First-order Catchment Research Articles

Dilution of 10Be in detrital quartz by earthquake-induced landslides: Implications for determining denudation rates and potential to provide insights into landslide sediment dynamics

The concentration of 10Be in detrital quartz (10Beqtz) from river sediments is now widely used to quantify catchment-wide denudation rates but may also be sensitive to inputs from bedrock landslides that deliver sediment with low 10Beqtz. Major landslide-triggering events can provide large amounts of low-concentration material to rivers in mountain catchments, but changes in river sediment 10Beqtz due to such events have not yet been measured directly. Here we examine the impact of widespread landslides triggered by the 2008 Wenchuan earthquake on 10Beqtz in sediment samples from the Min Jiang river basin, in Sichuan, China. Landslide deposit material associated with the Wenchuan earthquake has consistently lower 10Beqtz than in river sediment prior to the earthquake. River sediment 10Beqtz decreased significantly following the earthquake downstream of areas of high coseismic landslide occurrence (i.e., with greater than ∼0.3% of the upstream catchment area affected by landslides), because of input of the 10Be-depleted landslide material, but showed no systematic changes where landslide occurrence was low. Changes in river sediment 10Beqtz concentration were largest in small first-order catchments but were still significant in large river basins with areas of 104–105 km2. Spatial and temporal variability in river sediment 10Beqtz has important implications for inferring representative denudation rates in tectonically active, landslide-dominated environments, even in large basins. Although the dilution of 10Beqtz in river sediment by landslide inputs may complicate interpretation of denudation rates, it also may provide a possible opportunity to track the transport of landslide sediment. The associated uncertainties are large, but in the Wenchuan case, calculations based on 10Be mixing proportions suggest that river sediment fluxes in the 2–3 years following the earthquake increased by a similar order of magnitude in the 0.25–1 mm and the <0.25 mm size fractions, as determined from 10Beqtz mixing calculations and hydrological gauging, respectively. Such information could provide new insight into sediment transfer, with implications for secondary sediment-related hazards and for understanding the removal of mass from mountains.

Impacts of Three Silvicultural Prescriptions on Sediment Mobility and Water Quality in Headwater Streams of Forested Watersheds in the Upper Gulf Coastal Plain of Mississippi, USA

Headwater streams are very important contributors of water, sediment, and nutrients to the downstream fluvial environment because their source areas compose more than 50-80% of a catchment. We investi- gated the influence of headwater areas (ephemeral and intermittent) on sediment mobility and downstream water quality using selected water quality parameters (temperature, dissolved oxygen, conductivity, and pH) within three first-order catchments in the Upper Gulf Coastal Plain of Mississippi, USA. This study included one year of pre-harvest and two years of post-harvest observations for ephemeral-intermittent streams. Four treatments including two Best Management Practices (BMPs) for ephemeral streams were applied: standard BMP (SBMP) - removal of all merchantable stems while leaving understory intact with minimum surface soil disturbance; modified BMP (MBMP) - same as SBMP with the addition of logging debris to the drainage channel; Clearcut (CC) - total harvest with no BMPs applied; Reference (REF) - left uncut as a con- trol. Forest clearcutting in ephemeral-intermittent streams resulted in substantial impacts to net erosion/ deposition for both channel and hillslope positions and significantly increased total suspend sediment (TSS) within ephemeral-intermittent stream channels. There were no significant differences among BMPs and REF in net erosion/deposition and TSS. However, perennial stream water quality did not change significantly among treatments following harvest. Results imply that the impacts of timber harvesting were ameliorated by implementing two ephemeral-intermittent stream BMPs during harvesting activities. These findings support the use of either SBMP or MBMP for conserving ephemeral-intermittent streams. While the addi- tion of logging debris in the channel of MBMP did not substantially reduce erosion and sediment flux when compared to SBMP, there may be operational benefits to permitting using a natural sediment baffle because it reduces operational constraints allowing for a greater timber yield.

Debris-flow susceptibility analysis using fluvio-morphological parameters and data mining: application to the Central-Eastern Pyrenees

Based on debris-flow inventories and using a geographical information system, the susceptibility models presented here take into account fluvio-morphologic parameters, gathered for every first-order catchment. Data mining techniques on the morphometric parameters are used, to work out and test three different models. The first model is a logistic regression analysis based on weighting the parameters. The other two are classification trees, which are rather novel susceptibility models. These techniques enable gathering the necessary data to evaluate the performance of the models tested, with and without optimization. The analysis was performed in the Catalan Pyrenees and covered an area of more than 4,000 km2. Results related to the training dataset show that the optimized models performance lie within former reported range, in terms of AUC, although closer to the lowest end (near 70 %). When the models are applied to the test set, the quality of most results decreases. However, out of the three different models, logistic regression seems to offer the best prediction, as training and test sets results are very similar, in terms of performance. Trees are better at extracting laws from a training set, but validation through a test set gives results unacceptable for a prediction at regional scale. Although omitting parameters in geology or vegetation, fluvio-morphologic models based on data mining, can be used in the framework of a regional debris-flow susceptibility assessment in areas where only a digital elevation model is available.

How mineralogy and slope aspect affect REE release and fractionation during shale weathering in the Susquehanna/Shale Hills Critical Zone Observatory

Abstract To understand the factors that control rare earth elements (REE) release and fractionation during shale weathering, we investigate the REE contents in solid (bedrock, regolith, stream sediments), and natural waters (stream, and pore waters) from a first-order catchment developed entirely on gray shales in central Pennsylvania, USA. Up to 65% of the REE (relative to parent bedrock) is depleted from the weathering profiles in the acidic and organic-rich soils due to chemical leaching. In addition, newly formed fine particles were also lost along with the down-slope movement of soil waters. Weathering profiles on the south-facing slope show less depletion of REE than those on the north-facing slope (33% vs. 45% on average). We hypothesize that the different degrees of REE depletion on the two transects reflect a history of different chemical weathering rates and possible different surface erosion rates, controlled by contrasting slope aspect-induced microclimate conditions. In addition, weathering profiles, natural waters and sequential extractions all show a preferential removal of Middle REE (up to 22% more) relative to Light REE and Heavy REE during shale weathering, due to preferential release of MREE from rhabdophane. Furthermore, the long-term phosphate mineral dissolution rates (e.g., rhabdophane) were estimated at 10 − 15 to 10 − 14 mol m − 2 s − 1 under field conditions, based on REE depletion profiles. Strong positive Ce anomalies (average [Ce/Ce*] N value: 1.79) observed in the regolith, stream sediments, and regolith extractions point to the fractionation and preferential precipitation of Ce as compared to other REE, due to the generally oxidizing conditions during release, transport, and redistribution of REE in the surface and subsurface environments. Positive Eu anomalies (average [Eu/Eu*] N value: 1.30) observed in the natural waters of the catchment are attributed to weathering of plagioclases in the shale bedrock. This study highlights the use of REE as natural tracers for low-temperature geochemical processes.

Geochemical behaviors of different element groups during shale weathering at the Susquehanna/Shale Hills Critical Zone Observatory

► Statistical analysis on element concentrations of bedrock, regolith, and sediments. ► Element groups with different geochemical behaviors during soil formation. ► Mineralogical, cycling, and geochemical controls during chemical weathering. Major and trace element concentrations were measured in bedrock, regolith and stream sediments from a first-order catchment developed entirely on grey shale in central Pennsylvania, USA. These elements can be classified into five major groups based on statistical data analysis. The presence of different elemental groups is due to the mineralogical origin, cycling processes, and geochemical properties of these elements during soil formation. A better understanding of the behaviors of these elements during chemical weathering would allow for their possible use as natural tracers in Critical Zone processes.

Loss of genetic diversity in the North American mayfly Ephemerella invaria associated with deforestation of headwater streams

Summary 1. Terrestrial dispersal by aquatic insects increases population connectivity in some stream species by allowing individuals to move outside the structure of the stream network. In addition, individual survival and reproductive success (as well as dispersal) are tightly linked to the quality of the terrestrial habitat. 2. In historically forested catchments, deforestation and altered land use have the potential to interfere with mayfly dispersal or mating behaviours by degrading the quality of the terrestrial matrix among headwater streams. We hypothesised that loss of tree cover in first-order catchments would be associated with an increase in population substructure and a decrease in genetic diversity of mayfly populations. 3. To test this hypothesis, we investigated spatial patterns of genetic variation in the common mayfly Ephemerella invaria across a gradient of deforestation in the central piedmont region of eastern United States. Intraspecific genetic diversity and population substructure were estimated from data obtained using fluorescent amplified fragment length polymorphism (AFLP) markers. 4. We found that mayfly populations had low population substructure within headwater stream networks and that genetic diversity was strongly negatively correlated with mean deforestation of the first-order catchments. The large-scale pattern of population substructure followed a pattern of isolation by distance (IBD) in which genetic differentiation increases with geographical distance, but assignment tests placed a few individuals into populations 300 km away from the collection site. 5. Our results show that loss of genetic diversity in this widespread aquatic insect species is co-occurring with deforestation of headwater streams. 6. Most arguments supporting protection of headwater streams in the United States have centred on the role of these streams as hydrological and biogeochemical conduits to downstream waters. Our work suggests that headwater stream land use, and specifically tree cover, may have a role in the maintenance of regional genetic diversity in some common aquatic insect species.

A channel head stability criterion for first-order catchments

The present analysis derives a stability criterion for long-term equilibrium channel heads. The concept of finite perturbation analysis is presented, during which the surface is subjected to perturbations of a finite amplitude and resulting changes in flow path structure and slope are computed. Based on these quantities the analysis predicts whether the perturbed location is going to erode, be filled in or remain steady. The channel head is defined geometrically as the focus point of converging flow lines at the bottom of hollows. It is demonstrated that stability at the channel head grows out of the competition between the rate of flow path convergence and the degree of profile concavity. Analytical functions are derived to compute channel head-contributing area and -slope, flow path convergence and profile concavity as a function of perturbation depth, distance from the crest and the initial slope. In a numerical model these quantities point to the long-term equilibrium channel head position, which is shown to depend also on the width to length ratio of hollows. It is also demonstrated that the equilibrium channel head position is sensitive to the base-level lowering/non-dimensional slope length ratio and to the slope of the initial topography. Morphometrical measurements both in the field and on simulated topographies were used to test the theoretical predictions.

First-order catchment mass balance during the wet season in the Panama Canal Watershed

Summary Tropical hydrology is poorly understood for a number of reasons. Intense biological activity in the tropics introduces complexities to the hydrologic process. Bioturbation, rapid rates of decay, and intensive insect activity all tend to promote rapid flow paths in the upper soil. Aggressive weathering leads to clays depleted of light cations and deep soil profiles. Processes in the seasonal tropics are further complicated by seasonal transitions, and very large changes in catchment storage between seasons. Beginning in 2005, we installed a suite of hydrologic sensors in a 16.7 ha first-order catchment in the Panama Canal Watershed to observe hydrologic variables and identify the dominant streamflow generation processes. The site is located near the village of Gamboa, which is located on the east bank of the Panama Canal at the confluence of Lake Gatun and the Chagres River. The study catchment is located on the north side of a ridge off the eastern flank of a 230 m tall hill known as Cerro Pelado, and is covered by 70–120 year old re-growth triple-canopy forest. Measurements included: rainfall above the canopy, throughfall, stemflow, evapotranspiration, shallow groundwater levels and streamflow. Deep groundwater storage was not measured. This paper describes measurements made, data collected, and the worth of those data in estimating the mass balance closure of a first-order catchment during the wet season. We compare measurements of the different components of the water cycle with observations from other published studies from the tropics. Data analysis results indicate water balance closure errors of approximately 8%.

Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory

Located in the uplands of the Valley and Ridge physiographic province of Pennsylvania, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a tectonically quiescent, first-order catchment developed on shales of the Silurian Rose Hill Formation. We used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical/chemical weathering fluxes. About 25 m of bedrock was also drilled to estimate parent composition. Depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate-weathering front. Overlying this, extending from ∼6 m below the bedrock–soil interface up into the soil, is the feldspar dissolution front. In the soils, depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock define the illite and chlorite reaction fronts. When combined with a cosmogenic nuclide-derived erosion rate on watershed sediments, these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite (1–5 × 10 −17 mol m −2 s −1) and illite (2–9 × 10 −17 mol m −2 s −1) than observed in the laboratory. Mineral reactions result in formation of vermiculite, hydroxy-interlayered vermiculite, and minor kaolinite. During weathering, exchangeable divalent cations are replaced by Al as soil pH decreases. The losses of Mg and K in the soils occur largely as solute fluxes; in contrast, losses of Al and Fe are mostly as downslope transport of fine particles. Physical erosion of bulk soils also occurs: results from a steady-state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions. Chemical weathering losses of Mg, Na, and K are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters. Chemical weathering slows down in the valley floor and Al and Si even show net accumulation. The simplest model for the hillslope that is consistent with all observations is a steady-state, clay weathering-limited system where soil production rates decrease with increasing soil thickness.

Organic and inorganic carbon concentrations and fluxes from managed and unmanaged boreal first-order catchments

Seasonal and between stream variation (catchment dependent variation) in losses of organic and inorganic carbon via downstream transport and outgassing of CO 2 into the atmosphere were studied in 11 small boreal catchments situated in close proximity to each other. Of these catchments four were undrained peatland rich catchments, four drained peatland rich catchments and three managed mineral soil-dominated catchments. Downstream export of total inorganic carbon (TIC) varied between 870 and 1400 kg km − 2 a − 1 and was rather consistent between the catchments, except in the case of the mineral soil-dominated catchment Kangaslampi, where export was only 420 kg km − 2 a − 1 . The export of total organic carbon (TOC) varied between 2300 and 14,800 kg km − 2 a − 1 and was highest in peatland rich catchments. Peatland drainage decreased TIC and TOC concentrations in the long term, but did not affect lateral carbon export due to increased runoff from the catchments. Partial pressure of CO 2 in streams was the highest in undrained peatland rich catchments, but the outgassing of CO 2 into the atmosphere was also high from drained peatlands due to the higher discharge rate and long ditch networks. In mineral soil-dominated catchments both downstream export of carbon and emission into the atmosphere were low. TOC exports were compared in two climatically different years (2003 and 2007). The results indicate that climate change might alter the timing of the TOC export from the catchments, the importance of the spring ice melt diminishing and both snow cover and snow free period export increasing.

Long-term changes in climate, streamflow, and nutrient budgets for first-order catchments at the Experimental Lakes Area (Ontario, Canada)This paper is part of the series “Forty Years of Aquatic Research at the Experimental Lakes Area”.

We analyzed 36 years of records for climate, hydrology, and the chemistry of precipitation and runoff from the Lake 239 (Rawson Lake) catchment at the Experimental Lakes Area (Ontario, Canada). No evidence of significant periodicity was found for any variable in the Experimental Lakes Area records. The longer (67-year) record from nearby Kenora, Ontario, revealed a 31-year cycle in precipitation but not temperature. Significant increasing long-term trends at the Experimental Lakes Area were found for annual and winter temperature and summer precipitation and deposition of ammonium, total nitrogen (TN), and total dissolved nitrogen (TDN). Large (80%) increases in ammonium deposition appear to be related to higher emissions in the central United States. A significant negative trend for dissolved organic carbon (DOC) in precipitation cannot be explained. In runoff, significant positive trends were found for TDN and DOC in the northeast catchment and TN, TDN, ammonium, and DOC in the northwest. Significant negative trends were found for TDP and nitrate in the northeast. Significant increases were found for retention of ammonium in all catchments and DOC in the northeast and northwest. There was no apparent relationship between forest succession and retention of nutrients. Results are compared with those from three other long-term monitoring sites in eastern North America.

Impact of Sedimentation on Wetland Carbon Sequestration in an Agricultural Watershed

Landscape redistribution of soil C is common within agricultural ecosystems. Little is known about the effects of upland sediment deposition on C dynamics within riparian wetlands. To assess sedimentation impact, we obtained profile samples of wetland soil and used the combination of (137)Cs, (210)Pb, and (14)C chronological markers to determine rates of C sequestration and mineral deposition over the history of a wetland within a first-order catchment under agricultural management in the coastal plains of the United States. Substantial post settlement deposition in the wetland soil was evidenced in places by a 20- to 40-cm layer of mineral soil that buried the original histosol. Soil profiles contained a minimum in C content within the top 35 cm of the profile which originated from a rapid deposition from low C upland soils. Radiocarbon and radioisotope dating showed that increases in C above this minimum were the result of C sequestered in the past approximately 50 yr. Modeling the kinetics of modern C dynamics using the (137)Cs and (210)Pb markers within these surface profiles provides strong evidence for accelerated C sequestration associated with mineral sediment deposition in the ecosystem. These findings indicate that at the landscape scale, dilution of ecosystem C by import of low C upland sediment into wetlands stimulates C sequestration by pulling soil C content below some pedogenic equilibrium value for the ecosystem. They also indicate that over the history of the wetland, rates of C accretion may be linked to mineral soil deposition.

Spatial variability of CO 2 exchange for riparian and open grasslands within a first-order agricultural basin in Southern Ontario

Agriculture is one of the most widespread landuse types in Ontario, Canada, where cultivated lands are often located within small first-order catchments with extensive grass-dominated riparian areas. These grass-dominated areas may be significant to the overall catchment carbon functioning as they are often high in soil organic matter and elevated biological productivity due to the availability of agricultural nutrients. As such, this study examines midday CO 2 exchange between the atmosphere and two grass-dominated riparian areas (one adjacent to a cultivated disk-tilled cornfield, one adjacent to a no-till grassland) and an open area grassland (no-till field) in a predominantly agricultural basin in Southern Ontario using daytime dynamic closed-chamber measurements from May 7 to October 27, 2003. Soils in this catchment are generally comprised of 50% sand, 35% silt and 15% clay, with riparian areas containing an organic layer approximately 0.20–0.40 m deep. The influence of nearby agriculture on riparian areas is apparent, as larger rates of CO 2 exchange were correlated with elevated soil nutrients as well as above ground biomass and canopy height, which were larger in the riparian zone influenced by intensive agriculture (Rip-Corn) than the grassland-influenced riparian area (Rip-Grass) and the adjacent grassland field (Grass). Spatial patterns in soil and vegetation respiration ( R Tot), net ecosystem CO 2 exchange (NEE) and gross ecosystem productivity (GEP) for all sites showed strong positive relationships with soil characteristics such as total nitrogen, carbon–nitrogen ratio and above-ground biomass. Our results show that grass-dominated riparian and non-riparian areas, with similar vegetation that appear to be homogenous, located approximately 250–300 m from one another, may exhibit very different CO 2 fluxes. In general, CO 2 exchange is much greater in riparian areas than in the adjacent upland grasslands, and this is more exaggerated where those upland sites are tilled and receive inorganic fertilizers. Furthermore, our results indicate that the effects of land-use (i.e. agriculture) over-ride the effects of microclimate in controlling spatial patterns in CO 2 exchange in this watershed. This highlights the need to better assess CO 2 fluxes from heterogeneous agricultural landscapes, and emphasizes that estimates based on data grouped solely on soil or vegetation units can be rather conservative and may not capture the inherent spatial variability and small scale processes that drive CO 2 exchange.

Measurement and prediction of natural and anthropogenic sediment sources, St. John, U.S. Virgin Islands

A quantitative understanding of both natural and anthropogenic sediment sources is needed to accurately assess and predict the potentially adverse effects of land development on aquatic ecosystems. The main objective of this study was to quantify sediment production and delivery rates in a dry tropical environment on the island of St. John in the eastern Caribbean. One to three years of measurements were used to determine values and empirical functions for estimating sediment production from streambanks, treethrow, undisturbed hillslopes, zero-order sub-catchments, unpaved road surfaces, and road cutslopes. Sediment production also was measured from both undisturbed and roaded first-order sub-catchments. Among natural sources of sediment, streambanks had the highest mean erosion rate at 100 Mg ha − 1 yr − 1 . The uprooting of trees along stream margins is estimated to generate approximately of 0.2 Mg of sediment per kilometer of stream per year, or about 0.1 Mg ha − 1 yr − 1 for a stream corridor that consists of a 9-m wide channel and a 3-m wide buffer zone. Undisturbed 40 m 2 hillslope plots generated 0.01 to 0.27 Mg ha − 1 yr − 1 . Mean sediment yields from undisturbed zero- and first-order catchments were only 0.01 and 0.08 Mg ha − 1 yr − 1 , respectively. Unpaved roads that were graded at least every other year had sediment production rates ranging from 57 Mg ha − 1 yr − 1 for a road with a 2% slope to 580 Mg ha − 1 yr − 1 for a road with a 21% slope. Sediment production rates from ungraded roads were about 40% lower than those from recently graded roads, while production rates from steep abandoned roads were only 12 Mg ha − 1 yr − 1 . Cutslope sediment production rates ranged from 20 to 170 Mg ha − 1 yr − 1 , but their contribution to sediment yields at the road segment scale was relatively small. Since unpaved roads increase hillslope-scale sediment production rates by several orders of magnitude, the first-order catchments with unpaved roads had sediment yields that were at least five times higher than undisturbed catchments. The relative importance of each sediment source varies from catchment to catchment as a result of the abundance and spatial distribution of landscape types. The values and predictive functions developed in this study have been incorporated into a GIS-based model to predict catchment-scale sediment yields. Application of this model to three basins in St. John suggest that unpaved roads are currently the dominant sediment source, and that they are responsible for increasing watershed-scale sediment yields by 3–9 times relative to undisturbed conditions. Both the data from the present study and the GIS model can help estimate sediment production and catchment-scale sediment yields in similar environments.

Water and nitrogen processes along a typical water flowpath and streamwater exports from a forested catchment and changes after clear-cutting: a modelling study

Abstract. A two dimensional model, FEMMA, to describe water and nitrogen (N) fluxes within and from a forested first-order catchment (Kangasvaara in Eastern Finland) was constructed by linking the most significant processes affecting the fluxes of water, ammonium, nitrate and dissolved organic nitrogen along a hillslope from the water divide to the stream. The hillslope represents the average flowpath of water in the catchment and the model was used to estimate the N fluxes for a catchment in eastern Finland before and after clear-cutting. The simulated results were in reasonable agreement with the nitrate, dissolved organic N and dissolved total N measurements from the study catchment and with other results in the literature. According to the simulations, the major sinks of N after clear-cutting were immobilisation by soil microbes, uptake by ground vegetation and sorption to soil. These sinks increased downslope from the clear-cut area, indicating the importance of an uncut buffer zone between the stream and the clear-cut area in reducing N exports. The buffer zone retained 76% of the N flux coming from the clear-cut area. Nitrification was a key process in controlling the N export after clear-cutting and N increases were mainly as nitrate. Most of the annual N export took place during the spring flood, when uptake of N by plants was minimal.

Spatial Variability of Soil Phosphorus in Relation to the Topographic Index and Critical Source Areas

A measure of soil P status in agricultural soils is generally required for assisting with prediction of potential P loss from agricultural catchments and assessing risk for water quality. The objectives of this paper are twofold: (i) investigating the soil P status, distribution, and variability, both spatially and with soil depth, of two different first-order catchments; and (ii) determining variation in soil P concentration in relation to catchment topography (quantified as the "topographic index") and critical source areas (CSAs). The soil P measurements showed large spatial variability, not only between fields and land uses, but also within individual fields and in part was thought to be strongly influenced by areas where cattle tended to congregate and areas where manure was most commonly spread. Topographic index alone was not related to the distribution of soil P, and does not seem to provide an adequate indicator for CSAs in the study catchments. However, CSAs may be used in conjunction with soil P data for help in determining a more "effective" catchment soil P status. The difficulties in defining CSAs a priori, particularly for modeling and prediction purposes, however, suggest that other more "integrated" measures of catchment soil P status, such as baseflow P concentrations or streambed sediment P concentrations, might be more useful. Since observed soil P distribution is variable and is also difficult to relate to nationally available soil P data, any assessment of soil P status for determining risk of P loss is uncertain and problematic, given other catchment physicochemical characteristics and the sampling strategy employed.

Relationships of Land Use and Stream Solute Concentrations in the Ipswich River Basin, Northeastern Massachusetts

The relationships of land use/land cover (LULC) on major solute concentrations in stream water were investigated for the Ipswich River basin (404 km2) in northeastern Massachusetts. Stream water was sampled seven times during base flow in 43 first-order catchments and four times in 28 second- and third-order catchments. Regression analysis of the first-order catchment data indicates that NO3−, acid neutralizing capacity (ANC), Cl−, SO42−, and the base cations had positive, mostly exponential relationships with the increasing extent of urban + agricultural area (P < 0.05), whereas dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) had positive, exponential relationships with the increasing extent of wetland + open water (P < 0.05). Solute sources responsible for many of these relationships are human-derived constituents found in septic effluent, fertilizers, and road salts. In contrast to more conservative solutes, concentrations of NO3− in the first-order streams were commonly higher than in those of second- and third-order streams with similar proportions of urban + agricultural land use. Using LULC subclasses (e.g., high density residential), as well as the proportions of LULC in 50, 100, and 200 m concentric zones bordering streams, generally decreased the relationships (r2) determined above. Hence, the disturbed area of the entire subbasin was the best descriptor of streamwater solute concentrations. Potassium concentrations in stream water had stronger relationships than any other ion, yet these explained < 60% of the variability, indicating that there are a number of important ancillary factors that affect streamwater solute composition in the Ipswich River basin.

Sulphate and base cation concentrations and export in streams from unmanaged forested catchments in Finland

Sulphate and base cation (Ca 2+, Mg 2+, K +, Na +) concentrations and export, and their relationships with catchment characteristics, were studied in stream water flowing out of unmanaged forested catchments in Finland. Data were collected during 1997–1999 from 21 first-order catchments, and they were combined with a corresponding data from another 19 catchments monitored in other studies during periods ranging from 1978 to 1999. The results showed that the concentrations of SO 4 2− ,K + and Na decreased in streams on moving from the south to the north, and that the decrease was most pronounced for SO 4 2. In the export of elements from the catchments, a similar regional pattern was observed for SO 4 2− only. The south–north decrease in stream water concentrations was mainly explained by the level of atmospheric deposition of SO 4 2−. The average forest site type on the whole catchment area, which describes the productivity of the sites had a significant positive correlation with the concentrations and export of SO 4 2− and Ca 2+, Mg 2+, K + and Na +, probably reflecting higher base cation concentrations in the soil on fertile sites. There were exceptionally high concentrations and exports of sulphate and base cations in some of the streams compared to the general pattern for the whole country, probably reflecting special bedrock or soil types or the vicinity to local deposition sources.

Sulphate and base cation concentrations and export in streams from unmanaged forested catchments in Finland

Sulphate and base cation (Ca2+, Mg2+, K+, Na+) concentrations and export, and their relationships with catchment characteristics, were studied in stream water flowing out of unmanaged forested catchments in Finland. Data were collected during 1997–1999 from 21 first-order catchments, and they were combined with a corresponding data from another 19 catchments monitored in other studies during periods ranging from 1978 to 1999. The results showed that the concentrations of SO42− ,K+ and Na decreased in streams on moving from the south to the north, and that the decrease was most pronounced for SO42. In the export of elements from the catchments, a similar regional pattern was observed for SO42− only. The south–north decrease in stream water concentrations was mainly explained by the level of atmospheric deposition of SO42−. The average forest site type on the whole catchment area, which describes the productivity of the sites had a significant positive correlation with the concentrations and export of SO42− and Ca2+, Mg2+, K+ and Na+, probably reflecting higher base cation concentrations in the soil on fertile sites. There were exceptionally high concentrations and exports of sulphate and base cations in some of the streams compared to the general pattern for the whole country, probably reflecting special bedrock or soil types or the vicinity to local deposition sources.

Spatial distribution of surface conditions and runoff generation in small arid watersheds, Zin Valley Badlands, Israel

High rill density may be regarded as indicative of frequent and integrated runoff along a drainage network. However, field observations of soil development and rill geometry in small, first-order catchments (0.1 to 1 ha) of the Zin Valley Badlands, northern Negev, Israel, suggest a pattern of partial area contribution and frequent flow discontinuities along hillslopes and channels. Changing soil properties, associated with an increase of slope angle and slope length, appear to be responsible for high infiltration on the slopes, leading to nonuniform runoff generation within small drainage basins. Runoff observation, sprinkling tests, and soil analysis along ridges and sideslopes were carried out to test this hypothesis. The results confirmed that infiltration capacity on the sideslopes is significantly higher than on the ridges. Under current rainfall conditions, only extreme rainfall intensities are sufficient to generate runoff along entire slopes. The discontinuous nature of most runoff events causes erosion on ridges and deposition on slopes, which enhances soil development on the valley sideslopes, creating a positive feedback on infiltration rate and depth. This demonstrates that the links between within-storm rainfall conditions and spatial distribution of soil characteristics have to be incorporated into our understanding of landscape development in badlands.