Small Urban Watershed Research Articles

Concentrations and loads of PCBs, dioxins, PAHs, PBDEs, OC pesticides and pyrethroids during storm and low flow conditions in a small urban semi-arid watershed

Urban runoff has been identified in water quality policy documents for San Francisco Bay as a large and potentially controllable source of pollutants. In response, concentrations of suspended sediments and a range of trace organic pollutants were intensively measured in dry weather and storm flow runoff from a 100% urban watershed. Flow in this highly urban watershed responded very quickly to rainfall and varied widely resulting in rapid changes of turbidity, suspended sediments and pollutant concentrations. Concentrations of each organic pollutant class were within similar ranges reported in other studies of urban runoff, however comparison was limited for several of the pollutants given information scarcity. Consistently among PCBs, PBDEs, and PAHs, the more hydrophobic congeners were transported in larger proportions during storm flows relative to low flows. Loads for Water Years 2007–2010 were estimated using regression with turbidity during the monitored months and a flow weighted mean concentration for unmonitored dry season months. More than 91% of the loads for every pollutant measured were transported during storm events, along with 87% of the total discharge. While this dataset fills an important local data gap for highly urban watersheds of San Francisco Bay, the methods, the uniqueness of the analyte list, and the resulting interpretations have applicability for managing pollutant loads in urban watersheds in other parts of the world.

Investigating the Response of a Small Urbanized Watershed to Acute Toxicity Events via Analysis of High-frequency Environmental Data

This study, conducted over 10 weeks by an undergraduate student in an NSF REU (U.S. National Science Foundation Research Experiences for Undergraduates) program under mentorship of the co-authors, investigates responses of a small urban watershed, located in Blacksburg, Virginia, to acute toxicity events. High-frequency water and weather data were collected using monitoring equipment maintained by the Learning Enhanced Watershed Assessment System (LEWAS) lab, including a weather station, rain gauge, water quality sonde, and an Acoustic Doppler Current Profiler. The location and attributes of local stormwater catchments were surveyed and recorded using GIS software to determine the overall contributing area of the LEWAS lab watershed. This fieldwork resulted in the construction of an accurate stormwater network map, which in turn allowed the identification of sources of sedimentation and other pollution from runoff events. A case study presents the response of the watershed to a winter storm event that resulted in acute chloride toxicity from runoff containing road salts. KEYWORDS: Water; Stormwater, Urban Watersheds; Watershed Responses; Acute Toxicity; High-frequency Data; Data Analysis; Data Collection; Environmental Monitoring; Flow Measurements; Hydrologic Data.

Exploring storage and runoff generation processes for urban flooding through a physically based watershed model

AbstractA physically based model of the 14 km2 Dead Run watershed in Baltimore County, MD was created to test the impacts of detention basin storage and soil storage on the hydrologic response of a small urban watershed during flood events. The Dead Run model was created using the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) algorithms and validated using U.S. Geological Survey stream gaging observations for the Dead Run watershed and 5 subbasins over the largest 21 warm season flood events during 2008–2012. Removal of the model detention basins resulted in a median peak discharge increase of 11% and a detention efficiency of 0.5, which was defined as the percent decrease in peak discharge divided by percent detention controlled area. Detention efficiencies generally decreased with increasing basin size. We tested the efficiency of detention basin networks by focusing on the “drainage network order,” akin to the stream order but including storm drains, streams, and culverts. The detention efficiency increased dramatically between first‐order detention and second‐order detention but was similar for second and third‐order detention scenarios. Removal of the soil compacted layer, a common feature in urban soils, resulted in a 7% decrease in flood peak discharges. This decrease was statistically similar to the flood peak decrease caused by existing detention. Current soil storage within the Dead Run watershed decreased flood peak discharges by a median of 60%. Numerical experiment results suggested that detention basin storage and increased soil storage have the potential to substantially decrease flood peak discharges.

Flood Frequency Analysis for the Annual Peak Flows Simulated by an Event-Based Rainfall-Runoff Model in an Urban Drainage Basin

The proper assessment of design flood is a major concern for many hydrological applications in small urban watersheds. A number of approaches can be used including statistical approach and the continuous simulation and design storm methods. However, each method has its own limitations and assumptions being applied to the real world. The design storm method has been widely used for a long time because of the simplicity of the method, but three critical assumptions are made such as the equality of the return periods between the rainfall and corresponding flood quantiles and the selections of the rainfall hyetograph and antecedent soil moisture conditions. Continuous simulation cannot be applied to small urban catchments with quick responses of runoff to rainfall. In this paper, a new flood frequency analysis for the simulated annual peak flows (FASAP) is proposed. This method employs the candidate rainfall events selected by considering a time step order of five minutes and a sliding duration without any assumptions about the conventional design storm method in an urban watershed. In addition, the proposed methodology was verified by comparing the results with the conventional method in a real urban watershed.

A spatially distributed model for the assessment of land use impacts on stream temperature in small urban watersheds

Stream temperatures in urban watersheds are influenced to a high degree by changes in landscape and climate, which can occur at small temporal and spatial scales. Here, we describe a modelling system that integrates the distributed hydrologic soil vegetation model with the semi-Lagrangian stream temperature model RBM. It has the capability to simulate spatially distributed hydrology and water temperature over the entire network at high time and space resolutions, as well as to represent riparian shading effects on stream temperatures. We demonstrate the modelling system through application to the Mercer Creek watershed, a small urban catchment near Bellevue, Washington. The results suggest that the model was able to produce realistic streamflow and water temperature predictions that are consistent with observations. We use the modelling construct to characterize impacts of land use change and near-stream vegetation change on stream temperatures and explore the sensitivity of stream temperature to changes in land use and riparian vegetation. The results suggest that, notwithstanding general warming as a result of climate change over the last century, there have been concurrent increases in low flows as a result of urbanization and deforestation, which more or less offset the effects of a warmer climate on stream temperatures. On the other hand, loss of riparian vegetation plays a more important role in modulating water temperatures, in particular, on annual maximum temperature (around 4 °C), which could be mostly reversed by restoring riparian vegetation in a fairly narrow corridor – a finding that has important implications for management of the riparian corridor. Copyright © 2014 John Wiley & Sons, Ltd.

Adequacy of Drainage Channels in a Small Urban Watershed in Nigeria

This research presents an analysis for the adequacy of drainage channels. Topographical and location map was used as a guide to locate the drainage channels. Rainfall records for a period of four years (2008-2011) were used to develop the intensity bdurationB B frequency of the study area . Field survey and geometrical measurement of the drainage channels were carried out and data obtained from the measurement were used for the analysis of runoff from drainage channels. The time of concentration was determined using the Kirpich equation and modified version of Kirpich equation (new equation ofB time of concentration). The peak flow and maximum flow velocity from the drainage channels were determined using the rational model and manningbs equation.B Analysis of the results showed that most of the drainage channels had lesser runoff when the modified Kirpich equation was used. All the drainage channels of basin A had velocities ranging from 1 to 2m/s and the drain capacities are okay. All the drainage channels of basin B had velocities ranging from 2 to 4m/s and the drain capacities are okay. Channels 2 and 9 of basin C had drain capacity less than the actual discharge this makes the drain inadequate. Basin C revealed that, channels 1, 4, 5, 7, 8, 10, 12 and 13 have flow velocities less than 1m/s and this will cause deposition of sediment, a redesign of inadequate drainage channels with velocity less than 1m/s B was done and their velocities ranges from 1.21m/s to 1.54m/s. A redesign of drainage channels 2 and 9 of basin C showed that the drain capacity were greater than the actual discharge and this makes the drain adequate .This study suggests that; proper design, analysis, construction and maintenance of drainage channels be made. http://dx.doi.org/10.4314/njt.v33i4.20

Fate of selenium in a small urban watershed

A field study was conducted on a small urban watershed (residential and golf course dominated) in southern Nevada to assess the concentration and speciation of selenium (Se) in a series of drain lines and monitoring wells and to quantify the mass discharge of Se from the drain system. Water samples were collected on a monthly basis and analyzed for total Se, selenate (SeO4 (=)) and selenite (SeO3 (=)). In addition, where possible, flow was assessed as was, temperature, redox potential, pH, dissolved oxygen (DO) along with all major cations and anions. The data were then modeled with PhreeqC to identify selenium speciation. Results revealed a SeO4 (=) dominated system with SeO4 (=) concentrations ranging from 13 to 62 ppb. In the monitoring wells, 66 % of the variation in the total Se concentration could be described based on depth to groundwater, temperature and sulfate concentrations (P < 0.001). In particular, higher total Se concentrations were predicted for shallower depth to groundwater, suggesting the solubilization of Se evapo-concentrates near the surface could be reduced by lowering water tables. The highest of all correlations was found between SeO4 (=) concentrations (↑) and the sodium (↑) and DO (↑) concentrations in the monitoring wells (R (2) = 0.77, P < 0.001). An excellent curvilinear relationship was found between total Se and the electrical conductivity in the water (R (2) = 0.73, P < 0.001). Based on the Se data and time line identified in this study, high concentrations of Se could be expected to drain from this area for many years to come, with salinity acting as a good proxy for Se concentration. In the drain lines, Se concentrations were found to be invariant to flow (P > 0.05). Flow discharge from the main drain system to the Las Vegas Wash was estimated at 559 acre feet during the 1 year study period. This flow was estimated to carry 4,203 Mg of salts 6.71 Mg of nitrate-N and 27.1 kg of total Se.

Reconstruction of a century of landscape modification and hydrologic change in a small urban watershed in Pittsburgh, PA

Assessing the causes of stream impairments is challenging without a clear understanding of the spatiotemporal interactions among human infrastructure networks and hydrologic systems. Landscape change is often characterized using simplistic metrics that lump changes into generalized categories, such as impervious cover. We examined the evolution of human infrastructure in Panther Hollow, a small watershed in Pittsburgh, Pennsylvania to characterize the impacts of long-term (~100 years) landscape change on stream flow. Results show that impervious cover in the catchment grew from 3 % in 1900 to 27 % in 2010. Growth was non-linear, with 60 % of the development occurring between 1904 and 1930. We then compared two models that predict changes in annual water yield, one model based on watershed impervious cover and one based on human infrastructure arrangement. The model based solely on impervious cover predicts excessive amounts of surface runoff relative to the infrastructure model and monitored yield. This discrepancy occurs because the impervious model does not account for the diversion of 50 % of the watershed drainage through the combined sewer system to an adjacent basin. In the Panther Hollow watershed, hydrology is dominated by a reduction in water yield, contrasting typical hydrologic changes associated with urbanization. Our analysis reveals the value of quantifying additional landscape metrics, such as infrastructure pattern and connectivity, which provide a more complete understanding of how human development alters natural hydrology.

Comparison of pond-sedimentation data with a GIS-based USLE model of sediment yield for a small forested urban watershed

Abstract Urban land covers are complex and heavily fragmented. A better understanding of how different urban land-cover types influence sediment yields is required from a land management perspective given rapid global population growth. This paper examines the effect of urban forest covers on soil erosion within a small watershed in northeastern Ohio that contains a retention pond. Sediment volumes emplaced within the pond over a 40-year period are compared against USLE-based erosion-model estimates to elucidate an erosion factor for the unknown forest cover effect. The USLE is applied for its ease of use, GIS-compatibility, and data availability. High erosion factors are resolved for the urban forest cover based on the pond's sediment record. These values comply with forest covers in other settings, but fall within the high end of the spectrum. Results provide a baseline for assessing model shortcomings and developing refined model versions that can provide urban planners with a straightforward means of erosion planning. Model calibration is possible at small watershed scales and single land-cover types. The USLE should provide land managers a quick and easy-to-use tool for evaluating erosion potential in urban settings across regional scales provided individual urban land-cover types are well-constrained.

Case Study of Simulation of Heat Export by Rainfall Runoff from a Small Urban Watershed Using MINUHET

AbstractThe influence of urban development on the volume of storm water runoff and on water temperature, and the associated impact on streams in urban watersheds, has generated a need for tools to predict the temperature of urban storm water runoff, particularly for the assessment of future urban development. To address this need, the Minnesota Urban Heat Export Tool (MINUHET) model was developed to simulate the flow of heat and storm water through a small urban watershed for a rainfall-runoff event. The tool includes process-based hydrological and thermal models for runoff from developed, undeveloped, or vegetated land uses, for various components of drainage networks, and for some best management practices such as detention and infiltration ponds, storm water vaults, and rock cribs. The primary output of MINUHET is a time series of the flow rate and temperature of runoff, which can be used to determine the heat content of the runoff at the outlet of the modeled watershed and aid in the assessment of the...

Effects of Land Cover Disturbance on Stream Invertebrate Diversity and Metal Concentrations in a Small Urban Industrial Watershed

ABSTRACT The landscape surrounding Sudbury, Ontario, has been severely affected by 100 years of mining and forestry and a recent, large-scale ecological risk assessment found that terrestrial plant communities continue to be impaired by remnant metals and poor soil conditions. We investigated the risks of these adverse landscape conditions on a small headwater stream by digitizing land cover at a fine scale and relating it to benthic invertebrate diversity and metal concentrations at 13 sites in the system. The combination of historically barren landscape and modern impervious surfaces such as asphalt, roofs, and hard gravel was associated with decreased benthic invertebrate community diversity at all four watershed spatial scales measured. The same combination of barren bedrock and impervious surface was associated with increased levels of potentially toxic Ni, whereas increased Cu was most strongly associated with bedrock alone. Our results highlight previously undocumented relationships in this historically impacted area between unrestored landscapes, modern impervious surfaces, and their potential risks to aquatic life. We suggest regreening at different watershed scales as a risk mitigation tactic worthy of consideration and further study.

Bayesian uncertainty assessment of flood predictions in ungauged urban basins for conceptual rainfall-runoff models

Abstract. Urbanization and the resulting land-use change strongly affect the water cycle and runoff-processes in watersheds. Unfortunately, small urban watersheds, which are most affected by urban sprawl, are mostly ungauged. This makes it intrinsically difficult to assess the consequences of urbanization. Most of all, it is unclear how to reliably assess the predictive uncertainty given the structural deficits of the applied models. In this study, we therefore investigate the uncertainty of flood predictions in ungauged urban basins from structurally uncertain rainfall-runoff models. To this end, we suggest a procedure to explicitly account for input uncertainty and model structure deficits using Bayesian statistics with a continuous-time autoregressive error model. In addition, we propose a concise procedure to derive prior parameter distributions from base data and successfully apply the methodology to an urban catchment in Warsaw, Poland. Based on our results, we are able to demonstrate that the autoregressive error model greatly helps to meet the statistical assumptions and to compute reliable prediction intervals. In our study, we found that predicted peak flows were up to 7 times higher than observations. This was reduced to 5 times with Bayesian updating, using only few discharge measurements. In addition, our analysis suggests that imprecise rainfall information and model structure deficits contribute mostly to the total prediction uncertainty. In the future, flood predictions in ungauged basins will become more important due to ongoing urbanization as well as anthropogenic and climatic changes. Thus, providing reliable measures of uncertainty is crucial to support decision making.

Analyses of a long‐term, high‐resolution radar rainfall data set for the Baltimore metropolitan region

We introduce a long‐term, high‐resolution radar rainfall data set for the Baltimore metropolitan area covering the 10‐yr period from 2000–2009. Rainfall fields are developed at 15 min time interval and 1 km horizontal resolution for a 17,000‐km2 region centered on the Baltimore metropolitan area. The Hydro‐NEXRAD system is used as a platform for generating radar rainfall fields. We utilize the high‐resolution, 10‐yr data set to characterize striking spatial heterogeneities in rainfall for the Baltimore metropolitan region, both in terms of mean rainfall and rainfall extremes. The role of complex terrain (associated with urbanization, the Chesapeake Bay, and mountainous terrain) in controlling spatial heterogeneities of rainfall climatology for the Baltimore study region is discussed. We also characterize the seasonal and diurnal variation of rainfall over the study region using the 10‐yr rainfall data set, with particular focus on the diurnal variation of rainfall during the warm season. High‐resolution rainfall fields are especially useful for examining the distribution of rainfall from a drainage basin perspective, as illustrated through analyses of basin‐averaged rainfall rate for basins of contrasting drainage area and analyses of the duration of dry periods for small urban watersheds. Analyses and methodologies used to develop the long‐term Baltimore rainfall data set are broadly applicable to other regions of the United States and in settings around the world with long‐term, high‐quality radar data sets.

Phosphorus export across an urban to rural gradient in the Chesapeake Bay watershed

Watershed export of phosphorus (P) from anthropogenic sources has contributed to eutrophication in freshwater and coastal ecosystems. We explore impacts of watershed urbanization on the magnitude and export flow distribution of P along an urban‐rural gradient in eight watersheds monitored as part of the Baltimore Ecosystem Study Long‐Term Ecological Research site. Exports of soluble reactive phosphorus (SRP) and total P (TP) were lowest in small watersheds with forest and low‐density residential land use (2.8–3.1 kg−1 km−2 yr−1). In contrast, SRP and TP exports increased with watershed impervious surface coverage and reached highest values in a small urban watershed (24.5–83.7 kg−1 km−2 yr−1). Along the Gwynns Falls, a larger watershed with mixed land use, the greatest proportion of SRP (68%) and TP (75%) was contributed from the lower watershed, where urban areas were the dominant land use. Load duration curve analysis showed that increasing urbanization in watersheds was associated with shifts in P export to high‐flow conditions (&gt;2 mm d−1). SRP concentrations during low‐flow conditions at urban headwater sites were highest during summer and lowest during winter. This seasonal pattern was consistent with sediment incubation experiments showing that SRP release from sediments was temperature dependent. Our results suggest that shifts in streamflow and alterations in water temperatures owing to urbanization and climate can influence stream water P concentrations and P export from urban watersheds.

Development of Sub-Daily Erosion and Sediment Transport Algorithms for SWAT

New Soil and Water Assessment Tool (SWAT) algorithms for simulation of stormwater best management practices (BMPs) such as detention basins, wet ponds, sedimentation filtration ponds, and retention irrigation systems are under development for modeling small/urban watersheds. Modeling stormwater BMPs often requires time steps as small as minutes to realistically capture the instantaneous flow and sediment load coming from upland areas. SWAT2005 uses the Modified Universal Soil Loss Equation (MUSLE) for modeling upland erosion and sediment load. The MUSLE model is an empirical soil loss equation, which was formulated based on field observations rather than theoretically derived relationships to predict long-term average soil loss. This article presents modified physically based erosion models in SWAT for seamless modeling of erosion processes with the recently developed sub-hourly flow models. In the new algorithms, splash erosion is calculated based on the kinetic energy delivered by raindrops, adapted from the European Soil Erosion Model, and overland flow erosion is estimated using a physically based equation adapted from the Areal Nonpoint-Source Watershed Environment Response Simulation (ANSWERS) model. The Yang model and the Brownlie model were also modified for in-stream sediment routing. The SWAT model with the modified sub-daily sediment algorithms was calibrated and validated each for a one-year period at 15 min intervals with measured data from the USDA-ARS Riesel watersheds in Texas. Results show that SWAT with the sub-daily algorithms performed as well or better in terms of sediment yield prediction than SWAT with the current daily output structure. In addition, SWAT (sub-daily) was able to adequately represent the timing, peak, and duration of sediment transport events. Thus, this initial evaluation indicates that the new sub-daily flow and sediment structure in SWAT is a promising tool for water quality assessment studies in small watersheds or urban watersheds where sub-daily process are so important to quantify.

Potential contributions of asphalt and coal tar to black carbon quantification in urban dust, soils, and sediments

Measurements of black carbon (BC) using either chemical or thermal oxidation methods are generally thought to indicate the amount of char and/or soot present in a sample. In urban environments, however, asphalt and coal-tar particles worn from pavement are ubiquitous and, because of their pyrogenic origin, could contribute to measurements of BC. Here we explored the effect of the presence of asphalt and coal-tar particles on the quantification of BC in a range of urban environmental sample types, and evaluated biases in the different methods used for quantifying BC. Samples evaluated were pavement dust, residential and commercial area soils, lake sediments from a small urban watershed, and reference materials of asphalt and coal tar. Total BC was quantified using chemical treatment through acid dichromate (Cr 2O 7) oxidation and chemo-thermal oxidation at 375 °C (CTO-375). BC species, including soot and char/charcoal, asphalt, and coal tar, were quantified with organic petrographic analysis. Comparison of results by the two oxidation methods and organic petrography indicates that both coal tar and asphalt contribute to BC quantified by Cr 2O 7 oxidation, and that coal tar contributes to BC quantified by CTO-375. These results are supported by treatment of asphalt and coal-tar reference samples with Cr 2O 7 oxidation and CTO-375. The reference asphalt is resistant to Cr 2O 7 oxidation but not to CTO-375, and the reference coal tar is resistant to both Cr 2O 7 oxidation and CTO-375. These results indicate that coal tar and/or asphalt can contribute to BC measurements in samples from urban areas using Cr 2O 7 oxidation or CTO-375, and caution is advised when interpreting BC measurements made with these methods.

The identification of sediment sources in a small urban watershed in southern Brazil: An application of sediment fingerprinting

Soil particles eroded from the land surface and transported into rivers by runoff are considered one of the main components of non‐point source pollution in urban watersheds. These particles also serve as a vector for a wide variety of both organic and inorganic constituents. As a result, the identification of sediment sources in an urban watershed is necessary not only to understand erosion dynamics, but also to help implement more effective measures to control and/or remediate non‐point source pollution. The present study employs sediment ‘fingerprinting’ to determine the main sediment sources in a small residential urban watershed (0.83 km2) on the outskirts of Porto Alegre in southern Brazil. Based on an evaluation spanning 12 rainfall events, the results show that paved and unpaved roads and the stream channel itself contribute, on average, 46%, 23%, and 31%, respectively, to the suspended sediment flux in the watershed. Furthermore, the source contributions varied both between events and over the course of a single event. This appears to imply that source contributions, at least to some extent, depend on local precipitation patterns. The results from this study indicate that the level of uncertainty in source ascription tends to decline with increasing numbers of tracers; hence, successful sediment fingerprinting and source ascription in complex hydrologic environments, such as urban watersheds, may require the use of a large number of chemical and/or physical tracers.

Flash Flood Forecasting for Small Urban Watersheds in the Baltimore Metropolitan Region

Abstract The utility of distributed hydrologic models in combination with high-resolution Weather Surveillance Radar-1988 Doppler (WSR-88D) rainfall estimates for flash flood forecasting in urban drainage basins is examined through model simulations of 10 flood events in the 14.3 km2 Dead Run watershed of Baltimore County, Maryland. The hydrologic model consists of a simple infiltration model and a geomorphological instantaneous unit hydrograph–based representation of hillslope and channel response. Analyses are based on high-resolution radar rainfall estimates from the Sterling, Virginia, WSR-88D and observations from a nested network of 6 stream gauges in the Dead Run watershed and a network of 17 rain gauge stations in Dead Run. For the three largest flood peaks in Dead Run, including the record flood on 7 July 2004, hydrologic model forecasts do not capture the pronounced attenuation of flood peaks. Hydraulic controls imposed by valley bottom constrictions associated with bridges and bridge abutments are a dominant element of the extreme flood response of small urban watersheds. Model analyses suggest that a major limitation on the accuracy of flash flood forecasting in urban watersheds is imposed by storm water management infrastructure. Model analyses also suggest that there is potential for improving model forecasts through the utilization of information on initial soil moisture storage. Errors in the rainfall field, especially those linked to bias correction, are the largest source of uncertainty in quantitative flash flood forecasting. Bias correction of radar rainfall estimates is an important element of flash flood forecasting systems.

도시유역의 유출특성을 고려한 강우분포 모형의 개발

본 연구는 면적이 작은 도시유역에 적용하기 위한 강우의 시간분포방법을 제안하였다. 이를 위하여 유역의 특성을 고려한 Interevent Time Definition(ETD) 결정방법과 3시간 미만의 강우를 포함한 무차원 누가우량곡선을 제안하였다. 새롭게 제시된 IETD 결정방법은 호우 사상의 종료 시점으로부터 직접유출의 종료 시점까지의 기간을 IETD로 정의하였다. 제안된 IETD 결정방법을 사용하여 중랑 배수구역에 대한 면적-IETD 관계곡선을 유도하였다. 제안된 강우분포모형을 4개의 도시 유역에 적용하여 10년빈도 2시간 강우를 시간분포 시켰으며, SWMM을 사용하여 유출수문곡선을 산정하였다. 산정된 유출수문곡선을 Huff 방법으로 산정한 유출수문곡선과 비교하였다. 비교한 결과 새롭게 제시된 강우의 시간분포 방법에 의한 유역의 첨두 홍수량은 Huff 모형에 의한 결과들보다 <TEX>$11\sim15%$</TEX>크게 나타났으며, 따라서 기존의 Huff 모형을 도시유역에 그대로 적용하는 것은 유출을 과소평가 할 위험성을 내포하고 있다. This study suggests a new time distribution method of rainfall for small urban watersheds. IETD (Interevent Time definition) determination method considering basin characteristics and dimensionless accumulation rainfall curves involving rainfall events with shorter duration than 3-hours are suggested. A new definition of IETD is the time period from the end of a rainfall event to the end of a direct runoff. Using the method, we drive an area-IETD regression curve for the Joong-Rang basin. The rainfall event with 10 year-return periods, 2-hour duration is distributed and applied four urban watersheds. In the four watersheds, we calculate hydrographs for four watersheds using SWMM and compare them with ones of the Huff's distribution model. From the comparison, we find that peak flows resulted from the developed methodology are <TEX>$11\sim15%$</TEX> larger than ones from the Huff's model. As conclusion, the Huff method should be adopted for the urban watersheds with careful verification.

Microbial source tracking in a small southern California urban watershed indicates wild animals and growth as the source of fecal bacteria

Three independent microbial source tracking (MST) methods were applied to a small urban subwatershed in Orange County, California. Fifty-seven water samples collected over summer 2002 were analyzed for human adenovirus and enterovirus. Enterococci and E. coli were isolated for antibiotic resistance analysis (ARA) and for PCR identification of human- and animal-specific toxin genes, respectively. All water samples were PCR negative for human enteroviruses and E. coli human-specific toxin gene. E. coli toxin markers revealed the presence of toxin genes specific to bird, rabbit, and cow. Enterococci ARA results supported this conclusion and indicated that fecal bacteria from bird and wild animal feces as well as soil were the predominant source found in the watershed. An E. coli, isolated from the watershed and inoculated back into the heat-sterilized storm drain water, increased 4 log units within 6 days. Collectively, these results suggest that bird and wild animal feces, soil amendments, and/or fecal coliform growth in the storm drain are the major contributors to the fecal bacterial pollution in downstream areas. However, human adenoviruses were detected on two occasions. Fecal bacterial concentrations were not elevated on these two occasions, suggesting that the elevated levels of fecal indicator bacteria in this small watershed could be unrelated to the source of human adenovirus.