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Base Flow (base + flow)
Selected AbstractsUtilization of Decadal Tritium Variation for Assessing the Residence Time of Base FlowGROUND WATER, Issue 3 2007S. Rose An iterative algorithm is presented that allows the user to model the subsurface residence time of shallow ground water comprising stream base flow based on decadal scale variation of tritium concentrations. The algorithm accounts for the effects of radioactive decay, the shallow subsurface mixing of ground water with precipitation, and ground water flux. The inverse of the best-fitting modeled flux through the saturated zone is equivalent to the residence time. The data required for this model include at least two measurements of tritium in base flow for a given stream location made at least a decade apart and the long-term tritium input in precipitation for the region of interest. The model is sensitive to relatively small changes in tritium concentrations and is limited by analytic uncertainties to an accuracy of approximately ±5 years. The algorithm was applied to stream base flow for several basins in the Piedmont Province of Georgia in which tritium concentrations were measured during the early 1990s and again in the 2000s. The model results produced highly concordant residence times for three hydrogeologically similar basins in the Upper Ocmulgee Basin in North Central Georgia. The best estimate of the average residence time for ground water comprising base flow in this Piedmont basin using this new method is between approximately 14 and 18 years. These results are generally consistent with calculations made in previous studies, and these relatively long residence times can be attributed to the storage of water in the clay soils that dominate Piedmont Province watersheds. [source] Decline in the quality of suspended fine particulate matter as a food resource for chironomids downstream of an urban areaFRESHWATER BIOLOGY, Issue 5 2004Emma J. Rosi-MarshallArticle first published online: 16 APR 200 Summary 1. Urbanization and its associated contamination could degrade the quality of suspended fine particulate organic matter (SFPM) (20 ,m to 1 mm) as a food resource for aquatic insects. SFPM was collected at four sites along the main stem of the Chattahoochee River, which drains metropolitan Atlanta at base and high flow during four seasons. 2. Composition of SFPM was estimated using measures conventionally associated with food quality: bacteria, N/C ratio, caloric content, % inorganic, and % lipids, and metal (Cd, Cu, Pb, and Zn) concentration. In SFPM collected during base flow, % inorganic matter, calories, Cu, Pb, and Zn concentrations increased with cumulative permitted wastewater treatment discharge (an indicator of extent of urbanization upstream). In SFPM samples collected during high flow, % diatoms, Cu, Pb and Zn concentrations increased with urbanization. 3. A growth assay was used as an integrated and direct measure of SFPM quality as a food resource. The instantaneous growth rate (IGR) of chironomids fed SFPM collected during base flow declined downstream of the city. IGRs of chironomids fed SFPM collected at all sites during high flow were as low as the lowest IGR measured during base flow. 4. Insects fed SFPM collected from the Chattahoochee River had IGRs only 20% of those of chironomids fed SFPM collected from the Little Tennessee River, a relatively undisturbed river in North Carolina. The mortality rate of chironomids fed SFPM was not different among sites or rivers. While the decline in SFPM quality in the Chattahoochee River is probably attributable to some aspect of urbanization, the decline was not related to conventional measures of food quality or metal contamination. [source] Utilization of Decadal Tritium Variation for Assessing the Residence Time of Base FlowGROUND WATER, Issue 3 2007S. Rose An iterative algorithm is presented that allows the user to model the subsurface residence time of shallow ground water comprising stream base flow based on decadal scale variation of tritium concentrations. The algorithm accounts for the effects of radioactive decay, the shallow subsurface mixing of ground water with precipitation, and ground water flux. The inverse of the best-fitting modeled flux through the saturated zone is equivalent to the residence time. The data required for this model include at least two measurements of tritium in base flow for a given stream location made at least a decade apart and the long-term tritium input in precipitation for the region of interest. The model is sensitive to relatively small changes in tritium concentrations and is limited by analytic uncertainties to an accuracy of approximately ±5 years. The algorithm was applied to stream base flow for several basins in the Piedmont Province of Georgia in which tritium concentrations were measured during the early 1990s and again in the 2000s. The model results produced highly concordant residence times for three hydrogeologically similar basins in the Upper Ocmulgee Basin in North Central Georgia. The best estimate of the average residence time for ground water comprising base flow in this Piedmont basin using this new method is between approximately 14 and 18 years. These results are generally consistent with calculations made in previous studies, and these relatively long residence times can be attributed to the storage of water in the clay soils that dominate Piedmont Province watersheds. [source] Recharge Through a Regional Till Aquitard: Three-Dimensional Flow Model Water Balance ApproachGROUND WATER, Issue 3 2000Richard E. Gerber In southern Ontario, vertical leakage through a regionally extensive till is the primary source of recharge to underlying aquifers used for domestic and municipal water supply. Since leakage is largely controlled by the bulk hydraulic conductivity (K) of the aquitard, accurate estimates of K are necessary to quantify the resource. Considerable controversy exists regarding estimates of K for this aquitard, which vary according to the scale of the test method. For the till matrix, estimates from core samples and slug tests consistently range from 10,11 to 10,10 m/s. Isotopic evidence (3H), on the other hand, indicates that nonmatrix structures such as sand lenses, erosional surfaces, joints, and fractures significantly enhance till permeability. This is confirmed by slug test, pump test, recharge, and water balance studies, which show that K varies over seven orders of magnitude (10,12 to 10,5 m/s). To provide a regional estimate of bulk K and a reliable estimate of vertical recharge through the Northern Till, a numerical ground water flow model was constructed for the Duffins and Petticoat Creek drainage hasin. The model was calibrated to measurements of hydraulic head and estimates and measurements of base flow throughout the basin. This model demonstrates that the vertical hydraulic conductivity (Kv) for the Northern Till ranges from 5 × 10,10 to 5 × 10,9 m/s, values that are up to 2.5 orders of magnitude greater than matrix K estimates. Regional recharge through the Northern Till is estimated to range from 30 to 35 mm/a. [source] Investigation of the Mekong River basin hydrology for 1980,2000 using the YHyMHYDROLOGICAL PROCESSES, Issue 9 2008Hapu Arachchige Prasantha Hapuarachchi Abstract This study investigates the Mekong River basin hydrology for the 1980,2000 period using a grid-based distributed hydrological model called Yamanashi Hydrological Model (YHyM). The performance of the model is evaluated using data observed at different locations and the results justify the physical soundness of the model. The seasonal variations of climatic and hydrological characteristics of the basin such as soil moisture, ground water saturation deficit, runoff, precipitation, evapotranspiration, etc. are analysed. On the basis of the simulated results, it is noticeable that there is no significant trend in the precipitation, discharge, or soil moisture state of the basin during the simulated period, though there are some seasonal variations which seem to be natural. However the analysis on the precipitation elasticity (E) of the river flow shows that the E values for all sub-basins are greater than unity, which indicates that x% change in annual precipitation can cause > x% change in annual river flow. Further the basin hydrological responses are analysed for a long term synthetically-induced drought the results of which show the significance of the base flow of the Mekong River basin. Copyright © 2008 John Wiley & Sons, Ltd. [source] Estimation of irrigation flow by hydrograph analysis in a complex agricultural catchment in subtropical ChinaHYDROLOGICAL PROCESSES, Issue 10 2007Jia-Liang Tang Abstract Estimating the amount of irrigation water is challenging at the catchment scale because of the difficulties in direct measurement and interactions between the flow components. The objectives of the study were to characterize the catchment flows in an agricultural catchment with an irrigation system in subtropical China and to estimate catchment irrigation flow using hydrograph analysis methods. A weighting model and multiple regression models were established to estimate catchment irrigation outflow according to the hydrographs of the inflows and outflows of the catchment. The multiple regression models took into consideration the drainage time of base flow, resulting in better estimation on an event and annual basis. Using the MR-6d method, the estimated irrigation outflows amounted to 3700 mm, 2600 mm and 2760 mm during 2001, 2002 and 2003 respectively, which covered 70%, 60% and 64% respectively of the total catchment outflows in the corresponding years. Copyright © 2007 John Wiley & Sons, Ltd. [source] The zone of vegetation influence on baseflow revealed by diel patterns of streamflow and vegetation water use in a headwater basinHYDROLOGICAL PROCESSES, Issue 8 2002Barbara J. Bond Water use by vegetation can be closely linked to streamflow patterns on a variety of time scales. However, many of the details of these linkages are poorly understood. We compared diel (24 h) patterns of transpirational water use with streamflow patterns in a small headwater basin that displays a marked diel variation during summer months. The study site was in western Oregon. Our objectives were to: (1) determine the phase shift, i.e. the time lag between maximum transpiration and minimum streamflow, and the strength of the correlation at that time lag; (2) determine the amount of streamflow that is ,missing' during each diel cycle (i.e. the difference between base flow, defined by the daily maxima, and actual flow) and use it to estimate the zone, or area, of vegetation that influences daily streamflow patterns; (3) test and refine a conceptual model of how the coupling between vegetation water use and streamflow changes over the period of summer drought in this basin. We found that vegetation water use in the summer is coupled to streamflow over time scales of 4 to 8 h, and water-use-related fluctuations accounted for 1 to 6% of summer base flow. Direct evaporation from the channel was an order of magnitude less than the diel streamflow decrease. Transpiration within only 0·1 to 0·3% of the basin area accounted for the diel variation in streamflow. As the basin drained further through the summer, the coupling between vegetation and streamflow was diminished and occurred at longer time scales, and the zone of vegetation influence became smaller. This pattern is in accordance with our conceptual model, which attributes the summer decline in the strength of the vegetation,streamflow coupling to the increasing depth of plant-available water in the soil profile. Although this study is preliminary, we believe it is an important first step in describing better the coupling of vegetation water use to streamflow. Copyright © 2002 John Wiley & Sons, Ltd. [source] Effects of Watershed Impervious Cover on Dissolved Silica Loading in Storm Flow,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 4 2007Socratis Loucaides Abstract:, Dissolved silica (DSi) availability is a factor that affects the composition of algal populations in aquatic ecosystems. DSi cycling is tightly linked to the hydrological cycle, which is affected by human alterations of the landscape. Development activities that increase impervious cover change watershed hydrology and may increase the discharge of DSi-poor rainwater and decrease the discharge of DSi-rich ground water into aquatic ecosystems, possibly shifting algal community composition toward less desirable assemblages. In this study, DSi loadings from two adjacent coastal watersheds with different percent impervious cover were compared during four rain and five nonrain events. Loadings in the more impervious watershed contained a significantly larger proportion of surface runoff than base flow (ground-water discharge) and had lower [DSi] water during rain events than the less impervious watershed. Application of the Soil Conservation Service Curve Number (CN) method showed that the minimum rainfall height necessary to yield runoff was significantly lower for the more impervious watershed, implying that runoff volumes increase with impervious cover as well as the frequency of runoff-yielding events. Empirical data collected during this study and estimates derived from the CN method suggest that impervious cover may be responsible for both short-term DSi limitation during rain events as well as long-term reduction of DSi inputs into aquatic ecosystems. [source] MODELING METALS TRANSPORT AND SEDIMENT/WATER INTERACTIONS IN A MINING IMPACTED MOUNTAIN STREAM,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 6 2004Brian S. Caruso ABSTRACT: The U.S. Environmental Protection Agency (USEPA) Water Quality Analysis Simulation Program (WASP5) was used to model the transport and sediment/water interactions of metals under low flow, steady state conditions in Tenmile Creek, a mountain stream supplying drinking water to the City of Helena, Montana, impacted by numerous abandoned hard rock mines. The model was calibrated for base flow using data collected by USEPA and validated using data from the U.S. Geological Survey (USGS) for higher flows. It was used to assess metals loadings and losses, exceedances of Montana State water quality standards, metals interactions in stream water and bed sediment, uncertainty in fate and transport processes and model parameters, and effectiveness of remedial alternatives that include leaving contaminated sediment in the stream. Results indicated that during base flow, adits and point sources contribute significant metals loadings to the stream, but that shallow ground water and bed sediment also contribute metals in some key locations. Losses from the water column occur in some areas, primarily due to adsorption and precipitation onto bed sediments. Some uncertainty exists in the metal partition coefficients associated with sediment, significance of precipitation reactions, and in the specific locations of unidentified sources and losses of metals. Standards exceedances are widespread throughout the stream, but the model showed that remediation of point sources and mine waste near water courses can help improve water quality. Model results also indicate, however, that alteration of the water supply scheme and increasing base flow will probably be required to meet all water quality standards. [source] WATERSHED SCALING EFFECT ON BASE FLOW NITRATE, VALLEY AND RIDGE PHYSIOGRAPHIC PROVINCE,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 5 2001Bruce D. Lindsey ABSTRACT: A study of stream base flow and NO3 -N concentration was conducted simultaneously in 51 subwatersheds within the 116-square-kilometer watershed of East Mahantango Creek near Klingerstown, Pennsylvania. The study was designed to test whether measurable results of processes and observations within the smaller watersheds were similar to or transferable to a larger scale. Ancillary data on land use were available for the small and large watersheds. Although the source of land-use data was different for the small and large watersheds, comparisons showed that the differences in the two land-use data sources were minimal. A land use-based water-quality model developed for the small-scale 7.3-square-kilometer watershed for a previous study accurately predicted NO3 -N concentrations from sampling in the same watershed. The water-quality model was modified and, using the imagery-based land use, was found to accurately predict NO3 -N concentrations in the subwatersheds of the large-scale 116-square-kilometer watershed as well. Because the model accurately predicts NO3 -N concentrations at small and large scales, it is likely that in second-order streams and higher, discharge of water and NO3 -N is dominated by flow from smaller first-order streams, and the contribution of ground-water discharge to higher order streams is minimal at the large scale. [source] Geochemistry and source waters of rock glacier outflow, Colorado Front RangePERMAFROST AND PERIGLACIAL PROCESSES, Issue 1 2006M. W. Williams Abstract We characterize the seasonal variation in the geochemical and isotopic content of the outflow of the Green Lake 5 rock glacier (RG5), located in the Green Lakes Valley of the Colorado Front Range, USA. Between June and August, the geochemical content of rock glacier outflow does not appear to differ substantially from that of other surface waters in the Green Lakes Valley. Thus, for this alpine ecosystem at this time of year there does not appear to be large differences in water quality among rock glacier outflow, glacier and blockslope discharge, and discharge from small alpine catchments. However, in September concentrations of Mg2+ in the outflow of the rock glacier increased to more than 900,µeq,L,1 compared to values of less than 40,µeq,L,1 at all the other sites, concentrations of Ca2+ were greater than 4,000,µeq,L,1 compared to maximum values of less than 200,µeq,L,1 at all other sites, and concentrations of SO reached 7,000,µeq,L,1, compared to maximum concentrations below 120,µeq,L,1 at the other sites. Inverse geochemical modelling suggests that dissolution of pyrite, epidote, chlorite and minor calcite as well as the precipitation of silica and goethite best explain these elevated concentrations of solutes in the outflow of the rock glacier. Three component hydrograph separation using end,member mixing analysis shows that melted snow comprised an average of 30% of RG5 outflow, soil water 32%, and base flow 38%. Snow was the dominant source water in June, soil water was the dominant water source in July, and base flow was the dominant source in September. Enrichment of ,18O from ,10, in the outflow of the rock glacier compared to ,20, in snow and enrichment of deuterium excess from +,17.5, in rock glacier outflow compared to +,11, in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow. Copyright © 2005 John Wiley & Sons, Ltd. [source] COMPARISON OF PROCESS-BASED AND ARTIFICIAL NEURAL NETWORK APPROACHES FOR STREAMFLOW MODELING IN AN AGRICULTURAL WATERSHED,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 3 2006Puneet Srivastava ABSTRACT: The performance of the Soil and Water Assessment Tool (SWAT) and artificial neural network (ANN) models in simulating hydrologic response was assessed in an agricultural watershed in southeastern Pennsylvania. All of the performance evaluation measures including Nash-Sutcliffe coefficient of efficiency (E) and coefficient of determination (R2) suggest that the ANN monthly predictions were closer to the observed flows than the monthly predictions from the SWAT model. More specifically, monthly streamflow E and R2 were 0.54 and 0.57, respectively, for the SWAT model calibration period, and 0.71 and 0.75, respectively, for the ANN model training period. For the validation period, these values were ,0.17 and 0.34 for the SWAT and 0.43 and 0.45 for the ANN model. SWAT model performance was affected by snowmelt events during winter months and by the model's inability to adequately simulate base flows. Even though this and other studies using ANN models suggest that these models provide a viable alternative approach for hydrologic and water quality modeling, ANN models in their current form are not spatially distributed watershed modeling systems. However, considering the promising performance of the simple ANN model, this study suggests that the ANN approach warrants further development to explicitly address the spatial distribution of hydrologic/water quality processes within watersheds. [source] | ||||||||||