Runoff Sources Research Articles

Changes in suspended sediment to solute yield ratios from an alpine basin during the transition to winter, southern alps, new zealand

.Sediment yields from alpine environments are often assumed to significantly decrease during the transition from late summer to winter as declining temperatures reduce runoff and therefore sediment mobilization. We monitored suspended sediment and solute flux during this period in an 8.2 km2 catchment in the Southern Alps, New Zealand that contained a high elevation upper catchment and a smaller, low elevation subcatchment. Totals of 248 t of suspended sediment and 166 t of solute were exported over 91 days of measurement: a seasonal suspended sediment to solute yield ratio of 60:40 (or 1:0.67). This ratio differs from annual ratios of around 90:10 (or 1:0.11) reported elsewhere in the Southern Alps. Three phases of sediment transport were identified. Sediment yields in Phase 1 (28 days) and Phase 2 (30 days) were dominated by exports of suspended sediment (155 t) and less solute (94 t). Full winter conditions were experienced in Phase 3 (33 days) when upstream suspended sediment (17 t) and solute (29 t) yields decreased due to reduced discharge and freezing conditions. With this decline, the lower elevation subcatchment became the dominant runoff source and downstream suspended sediment (93 t) and solute (92 t) yields increased. We estimate that 20% of the annual solute yield was transferred during the transition to winter and 10% of the combined annual suspended sediment and solute yield. Overall, the results from this study suggest that suspended sediment to solute yield ratios are: 1) sensitive to seasonal changes in meteorological conditions, and 2) more significant during the transition to winter than previously recognised.

Ecogeomorphology in the Australian drylands and the role of biota in mediating the effects of climate change on landscape processes and evolution

Abstract Australian dryland landscape has developed under the influence of aridity, low relief, tectonic stability and biota adapted to nutrient and water scarcity. The biota in general, but notably the plants, mediates the impact of water scarcity and of climate change on ecohydrological and geomorphological processes. It reduce the partitioning of rain into overland flow, and so limit soil erosion, notably through the development of patch structures that partition the landscape into local runoff sources and runon sinks. In large rain events, when flow does reach ephemeral streams, channel-associated plants again modify flow conditions, reducing flow speeds and flow competence. Given the diverse influences of biota on landscape processes, it is argued that it likewise moderated the effects of Quaternary and Holocene climate change. Field evidence from Australian and other drylands suggests that the effect of changing land surface properties on runoff and erosion may exceed the effect of moderate climate change. Knowledge of the role of dryland biota and its role in land surface change is therefore a prerequisite to understanding the responses of landscapes to climate change, to understanding the complex spatio-temporal variability in landscape development, and to developing the ability to correctly interpret the alluvial record of changing geomorphological processes in terms of changes in climate and other external drivers.

Factors and interactions controlling infiltration, runoff, and soil loss at the microscale in a patchy Mediterranean semiarid landscape

AbstractIn semiarid ecosystems, the transfer of water, sediments, and nutrients from bare to vegetated areas is known to be crucial to ecosystem functioning. Rainfall simulation experiments were performed on bare‐soil and vegetated surfaces, on both wet and dry soils, in semiarid shrub‐steppe landscapes of SE Spain to investigate the spatial and temporal factors and interactions that control the fine‐scale variation in water infiltration, runoff and soil loss, and hence the water and sediment flows in these areas. Three types of shrub‐steppe landscapes varying in plant community and physiography, and four types of plant patches (oak shrub, subshrub, tussock grass, and short grass mixed with chamaephytes) were studied. Higher infiltration and lower runoff and soil loss were measured on vegetation patches than on bare soils, for both dry and wet conditions. The oak‐shrub patches produced no runoff, while the subshrub patches showed the highest runoff and soil loss. Despite these differences among patch types, the influence of vegetation patch type on the variables analysed was not significant. The response of bare soil surfaces clearly varied between landscape types, yet the differences were only relevant under dry soil conditions. Stone cover, particularly the cover of embedded stones, and crust cover, were the key explanatory variables for the hydrological behaviour of bare soils. The study documents quantitatively how bare soils and vegetation patches function as runoff sources and runoff sinks, respectively, for a wide range of soil moisture conditions, and illustrates that landscape‐type effects on bare‐soil runoff sources may also exert an important control on the site hydrology, while the role of the vegetation patch type is less important. The effects of the control factors are modulated by antecedent soil moisture, with dry soils showing the most contrasting soil water infiltration between landscapes and surface types. Copyright © 2009 John Wiley & Sons, Ltd.

Sources of land-derived runoff to a coral reef-fringed embayment identified using geochemical tracers in nearshore sediment traps

Geochemical tracers, including Ba, Co, Th, 7Be, 137Cs and 210Pb, and magnetic properties were used to characterize terrestrial runoff collected in nearshore time-series sediment traps in Hanalei Bay, Kauai, during flood and dry conditions in summer 2006, and to fingerprint possible runoff sources in the lower watershed. In combination, the tracers indicate that runoff during a flood in August could have come from cultivated taro fields bordering the lower reach of the river. Land-based runoff associated with summer floods may have a greater impact on coral reef communities in Hanalei Bay than in winter because sediment persists for several months. During dry periods, sediment carried by the Hanalei River appears to have been mobilized primarily by undercutting of low 7Be, low 137Cs riverbanks composed of soil weathered from tholeiitic basalt with low Ba and Co concentrations. Following a moderate rainfall event in September, high 7Be sediment carried by the Hanalei River was probably mobilized by overland flow in the upper watershed. Ba-desorption in low-salinity coastal water limited its use to a qualitative runoff tracer in nearshore sediment. 210Pb had limited usefulness as a terrestrial tracer in the nearshore due to a large dissolved oceanic source and scavenging onto resuspended bottom sediment. 210Pb-scavenging does, however, illustrate the role resuspension could play in the accumulation of particle-reactive contaminants in nearshore sediment. Co and 137Cs were not affected by desorption or geochemical scavenging and showed the greatest potential as quantitative sediment provenance indicators in material collected in nearshore sediment traps.

GIS‐Based Predictive Models of Hillslope Runoff Generation Processes1

Abstract: Successful nonpoint source pollution control using best management practice placement is a complex process that requires in‐depth knowledge of the locations of runoff source areas in a watershed. Currently, very few simulation tools are capable of identifying critical runoff source areas on hillslopes and those available are not directly applicable under all runoff conditions. In this paper, a comparison of two geographic information system (GIS)‐based approaches: a topographic index model and a likelihood indicator model is presented, in predicting likely locations of saturation excess and infiltration excess runoff source areas in a hillslope of the Savoy Experimental Watershed located in northwest Arkansas. Based on intensive data collected from a two‐year field study, the spatial distributions of hydrologic variables were processed using GIS software to develop the models. The likelihood indicator model was used to produce probability surfaces that indicated the likelihood of location of both saturation and infiltration excess runoff mechanisms on the hillslope. Overall accuracies of the likelihood indicator model predictions varied between 81 and 87% for the infiltration excess and saturation excess runoff locations respectively. On the basis of accuracy of prediction, the likelihood indicator models were found to be superior (accuracy 81‐87%) to the predications made by the topographic index model (accuracy 69.5%). By combining statistics with GIS, runoff source areas on a hillslope can be identified by incorporating easily determined hydrologic measurements (such as bulk density, porosity, slope, depth to bed rock, depth to water table) and could serve as a watershed management tool for identifying critical runoff source areas in locations where the topographic index or other similar methods do not provide reliable results.

Modelling variable source area dynamics in a CEAP watershed

AbstractIn the Northeast US, saturation excess is the most dominant runoff process and locations of runoff source areas, typically called variable source areas (VSAs), are determined by the available soil water storage and the landscape topographic position. To predict runoff generated from VSAs some water quality models use the Soil Conservation Service Curve Number equation (SCS‐CN), which assumes a constant initial abstraction of rainfall is retained by the watershed prior to the beginning of runoff. We apply a VSA interpretation of the SCS‐CN runoff equation that allows the initial abstraction to vary with antecedent moisture conditions. We couple this modified SCS‐CN approach with a semi‐distributed water balance model to predict runoff, and distribute predictions using a soil topographic index for the Town Brook watershed in the Catskill Mountains of New York State. The accuracy of predicted VSA extents using both the original and the modified SCS‐CN equation were evaluated for 14 rainfall‐runoff events through a comparison with average water table depths measured at 33 locations in Town Brook from March–September 2004. The modified SCS‐CN equation captured VSA dynamics more accurately than the original equation. However, during events with high antecedent rainfall VSA dynamics were still under‐predicted suggesting that VSA runoff is not captured solely by knowledge of the soil water deficit. Considering the importance of correctly predicting runoff generation and pollutant source areas in the landscape, the results of this study demonstrate the feasibility of integrating VSA hydrology into water quality models to reduce non‐point source pollution. Copyright © 2009 John Wiley & Sons, Ltd.

Multivariate statistical study of organic pollutants in Nanjing reach of Yangtze River

Multivariate statistical approach was used to identify the source apportionment of 25 semi-volatile compounds (SVOCs) in surface water in Nanjing reach of Yangtze River, China. Surface water samples were collected from six sites in January 2007. Concentrations of SVOCs were determined by gas chromatography equipped with a mass spectrometry detector (GC–MS). Cluster analysis (CA) and principal component analysis-multiple linear regression (PCA-MLR) were used for evaluation of contamination status, distribution and source apportionment. The results of CA showed that the six sites in this study could be divided into two significant groups, i.e. low and high pollutant groups. PCA-MLR results suggested that four source types were identified, explaining about 84.5% of the total variance. Of four sources, industrial and domestic sewage sources contributed 43.5–64.8% of various SVOCs in six sites, precipitance and runoff sources 4.22–42.87%, shipping sources 4.22–23.49%, and other miscellaneous sources 1.05–8.97%. These results indicated that point source pollutants affected the water quality of this region. High correlation coefficient ( R 2 = 0.973) between the measured and predicted concentrations of SVOCs suggested the applicability of the PCA-MLR for estimation of source contribution to the SVOCs in water.

Hydrologic connectivity between landscapes and streams: Transferring reach‐ and plot‐scale understanding to the catchment scale

The relationship between catchment structure and runoff characteristics is poorly understood. In steep headwater catchments with shallow soils the accumulation of hillslope area (upslope accumulated area (UAA)) is a hypothesized first‐order control on the distribution of soil water and groundwater. Hillslope‐riparian water table connectivity represents the linkage between the dominant catchment landscape elements (hillslopes and riparian zones) and the channel network. Hydrologic connectivity between hillslope‐riparian‐stream (HRS) landscape elements is heterogeneous in space and often temporally transient. We sought to test the relationship between UAA and the existence and longevity of HRS shallow groundwater connectivity. We quantified water table connectivity based on 84 recording wells distributed across 24 HRS transects within the Tenderfoot Creek Experimental Forest (U.S. Forest Service), northern Rocky Mountains, Montana. Correlations were observed between the longevity of HRS water table connectivity and the size of each transect's UAA (r2 = 0.91). We applied this relationship to the entire stream network to quantify landscape‐scale connectivity through time and ascertain its relationship to catchment‐scale runoff dynamics. We found that the shape of the estimated annual landscape connectivity duration curve was highly related to the catchment flow duration curve (r2 = 0.95). This research suggests internal catchment landscape structure (topography and topology) as a first‐order control on runoff source area and whole catchment response characteristics.

Using nitrate dual isotopic composition (δ15N and δ18O) as a tool for exploring sources and cycling of nitrate in an estuarine system: Elkhorn Slough, California

Nitrate (NO3−) concentrations and dual isotopic composition (δ15N and δ18O) were measured during various seasons and tidal conditions in Elkhorn Slough to evaluate mixing of sources of NO3− within this California estuary. We found the isotopic composition of NO3− was influenced most heavily by mixing of two primary sources with unique isotopic signatures, a marine (Monterey Bay) and terrestrial agricultural runoff source (Old Salinas River). However, our attempt to use a simple two end‐member mixing model to calculate the relative contribution of these two NO3− sources to the Slough was complicated by periods of nonconservative behavior and/or the presence of additional sources, particularly during the dry season when NO3− concentrations were low. Although multiple linear regression generally yielded good fits to the observed data, deviations from conservative mixing were still evident. After consideration of potential alternative sources, we concluded that deviations from two end‐member mixing were most likely derived from interactions with marsh sediments in regions of the Slough where high rates of NO3− uptake and nitrification result in NO3− with low δ15N and high δ18O values. A simple steady state dual isotope model is used to illustrate the impact of cycling processes in an estuarine setting which may play a primary role in controlling NO3− isotopic composition when and where cycling rates and water residence times are high. This work expands our understanding of nitrogen and oxygen isotopes as biogeochemical tools for investigating NO3− sources and cycling in estuaries, emphasizing the role that cycling processes may play in altering isotopic composition.

Retrofit Storm Water Retention Volume for Low Impact Development

The concept of low impact development (LID) applies decentralized on-site runoff source control to storm water management. LID is an integration of bioretention and vegetated landscapes to maintain a catchment's hydrologic and ecological functions. In current practice, the LID implementation is quantified for the specified watershed development. During the dynamic development process, the existing LID facilities have to be improved according to the incremental changes in the watershed. This technical note presents an on-site hydrologic approach to relate the required incremental storm water retention volume to the alteration of surface imperviousness in the tributary area. This approach allows the storm water retention volume to be tailored according to the stage of the watershed development. Cumulatively, the total storage volume can be achieved though multiple stages of the watershed development. The incremental retention volume is found to be related to the local average event rainfall depth. Design charts were produced and normalized by the local average rainfall event depth for generalized applicability.

Measurement of the connectivity of runoff source areas as determined by vegetation pattern and topography: A tool for assessing potential water and soil losses in drylands

The connectivity of runoff sources is considered one of the main factors controlling the hydrology of sparsely vegetated landscapes. However, the empirical demonstration of this role is very limited, partly because of the scarcity of suitable connectivity metrics. In this work, we derived and tested a spatial metric, Flowlength, for quantifying the connectivity of runoff source areas considering both vegetation pattern and topography. Flowlength is calculated as the average of the runoff pathway lengths from all the cells in a raster‐based map of the target site. We evaluated the relationships between the connectivity of runoff sources, measured with Flowlength, and the runoff and sediment yields from six plots and three catchments in semiarid southeast Spain. Flowlength distinguished varying degrees of connectivity between differing vegetation patterns with similar vegetation cover. The connectivity increased with the grain size of the bare areas and was positively related to plot runoff and sediment yields. Flowlength also correctly ranked the three catchments according to total runoff yielded during the study period. The inclusion of microtopographic information in the quantification of Flowlength improved the relationships between the pattern of runoff sources and the measured fluxes, highlighting the importance of topographic features in the connectivity of surface flows. In general, the microtopography had a net decreasing effect on the connectivity, which was mainly attributed to an increase in the amount of runoff sink areas caused by the sediment terracettes developed upslope of plants. Our results confirm that the connectivity of runoff sources is a key factor controlling runoff and erosion in semiarid lands and support the potential of Flowlength as a surrogate for the hydrological functioning of ecosystems with patchy vegetation.

Delineating runoff processes and critical runoff source areas in a pasture hillslope of the Ozark Highlands

AbstractThe identification of runoff contributing areas would provide the ideal focal points for water quality monitoring and Best Management Practice (BMP) implementation. The objective of this study was to use a field‐scale approach to delineate critical runoff source areas and to determine the runoff mechanisms in a pasture hillslope of the Ozark Highlands in the USA. Three adjacent hillslope plots located at the Savoy Experimental Watershed, north‐west Arkansas, were bermed to isolate runoff. Each plot was equipped with paired subsurface saturation and surface runoff sensors, shallow groundwater wells, H‐flumes and rain gauges to quantify runoff mechanisms and rainfall characteristics at continuous 5‐minute intervals. The spatial extent of runoff source areas was determined by incorporating sensor data into a geographic information‐based system and performing geostatistical computations (inverse distance weighting method). Results indicate that both infiltration excess runoff and saturation excess runoff mechanisms occur to varying extents (0–58% for infiltration excess and 0–26% for saturation excess) across the plots. Rainfall events that occurred 1–5 January 2005 are used to illustrate the spatial and temporal dynamics of the critical runoff source areas. The methodology presented can serve as a framework upon which critical runoff source areas can be identified and managed for water quality protection in other watersheds. Copyright © 2008 John Wiley & Sons, Ltd.

Inferring the effect of catchment complexity on mesoscale hydrologic response

The effect of catchment complexity on hydrologic and hydrochemical catchment response was characterized in the mesoscale Dill catchment (692 km2), Germany. This analysis was developed using multivariate daily stream concentration and discharge data at the basin outlet, in connection with less frequently sampled catchment‐wide end‐member chemistries. The link between catchment‐wide runoff sources and basin output was observed through a combination of concentration‐discharge (C‐Q) analysis and multivariate end‐member projection. Subsurface stormflow, various groundwater and wastewater sources, as well as urban surface runoff emerged in catchment output chemistry. Despite the identification of multiple sources, several runoff sources observed within the catchment failed to display consistent links with the output chemistry. This failure to associate known source chemistry with outlet chemistry may have resulted from a lack of hydraulic connectivity between sources and basin outlet, from different arrival times of subbasin‐scale runoff contributions, and also from an overlap of source chemistries that subsumed discrete runoff sources in catchment output. This combination of catchment heterogeneity and complexity simply suggests that the internal spatial organization of the catchment impeded the application of lumped mixing calculations at the 692 km2 outlet. Given these challenges, we suggest that in mesoscale catchment research, the potential effects of spatial organization should be included in any interpretation of highly integrated response signals, or when using those signals to evaluate numerical rainfall‐runoff models.

Evaluating long-term hydrological impacts of regional urbanisation in Hanyang, China, using a GIS model and remote sensing

China has experienced a rapid urbanisation, especially since the 1980s; however, the environmental impacts of this process are not fully investigated. Hanyang (Hubei Province, south China) was selected as a typical case to investigate runoff and non-point source (NPS) pollution impacts of urbanisation. A water quality simulation model (L-THIA) was applied to determine the long-term implications of different degrees of regional urbanisation impacts on NPS pollutants. Land-use patterns in 1987, 1998 and 2003 were analysed to evaluate the temporal variation of urbanisation, and the precipitation dataset from 1975 to 2003 was used to estimate the mean annual runoff and NPS pollutants. The contributions of different land-use categories to average annual runoff and NPS pollutant production were assessed by the means of a regression model. Results show that urban/impervious lands increased by 30.4% between 1987 and 2003, with the most increase occurring in 1998–2003. Industrial and forestlands have the most and least impact, respectively, on mean annual runoff and NPS pollutants. A combination of L-THIA and the regression model was found to be useful as a decision support tool for regional and urban planning from the perspectives of water quality control.

Monitoring water quality in the coastal area of Tripoli (Lebanon) using high-resolution satellite data

Water quality in the coastal area of Tripoli (Lebanon) was assessed using Landsat 7 ETM+ data, to provide a first baseline for coastal resources management. The data were geometrically rectified to a standard geographical grid and brightness values were converted to reflectance through radiometric correction. Sea-truth data, collected in the field within 6 hours before/after the time of the satellite overpass, were used to derive empirical algorithms for chlorophyll-a concentration, Secchi disk depth and turbidity. Then, maps of the distribution of the selected water quality parameters were generated for the entire area of interest, and compared with analogous results obtained from SeaWiFS data. The maps indicate that the Tripoli coastal area is exposed to moderate eutrophic conditions, along most of its shoreline (in particular along the northern stretch), in correspondence with fluvial and wastewater runoff sources. The Landsat 7 ETM+ data proved useful for the intended application, and will be used to start a national database on water quality in the Lebanese coastal environment.

Volume-Based Imperviousness for Storm Water Designs

The concept of low impact development (LID) is to apply decentralized on-site runoff source control to preserve the watershed hydrologic and ecological functions. An integrated layout using bioretention and vegetated landscape can decrease the developed runoff volume and peak flow. One of the key factors in urban hydrology is the imperviousness in the watershed. The conventional approach is to weight the imperviousness by the subareas in the watershed. Obviously, the area-weighted method has become inadequate when coping with LID because LID takes the flow path into consideration. When an impervious area directly drains onto a pervious area, the area-weighted method fails to count for the additional soil infiltration loss. In this study, the conventional area-weighted method is modified with a paved area reduction factor that converts the area-weighted imperviousness to its effective imperviousness. A family of curves was also developed for engineering applications. When a cascading process is introduced to the runoff flow path, the paved area reduction factor can be determined using the impervious to pervious area ratio and the infiltration to rainfall intensity ratio.

GIS-based modeling of runoff source areas and pathways

Abstract. The application of runoff models that rely on calibration to future land use and climate conditions is restricted to situations where the reaction of Hydrologic Response Units to environmental change is known. This limitation and the ensuing uncertainty of model results can be avoided when a risk-based approach to landscape and runoff analysis is taken. GIS-based landscape analysis provides the possibility of assessing the risks associated with non-linear responses of Hydrologic Response Units to changing rainfall and land use. In this paper, a runoff module designed for IDRISI-Andes to calculate runoff amount and routing for single or multiple rainfall events on a hillslope at small catchment scale is presented. The module is raster-based and uses layers with topographic and hydrological parameters to calculate a spatially distributed output layer of surface runoff. Conceptually, the module extrapolates point data of infiltration capacity onto a field or hillslope. A spatially distributed runoff map is calculated based on the addition of layers with rainfall data and the routing of runoff through pathways connecting pixels in a digital elevation model. Unlike outlet-based runoff modeling, the need for parameterization of the catchment is kept to a minimum. The application of the RUNOFF module in a test area in the Eifel region of Germany indicated that runoff from grassland is sensitive to small increases in rainfall intensity and soil compaction. The spatial patterns of infiltration capacity also contribute significantly to the non-linearity of the test area reaction to changing rainfall and soil hydrologic properties.

Identifying dissolved phosphorus source areas and predicting transport from an urban watershed using distributed hydrologic modeling

A reduction in surface water quality in urban watersheds due to nonpoint source phosphorus (P) loading has prompted municipalities to consider management practices to reduce P loss from landscapes. However, locating P source areas can be time consuming and expensive. Use of distributed models allows delineation of P source areas and focused management strategies. Using the spatially distributed soil moisture distribution and routing model, we adapt and validate a dissolved P (DP) loading model for application to an urban watershed, in Ithaca, New York, to identify P source areas. The model calculates DP loss separately for base flow, impervious surfaces, plant‐soil complex, and fertilized areas. The load at the outlet is the sum of P loss from the four components distributed throughout the watershed. Both stream and distributed DP loss were well predicted as indicated by comparison with measured data. The model predicted the largest contribution from plant‐soil complexes (36%). Impervious surfaces contributed 10% of the total load but as much as 17% in the winter. More important, the impervious surfaces increased DP losses from the adjacent areas due to runoff from the impervious surfaces saturating the soil, thus increasing runoff losses. Fertilizer contributed substantially following application but decreased rapidly thereafter, a result of conversion from soluble to insoluble P. However, fertilization increased soil P levels, and thus DP losses were higher as a whole (19%). Results demonstrate that correctly predicting the coincidence of P and runoff source areas can be a powerful tool to identify and mitigate contamination of surface waters.

Re-conceptualizing the soil and water assessment tool (SWAT) model to predict runoff from variable source areas

Many water quality models use some form of the Natural Resources Conservation Services (formerly Soil Conservation Service) curve number (CN) equation to predict storm runoff in ways that implicitly assume an infiltration-excess response to rainfall. Because of this, these models may fail to predict variable source areas (VSAs) correctly, i.e. where runoff is typically generated in rural, humid regions. In this study, the Soil and Water Assessment Tool (SWAT) model was re-conceptualized to distribute overland flow in ways consistent with VSA hydrology by modifying how the CN and available water content were defined; the new modeling approach is called SWAT-VSA. Both SWAT and SWAT-VSA were applied to a sub-watershed in the Cannonsville basin in upstate New York to compare model predictions of integrated and distributed responses, including surface runoff, shallowly perched water table depth, and stream phosphorus loads against direct measures. Event runoff was predicted similarly well for SWAT-VSA and SWAT. However, the distribution of shallowly perched water table depth was predicted better by SWAT-VSA and it is this shallow groundwater that governs VSAs. Event based dissolved phosphorus export from the watershed was also predicted better by SWAT-VSA, presumably because the distribution of runoff source areas was better predicted particularly from areas where manure was applied. This has important consequences for using models to evaluate and guide watershed management because correctly predicting where runoff is generated is critical to locating best management practices to control non-point source pollution.

Land management impacts on runoff sources in small Amazon watersheds

AbstractForest 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.