Dry Antecedent Conditions Research Articles

Performance of Denitrifying Stormwater Biofilters Under Intermittent Conditions

Denitrifying stormwater biofiltration systems that include submerged zones containing organic carbon sources can alleviate eutrophication by reducing nitrogen loadings from stormwater runoff. The objective of this research was to evaluate performance of denitrifying biofilters containing wood chip medium under varying dynamic loading rates and antecedent dry conditions (ADCs). Results showed that during ADCs, dissolved organic carbon (DOC) concentrations increased in pore water due to dissolution of the wood chip medium. During a storm event, nitrate (NO3−) removal was enhanced due to high pore water DOC concentrations and mixing of NO3− from the influent with pore water; however, as DOC was gradually flushed out of the biofilter, higher effluent NO3− concentrations were observed over time. At lower hydraulic loading rates, effluent NO3− concentrations were low due to sufficient hydraulic residence time for denitrification. For all storm events tested, an overall NO3− mass removal efficiency of 85% was achieved. Sulfate reduction was observed at long ADCs (up to 30 days). Phosphate release was observed when NO3− was absent from the effluent, possibly due to dissolution of iron oxyhydroxides under low redox conditions. Findings from this study are being used to develop a mathematical model of nitrogen removal in denitrifying stormwater biofilters.

Hydrological connectivity inferred from diatom transport through the riparian-stream system

Abstract. Diatoms (Bacillariophyta) are one of the most common and diverse algal groups (ca. 200 000 species, ≈ 10–200 μm, unicellular, eukaryotic). Here we investigate the potential of aerial diatoms (i.e. diatoms nearly exclusively occurring outside water bodies, in wet, moist or temporarily dry places) to infer surface hydrological connectivity between hillslope-riparian-stream (HRS) landscape units during storm runoff events. We present data from the Weierbach catchment (0.45 km2, northwestern Luxembourg) that quantify the relative abundance of aerial diatom species on hillslopes and in riparian zones (i.e. surface soils, litter, bryophytes and vegetation) and within streams (i.e. stream water, epilithon and epipelon). We tested the hypothesis that different diatom species assemblages inhabit specific moisture domains of the catchment (i.e. HRS units) and, consequently, the presence of certain species assemblages in the stream during runoff events offers the potential for recording whether there was hydrological connectivity between these domains or not. We found that a higher percentage of aerial diatom species was present in samples collected from the riparian and hillslope zones than inside the stream. However, diatoms were absent on hillslopes covered by dry litter and the quantities of diatoms (in absolute numbers) were small in the rest of hillslope samples. This limits their use for inferring hillslope-riparian zone connectivity. Our results also showed that aerial diatom abundance in the stream increased systematically during all sampled events (n = 11, 2011–2012) in response to incident precipitation and increasing discharge. This transport of aerial diatoms during events suggested a rapid connectivity between the soil surface and the stream. Diatom transport data were compared to two-component hydrograph separation, and end-member mixing analysis (EMMA) using stream water chemistry and stable isotope data. Hillslope overland flow was insignificant during most sampled events. This research suggests that diatoms were likely sourced exclusively from the riparian zone, since it was not only the largest aerial diatom reservoir, but also since soil water from the riparian zone was a major streamflow source during rainfall events under both wet and dry antecedent conditions. In comparison to other tracer methods, diatoms require taxonomy knowledge and a rather large processing time. However, they can provide unequivocal evidence of hydrological connectivity and potentially be used at larger catchment scales.

Modelling of Spatially Distributed Surface Runoff and Infiltration in the Olifants River Catchment/Water Management Area Using GIS

This study aimed to model the amount of rainfall leaving the Olifants River Catchment area as surface runoff and entering into the subsurface as infiltration (as part of these waters may contribute to the water ingress in the abandoned/closed mines) using the RINSPE model implemented in ArcView GIS 3.3. The runoff and infiltration depths and the total volumes were calculated by the model for 7 scenarios using National Land-Cover 2000 Dataset. Scenarios 1 & 2: used one inch uniform rainfall to predict the expected runoff and infiltration at any location in the catchment for average and dry antecedent moisture conditions (AMCII & AMCI); scenario 3 used annual rainfall and assumed 40 rainfall events in a year for AMCII; scenarios 4 to 6 used annual rainfall and assumed 35, 40 and 46 rainfall events in a year for AMCI, and scenario 7 assumed 40 rainfall events in a year for AMCI and excluded the catchments areas of Letaba and Shingwedzi Rivers. Scenarios 1 and 2 show that runoff and infiltration are respectively 11.36 & 59.83% and 6.16% & 65.03% of the rainfall. Scenario 3 predicted a total infiltration of 6,449.793 million cubic meters or MCM (14.46% of total rainfall); the total runoff predicted is 16,589.1 MCM (37.2% of total rainfall). Scenarios 4, 5, 6 and 7 showed infiltrations of respectively 22.24%, 24.57%, 19.3% and 22.71% of total rainfall whereas the surface runoff predicted are respectively 13,120.85, 14,776.52, 11,310.57 and 10,748.07. MCMs (29.43%, 33.14%, 25.37% and 31.49% of total rainfall). Spatially distributed runoff and infiltration maps will help to understand the amount of rainfall leaving mined areas as polluted runoff and the amount of water infiltrating into the subsurface horizons, which may later contribute to water ingress or appear as part of the acid mine drainage formed in the catchment.

High-frequency monitoring of nitrogen and phosphorus response in three rural catchments to the end of the 2011–2012 drought in England

Abstract. This paper uses high-frequency bankside measurements from three catchments selected as part of the UK government-funded Demonstration Test Catchments (DTC) project. We compare the hydrological and hydrochemical patterns during the water year 2011–2012 from the Wylye tributary of the River Avon with mixed land use, the Blackwater tributary of the River Wensum with arable land use and the Newby Beck tributary of the River Eden with grassland land use. The beginning of the hydrological year was unusually dry and all three catchments were in states of drought. A sudden change to a wet summer occurred in April 2012 when a heavy rainfall event affected all three catchments. The year-long time series and the individual storm responses captured by in situ nutrient measurements of nitrate and phosphorus (total phosphorus and total reactive phosphorus) concentrations at each site reveal different pollutant sources and pathways operating in each catchment. Large storm-induced nutrient transfers of nitrogen and or phosphorus to each stream were recorded at all three sites during the late April rainfall event. Hysteresis loops suggested transport-limited delivery of nitrate in the Blackwater and of total phosphorus in the Wylye and Newby Beck, which was thought to be exacerbated by the dry antecedent conditions prior to the storm. The high rate of nutrient transport in each system highlights the scale of the challenges faced by environmental managers when designing mitigation measures to reduce the flux of nutrients to rivers from diffuse agricultural sources. It also highlights the scale of the challenge in adapting to future extreme weather events under a changing climate.

Effect of antecedent groundwater conditions on the triggering of static liquefaction landslides

Real-time early warning systems for shallow landslides are typically built upon real-time measurements and forecasts of rainfall and empirical correlations between past patterns of rainfall and landslide occurrence. Whereas these relationships describe whether certain combinations of rainfall and preexisting groundwater levels are of elevated risk of landslide triggering, not all combinations leading to landslide events necessarily have the same consequences in terms of landslide mobility (velocity and distal reach of the landslide). In this paper, the technique of geotechnical centrifuge modeling is used to quantitatively evaluate the hypothesis that the mobility of a landslide triggered under elevated antecedent groundwater conditions is higher than scenarios under drier antecedent conditions. Five identical slope models with a shallow depth to bedrock were subjected to different antecedent conditions ranging from zero groundwater flux to nearly saturated conditions prior to rainfall. The results from these scenarios show that higher antecedent groundwater conditions can result in landslides with velocities about three times higher and travel distances about eight times higher than low antecedent conditions due to static liquefaction of the soil at the base of the slope.

Dust storm frequency and impact over Eastern Australia determined by state of Pacific climate system

Dust storms resulting from synoptic-scale mid-latitude frontal systems affect inland, eastern Australia, predominantly through spring and summer but extend to the east coast only rarely when strong, frontal westerly winds crossing the continent are sustained over the coast. Here it is shown that extreme westerly wind dust events are anomalous to the more pervasive southerly winds that have shaped the sand dunes over inland eastern Australia. It is also shown that while antecedent dry conditions are very important, higher SON and DJF dust storm frequencies from 1957 to the mid-1970s occurred due to both anomalously strong, southerly winds existing on the western side of a cyclonic anomaly adjacent to the east Australian coast, which resulted from the state of the Pacific climate system, and an anticyclonic anomaly at the top of the Great Australian Bight. A change in BoM observing practice, after 1973, is unlikely to be the major cause of changes in total dust frequency from the mid-1970s. Rather, extreme rainfall years are more likely to have been a major contributing factor to the large decreases in dust occurrence from 1973 to 1976, in addition to the other La Niña periods of 2000/01 to 2010/11. Synoptic-scale frontal systems in the westerlies that result in the transport of dust remained low in frequency throughout the whole period from 1957 to 2011. However, those dust storms in the westerlies that do reach the east coast, although infrequent, tend to occur during El Niño-dominated years. On the other hand, they occur during both negative and positive phases of the southern annular mode (SAM). This ambiguity with the SAM phase is consistent with the fact that the mid-latitude westerlies and associated frontal systems are usually at their most equatorward position in Australian longitudes in late winter/spring regardless of the SAM phase. This suggests little change is likely in the frequency of westerly induced dust storms in late winter/spring over central/eastern Australia even though the seasonal westerly winds are expected to contract further poleward under increased global warming. However, a complicating factor is that the increasing likelihood of longer dry spells under climate change would imply reduced vegetation for longer periods over the dust-prone source region of Lake Eyre basin, which could worsen the impact of dust storm events. Furthermore, longer dry spells throughout the year would increase the risk of more frequent westerly dust storm events in other seasons when strong, westerly frontal systems can also occur.

Reduced transpiration response to precipitation pulses precedes mortality in a piñon–juniper woodland subject to prolonged drought

Global climate change is predicted to alter the intensity and duration of droughts, but the effects of changing precipitation patterns on vegetation mortality are difficult to predict. Our objective was to determine whether prolonged drought or above-average precipitation altered the capacity to respond to the individual precipitation pulses that drive productivity and survival. We analyzed 5yr of data from a rainfall manipulation experiment in piñon-juniper (Pinus edulis-Juniperus monosperma) woodland using mixed effects models of transpiration response to event size, antecedent soil moisture, and post-event vapor pressure deficit. Replicated treatments included irrigation, drought, ambient control and infrastructure control. Mortality was highest under drought, and the reduced post-pulse transpiration in the droughted trees that died was attributable to treatment effects beyond drier antecedent conditions and reduced event size. In particular, trees that died were nearly unresponsive to antecedent shallow soil moisture, suggesting reduced shallow absorbing root area. Irrigated trees showed an enhanced response to precipitation pulses. Prolonged drought initiates a downward spiral whereby trees are increasingly unable to utilize pulsed soil moisture. Thus, the additive effects of future, more frequent droughts may increase drought-related mortality.

Variaciones del quimismo del arroyo durante las crecidas en una cuenca de encinar montano

Streamwater changes during storm events are investigated in a small evergreen-oak forested watershed in the Montseny mountains (NE Spain). Results from two storms of different intensities that took place under dry antecedent conditions are discussed in terms of ion sources and runoff mechanisms. Dilution effects were found for conductivity, pH, HC03, Ca2+, Na+ and Mg2+ while K+ and N03 usually increased with discharge, sometimes dramatically. Substantial and sustained in creases of S04 - and elin storm runoff were also observed. A principal component analysis helps in summarizing the data.

Defining Spatial Heterogeneity of Hillslope Infiltration Characteristics Using Geostatistics, Error Modeling, and Autocorrelation Analysis

AbstractProcedures that can characterize spatial correlation and interpolate values at unknown locations can be valuable in capturing spatial trends, may increase sampling accuracy, and reduce error. This study sought to compare three common methods of measuring infiltration, the double ring (DR), sprinkler (SPR), and frame (FRM) methods, on a hillslope to characterize the spatial structure of the infiltration measurements. All infiltration method measurements were log normally distributed; therefore, universal kriging estimators were developed in conjunction with semivariogram analysis and an error model to interpolate infiltration rates under wet and dry antecedent soil conditions. Variability in measured infiltration rates were greatest from the DR and SPR methods, coefficients of variation (CV) ranged from 42–53%, whereas the CV of the FRM method was 27%. This, however, is not unexpected, as the DR and SPR methods had highly significant directional trends and captured a much larger range in infiltrati...

Successional stage of biological soil crusts: an accurate indicator of ecohydrological condition

ABSTRACTBiological soil crusts are a key component of many dryland ecosystems. Following disturbance, biological soil crusts will recover in stages. Recently, a simple classification of these stages has been developed, largely on the basis of external features of the crusts, which reflects their level of development (LOD). The classification system has six LOD classes, from low (1) to high (6). To determine whether the LOD of a crust is related to its ecohydrological function, we used rainfall simulation to evaluate differences in infiltration, runoff, and erosion among crusts in the various LODs, across a range of soil depths and with different wetting pre‐treatments. We found large differences between the lowest and highest LODs, with runoff and erosion being greatest from the lowest LOD. Under dry antecedent conditions, about 50% of the water applied ran off the lowest LOD plots, whereas less than 10% ran off the plots of the two highest LODs. Similarly, sediment loss was 400 g m−2 from the lowest LOD and almost zero from the higher LODs. We scaled up the results from these simulations using the Rangeland Hydrology and Erosion Model. Modelling results indicate that erosion increases dramatically as slope length and gradient increase, especially beyond the threshold values of 10 m for slope length and 10% for slope gradient. Our findings confirm that the LOD classification is a quick, easy, nondestructive, and accurate index of hydrological condition and should be incorporated in field and modelling assessments of ecosystem health. Published in 2012. This article is a U.S. Government work and is in the public domain in the USA.

Phosphorus dynamics in an ephemeral wetland ecosystem after re-flooding

To test whether large amounts of phosphorus (P) are accumulated in the floodplain during dry antecedent conditions, and whether they would be released as the predominant source of P in the overlaying floodwater, in accord with the Flood Pulse Concept, we calculated the mass balance of P in an intermittent floodplain wetland after environmental water application. The P mass balance was calculated by combining a wetland water balance model and P releasing dynamics that were estimated from glasshouse sediment inundation experiments. Upon receiving environmental water, which inundated 412 ha of swamps and river red gum (Eucalyptus camaldulensis) woodlands, our results showed that 394.5 kg of P was mobilised in the system (342.0 kg from 368.4 ha of woodland and 52.5 kg from 43.6 ha of swamp and channels). In addition, the mass of P in incoming water was 74.0 kg, giving a peak in situ mass of 468.5 kg P in the water column. The estimation was verified using the water column P calculated from field samples. Our results indicated that the majority of P (84.2%) was internal loading, and the floodplain may be a source of P enrichment for adjacent water bodies or groundwater if floodwater is discharged rapidly. However, our modelling results also suggested that the high concentration of P in the water column was not sustained most probably because soils re-adsorbed the dissolved P. Approximately 110 days for the woodland and 39 days for the swamp were needed to reduce the P levels to 20% of their peak values respectively. Environmental water managers must decide how to manage P-enriched floodwater; retaining floodwater in the floodplain for a long time could reduce the risk of P enrichment downstream receiving water bodies; or, keeping internal P loading in the wetland could cause eutrophication in the system that was initially targeted for restoration. Best management practices, such as staged flooding and flooding timing are discussed.

Responses of trace gases to hydrologic pulses in desert floodplains

Pulsed hydrologic inputs interact with antecedent moisture conditions to shape biogeochemical dynamics in many ecosystems, but the outcomes of these interactions remain difficult to predict. Hydrologic pulses may influence biogeochemical activity through several mechanisms: by providing water as a resource, providing limiting nutrients or substrates that fuel particular biogeochemical pathways, or determining redox conditions. Antecedent moisture conditions may modify the relative importance of each of these potential mechanisms, by influencing accumulation of labile carbon and nutrients, the severity of water limitation to biological processes, and longer‐term effects on abiotic conditions, including redox. We experimentally applied hydrologic pulses of different sizes (1‐cm and 20‐cm events) to soils of desert floodplains and assessed responses of trace gases (CO2, CH4, NO, and N2O) in dry and monsoon seasons to test these mechanisms. Size of the hydrologic pulse strongly interacted with antecedent soil‐moisture conditions to determine emissions of some trace gases. Following dry antecedent conditions, water addition stimulated emissions of CO2, CH4, and NO, but not N2O, and larger experimental pulses resulted in larger fluxes. In the monsoon season, responses to water addition were muted and size of the hydrologic pulse had no effect, except for CH4emission, which increased in response to the 20‐cm event. Seasonal contrasts indicated that antecedent moisture conditions constrain the effects of hydrologic pulses on biogeochemical processes, whereas contrasts among responses of different trace gases demonstrated that mechanisms controlling emissions of particular gases are water limitation (CO2), in situ production of nitrogen substrates (NO), or redox conditions (CH4). Strong and predictable interactive effects of water inputs and antecedent conditions indicate that extended droughts may cause elevated emissions of gaseous C and NO following the return of precipitation, whereas larger floods or longer wet seasons are expected to dampen gaseous fluxes, which may contribute to conserving soil C and nutrients within floodplains.

Subsurface flow processes in sloping cropland of purple soil

Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin. However, it remains difficult to identify and quantify. In this study, in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry (TDR) together with overland flow and subsurface flow using isolated collecting troughs. Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties, topography, rainfall intensity, initial wetness, and tillage. Results showed that subsurface flow was ubiquitous in purple soil profiles due to well-developed macropores, especially in surface soils while frequency of preferential flow occurrence was very low (only 2 cases in plot C) during all 22 rainfall events. Dry antecedent moisture conditions promoted the occurrence of preferential flow. However, consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms. Rainfall intensity, tillage operation, and soil properties were recognized as main controls of subsurface flow in the study area, which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region.

The Fill–Spill Hydrology of Prairie Wetland Complexes during Drought and Deluge

AbstractThe fill–spill of surface depressions (wetlands) results in intermittent surface water connectivity between wetlands in the prairie wetland region of North America. Dynamic connectivity between wetlands results in dynamic contributing areas for runoff. However, the effect of fill–spill and the resultant variable or dynamic basin contributing area has largely been disregarded in the hydrological community.Long‐term field observations recorded at the St. Denis National Wildlife Area, Saskatchewan, allow fill–spill in the basin to be identified and quantified. Along with historical water‐level observations dating back to 1968, recent data collected for the basin include snow surveys, surface water survey and production of a light detection and ranging–derived digital elevation model. Data collection for the basin includes both wet and dry antecedent basin conditions during spring runoff events.A surface water survey at St. Denis in 2006 reveals a disconnected channel network during the spring freshet runoff event. Rather than 100% of the basin contributing runoff to the outlet, which most hydrological models assume, only approximately 39% of the basin contributes to the outlet. Anthropogenic features, such as culverts and roads, were found to influence the extent and spatial distribution of contributing areas in the basin. Historical pond depth records illustrate the effect of antecedent basin conditions on fill–spill and basin contributing area. A large pond at the outlet of the St. Denis basin, which only receives local runoff during dry years when upstream surface storage has not been satisfied, has pond runoff volumes that increase by a factor of 20 or more during wet years when upstream antecedent basin surface storage is satisfied and basin‐wide runoff contributes to the pond. Copyright © 2011 John Wiley & Sons, Ltd.

Dissecting the variable source area concept – Subsurface flow pathways and water mixing processes in a hillslope

Summary This study uses an instrumented (trenched) 0.5 ha hillslope in the southern tier of New York State, USA, to provide new data and insights on how variable source areas and associated flow pathways form and combine to connect rainfall with downstream water flows across a hillslope. Measurements of water fluxes in the trench, upslope water table dynamics, surface and bedrock topography, and isotopic and geochemical tracers have been combined for a four-dimensional (space–time) characterization of subsurface storm flow responses. During events with dry antecedent conditions infiltrating rainwater was found to percolate through a prevailing fragipan layer to deeper soil layers, with much (33–71%) of the total discharge of the hillslope originating from deeper water flow below the fragipan. During storm events with wet antecedent conditions and large rainfall amounts, shallow lateral flow of event and pre-event water above the fragipan occurred and was one magnitude greater than the deeper water flow contribution. Spatial surface and subsurface water quality observations indicate that water from a distance of up to 56 m contributed runoff from the hillslope during storm events. In addition, mobilization of total dissolved phosphorus (TDP) with subsurface flow played a greater role than with overland or near-surface flow. During all events TDP loads were highest in the total discharge during peak flows (8–11.5 kg ha −1 d −1 ), except during the largest storm event, when TDP concentrations were highly diluted. These results have implications for strategies to protect streams and other downstream water recipients from waterborne nutrient and pollutant loading.

Hydrologic response of a small forested swamp complex, Seymour Valley, British Columbia

Wetlands are common in Coastal Western Hemlock forests yet the hydrologic processes that generate runoff from small swamps are not completely understood. Direct field observations, hydrologic, and electrical conductivity data were collected from a gently sloping forested swamp complex from July to November 2009. Swamps occupied depressions between raised mounds (0.1 to 3 m high) and were connected by an ephemeral creek. Runoff was controlled by antecedent moisture conditions and influenced by basin microtopography. Two hydrologic regimes occurred during the study period and different runoff processes dominated each regime. Runoff was generated by subsurface flow during dry antecedent conditions as swamps remained hydrologically disconnected from each other. Runoff was generated by surface outflow from hydrologically connected swamps during wet antecedent conditions as ponded water spilled out of the depressions. The forested swamp complex produced a faster but limited hydrologic response during dry antecedent conditions compared to a slower but greater hydrologic response during wet antecedent conditions. Stormwater runoff and runoff ratios were up to two orders of magnitude higher in wet conditions than during similarly sized events in dry conditions. These factors should be considered when designing monitoring programs or runoff models in forested swamps with significant microtopography. Field surveys and estimates of hydrologic inputs and outputs may be useful in predicting the potential hydrologic connectivity of isolated forested swamps.

Effect of spatial heterogeneity of runoff generation mechanisms on the scaling behavior of event runoff responses in a natural river basin

This paper presents a theoretical investigation of the effects of spatial heterogeneity of runoff generation on the scaling behavior of runoff timing responses. A previous modeling study on the Illinois River Basin in Oklahoma had revealed a systematic spatial trend in the relative dominance of different runoff generation mechanisms, attributable to corresponding systematic trends in landscape properties. Considering the differences in the timing of hillslope responses between the different runoff mechanisms, this paper explores their impacts on the catchment‐scale runoff routing responses, including how they change with spatial scale. For this purpose we utilize a distributed, physically based hydrological model, with a fully hydraulic stream network routing component. The model is used to generate instantaneous response functions (IRF) for nested catchments of a range of sizes along the river network and quantitative measures of their shape, e.g., peak and time to peak. In order to separate the effects of soil heterogeneity from those due to basin geomorphology, the model simulations are carried out for three hypothetical cases that make assumptions regarding landscape properties (uniform, a systematic trend, and heterogeneity plus the trend), repeating these simulations under wet and dry antecedent conditions. The simulations produced expected and also surprising results. The power law relationship between the peak of the IRF and drainage area is shown to be flatter under wet conditions than under dry conditions, even though the (faster) saturation excess mechanism is more dominant under wet conditions. This result appears to be caused by partial area runoff generation: under wet conditions, the fraction of saturation area is about 30%, while under dry conditions it is less than 10% for the same input of rainfall. This means travel times associated with overland flow (which mostly contributes to the peak and time to peak) are, in fact, longer during wet conditions than during dry conditions. The power law relationship between peak and drainage area also exhibits a scaling break at around 1000 km2, which can be shown to be related to the peculiar geomorphology of the catchment.

Effect of antecedent soil moisture on preferential flow in a texture-contrast soil

The effect of soil moisture status on preferential flow in a texture-contrast soil was investigated by applying 25 mm Brilliant Blue dye tracer to soil profiles at high and low antecedent soil moisture. Differences in soil morphology and chemistry between soil profiles had little effect on the depth of dye infiltration and dye distribution down the profile. Antecedent soil moisture strongly influenced the type, depth and rate of dye tracer movement. In the wet treatment, the dye tracer infiltrated to depths between 0.24 and 0.40 m, at an average rate of 120 mm h−1. Whilst in the dry treatment, the same volume of dye tracer infiltrated to between 0.85 and 1.19 m depth at an average rate of 1160 mm h−1. In dry antecedent conditions, finger flow developed in the A1 horizon as a result of water repellency. In the wet treatment, the wetting front developed permutations but did not break into fingers. Despite similar particle size distributions, flow in the A2e was slower than the A1 horizon, due to the absence of macropores. In the dry treatment, the dye tracer ponded on the upper surface of the B21 horizon, which then spilled down the sides of the large clay columns as rivulets, at rates of between 2000 and 3000 mm h−1. The dye tracer accumulated at the base of the columns resulting in backfilling of the inter column shrinkage cracks, at an estimated rate of 750 mm h−1. In the subsoil, water movement occurred via shrinkage cracks which resulted in flow by-passing 99% of the soil matrix in the B21 horizon and 94% of the soil matrix in the B22 horizon. Evidence of rapid and deep infiltration in ‘dry’ texture-contrast soils has implications for water and solute management. This knowledge could be used to: (i) improve irrigation and fertilizer efficiency (ii) explain variations in crop yield (iii) reduce salinity through improved leaching practices, (iv) reduce the risk of agrochemicals contaminating shallow groundwater.

Source-to-stream connectivity assessment through end-member mixing analysis

Summary Streamflow sources across various hydrologic conditions were examined in a 5.1 ha temperate humid forested catchment (Laurentians, Canada). In that system, the relationship between rainfall and runoff is nonlinear, thus hinting towards complex processes involving critical, transient source areas and a changing catchment internal state of connectivity. Multiyear daily stream chemistry data were broken down into several hydrologic scenarios reflecting different conditions with respect to stream discharge and antecedent catchment wetness. End-member mixing analysis and mass balance calculations were performed to: (1) compare the dimensionality of the mixing spaces (i.e. the number of streamflow sources) obtained under 64 different hydrologic scenarios; (2) screen independently sampled end-members (i.e. the nature of sources) to assess catchment connectivity from a spatial perspective; and (3) estimate the relative contributions of end-members to streamflow to characterize hydrological connectivity from a volumetric standpoint. Mixing space dimensionality did not vary significantly among the tested hydrologic scenarios, as three end-members were generally required to account for most of the variance in stream geochemistry. Differences were significant in the ability of the tested end-members to fit in mixing spaces; for instance, throughfall and organic soil water end-members better fitted in mixing spaces associated with high rather than low discharges. The relative contributions of end-members to streamflow were highly variable in time. Scenarios involving low discharges and dry antecedent conditions were mostly associated with baseflow, while scenarios involving high discharges and wet antecedent conditions were associated with increased proportions of throughfall and organic soil water from downstream downslope and downstream upslope areas. These results suggest a cautious evaluation of the predictive power of one single mixing space with regards to the nature of streamflow sources across hydrologic conditions.

The Hydrological Effects of Lateral Preferential Flow Paths in a Glaciated Watershed in the Northeastern USA

Despite observational evidence of lateral preferential flow paths in northeastern U.S. watersheds, their effects on the sources of runoff remain unclear. An intense field survey was undertaken during the 2007 growing season to determine the sources of stream runoff from a 2.51 km2 watershed in the Catskill Mountains, New York State. Lateral preferential flow paths are caused by groundwater springs and soil piping in this region. A two‐component hydrograph separation using δ18O showed that event water (rain water) was a significant source of runoff during nine rainfall events (from July to October). With these rainfall events, 14 to 37% of the volume and 18 to 49% of the peak streamflow was attributable to event water. Further, end‐member mixing analysis (EMMA), using δ18O, Si, and dissolved organic carbon (DOC), showed that saturated areas accounted for 2 to 24% of the total volume and 4 to 59% of peak streamflow but that groundwater was the dominant source of runoff volume during all events. Field surveys of saturated areas also suggested that near‐stream areas were insufficient to generate the observed stream chemistry during rainfall events larger than 8 mm. A connection with the hillside saturated areas was therefore required to explain the results of the hydrograph separations, which were corroborated by the timing of the transient (perched) groundwater and overland flow. The hydrometric measurements confirmed that hillside lateral preferential flow paths rapidly transported water to near‐stream saturation areas during runoff events under relatively dry antecedent conditions. A qualitative comparison with conventional techniques for distributing variable saturation areas (VSA) using surface topography and soil transmissivity (i.e., topographic index and soil topographic index), which do not consider the effects of lateral preferential flow paths, demonstrated that typical parameterizations (on the order of <10−1 m) would not have the spatial resolution to represent the measured lateral preferential flow paths (on the order of <10−3 m). Overall, the results suggest that the lateral redistribution of water from hillside areas reduces the influence of surface topography and channel topology on the sources of stream runoff, a finding that is consistent with recent ones from other landscapes where glacial soils have coevolved with the terrestrial hydrology.