree of infiltration rate reduction depending on the specific design of coating materials while none of the application methods decreased the infiltration rates below levels applicable for standard hydrological design. When pervious concrete was compared to traditional concrete, pervious concrete showed higher NO reductions.

Infiltration and runoff losses under fallowing and conservation agriculture practices on contrasting soils, Zimbabwe

Fallowing and conservation agriculture are sustainable farming practices that can be used for soil and water conservation. The objectives of the study were to evaluate the effects of different conservation agriculture practices on rainfall infiltration and soil and water losses across 4 sites, using simulated rainfall. The study was carried out at Domboshawa and the Institute of Agricultural Engineering and Chikwaka smallholder farming areas, 4 sites with different soil types. Conservation agriculture practices evaluated were mulch reaping (MR) and clean reaping (CR) at Domboshawa with 5% clay and the Institute of Agricultural Engineering (IAE) with 50% clay. The study also evaluated runoff losses from fallow plots subjected to no tillage (NT) and conventional tillage (CT) at ICRAF Domboshawa site (20% clay) and fallows subjected to CT in Chikwakasmallholder farming areas (4% clay). Infiltration rates were greater under conservation agriculture practices (&gt;35 mm&#183;h-1) when compared to CT (&lt;27-29 mm&#183;h-1). On fallows infiltration rates ranged from 24-35 mm&#183;h-1 when compared to &lt;15 mm&#183;h-1 in maize under CT. Runoff losses were highest under CT at both Domboshawa and IAE sites, and were 21.5 and 15% respectively, while there was no runoff under MR and CR. At the ICRAF Domboshawa site, runoff ranged between 0-31% in fallows and was 57% in maize under CT. At Chikwaka runoff in CT maize was 58%, while in fallow plots runoff ranged 37-44%. Soil losses ranged from 0.2-0.3 t&#183;ha-1 per rainfall event in maize, while in fallows, soil loss ranged from 0-0.1 t&#183;ha-1. The results showed that CT resulted in reduced infiltration rates, increased soil and water loss when compared to fallowing and conservation agriculture across different range of soils. Conservation agriculture practices and fallowing are potential sustainable cropping practices that reduce soil and water loss and increase water use efficiency.

The impact of subterranean termite activity on water infiltration and topsoil properties in Burkina Faso

ABSTRACTFew quantitative experimental studies have been carried out on the influence of subterranean termite activity on the water infiltration capacity of crusted soils in the semi‐arid Sahelian region. These studies found increased infiltration rates on soils that were affected by foraging galleries of subterranean termites. In this paper, remarkable results are presented from crusted agricultural fields in the Sanmatenga region in Burkina Faso with clear termite activity compared to reference fields without termite activity. Fine‐scale rainfall experiments were carried out and general topsoil (upper 5 cm) characteristics were measured. Infiltration rates were found to be significantly slower on plots affected by subterranean termite activity. These results are contradictory to the findings reported in general literature. On the basis of topsoil property measurements, the reduced infiltration rates cannot be indisputably explained, and we suggest that the main responsible factor for a reduction of infiltration was related to differences in topsoil surface (upper 3 mm) properties. Two hypotheses are formulated in an attempt to explain the reduced water infiltration rates on termite plots. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Pollution profiling and particle size distribution within permeable paving units – with and without a geotextile

PurposeThe aim of this paper is to investigate the sediment and pollution profiling and particle size distribution with depth within permeable paving structures, both with and without a geotextile.Design/methodology/approachTest rigs set up in an earlier pilot study were used for four separate but linked studies. Street dust was applied to rigs C and D to determine the retention of sediments. Heavy metals together with street dust were applied to rig A and B (previously control rigs) to determine the effect of sediment on the pollution removal capabilities of the test rigs. Following the application of sediment, rigs A and B were carefully taken apart to determine pollution profiles and particle size distributions.FindingsThe findings reveal that sediment does have an effect on the metal removal capabilities of permeable paving systems, but there was minimal difference between the rigs with a geotextile and without. Pollution profiling within the test rigs identified that the greatest concentration of metals was in the surface sediment and that both rigs removed similar percentages but the concentrations of metals were distributed differently. It was also found that after between 10 and 15 years of sediment application paving rigs began to “block” causing reduced infiltration rates. Unfortunately it is difficult to explain the variations of particle size distributions found with depth in the paving structure. It had been expected to find progressively higher proportions of finer material with depth. However, the reverse was true, with a higher proportion of fines in the sediment applied to the surface and a progressive increase in the proportion of coarse material with depthPractical implicationsThe conclusions confirm the effects of sediment within a permeable paving structure and indicate the loading at which the infiltration rates become affected. These results may help to determine a maintenance programme, however this would require further research.Originality/valueThe paper provides a comparative study on pollution and sediment profiling within a permeable paving structure both with and without the inclusion of an upper geotextile. It provides valuable insight into the amount of sediment on paving before it becomes blocked. The methodology and results reported in this research could be used for further studies to provide more evidence as to whether the inclusion of an upper geotextile is beneficial.

Transient Soil Surface Sealing and Infiltration Model for Bare Soil under Droplet Impact

The marked reduction in infiltration rate caused by formation of a soil surface seal due to water droplet impact on bare soil is a well known phenomenon but is rarely considered in infiltration models, especially under center-pivot irrigation. Water application rates under center-pivot irrigation commonly exceed the infiltration rate, especially near the end of the system lateral. This can lead to off-site runoff and erosion, but more importantly results in field-scale non-uniform water infiltration that can substantially reduce water use efficiency of these irrigation systems. The objective of this study was to develop a sealing soil infiltration model that considers transient soil seal formation on a 30 min or less time scale and can potentially be applied to center-pivot sprinkler irrigation systems. A sealing soil infiltration model was developed using an explicit finite difference solution scheme with a transient soil seal formation model, which is unique from other studies in that it explicitly uses droplet specific power as the driving factor for formation of a soil surface seal. The form of the transient seal formation model is also unique in that it is expressed as a rational function of specific power rather than an exponential decay function of droplet kinetic energy. The model was applied to published runoff data from two rainfall simulation studies with varying droplet kinetic energies and application rates on three soils. The sealing soil infiltration model represented the measured infiltration rates very well for all rainfall simulator tests. The transient soil seal formation model uses three parameters, one of which is an empirical parameter representing the susceptibility of the soil to aggregate breakdown that was constant for a given soil. A second model parameter, final saturated hydraulic conductivity of the surface seal, was well correlated to droplet specific power for a given soil. Application of the model to center-pivot irrigation will require the development of a model for estimating droplet specific power and application rate profiles from center-pivot sprinklers for a range of sprinkler designs, flow rates, operating pressures, spacings, and heights.

Crust Composition and Disturbance Drive Infiltration Through Biological Soil Crusts in Semiarid Ecosystems

Soil crusts influence many soil parameters that affect how water moves into and through the soil, and therefore, critically influence water availability, erosion processes, nutrient fluxes, and vegetation distribution patterns in semiarid ecosystems. Soil crusts are quite sensitive to disturbance, and their alteration can lead to modification of the local hydrological regime, thus affecting general functioning of the ecosystem. The aim of this study was to analyze the influence of different types of soil crusts, physical, and biological in different developmental stages, as well as the impact of their disturbance, on infiltration. This was assessed by means of rainfall simulations conducted in two semiarid ecosystems in southeast Spain characterized by different lithologies, topographies, and soil crust distributions. Two consecutive rainfall simulation experiments (50 mm h−1 rainfall intensity), the first on dry soil and the second on wet soil, were carried out in microplots (0.25 m2) containing the most representative soil crust types at each site, each crust type subjected to three disturbance treatments: (a) undisturbed, (b) trampling, and (c) removal. Infiltration in the crusts was higher on coarse- than on fine-textured soils and almost two times greater on dry than on wet soil. Biological soil crusts (BSC) showed higher infiltration rates than physical soil crusts (PSC). Within BSC, infiltration increased as cyanobacterial biomass increased and was the highest in moss crusts. However, late-successional crustose and squamulose lichen crusts showed very low infiltration rates. Trampling reduced infiltration rates, especially when soil was wet, whereas crust removal enhanced infiltration. But this increase in infiltration after removing the crust decreased over time as the soil sealed again due to raindrop impact, making runoff rates in the scraped microplots approach those registered in the respective undisturbed crust types. Our results demonstrate that water redistribution in semiarid ecosystems strongly depends on the type of crusts that occupy the interplant spaces and the characteristics of the soils which they overly, as well as the antecedent moisture conditions of the soil. Disturbance of these crust patches results in increased runoff and erosion, which has important consequences on general ecosystem functioning.

Impacts of soil damage by grazing livestock on crop productivity

Soil damage by livestock on cropped land is a concern for mixed crop-livestock producers. The hypothesis that livestock-induced soil damage does not effect subsequent crop productivity was investigated in three ways; a review of existing literature; two experimental studies in southern Australia; and based on these, use of a farming system model to simulate likely consequences of changes to soil conditions on crop productivity under a variety of seasonal conditions. The literature suggests that treading by livestock increases soil strength and bulk density and reduces macro-porosity and infiltration rate. Yet, the effects are confined to the soil surface (0–0.1 m) and are short-lived due to amelioration through natural processes or tillage. Despite the potential for soil damage, few studies report yield penalties in following crops, possibly because effects are too small in magnitude or depth to influence plant growth significantly, or because of the highly season-dependent nature of crop response. Two recent field experiments in southern Australia also found no effect of sheep grazing on subsequent crop growth or yield, despite evidence of surface compaction and reduced infiltration rate. Using reviewed literature and this experimental evidence, we adjusted soil parameters in APSIM (Agricultural Production Systems Simulator) to reflect mild, moderate and severe reductions in surface root growth and surface water infiltration. The predicted sensitivity of wheat crop productivity to these simulated impacts was investigated at 6 locations in eastern Australia. Consistent with experimental evidence, predicted average grain yield was reduced by <10% (+3 to −10%) under mild and moderate scenarios typical of those reported experimentally. Crop yields were reduced by lower rainfall infiltration and fallow efficiency in locations with summer-dominant-rainfall, and by reduced root exploration at locations with winter-dominant-rainfall. Lower residue cover levels amplified reductions in rainfall infiltration, especially reducing the accumulation of soil water during summer fallow. Predicted grain yield was only reduced substantially (up to 43% on average) when severe changes in soil parameters were implemented (e.g. 90% reduction in rate of root exploration in surface layers). Overall, most experiments support our hypothesis that grazing livestock have little effect on subsequent crop yields. Long-term simulations suggest that soil impacts generating large reductions in root growth and infiltration rate are required to reduce subsequent crop yields substantially. Such impacts are unlikely where current best-practice grazing management occurs, but would be possible on structurally degraded soils where surface cover is allowed to fall below critical levels.

Synergistic effects of water repellency and macropore flow on the hydraulic conductivity of a burned forest soil, south‐east Australia

AbstractResearch shows that water repellency is a key hydraulic property that results in reduced infiltration rates in burned soils. However, more work is required in order to link the hydrological behaviour of water repellent soils to observed runoff responses at the plot and hillslope scale. This study used 5 M ethanol and water in disc infiltrometers to quantify the role of macropore flow and water repellency on spatial and temporal infiltration patterns in a burned soil at plot (&lt;10 m2) scale in a wet eucalypt forest in south‐east Australia. In the first summer and winter after wildfire, an average of 70% and 60%, respectively, of the plot area was water repellent and did not contribute to infiltration. Macropores (r &gt; 0·5 mm), comprising just 5·5% of the soil volume, contributed to 70% and 95%, respectively, of the field‐saturated and ponded hydraulic conductivity (Kp). Because flow occurred almost entirely via macropores in non‐repellent areas, this meant that less than 2·5% of the soil surface effectively contributed to infiltration. The hydraulic conductivity increased by a factor of up to 2·5 as the hydraulic head increased from 0 to 5 mm. Due to the synergistic effect of macropore flow and water repellency, the coefficient of variation (CV) in Kp was three times higher in the water‐repellent soil (CV = 175%) than under the simulated non‐repellent conditions (CV = 66%). The high spatial variability in Kp would act to reduce the effective infiltration rate during runoff generation at plot scale. Ponding, which tend to increase with increasing scale, activates flow through macropores and would raise the effective infiltration rates at larger scales. Field experiments designed to provide representative measurements of infiltration after fire in these systems must therefore consider both the inherent variability in hydraulic conductivity and the variability in infiltration caused by interactions between surface runoff and hydraulic conductivity. Copyright © 2010 John Wiley &amp; Sons, Ltd.