Mechanisms responsible for streamflow generation on a small,salt-affected and deeply weathered hillslope

This paper considers the contributions of overland flow, throughflow and deep seepage to the generation of streamflow in a salt-affected, deeply weathered landscape. Runoff mechanisms on a small hillslope in south-western Australia were dependent on the extent and development of variable source areas. In winter, streamflow generation was controlled by returnflow, saturation overland flow and throughflow. In summer, post-ponding, infiltration-excess and saturation overland flow dominated. The extent of the variable source area and the magnitude of streamflow were due to antecedent soil moisture, rainfall and slope morphology. Concave hillslope sections accumulated soil moisture due to both saturated and unsaturated lateral flow processes. Throughflow provided the mechanism and vehicle for solute movement from the groundwater discharge area to the stream. However, discharge from the deep aquifer was the primary mechanism responsible for soil salinity and maintaining the core of the variable source area. Estimates of throughflow which only take account of soil-water movement and disregard returnflow, will underestimate the magnitude of throughflow.     

Physically‐based modelling of double‐peak discharge responses at Slapton Wood catchment

Heavy winter rainfall produces double‐peak hydrographs at the Slapton Wood catchment, Devon, UK. The first peak is saturation‐excess overland flow in the hillslope hollows and the second (i.e. the delayed peak) is subsurface stormflow. The physically‐based spatially‐distributed model SHETRAN is used to try to improve the understanding of the processes that cause the double peaks. A three‐stage (multi‐scale) approach to calibration is used: (1) water balance validation for vertical one‐dimensional flow at arable, grassland and woodland plots; (2) two‐dimensional flow for cross‐sections cutting across the stream valley; and (3) three‐dimensional flow in the full catchment. The main data are for rainfall, stream discharge, evaporation, soil water potential and phreatic surface level. At each scale there was successful comparison with measured responses, using as far as possible parameter values from measurements. There was some calibration but all calibrated values at one scale were used at a larger scale. A large proportion of the subsurface runoff enters the stream from three dry valleys (hillslope hollows), and previous studies have suggested convergence of the water in the three large hollows as being the major mechanism for the production of the delayed peaks. The SHETRAN modelling suggests that the hillslopes that drain directly into the stream are also involved in producing the delayed discharges. The model shows how in the summer most of the catchment is hydraulically disconnected from the stream. In the autumn the catchment eventually ‘wets up’ and shallow subsurface flows are produced, with water deflected laterally along the soil‐bedrock interface producing the delayed peak in the stream hydrograph. Copyright © 2007 John Wiley & Sons, Ltd.     

Characteristics of surface runoff and throughflow in a purple soil of Southwestern China under various rainfall events

In order to harvest runoff to palliate water disaster as well as effectively manage irrigation and fertilizer application in the studied region, it is necessary to better understand the runoff processes. A newly designed runoff collection system for a plot scale was used to partition runoff under contrasting rainfall events into surface flow and subsurface flow to obtain characteristics of surface runoff and throughflow in a purple soil (Regosols in FAO taxonomy, Entisol in USDA taxonomy) of Sichuan, China. Under small rainfall (shower and drizzle), only surface runoff was observed. It is noted that, under shower, particularly with antecedent dry soil conditions, the highest peak surface runoff significantly lagged behind that of rainfall, because air‐locked soil pores of the top layer appeared temporally. Under rainstorm and downpour, surface runoff and throughflow both commenced and showed hysteresis. The hydrograph of surface runoff better resembled that of rainfall than throughflow did. The durations of throughflow discharge of post‐rainfall‐end were near the same (within 24 h) under various rainfalls and rather dependent upon the soil properties than the rainfall characteristics. Throughflow is about 60–90% of total runoff, and especially significant in a ploughed layer under downpour. The chloride concentration of throughflow was over twice that of surface runoff and rainfall, implying that throughflow contains more nutrients than surface runoff. Presumably, surface runoff was primarily governed by an infiltration‐excess or saturated excess‐infiltration mechanism under unsaturated or saturated soil conditions. Therefore, the management of water and fertilizer, and the harvesting of water flow in the ploughed soil layer, should be emphasized in this region. Copyright © 2005 John Wiley & Sons, Ltd.     

Evaluating the hydrological and hydrochemical responses of a High Arctic catchment during an exceptionally warm summer

The Arctic has experienced substantial warming during the past century with models projecting continued warming accompanied by increases in summer precipitation for most regions. A key impact of increasing air surface temperatures is the deepening of the active layer, which is expected to alter hydrological processes and pathways. The aim of this study was to determine how one of the warmest and wettest summers in the past decade at a High Arctic watershed impacted water infiltration and storage in deeply thawed soil and solute concentrations in stream runoff during the thaw period. In June and July 2012 at the Cape Bounty Watershed Observatory, we combined active layer measurements with major ion concentrations and stable isotopes in surface waters to characterize the movement of different runoff sources: snowmelt, rainfall, and soil water. Results indicate that deep ground thaw enhanced the storage of infiltrated water following rainfall. Soil water from infiltrated rainfall flowed through the thawed transient layer and upper permafrost, which likely solubilized ions previously stored at depth. Subsequent rainfall events acted as a hydrological flushing mechanism, mobilizing solutes from the subsurface to the surface. This solute flushing substantially increased ion concentrations in stream runoff throughout mid to late July. Results further suggest the importance of rainfall and soil water as sources of runoff in a High Arctic catchment during mid to late summer as infiltrated snowmelt is drained from soil following baseflow. Although there was some evaporation of surface water, our study indicates that flushing from solute stores in the transient layer was the primary driver of increased ion concentrations in stream runoff and not evaporative concentration of surface water. With warmer and wetter summers projected for the Arctic, ion concentrations in runoff (especially in the late thaw season), will likely increase due to the deep storage and subsurface flow of infiltrated water and subsequent flushing of previously frozen solutes to the surface.     

Sources and sinks of nutrients in a New Zealand hill pasture catchment I. Stormflow generation

The processes of stormflow generation were studied in a hill pasture catchment near Hamilton, New Zealand. Although rainfall was relatively evenly distributed throughout the year, stormflow was highly seasonal and over 65 per cent occurred during the winter. Three main processes contributing to stormflow were identified which could be related to soil type and physiographic position. On gleyed soils derived from rhyolitic colluvium, saturation overland flow was the dominant process. Hydrographs from ‘Whipkey’ throughflow troughs also indicated that there was a subsurface response (saturated wedge) from this soil type. On steeper convex slopes, more permeable soils were derived from weathered greywacke. The presence of ephemeral springs on the hillslopes and direct observation during storm events indicated that storm runoff was generated as return flow from this soil. It was noted that nitrate concentrations from subsurface sources were 5–10 times higher than surface runoff. This difference in concentration was utilized in a chemical mixing equation which partitioned stormflow sources. This was compared with the stormflow predicted from rain falling on to saturated areas. There was good agreement between the two models for winter-spring events with respect to the volumes of surface runoff predicted, however the saturated areas model underestimated total stormflow. The results of the study are briefly discussed in terms of the potential for water quality management.     

Seasonal and event-scale variations in solute chemistry for four Sierra Nevada catchments

Hydrobiogeochemical processes controlling stream water chemistry were examined in four small (<5 km²) catchments having contrasting bedrock lithologies in the western Sierra Nevada foothills of California. The Mediterranean climate with its cool/wet and hot/dry cycle produces strong seasonal patterns in hydrological, biological and geochemical processes. Stream water solutes fall into three general groups according to seasonal fluctuation in concentration: strong, rainy season minimum–dry season maximum (Cl⁻, SO₄²⁻, base cations); weak, rainy season minimum–dry season maximum (Si); and rainy season maximum–dry season minimum (NO₃⁻ and K⁺). Solute dynamics in soil solutions and stream water suggest that mixing of drainage waters from bedrock and soil sources regulate stream water solute concentrations. Patterns are further altered by the leaching of solutes accumulated in the soil over the summer period of desiccation and the temporal discoupling of nutrient cycles that occurs due to differences in the timing between vegetation growth (late spring) and leaching (early winter). Solute concentrations are remarkably similar between watersheds with varying bedrock types, with the exception of nitrate, sulfate and bicarbonate. Three watersheds have nitrogen-bearing metasedimentary bedrock that contributes to elevated nitrate concentrations in stream waters. Watersheds whose bedrock includes mineralized veins of sulfide and carbonate minerals similarly have greater sulfate and bicarbonate concentrations in stream water. Hydrobiogeochemical processes are highly dynamic at the seasonal and storm-event temporal scales and spatially complex at the watershed scale making management of stream water chemical composition, such as nitrate concentrations, very challenging.     

Nutrient budget of a montane evergreen broad‐leaved forest at Ailao Mountain National Nature Reserve,Yunnan, southwest China

Hydrological fluxes and associated nutrient budget were studied during a 2 year period (1998–99) in a montane moist evergreen broad‐leaved forest at Ailao Mountain, Yunnan. Water samples of rainfall, throughfall, and stemflow, and of surface runoff, soil water, and stream flow were collected bimonthly to determine the concentration and fluxes of nutrients. Soil budgets were determined from the difference between precipitation input (including nutrient leaching from canopy) and output via runoff and drainage. The forest was characterized by low canopy interception and surface runoff, and high percolation and stream flow. Concentrations of nutrients were increased in throughfall and stemflow compared with precipitation. Surface runoff and drainage water had higher nutrient concentrations than precipitation and stream water. Total nitrogen and NH₄⁺‐N concentrations were higher in soil water than stream water, whereas K⁺, Ca²⁺, and Mg²⁺ concentrations were lower in the former than the latter. Annual nutrient fluxes decreased with soil depth following the pattern of water flux. Annual losses of most nutrient elements via stream flow were less than the corresponding inputs via throughfall and stemflow, except for calcium, for which solute loss was greater than the inputs via precipitation. Leaching losses of that element may be compensated by weathering. Losses of nitrogen, phosphorus, potassium, magnesium, sodium, and sulphur could be replaced through atmospheric inputs. Copyright © 2002 John Wiley & Sons, Ltd.     

Stratification response of soil water content during rainfall events under different rainfall patterns

Many researchers have studied the influence of rainfall patterns on soil water movement processes using rainfall simulation experiments. However, less attention has been paid to the influence under natural condition. In this paper, rainfall, soil water content (SWC), and soil temperature at 10‐, 20‐, 30‐, 40‐, and 50‐cm depths were simultaneously monitored at 1‐min intervals to measure the variation in SWC (SWCv) in response to rainfall under different rainfall patterns. First, we classified rainfall events into four patterns. During the study period, the main pattern was the advanced rainfall pattern (38% of all rainfall events), whereas the delayed, central, and uniform rainfall patterns had similar frequencies of about 20%. During natural rainfall, rainwater rapidly passed through the top soil layers (10–40 cm) and was accumulated in the bottom layer (50 cm). When a high rainfall pulse occurred, the water storage balance was disturbed, resulting in the drainage of initial soil water from the top layers into the deeper layers. Therefore, the critical function of the top layers and the bottom layers was infiltration and storage, respectively. The source of water stored in the bottom layer was not only rainfall but also the initial soil water in the upper soil layers. Changes in soil temperature at each soil depth were comonitored with SWCv to determine the movement characteristics of soil water under different rainfall patterns. Under the delayed rainfall pattern, preferential flows preferred to occur. Under the other rainfall patterns, matrix flow was the main form of soil water movement. Rainfall amount was a better indicator than rainfall intensity for SWCv in the bottom layer under the delayed rainfall pattern. These results provide insights into the responses of SWCv under different rainfall patterns in northern China.     

The distribution of solute processes on an acid hillslope and the delivery of solutes to a stream: I. Exchangeable bases

This paper aims to identify the spatial distribution of exchangeable base cations in soils on an acid hillslope and to investigate possible cation release processes from slope soils to the stream. The basic assumption underlying this research is that the amount of exchangeable cations in soils reflects the nutrient stores and cation leaching processes across the slope where vegetation and parent materials are similar. The distribution of exchangeable Ca²⁺, Mg²⁺, K⁺ and Na⁺ has been investigated on a three-dimensional hillslope on the Quantock Hills, Somerset, UK. A two-way ANOVA shows that soil depth is predominant in explaining the total variance of exchangeable bases, despite the steep slope gradient and clear podzolic catena development. Major nutrient base cations, such as Ca²⁺, Mg²⁺ and K⁺, display homogeneous topsoil storage right across the slope. This spatial pattern may indicate that the spatial distribution of major nutrient cations is tightly controlled by the soil–vegetation system in nutrient-poor heathland environments. Na⁺ is an exception to this vegetation-controlled spatial distribution, because of its small involvement in the soil–vegetation and soil exchangeable systems. In subsurface soils, cations liberated from the soil–vegetation system are subject to redistribution over the slope according to the hydrological flowpaths operating on the slope, with some eventually released into the stream. The saturated wedge developed at the base of the slope plays a key role in the storage and release processes of base cations from slope soils to the stream. Ca²⁺, Mg²⁺ and Na⁺ carried by throughflow are stored in the saturated wedge and gradually released into the stream at times of high flow. K⁺, however, shows an apparently different spatial behaviour, being deficient in the saturated wedge. Copyright © 1999 John Wiley & Sons, Ltd.     

Transformations of runoff chemistry in the Arctic tundra,Northwest Territories,Canada

The transformation of snowmelt water chemical composition during melt, elution and runoff in an Arctic tundra basin is investigated. The chemistry of the water flowing along pathways from the surface of melting snow to the 95·5 ha basin outlet is related to relevant hydrological processes. In so doing, this paper offers physically based explanations for the transformation of major ion concentrations and loads of runoff water associated with snowmelt and rainfall along hydrological pathways to the stream outlet. Late‐lying snowdrifts were found to influence the ion chemistry in adjacent reaches of the stream channel greatly. As the initial pulse of ion‐rich melt water drained from the snowdrift and was conveyed through hillslope flowpaths, the concentrations of most ions increased, and the duration of the peak ionic pulse lengthened. Over the first 3 m of overland flow, the concentrations of all ions except for NO increased by one to two orders of magnitude, with the largest increase for K⁺, Ca²⁺ and Mg²⁺. This was roughly equivalent to the concentration increase that resulted from percolation of relatively dilute water through 0·25 m of unsaturated soil. The Na⁺ and Cl^? were the dominant ions in snowmelt water, whereas Ca²⁺ and Mg²⁺ dominated the hillslope runoff. On slopes below a large melting snowdrift, ion concentrations of melt water flowing in the saturated layer of the soil were very similar to the relatively dilute concentrations found in surface runoff. However, once the snowdrift ablated, ion concentrations of subsurface flow increased above parent melt‐water concentrations. Three seasonally characteristic hydrochemical regimes were identified in a stream reach adjacent to late‐lying snowdrifts. In the first two stages, the water chemistry in the stream channel strongly resembled the hillslope drainage water. In the third stage, in‐stream geochemical processes, including the weathering/ion exchange of Ca²⁺ and Mg²⁺, were the main control of streamwater chemistry. Copyright © 2006 John Wiley & Sons, Ltd.     

INFERRING HYDROLOGICAL PROCESSES IN A TEMPERATE BASIN USING ISOTOPIC AND GEOCHEMICAL HYDROGRAPH SEPARATION: A RE-EVALUATION

Simultaneous monitoring of conservative and non-conservative tracers in streamflow offers a valuable means of obtaining information on the age and flow paths of water reaching the basin outlet. Previous studies of stormflow generation in a small forested basin on the Canadian Shield used isotopic (IHS) and geochemical hydrograph separations (GHS) to infer that some event water during snowmelt reaches the stream via subsurface pathways, and that surface water runoff is generated by direct precipitation on to saturated areas (DPSA) in the stream valley. These hypotheses were tested for rainfall inputs using simultaneous IHS (¹⁸O) and GHS (dissolved silica) of basin stormflow, supplemented by hydrochemical and hydrometric data from throughflow troughs installed on basin slopes. Comparison of pre-event and subsurface water hydrographs did not provide conclusive evidence for subsurface movement of event water to the stream, owing to the appreciable uncertainty associated with the hydrograph separations. However, IHSs of runoff at the soil–bedrock interface on basin slopes indicated that event water comprised 25–50% of total runoff from areas with deep soil cover, and that these contributions supplied event water flux from the basin in excess of that attributable to DPSA. The surface water component of stormflow estimated from the GHS was also largely the result of DPSA. GHS assumes that dissolved silica is rapidly and uniformly taken up by water infiltrating the soil and that water moving via surface pathways retains the low dissolved silica level of rainfall; however, neither assumption was supported by the hillslope results. Instead, results suggest that the observed depression of silica levels in basin stormflow previously attributed to dilution by DPSA was partly a function of transport of dilute event water to the channel via preferential pathways. Implications of these results for the general use of simultaneous IHS and GHS to infer hydrological processes are discussed. © 1997 by John Wiley & Sons, Ltd.     

Field investigation and modelling of coupled stream discharge and shallow water‐table dynamics in a small Mediterranean catchment (Sardinia)

In the semi‐arid Mediterranean environment, the rainfall–runoff relationships are complex because of the markedly irregular patterns in rainfall, the seasonal mismatch between evaporation and rainfall, and the spatial heterogeneity in landscape properties. Watersheds often display considerable non‐linear threshold behavior, which still make runoff generation an open research question. Our objectives in this context were: to identify the primary processes of runoff generation in a small natural catchment; to test whether a physically based model, which takes into consideration only the primary processes, is able to predict spatially distributed water‐table and stream discharge dynamics; and to use the hydrological model to increase our understanding of runoff generation mechanisms. The observed seasonal dynamics of soil moisture, water‐table depth, and stream discharge indicated that Hortonian overland‐flow was negligible and the main mechanism of runoff generation was saturated subsurface‐flow. This gives rise to base‐flow, controls the formation of the saturated areas, and contributes to storm‐flow together with saturation overland‐flow. The distributed model, with a 1D scheme for the kinematic surface‐flow, a 2D sub‐horizontal scheme for the saturated subsurface‐flow, and ignoring the unsaturated flow, performed efficiently in years when runoff volume was high and medium, although there was a smoothing effect on the observed water‐table. In dry years, small errors greatly reduced the efficiency of the model. The hydrological model has allowed to relate the runoff generation mechanisms with the land‐use. The forested hillslopes, where the calibrated soil conductivity was high, were never saturated, except at the foot of the slopes, where exfiltration of saturated subsurface‐flow contributed to storm‐flow. Saturation overland‐flow was only found near the streams, except when there were storm‐flow peaks, when it also occurred on hillslopes used for pasture, where soil conductivity was low. The bedrock–soil percolation, simulated by a threshold mechanism, further increased the non‐linearity of the rainfall–runoff processes. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulation of field‐scale water flow and bromide transport in a cracked clay soil

Single domain models may seriously underestimate leaching of nutrients and pesticides to groundwater in clay soils with shrinkage cracks. Various two‐domain models have been developed, either empirical or physically based, which take into account the effects of cracks on water flow and solute transport. This paper presents a model concept that uses the clay shrinkage characteristics to derive crack volume and crack depth under transient field conditions. The concept has been developed to simulate field average behaviour of a field with cracks, rather than flow and transport at a small plot. Water flow and solute transport are described with basic physics, which allow process and scenario analysis. The model concept is part of the more general agrohydrological model SWAP, and is applied to a field experiment on a cracked clay soil, at which water flow and bromide transport were measured during 572 days. A single domain model was not able to mimic the field‐average water flow and solute transport. Incorporation of the crack concept considerably improved the simulation of water content and bromide leaching to the groundwater. Still deviations existed between the measured and simulated bromide concentration profiles. The model did not reproduce the observed bromide retardation in the top layer and the high bromide dispersion resulting from water infiltration at various soil depths. A sensitivity analysis showed that the amounts of bromide leached were especially sensitive to the saturated hydraulic conductivity of the top layer, the solute transfer from the soil matrix to crack water flow and the mean residence time of rapid drainage. The shrinkage characteristic and the soil hydraulic properties of the clay matrix showed a low sensitivity. Copyright © 2000 John Wiley & Sons, Ltd.     

Hydrological control of dissolved organic carbon dynamics in a forested headwater catchment,Kiryu Experimental Watershed,Japan

To evaluate the influence of hydrological processes on dissolved organic carbon (DOC) dynamics in a forested headwater catchment, DOC concentration was observed along the flow path from rainfall to stream water via throughfall, soil water, groundwater, and spring water for 4 years, and DOC flux through the catchment was calculated. The spatial and temporal variations in DOC concentration and flux were compared with physical hydrological observations and the mean residence time of water. In the upslope soil layer, DOC concentrations were not significantly correlated with water fluxes, suggesting that DOC concentrations were not strictly controlled by water fluxes. In the upslope perennial groundwater, DOC concentration was affected by the change in the amount of microbial degradation of DOC produced by changes in the mean residence time of water. In stream water, the temporal variation in DOC concentration was usually affected by changes in DOC concentration of the inflow component via vertical infiltration from above the perennial groundwater. During dry periods, however, the component from inflow via vertical infiltration was negligible and DOC in the upslope perennial groundwater became the major component of stream water DOC. The temporal variation in stream water DOC concentration during baseflow was affected by rainfall patterns over several preceding months. Therefore, records of rainfall over several preceding months are one of the most important factors for predicting changes in DOC concentration on a catchment scale. Copyright © 2007 John Wiley & Sons, Ltd.     

The distribution of solute processes on an acid hillslope and the delivery of solutes to a stream: II. Exchangeable Al3+

In order to identify the distribution of aluminium (Al) within an acid hillslope and its release to a stream, the spatial distribution of acid ammonium oxalate extractable Al (Al_o) and exchangeable Al³⁺ have been investigated on a podzolized hillslope in Bicknoller Combe, Somerset, UK. The eluviated Al from topsoils is mainly deposited in the lower soil horizons forming podzolic B horizons, but some Al flows downslope carried by lateral throughflow. Al oxides may provide the main source of exchangeable Al³⁺ on the study slope due to high soil acidity. Examination of the spatial distribution of exchangeable Al³⁺ suggests that the slope hollow, where active convergent throughflow occurs, and the saturation wedge at the base of the slope are the main delivery routes of dissolved Al³⁺ to the stream. Divalent base cations (Ca²⁺ and Mg²⁺), supplied from atmospheric input and organic decomposition and carried by throughflow, exchange Al³⁺ via cation exchange reactions under high water content. Laterally illuviated Al oxides in the lower hollow adjacent to the saturation wedge probably provide a pool for continuous delivery of Al either as soluble or complexed forms to the stream via the saturated wedge. Copyright © 1999 John Wiley & Sons, Ltd.     

Land management impacts on runoff sources in small Amazon watersheds

Forest clearing and conversion to cattle pasture in the lowland Amazon region has been linked to soil compaction and increased soil water storage, which combine to diminish soil infiltration, enhance quick lateral flows and increase the stream flow response to precipitation. Quantifying the importance of quick surficial flow in response to this land use change requires identification of water sources within catchments that contribute to stream flow. Using an end member mixing analysis approach, potential contributing sources of stream flow were evaluated during an entire rainy season in a forest and a pasture watershed drained by ephemeral‐to‐intermittent streams in the south‐western Amazon. Water yield was 17% of precipitation in the pasture and 0·8% of precipitation in the forest. During the early rainy season, throughfall, groundwater, and soil water contributed 79%, 18%, and 3%, respectively, to total forest stream flow. Over the entire rainy season, throughfall, groundwater, and shallow soil water provided 57%, 24%, and 19%, respectively, of stream flow. In the pasture watershed, overland flow dominated stream flow both in the early (67%) and late (57%) rainy season, with a mean contribution of 60% overland flow, 35% groundwater, and 5% soil water. The uncertainty associated with those estimates was studied using a Monte Carlo approach. In addition to large changes in total surface flow, marked differences were found in the proportions of total stream flow in the second half of the rainy season between the forest and pasture watershed. These results suggest that (1) there is great potential for alteration of the hydrological budgets of larger watersheds as the proportion of deforested land in the Amazon increases, and (2) as more rainfall is diverted into fast flowpaths to streams in established pastures, the potential to deliver water with higher solute concentrations generated by erosion or by bypassing sites of solute removal increases. Copyright © 2007 John Wiley & Sons, Ltd.     

PRINCIPES NOUVEAUX POUR DISTINGUER CE QUI EST Dû A L'ÉCOULEMENT SOUTERRAIN SUR L'HYDROGRAPHE DES FLEUVES

Abstract

Throughflow has been measured from three soil horizons on a 12 slope with impermeable, bedrock. Storm flow comes from the 10–45 cm horizon and is controlled by the upslope extent of saturated conditions. Base flow comes from the 45–75 cm horizon and is supplied by slow unsaturated flow from the whole soil mass to a small constant zone of saturation.

Differences between input and output stream hydrographs over 270 metres of channel are attributed to throughflow and correlate well with measured values providing a basis for separating throughflow components from the stream hydrograph. Observed stream flows contain no overland flow or ground water flow components. The main basin flood peak is not generated within this control section of channel but is produced in the headwater zone (0.1 km²) by the faster runoff characteristics of the soils in that area and by topographic factors which lead to rapid channel extension.     

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon,nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm-humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km² forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral-derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in-stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.     

Multi-functional heat pulse probe measurements of coupled vadose zone flow and transport

Simultaneous measurement of coupled water, heat, and solute transport in unsaturated porous media is made possible with the multi-functional heat pulse probe (MFHPP). The probe combines a heat pulse technique for estimating soil heat properties, water flux, and water content with a Wenner array measurement of bulk soil electrical conductivity (EC_bulk). To evaluate the MFHPP, we conducted controlled steady-state flow experiments in a sand column for a wide range of water saturations, flow velocities, and solute concentrations. Flow and transport processes were monitored continuously using the MFHPP. Experimental data were analyzed by inverse modeling of simultaneous water, heat, and solute transport using an adapted HYDRUS-2D model. Various optimization scenarios yielded simultaneous estimation of thermal, solute, and hydraulic parameters and variables, including thermal conductivity, volumetric water content, water flux, and thermal and solute dispersivities. We conclude that the MFHPP holds great promise as an excellent instrument for the continuous monitoring and characterization of the vadose zone.     

Experimental study and mathematical modelling of soluble chemical transfer from unsaturated/saturated soil to surface runoff

A one‐dimensional, two‐layer solute transport model is developed to simulate chemical transport process in an initially unsaturated soil with ponding water on the soil surface before surface runoff starts. The developed mathematical model is tested against a laboratory experiment. The infiltration and diffusion processes are mathematically lumped together and described by incomplete mixing parameters. Based on mass conservation and water balance equations, the model is developed to describe solute transport in a two‐zone layer, a ponding runoff zone and a soil mixing zone. The two‐zone layer is treated as one system to avoid describing the complicated chemical transport processes near the soil surface in the mixing zone. The proposed model was analytically solved, and the solutions agreed well with the experimental data. The developed experimental method and mathematical model were used to study the effect of the soil initial moisture saturation on chemical concentration in surface runoff. The study results indicated that, when the soil was initially saturated, chemical concentration in surface runoff was significantly (two orders of magnitude) higher than that with initially unsaturated soil, while the initial chemical concentrations at the two cases were of the same magnitude. The soil mixing depth for the initially unsaturated soil was much larger than that for the initially saturated soil, and the incomplete runoff mixing parameter was larger for the initially unsaturated soil. The higher the infiltration rate of the soil, the greater the infiltration‐related incomplete mixing parameter. According to the quantitative analysis, the soil mixing depth was found to be sensitive for both initially unsaturated and saturated soils, and the incomplete runoff mixing parameter was sensitive for initially saturated soil but not for the initially unsaturated soil; the incomplete infiltration mixing parameter behaved just the opposite. Some suggestions are made for reducing chemical loss from runoff. Copyright © 2010 John Wiley & Sons, Ltd.