Stormflow Periods Research Articles

Combined effects of rainfall‐runoff events and antecedent soil moisture on runoff generation processes in an upland forested headwater area

AbstractIn this study, we investigate the combined effect of different rainfall‐runoff event types and antecedent soil moisture (ASM) on runoff processes in the headwater elementary discharge area of a small forested upland catchment. The study focuses on (i) the relationship between soil moisture thresholds and runoff generation; (ii) the combined effect of ASM and tree vicinity and (iii) the relationship between different rainfall‐runoff event types and different types of runoff (baseflow and stormflow). The results suggest that ASM has a strong impact on local runoff generation processes. Soil water content (35%–36%) threshold exceedance was related to stormflow runoff generation caused by the activation of quick preferential flow paths in the soil during storm events, especially in the upper and the deepest soil layers. At the same time, unexpected non‐linear increases in baseflow runoff ratios were documented during dry, precipitation‐free, periods and when the 31%–34% soil moisture threshold was exceeded, presumably due to the hydrological connection of farther slope areas during these conditions. Multiple stormflow periods, which exhibited the lowest runoff coefficient, were the most significant events in terms of water retention and soil water recharge due to increased vertical hydrological connectivity enabling more rapid transport to deeper soil layers. However, this rainfall type occurred least often over the study period. The important role of forest stands (individual trees) in creating spatial patterns of soil moisture and preferential infiltration paths to deeper soil layers was also confirmed. These results contribute towards a better conceptualisation of hydrological behaviour in elementary headwater discharge areas and highlight the potential dangers associated with expected increases in extreme weather events.

Carbon Dioxide and Methane Dynamics in a Peatland Headwater Stream: Origins, Processes and Implications

AbstractFirst‐order streams flowing through peatlands receive, carry and transform large amounts of organic carbon, methane (CH4) and carbon dioxide (CO2) but remain poorly documented. The objectives of this study were to (a) identify the origins of CO2 and CH4 (thereafter C‐GHG for carbon greenhouse gases) in a peatland headwater stream, (b) determine the environmental factors driving C‐GHG export and emissions, and (c) quantify C‐GHG losses from this system and discuss its implications. Data were collected from eight sampling sites along a 3 km boreal peatland headwater stream (Eastern Canada) over the growing seasons 2019 and 2020. The studied stream was oversaturated in pCO2 [min: 2,044; max: 23,306 μatm] and pCH4 [14; 17,614 μatm]. A mass balance model showed that ∼81% of in‐stream CO2 originated from porewater seepage while the remaining 17% and 2% originated from in‐stream productivity and methane oxidation, respectively. Porewater seepage was concluded to be the primary source of CH4. Seasonal dissolved C‐GHG concentrations were negatively correlated with the peatland water table depth, suggesting an active release of carbon‐rich peat porewater during the base flow. Nevertheless, greater C‐GHG losses occurred during stormflow periods which acted as pulses with most of the C‐GHG being shunted downstream. The sum of C‐GHG export and emissions at our site was 8.08 gC m−2 y−1 with 86% being released to the atmosphere and 14% being exported downstream. Our study demonstrates that peatland headwater streams act as large sources of C‐GHG and that precipitation events and topography control the magnitude of the fluxes.

Factors affecting dominant peak‐flow runoff‐generation mechanisms among five neighbouring granitic headwater catchments

AbstractWe examined how and why dominant peak‐flow runoff‐generation mechanisms differ among neighbouring headwater catchments. We monitored runoff and groundwater levels and performed terrain analyses in a granitic second‐order catchment and its four neighbouring subcatchments in the Kiryu Experimental Watershed in Japan. Our analysis of lag times from peak rainfall to peak runoff suggests differences in the dominant peak‐flow runoff‐generation mechanisms among the five catchments. For two of the three zero‐order catchments, with few perennial groundwater bodies, subsurface flow from hillslopes was the dominant mechanism at some events. However, the dominant mechanisms were channel precipitation and riparian runoff at almost all events in first‐ and second‐order catchments and in the third zero‐order catchment, which has a large perennial groundwater body over a bedrock depression in the riparian zone. In this zero‐order catchment, the quick‐flow ratio was the smallest of the five catchments because subsurface flow from the hillslope was buffered at the riparian zone. These facts suggest that the channel length, riparian buffering, and hillslope connectivity were the factors governing the different dominant peak‐flow runoff‐generation mechanisms among the catchments. Riparian buffering was affected, not only by surface topography, but also by bedrock topography and bedrock groundwater (BGW) dynamics. Our findings indicate that both of BGW dynamics and topography are important for catchment classification, and the relative importance of topography increases with the change from baseflow to stormflow. Furthermore, mismatching between a geographic source and a flow path resulted in different catchment classifications depending on the approach. Therefore, multiple approaches during both baseflow and stormflow periods are necessary for catchment classification to apply information obtained from one headwater catchment to other headwater catchments within the same region.

Effect of forest thinning on hydrologic nitrate exports from a N-saturated plantation

Although several studies have assessed the effects of thinning on water quality, including nitrogen (N) exports, few have examined N-saturated plantations. This study assessed the short-term effect of thinning on N exports from a N-saturated plantation forest in northern Kyushu, western Japan, that was thinned (43% of basal area) during January–March 2012. Water levels at the gauging station were continually measured, and stream-water baseflow and stormflow samples were collected before (2011) and after (2013) forest thinning. Annual precipitation before (2469 mm) and after (2418 mm) forest thinning differed little, and annual water discharge after thinning (1641 mm) was similar to that before thinning (1609 mm). However, direct flow during stormflow periods was higher after thinning (260 mm) than before (153 mm). The concentrations of major ions in stream water did not differ before and after thinning. As a result, the high annual dissolved inorganic N (DIN) exports differed little before (35.8 kg N ha−1 a−1) and after (36.5 kg N ha−1 a−1) forest thinning. DIN exports during stormflow periods were slightly higher after (12.2 kg N ha−1 a−1) than before thinning (10.5 kg N ha−1 a−1) and were proportional to the increased direct flow after thinning (561 mm after vs. 470 mm before thinning). We concluded that thinning does not affect annual N exports from the N-saturated plantation forest, but it can increase the proportion of N exported during stormflow periods in proportion to increased water volume of direct flow after thinning.

How does rapidly changing discharge during storm events affect transient storage and channel water balance in a headwater mountain stream?

[1] Measurements of transient storage in coupled surface-water and groundwater systems are widely made during base flow periods and rarely made during storm flow periods. We completed 24 sets of slug injections in three contiguous study reaches during a 1.25 year return interval storm event (discharge ranging from 21.5 to 434 L s−1) in a net gaining headwater stream within a steep, constrained valley. Repeated studies over a 9 day period characterize transient storage and channel water from prestorm conditions through storm discharge recession. Although the valley floor was always gaining from the hillslopes based on hydraulic gradients, we observed exchange of water from the stream to the valley floor throughout the study and flow conditions. Interpretations of transient storage and channel water balance are complicated by dynamic in-stream and near-stream processes. Metrics of transient storage and channel water balance were significantly different (95% confidence level) between the three study reaches and could be identified independently of stream discharge via analysis of normalized breakthrough curves. These differences suggest that the morphology of each study reach was the primary control on solute tracer transport. Unlike discharge, metrics of transient storage and channel water balance did not return to the prestorm values. We conclude that discharge alone is a poor predictor of tracer transport in stream networks during storm events. Finally, we propose a perceptual model for our study site that links hydrologic dynamics in 3-D along the hillslope-riparian-hyporheic-stream continuum, including down-valley subsurface transport.

Hot moments and hot spots of nutrient losses from a mixed land use watershed

Summary Non-point nitrogen (N) and phosphorus (P) pollution from agriculture has increasingly received more public attention. In this study, NO 3 –N, dissolved P (DP) and particulate P (PP) concentrations and loads were investigated for four sub-basins (labeled 1–4 going up the watershed) within a mixed land use watershed (39.5 ha) in the Appalachian Valley and Ridge Physiographic Province. The hot moments of NO 3 –N concentration and load occurred in base flow and during the non-growing season. Great and temporally variable DP and PP concentrations were observed in storm flow. The hot moments of DP concentration and load were in storm flow from May to December and from September to Nov, respectively, while the hot moments of PP concentration and load were in storm flow from January to June. The NO 3 –N, DP, and PP loads were compared for all four sub-basins on a loss per length of stream reach basis to determine the hot spots and their corresponding losses. The hot spots and hot moments of NO 3 –N loads were in Sub-basins 1 and 4 during the non-growing season base flow period and Sub-basin 2 during the post-growing season base flow period (>110 g m −1 mo −1 ). The hot spots of DP loads were also in Sub-basins 1 and 4, but during the growing and post-growing season storm flow period (>1.4 g m −1 mo −1 ). In contrast, the hot spots and hot moments of PP load were in Sub-basin 3 during the pre-growing and growing season storm flow, as much as 13.4 and 14.1 g m −1 mo −1 , respectively. Controlling factors of nutrient export were discussed in this study, including season, hydrology (base flow, storm flow, surface and subsurface runoff), and land use. Although different hot moments and hot spots within the watershed were identified for NO 3 –N, DP, and PP losses, the implementation of a couple of management practices (cover crops and no-till) might be sufficient to effectively reduce nutrient losses from this and similar Valley and Ridge watersheds.

Carbon isotopes as tracers of dissolved organic carbon sources and water pathways in headwater catchments

Summary Stable carbon isotopes (δ 13 C) are assessed in further detail for their potential to (i) trace the relationship between spatial variations in the source of dissolved organic carbon (DOC) in soils and temporal variability of both DOC concentration and composition in streams, and (ii) elucidate water pathway changes during storm events in headwater catchments. For this purpose, we investigated δ 13 C DOC values in a wetland soil (0–50 cm), in deep groundwater (until 6 m) and during a storm flow event with high-resolution monitoring (⩽hourly basis) in a small, lowland catchment in western France (Kervidy-Naizin catchment). The results show a combined increase of stream DOC concentration (from 4 to 14 mg L −1 ) and decrease of stream δ 13 C DOC (from −27 to −29‰) with increasing discharge, suggesting a change in DOC sources between base flow and storm flow periods. Such an interpretation is consistent with the δ 13 C DOC values in soils that show a 6‰ vertical variation, with δ 13 C DOC values of the uppermost soil horizons (0–10 cm) of the wetland domains being close to those measured in the stream channel during the ascending limb of the hydrograph. Overall, the results presented in this study are consistent with a model in which the water-table rise and wetland runoff caused by rainfall lead to a flushing of the DOC stored in the uppermost soil horizons of the wetland domains near the channel network. Subsequently, these wetland soils become the dominant DOC source during storm events (ca. 70% of the total DOC flux). In this way, the stream DOC isotopic composition reflects the combined effects of the vertical variation of soil organic matter composition as well as the changes in water routing through time. This study demonstrates the ability of the stable isotopes of carbon to serve not only as a tool for the location of stream DOC sources in landscapes but also the reconstruction of water pathways in headwater catchments.

Effects of storm flow samplings on the evaluation of inorganic nitrogen and sulfate budgets in a small forested watershed

AbstractTo investigate the effects of storm flow samplings on the evaluation of DIN and SO42− budgets, we conducted storm flow samplings and discrete interval (weekly) samplings of stream water, throughfall, and stemflow. This investigation lasted for three years and took place in a small forested watershed in western Japan. Annual exports were calculated by the sum of the load (ΣLtotal)‐ sum of the discharge (ΣQtotal) relationships obtained by storm flow samplings. These were then compared to calculated ones that were based on LQ relationships obtained by discrete interval sampling. The results of discrete interval sampling showed that the relationships of DIN or SO42− concentration to the discharge amount found in stream water differed from those of storm flow sampling. This was especially notable in years with higher annual precipitation and subsequent higher annual discharges. Also, exports of DIN and SO42− during storm flow periods accounted for 83% and 78% of annual exports on 3‐year averages, respectively. These results confirmed that storm flow samplings are essential for estimating annual exports of DIN and SO42−. Annual exports that were calculated based on just discrete interval sampling (46% and 17%, respectively) overestimated DIN and SO42− from discrete interval and storm flow samplings on a 3‐year average. These overestimates were pronounced in years of higher discharge, and critically affected the rates of net‐retention or export of DIN and SO42−. The relationship of smaller input than output of DIN was reversed if storm flow sampling was included. Copyright © 2009 John Wiley & Sons, Ltd.

Growing season surface water loading of fecal indicator organisms within a rural watershed

The loading of microbial contaminants was examined within the Thomas Brook watershed, a 784 ha mixed land-use catchment located in the headwaters of the Cornwallis River drainage basin (Nova Scotia, Canada). The objectives were to: (i) examine spatial and temporal characteristics of fecal bacteria loading during the growing season from five subwatersheds, and (ii) develop areal fecal indicator organism export coefficients for rural landscapes. Fecal coliform, Escherichia coli, total suspended solids (TSS) concentrations and stream flow were monitored at five locations in the watershed over six consecutive growing seasons (May–Oct, 2001–2006). A nested watershed monitoring approach was used to determine bacterial loading from distinct source types (residential vs. agricultural) during both baseflow and stormflow periods. Areal bacterial loading rates increased in each nested watershed moving downstream through the watershed and were highest in the three subcatchments dominated by agricultural activities. Upper watershed bacterial loading throughout the growing season from an agricultural subcatchment (Growing Season Avg 8.92 × 10 10 CFU ha −1) was consistently higher than a residential subcatchment (Growing Season Avg 8.43 × 10 9 CFU ha −1). As expected, annual average stormflow bacterial loads were higher than baseflow loads, however baseflow loads still comprised between 14 and 35% of the growing season bacterial loads in the five subwatersheds. Fecal bacteria loads were greater during years with higher annual precipitation. A positive linear relationship was observed between E. coli and TSS loading during the 2005 and 2006 growing seasons when both parameters were monitored, indicating that the processes of sediment transport and bacterial transport are linked. It is anticipated that computed areal microbial loading coefficients will be useful in developing watershed management plans. More intensive sampling during stormflow events is recommended for improving these coefficients.

Arsenic retention and release in ombrotrophic peatlands

Organic matter can play an important role in the mobility and fate of As in the environment, but there is a lack of data on As biogeochemistry in ombrotrophic peatlands. The aim of this study was to investigate As retention and release in atmospherically contaminated ombrotrophic peat soils in the Peak District National Park (UK). Solid phase As concentrations in the peat soils exceed 25 mg kg − 1 . Solid phase As and Fe concentrations are closely correlated at sites where the peat is subjected to drying and oxic conditions. In a wetter zone of the bog, solid phase As and Fe distributions are decoupled, suggesting that As retention in these systems is not solely controlled by the presence of Fe oxides. Comparison of solid phase As and Pb distributions reveals that As has been subjected to post-depositional mobility in areas of water table fluctuation. Conversely, at permanently waterlogged locations As is immobile. Detailed stream water sampling reveals that As is released from the organic-rich uplands soils into the fluvial system. Dissolved As concentrations are highly variable, with values ranging from 0.20 to 7.28 μg l − 1 . Stream water As concentrations are elevated during late summer stormflow periods when there has been re-wetting of the peat after significant water table draw-down. Dissolved As is strongly correlated to dissolved organic carbon under stormflow and baseflow. The results of this study suggest that organic matter plays an important role in As dynamics in ombrotrophic peatlands, but further work is needed to identify the exact As binding and release mechanisms. Drying and re-wetting of ombrotrophic peat soils and associated changes in redox status has the potential to lead to increased As mobility. Further work is needed to provide information on how predicted climate change will influence As cycling at sites containing a legacy of atmospheric contamination.

DOM in Recharge Waters of the Santa Ana River Basin

Assessment of the composition, reactivity, and potential health effects of dissolved organic matter (DOM) is an important issue in the regulation and operation of groundwater recharge projects. The composition of DOM in various natural surface waters and reclaimed waters used to recharge the alluvial aquifers of the lower Santa Ana River Basin in California was derived primarily from terpenoid hydrocarbons and amino sugars. Lesser amounts of aromatic sulfonate and alkylphenol polyethoxylate metabolites of anionic and neutral surfactants were detected in reclaimed water, and these metabolites persisted in waters of the Santa Ana River, the Prado Wetland, and the recharge basins. Only small amounts of humic substances derived from tannins and lignins were deposited during storm flow periods in the Prado Wetland, but these amounts were responsible for significant increases in disinfection by‐product formation potential in these waters. Colloids composed of bacterial cell wall fragments showed large percentage increases in all waters during storm flow periods.

Nutrient runoff from forested watersheds in central Japan during typhoon storms: implications for understanding runoff mechanisms during storm events

AbstractWe investigated the characteristics of nutrients (N and P) and dissolved organic carbon (DOC) concentrations during high, intense typhoon storm and base flow periods from three watersheds (W1, W2 and W3) in central Japan. Concentrations in base flow were low and relatively constant. In contrast, concentrations of N, P, and DOC in runoff during typhoon storms became highly dynamic. Particulate nitrogen (PN) and particulate phosphorus (PP) concentrations in runoff were 4–50 times greater than those in base flow for all storm events. Because PN and PP concentrations were significantly correlated with suspended sediment concentrations, particulate nutrient transport may be associated with overland flow in forested hillslopes. Dissolved organic nitrogen (DON), NO3‐N, DOC and dissolved phosphorus (DP) concentrations in runoff were 1–10 times greater than those in base flow. All nutrients showed a clockwise, discrete hysteresis pattern, which implied that the sources of nutrients were from forested hillslopes. Additionally, the time constant (t1/e) calculated from a decreasing function suggested that particulate nutrients were mainly sourced from surface flow, whereas dissolved nutrients were provided from both surface flow and subsurface flow. The ratios of DOC:nitrate and UV:DOC suggested that higher values occurred in the early period of a storm event and was potentially related to the occurrence of overland flow. Our findings suggested that investigation of the parameters (e.g. DOC:nitrate and UV:DOC) were potentially useful for identifying sources of nutrients and potential applications of hydrological separation. Our results also contribute towards understanding the nutrient dynamics during intense storm events when various hydrological processes such as surface and subsurface runoffs govern the nutrient concentrations in headwater streams. Copyright © 2007 John Wiley & Sons, Ltd.

出水時における森林河川の溶存有機炭素濃度と斜面の水文過程

On the basis of hydrological processes in a well-characterized hillslope (0.65 ha), we analyzed the effects of flow paths on dissolved organic carbon (DOC) concentrations in a headwater stream (catchment area, 103 ha) during storm flow periods. In the hillslope, our previous findings of water flow paths and an analysis of the relationship between soil pore water pressure and hillslope discharge enabled us to classify the hillslope discharge representing water flow paths. During storm flow periods, the occurrence of upslope-derived water had the effect of keeping the stream DOC concentration low, reflecting that the DOC concentration of the upslope-derived water was very low. However, this effect was not observed during nonstorm flow periods, when the stream DOC concentration was nearly constant regardless of the occurrence of upslope-derived water. We inferred that the major reason that the effects of upslope-derived water on stream DOC concentration were different between storm and nonstorm flow periods was the difference in the timing of DOC export from a riparian area. We concluded that information on hydrological processes in hillslopes is helpful for elucidating the formation mechanism of stream DOC concentration in forested headwater catchments.

多雪山地流域の融雪期における河川水の水質形成過程

In a snowy watershed, the response of streamwater chemistry to snowmelt depends not only on meltwater chemistry but on the hydrochemical processes of the subsurface water. In this study, the δ18O values and solute concentrations of the precipitation, meltwater, subsurface water and surface water at the Moshiri experimental watershed located in the northern part of Hokkaido, Japan were obtained, and were discussed here in order to clarify the subsurface runoff processes and the formational mechanisms of the subsurface and surface water chemistry during the snowmelt season. A groundwater with a constant δ18O value (-12.3‰) and constant Cl- concentration (200 μeq/L) was widely found in the watershed and in the shallow saturated water zone. Hydrograph separations with high precision were conducted using δ18O early in the snowmelt season (Mar. 23-Apr. 14), and using a Cl- concentration during the active snowmelt season (Apr. 21-May 1). The contributions of the old water to the streamwater discharge were 82% (Mar. 23-Apr. 14) and 76% (Apr. 21-May 1). The contributions of old water were very high throughout the snowmelt season, which means that the groundwater reservoir must be significantly large. Subsequently, the temporal and spatial variations of the SiO2 concentrations of the subsurface water and surface water were observed to clarify the chemistry of the subsurface water, and revealed that there were two geographic sources (shallow groundwater and deep groundwater) in the ground. The SiO2 concentration of the deep groundwater was higher than that of the shallow groundwater. The sources of two spring waters changed from the deep groundwater to the shallow groundwater, and the SiO2 concentrations of these waters decreased while the discharges of these waters increased. The SiO2 concentration of old water was estimated using the results of isotopic and chloride hydrograph separation. Then, the result of the relationship between the amount of old water discharge and the SiO2 concentration of old water showed that the source of old water during the low outflow period was deep groundwater, and the source during the low outflow period was shallow groundwater. As the discharge of the old water reached over 0.08m3/s, the estimated SiO2 concentration of the old water became constant. These results indicated that a large amount of the old water flowing out to the river in the active snowmelt season was discharged from shallow groundwater. By observing the snowmelt runoff processes in a nival watershed, it was clarified that the old water is composed of a combination of two sources of groundwater and that the shallow groundwater chemistry greatly affects the streamwater chemistry during the large stormflow period.

Phosphorus dynamics observed through increasing scales in a nested headwater-to-river channel study

The hypothesis that the dynamic patterns of phosphorus (P) transport at plot scale are similar to the patterns that could be observed quasi-simultaneously (i.e., approximately at the same time) at a river basin scale, in terms of inputs and dilution of P, across a range of rainfall and runoff conditions, was tested. From this information, it was hoped to be able to make some simple inferences about the connectivity or mass flux of P transport between the different scales of observation. An intensive study using 30-m 2 plots, 1-ha plots and nested river channel sites ranging in scale from 20 ha up to a maximum of 834 km 2 in the River Taw basin, South West England, UK, was conducted with three campaigns under differing flow conditions: (1) a zero rainfall base flow period, (2) a 10-mm rainfall residual flow period, and (3) a 42-mm rainfall storm flow period. The mass flux of total P ranged from 49 kg during base flow to 4 tonnes during the storm period at the largest 834 km 2 scale. During base flow conditions, total phosphorus (TP) concentrations from diffuse sources were low (26 μg L −1 in the upper catchment) and reactive P forms dominated the fractions filtered <0.45 μm. During storm flow, concentrations of TP increased at all scales within the drainage basin, to a maximum of 500 μg L −1 and were sufficient to override the effect of any point source inputs of P. Unreactive (i.e., mostly ‘organic’) forms of P dominated the fractions that were >0.45 μm during residual flows and storm flows. Spatially normalised discharge apparently decreased with increasing scale, most notably during storm flow conditions and this implies that there is some storage of water through the catchment and in part may reflect varying velocities of water in different pathways. Most attenuation and dilution of P appeared to occur at larger scales, whilst the hydrological connectivity between source areas and receiving waters was greatest at smaller scales (<20 ha), and during the highest flows. The importance of diffuse agricultural sources in contributing to P export through the basin was dominated by dynamic temporal changes in hydrological activity, with a ‘piston pushing’ effect of particulate associated P through the basin as it wets up in response to rainfall input. We concluded that the processes of P transfer are different at different scales. However, some uncertainties of spatial heterogeneity around the catchment underlie the difficulties in dealing with scale and thus more data and studies of this nature are required.

Persistence of enteric bacteria in alluvial streams

A tracer-bacteria was used to study the persistence of enteric bacteria in three alluvial streams (Carroll Creek, Lutteral Creek, Eramosa River) located in Southern Ontario, Canada. Within each stream, a 1.1 m2 section of the bed was seeded with a strain of Escherichia coli resistant to nalidixic acid (E. coli NAR). The survival of the tracer-bacteria within the stream bed and the release of the tracer-bacteria to the water column were monitored for approximately 3 weeks. Survival dynamics were also studied in each stream using dialysis tubes. In Carroll Creek, where water temperatures were typically lower than 16 °C, the inactivation of the tracer-bacteria did not follow a first order decay. The number of tracer-bacteria within the stream bed did not decrease for the first 5 d of the experiment. Inactivation of the tracer-bacteria within bed sediments of Lutteral Creek and the Eramosa River resembled typical first order decay, preceded by a 24-h lag phase. Water temperatures in Lutteral Creek and the Eramosa River were generally 10 °C higher than in Carroll Creek. Downstream water quality monitoring indicated that the tracer-bacteria was being released from the seeded bed sediments during both baseflow and stormflow periods in Carroll and Lutteral Creeks. However, in the Eramosa River, where bed sediments possessed an organic matter content of 9.5%, the tracer-bacteria was rarely recovered in downstream water samples. Survival dynamics observed in dialysis tubes resembled those observed in bed seeding experiments. The experimental approach employed in this study could be used to further investigate the survival and transport characteristics of sediment-associated bacteria. Key words: E. coli, sediments, survival, streams, tracer-bacteria.

18O isotopic separation of stream nitrate sources in mid-Appalachian forested watersheds

The δ 18O values of atmospheric nitrate deposition, microbe-produced nitrate, and stream nitrate were measured to determine the dominant source of stream nitrate in 27 mid-Appalachian headwater forested watersheds (12–771 ha) with varying bedrock geologies, land disturbance histories, and stand ages. The 12 monthly composite nitrate δ 18O values of wet deposition and throughfall exhibited similar pronounced seasonal trends, with relatively depleted δ 18O values during the summer. Wet deposition and throughfall nitrate δ 18O values were not significantly different between northern (Leading Ridge, PA) and southern (Fernow, WV) regional sampling sites, indicating that δ 18O values were spatially similar across the study area. Atmospheric nitrate δ 18O values were significantly greater than microbe-produced nitrate δ 18O values, allowing the two sources of stream nitrate to be separated. During four baseflow and three stormflow sampling periods, microbe-produced nitrate was the dominant (>70%) source of nitrate in the study streams. This result does not mean atmospheric nitrogen deposition should be discounted as a source of forested stream nitrate, because atmospheric deposition is the primary external contributor to the long-term soil nitrogen pool that ultimately drives soil nitrate production rates. Stream nitrate δ 18O values were greater during stormflow periods compared to baseflow periods, indicating greater contributions of atmospheric nitrate during storm events. Neither microbe-produced nitrate δ 18O values from incubated forest soil samples nor stream nitrate δ 18O values showed strong relationships with land disturbance history or stand age. However, watersheds dominated by Pottsville/Allegheny bedrock and associated extremely acid soils had greater summer stream nitrate δ 18O values than watersheds containing predominantly Catskill/Chemung/Pocono and Mauch Chunk/Greenbrier bedrock. Inhibited microbial nitrate production by low soil pH could account for the greater proportions of atmospheric nitrate deposition in streams draining Pottsville/Allegheny bedrock.

Continuous modeling of intermittent stormflows on a semi-arid agricultural catchment

The major barriers to modeling hydrological processes in semi-arid and arid areas are a lack of understanding and model representations of the distinctive features and processes associated with runoff generation in those regions and a paucity of field data. In this study, TOPOG, developed by CSIRO in Australia, was modified by adding model representations of some of the predominant features and processes identified through field and experimental investigations on a semi-arid agricultural catchment with complex-terrain and mixed land use in the Loess Plateau of China. The modified models were then used to continuously simulate both slowly changing hydrologic states during interstorm periods and fast-responding overland and tunnel flows during stormflow periods. Finally, the effectiveness of model simulation under a wide range of rainfall and spatio-temporally varied land cover conditions was tested by comparison of simulated and observed stormflow discharges from both catchment outlet and internal plots. Considerable variability in simulation accuracy was found among storm events and within the catchment. The simulation results highlight the critical importance of crusting and some land management practices on stormflow generation and its spatial variation in this area. Overall, the simulations showed that the model represents reasonably well stormflows generated by rainfall events with recurrence intervals >2 years, which account for more than 60% of runoff and 70% of sediment leaving this area.

Hydrogeochemical responses to rainfall inputs in a small rainforest basin: Rio de Janeiro, Brazil

This work provides basic information about hydrogeochemical contributions to different pathways of water in a small tropical rainforest catchment (3.5 km2) in Rio de Janeiro, Brazil. Precipitation and discharge measurements were conducted at the basin outlet, and at five other stream sites that were selected according to their hydrological behaviour and lithology. Precipitation and streamflow samples were collected weekly, over seven months, and continuously during one storm event. These streamflow samples were analyzed for Ca2+, Mg2+ Na+, K+ and SiO2. Precipitation, throughfall and topsoil water samples were also collected after various storm events, over six months, within a small sub-catchment and were analysed for Ca2+, Mg2+, K+, Na+ and pH. Results from this research showed that rainfall was acid, with low cation content. Throughfall was neutral and cations enriched. Analysis of topsoil water indicated that litter was an important source of Ca2+ and K+ by means of saturation overland flow. In relation to throughfall, streamflow output was characterized by lower concentrations of K+, Ca2+, and Mg2+. Biotite gneiss weathering was the main source of Mg2+ to streamflow. During stormflow periods SiO2 and Na+ concentrations decreased due to Hortonian overland flow contributions to the stream channel. The increasing concentrations of Ca2+ and K+ were due to litter leaching, especially in the zones producing saturation overland flow. Streamflow hydrogeochemistry was not only a product of weathering processes, but also of biotic processes.

Streamflow generation on a small agricultural catchment during autumn recharge: I. Nonstormflow periods

Some sources and controls on nonstorm streamflow from a 19.8 ha agricultural hill land catchment located in east-central Pennsylvania were hypothesized and identified based on 18O, Si, NO 3, Cl and SO 4 concentration patterns. Streamflow, springflow, soil water, seepage and shallow groundwater were sampled routinely from 16 October to 20 November 1989, which included three storms (17–20 October, 9 November and 14 November). The first storm caused this catchment to shift from a low to high flow regime typical of the late autumn period. The chemical patterns in streamflow followed a three-part sequence — stormflow, early post-storm drain-down period, and subsequent post-storm base flow period. The stormflow period was characterized by dilution of NO 3, Cl, SO 4, Si and 18O, whereas the highest NO 3, Cl and SO 4 concentrations and flow rates occurred during the early post-storm period, suggesting that additional subsurface sources were temporarily activated by the storms. The chemical patterns and concentrations during base flow periods tended to be similar irrespective of preceding rainfall, which suggests that subsurface storage is a key control on streamflow chemistry. Nonstorm streamflow included discharge from springs, and near-stream seepage, and groundwater sampling sites located upstream. The spring and stream chemistry were most similar, but differed from the shallow groundwater and seepage, which were nearly twice as concentrated in NO 3. Thus, these sampling sites did not represent all nonstorm streamflow sources. The chemical indices most useful for characterizing the sources and behavior of nonstorm streamflow were NO 3 (agricultural land), SO 4, Cl and NO 3 (storm-activated subsurface source areas), SO 4 (spring contributions), Cl and 18O (dilution or mixing), Si (geochemically controlled and stable — latter nonstorm period; dilution controlled — storm period; kinetically controlled — early post-storm). The ratios of these chemical parameters were examined as indices; Si/Cl and SO 4 NO 3 were the most interesting and provided additional tools and insights for characterizing hydrologic source areas.