Overland Flow In Areas Research Articles

Process-based eco-hydrological modeling in an Eastern Himalayan watershed using RHESSys

Hydro-ecological models are important tools used to estimate streamflow from mountainous watersheds as driven by precipitation and snowmelt under the influence of different spatially variable hydro-ecological response variables. These models enable us to gain insight into hydrological and ecological processes occurring in the watershed. In the present study, a hydro-ecological model, Regional Hydro-Ecological Simulation System (RHESSys) was set up for a small representative watershed in Eastern Himalayan region of Arunachal Pradesh to investigate spatial variation in saturation deficit during snow depletion period. RHESSys model is capable of estimating variations in evaporation, transpiration, streamflow, overland flow, saturation deficit, etc. from the landscape. Studies related to hydrological and ecological sustainability of the Eastern Himalayan Rivers are limited, although a major part of the region is covered with evergreen forests, where ecological parameters may have a great impact on watershed hydrology. Nuranang watershed, which is located in Tawang district of Arunachal Pradesh, India, was selected as the study area. The hydro-meteorological data from 2004 to 2008 were collected from Central Water Commission (CWC). Digital Elevation Model (DEM) with 30 m spatial resolution was downloaded from NASA website. Calibration of the RHESSys model was performed for the depletion period of 2004 (13th April–21st August) and 2005 (17th April–4th September) and the model was validated for the years 2008 (17th April–20th September) and 2009 (13th April–23rd September) using observed streamflow at the watershed outlet. Comparison plot of the actual and log values of observed and simulated daily discharges showed that the model captured the variations in total observed discharge and subsurface flows reasonably well for both the calibration and validation years. Daily time-series outputs of various hydrological variables, i.e., subsurface flow, overland flow, streamflow, saturation deficit, and percentage saturated area, were obtained for both calibration and validation simulation. For simulation years, overland flow dominated stream discharge in all the months, and contributed around 60% of the total streamflow, whereas subsurface flow contributed around 40% in all the months. Comparatively, saturation deficit was more in the month of April due to less precipitation and it was reduced in the month of May because of pre-monsoon rainfall. It increased again in the month of June as the precipitation in the month was less compared to May. Then, it decreased again in the subsequent months of July and August due to high rainfall during the monsoon season and increased in the month of September. Average saturation deficit was estimated to be 389.69 mm and average percentage saturated area was estimated as 9.52%. The number of saturated pixels was found to be minimum in the month of April, while a maximum number of saturated pixels were observed in the month of August indicating the existence of inverse relationship between precipitation and saturation deficit for the study area.

Determination of runoff response to variation in overland flow area by flow routes using UAV imagery

Surface runoff can be routed to both pervious areas (PAs) and drainage systems during an overland flow. Excessive runoff from an impervious area (IA) flowing into a drainage system causes an overload, which can be relieved by diverting runoff to PAs. However, the hydrological link between IAs and PAs, especially the runoff response to variation in overland flow areas (OFAs), has not well been considered in runoff simulations due to the complexity of routing. To understand how the OFA within an IA contributes to runoff generation, a novel classification approach was applied to categorize the IA in a study area in Nanjing University, Xianlin campus, China into directly connected impervious area (DCIA) and indirectly connected impervious area (ICIA) by flow routes using high-resolution ground-based images (0.5 m) from an unmanned aerial vehicle. The OFAs then include DCIA and the total impervious area (TIA), which is the sum of DCIA and ICIA. The runoff simulations were supported by Storm Water Management Model (SWMM) calibrated with observed rainfall and runoff data. The resulting proportions of DCIA and ICIA of the study area were 34.13% and 10.99%, respectively. The spatial distributions of DCIA and ICIA are characterized by the subcatchment landscape heterogeneity resulting from vegetation, imperviousness, and slope. The observed runoff coefficients and peak flows were positively correlated with the percentages of DCIA and TIA. The runoff coefficient was significantly correlated with the DCIA in a light rainfall event of 18.4 mm (R2 =0.82) and with the TIA in a heavy rainfall event of 119 mm (R2 =0.92). Runoff generation is affected by both the characteristics of the rainfall event and the accompaning variations in OFAs. Results indicate that increasing of the flow connectivity from IAs to PAs and increasing the water retention capacity of PAs may be effective strategies for optimizing landscape patterns for stormwater management.

How do stream processes affect hazard exposure on alluvial fans? Insights from an experimental study

Alluvial fans are among the most privileged settlement areas in many mountain regions. These landforms are particularly dynamic being episodically affected by distributary processes generated by extreme flood events. Addressing risk assessment entails determining hazard exposure and unravelling how it might be related to process loading and to process dynamics once the flow becomes unconfined on the surface of alluvial fans. By following a ‘similarity of process concept’, rather than by attempting to scale a real-world prototype, we performed a set of 72 experimental runs on an alluvial fan model. Thereby, we considered two model layouts, one without a guiding channel and featuring a convex shape and the other one with a guiding channel, a bridge, and inclined but planar overland flow areas as to mirror an anthropic environment. Process magnitude and intensity parameters were systematically varied, and the associated biphasic distributary processes video recorded. For each experiment, the exposure was detected by mapping the exposed area in a GIS, thereby discerning between areas exposed to biphasic flows and the associated depositional phenomena or to the liquid flow phase only. Our results reveal that total event volume, sediment availability and stream power in the feeding channel, as well as depositional effects, avulsion, and channelization on the alluvial fan concur to determine the overall exposure. Stream process loading alone, even when rigorously defined in terms of its characterizing parameters, is not sufficient to exhaustively determine exposure. Hence, further developing reliable biphasic simulation models for hazard assessment on settled alluvial fans is pivotal.

Modeling the effects of sediment concentration on the propagation of flash floods in an Andean watershed

Abstract. Rain-induced flash floods are common events in regions near mountain ranges. In peri-urban areas near the Andes the combined effects of the changing climate and El Niño–Southern Oscillation (ENSO) have resulted in an alarming proximity of populated areas to flood-prone streams, increasing the risk for cities and infrastructure. Simulations of rapid floods in these watersheds are particularly challenging, due to the complex morphology, the insufficient hydrometeorological data, and the uncertainty posed by the variability of sediment concentration. High concentrations produced by hillslope erosion and rilling by the overland flow in areas with steep slopes and low vegetational covering can significantly change the dynamics of the flow as the flood propagates in the channel. In this investigation, we develop a two-dimensional finite-volume numerical model of the nonlinear shallow water equations coupled with the mass conservation of sediment to study the effects of different densities, which include a modified version of the quadratic stress model to quantify the changes in the flow rheology. We carry out simulations to evaluate the effects of the sediment concentration on the floods in the Quebrada de Ramón watershed, a peri-urban Andean basin in central Chile. We simulate a confluence and a total length of the channel of 10.4 km, with the same water hydrographs and different combinations of sediment concentrations in the tributaries. Our results show that the sediment concentration has strong impacts on flow velocities and water depths. Compared to clear-water flow, the wave-front velocity slows down more than 70 % for floods with a volumetric concentration of 60 % and the total flooded area is 36 % larger when the sediment concentration is equal to 20 %. The maximum flow momentum at cross sections in the urban area increases 14.5 % on average when the mean concentration along the main channel changes from 30 % to 44 %. Simulations also show that other variables such as the arrival time of the peak flow and the shape of the hydrograph at different locations along the channel are not significantly affected by the sediment concentration and depend mostly on the steep channel morphology. Through this work we provide a framework for future studies aimed at improving hazard assessment, urban planning, and early warning systems in urban areas near mountain streams with limited data and affected by rapid flood events.

Long-term changes in nitrogen and phosphorus emission into the Vistula and Oder catchments (Poland)\u2014modeling (MONERIS) studies

Modeling (MONERIS) studies allowed calculation of nitrogen (N) and phosphorus (P) emission into the Vistula and Oder basins (Poland), and facilitated estimation of N and P retention in these catchments in 1995–2015. In the discussion of results, data of other authors were used in order to get an insight into N (1880–2015) and P emission (1955–2015) into the Oder basin. Population growth and agricultural intensification were responsible for respective 5.3-fold and 3.5-fold increase in N and P emission into the Oder basin, with the maximum (135,000 tons N year−1; 14,000 tons P year−1) observed at the turn of the 1980s/1990s. Pro-ecological activities during the economic transition period (since 1989) covered various sectors of the economy including agriculture, environmental protection related to, e.g., construction of a large number of waste water treatment plants (WWTPs). Consequently, in 1985–2015, the emission into the Oder basin decreased from the abovementioned maxima to 94,000 tons N year−1 and to 5000 tons P year−1, whereas in 1995–2015, the emission into the Vistula basin decreased from 170,000 to 140,000 tons N year−1 and from 14,200 to 10,600 tons P year−1. In 1995–2015, groundwater, tile drainage, and WWTPs played a key role in N emission, while erosion, overland flow, WWTPs, and urban areas played a predominant role in P emission. The relative shares of nutrient emission pathways in overall N and P emission were considerably changing over time. Extreme weather conditions have a great impact on increased (floods) or decreased (droughts) nutrient emission; particularly, N emission is susceptible to variable weather conditions. In total, approximately 91,000 tons of N and 7600 tons of P were retained annually in the river basins.

Long-term purification efficiency and factors affecting performance in peatland-based treatment wetlands: An analysis of 28 peat extraction sites in Finland

Peatland-based treatment wetlands that purify incoming water by means of natural physical, chemical and biological processes belonging to the peatland ecosystem are widely used at Finnish peat extraction sites. They can comprise either undrained or drained overland flow areas (OFAs or DOFAs), with the OFAs representing the best available technology (BAT) for peat extraction. We analyse here the long-term treatment performance of these OFAs and DOFAs and factors affecting this performance. Data on 14 OFAs and DOFAs in different parts of Finland were taken from the extensive long-term environmental pollution control databases. Nearly half of these wetlands had been monitored for at least 4 years and seven for 8–23 years. The results indicated that peatland-based treatment wetlands purify drainage water as efficiently as other natural treatment wetlands on soils in general, the common challenge being phosphorus retention. Iron was also efficiently retained. The average reductions were highest in OFAs with good hydraulic function, and these also showed long-term water protection performance. An important factor affecting purification efficiency was the hydraulic loading rate. Important system design elements in this regard were the size of the wetland in relation to its catchment area, its gradient, the length of the water flow route within the wetland, and the efficient flow area of the wetland. The results regarding the latter two design elements strongly indicate that not all the areas potentially suitable in DOFAs for water purification are yet being used efficiently.

Can treatment wetlands be constructed on drained peatlands for efficient purification of peat extraction runoff?

Peat extraction for energy purposes causes major changes in the aquatic and terrestrial environment. According to national strategies for extracting peat in Finland, new peat extraction areas should be established on previously drained peatlands. On such areas it is difficult to find the natural, intact peatland required for treatment wetlands or so-called overland flow areas, which are considered the best available technology for runoff water purification. Therefore treatment wetlands must be constructed on drained peatland. It is known that drainage causes physical, biogeochemical and hydraulic changes in the peat layer, as well as changes in vegetation. It is probable that these changes affect the purification efficiency of wetland treatment systems in many ways. This study evaluated the function and characteristics of drained peatland areas for purification of peat extraction runoff water. Study sites were established on 11 drained wetlands and their purification efficiency was evaluated. Detailed measurements of peat physical properties and hydraulic conductivity, as well as studies on vegetation, were also made in the study areas. The results showed that wetlands constructed on drained peatland areas can purify peat extraction runoff. However, leaching of phosphorus (P) and iron (Fe) was observed in some areas. Leaching is influenced e.g. by pH and the soil P pool. The chemical oxygen demand was also observed to increase in runoff water from the wetland. The results indicated that low (Fe+aluminium (Al)+manganese (Mn))/P ratio (≤25) and quite high P content (>1200mg/kg) in the surface peat characterised those areas where P leaching was observed. The presence of a dense tree stand in a drained peatland area also seemed to indicate release of nutrients from the area after its rewetting and use as a treatment wetland. Thus, potential nutrient release from a drained peatland area intended for use as a treatment wetland can be assessed by studying the characteristics of the peatland area, especially the peat mineral content, and the vegetation, especially tree stand density in the area. Using the findings obtained, a conceptual decision tree was drawn up in order to help to establish and design wetlands in previously drained peatland areas.

Interpreting streamflow generation mechanisms from integrated surface-subsurface flow models of a riparian wetland and catchment

[1] The understanding of streamflow generation processes is vitally important in the management of water resources. In the absence of the data required to achieve this, Integrated Surface-Subsurface Hydrological Models (ISSHM) can be used to assist with the development of this understanding. However, the standard outputs from these models only enable elicitation of information about hydrological drivers and hydrological responses that occur at the same time. This generally limits the applicability of ISSHMs for the purposes of obtaining an improved understanding of streamflow generation processes to catchment areas that do not exhibit significant storage, travel times or flow depletion mechanisms. In order to overcome this limitation, a previously published Hydraulic Mixing-Cell (HMC) method is improved so that it can be used to follow surface water derived from direct rainfall and groundwater discharge to the stream and adjacent overland flow areas. The developed approach was applied to virtual experiments (based on the Lehstenbach catchment in southeastern Germany), which are composed of two ISSHMs of contrasting scales: (1) a riparian wetland of area 210 m2 and (2) a catchment of area 4.2 km2. For the two models, analysis of modeling results for a large storm event showed complex spatiotemporal variability in streamflow generation and surface water-groundwater interaction. Further analysis with the HMC method elucidated in-stream and overland flow generation mechanisms. This study showed within a modeling framework that identification and quantification of in-stream and overland flow generation better informed understanding of catchment functioning through decomposition of streamflow hydrographs, and analysis of spatiotemporal variability of flow generation mechanisms.

Depression storage and infiltration effects on overland flow depth-velocity-friction at desert conditions: field plot results and model

Abstract. Water infiltration and overland flow are relevant in considering water partition among plant life forms, the sustainability of vegetation and the design of sustainable hydrological models and management. In arid and semi-arid regions, these processes present characteristic trends imposed by the prevailing physical conditions of the upper soil as evolved under water-limited climate. A set of plot-scale field experiments at the semi-arid Patagonian Monte (Argentina) were performed in order to estimate the effect of depression storage areas and infiltration rates on depths, velocities and friction of overland flows. The micro-relief of undisturbed field plots was characterized at z-scale 1 mm through close-range stereo-photogrammetry and geo-statistical tools. The overland flow areas produced by controlled water inflows were video-recorded and the flow velocities were measured with image processing software. Antecedent and post-inflow moisture were measured, and texture, bulk density and physical properties of the upper soil were estimated based on soil core analyses. Field data were used to calibrate a physically-based, mass balanced, time explicit model of infiltration and overland flows. Modelling results reproduced the time series of observed flow areas, velocities and infiltration depths. Estimates of hydrodynamic parameters of overland flow (Reynolds-Froude numbers) are informed. To our knowledge, the study here presented is novel in combining several aspects that previous studies do not address simultaneously: (1) overland flow and infiltration parameters were obtained in undisturbed field conditions; (2) field measurements of overland flow movement were coupled to a detailed analysis of soil microtopography at 1 mm depth scale; (3) the effect of depression storage areas in infiltration rates and depth-velocity friction of overland flows is addressed. Relevance of the results to other similar desert areas is justified by the accompanying biogeography analysis of similarity of the environment where this study was performed with other desert areas of the world.

Quantification of genes encoding resistance to aminoglycosides, 𝛃-lactams and tetracyclines in wastewater environments by real-time PCR

In this study real-time PCR assays, based on the LUX™-technique, were developed for quantification of genes mediating resistance to aminoglycosides [aac(6´′)-Ie + aph(2´′´′)], β-lactams (mecA), and tetracyclines (tetA and tetB), for use in wastewater environments. The developed assays were applied on DNA extracted from three wastewater-associated environments: soil from an overland flow area treating landfill leachates, biofilm from a municipal wastewater treatment plant, and sludge from a hospital wastewater pipeline. The highest concentration of all genes was observed in the hospital pipeline and the lowest in the overland flow system. TetA and aac(6´′)-Ie + aph(2´′´′) could be detected in all environments. The tetB gene was detected in the overland flow area and the hospital wastewater pipeline and mecA was detected in the wastewater treatment plant and the hospital pipeline. The developed LUX™ real-time PCR assays were shown to be fast and reproducible tools for detection and quantification of the four genes encoding antibiotic resistance in wastewater.

Application of a fully‐integrated surface‐subsurface flow model at the watershed‐scale: A case study

Results are presented in which a physically‐based, three‐dimensional model that fully integrates surface and variably‐saturated subsurface flow processes is applied to the 75 km2 Laurel Creek Watershed within the Grand River basin in Southern Ontario, Canada. The primary objective of this study is to gauge the model's ability to reproduce surface and subsurface hydrodynamic processes at the watershed scale. Our objective was first accomplished by calibrating the steady‐state subsurface portion of the system to 50 observation wells where hydraulic head data were available, while simultaneously matching the stream baseflow discharge. The level of agreement between the observed and computed subsurface hydraulic head values, baseflow discharge and the spatial pattern of the surface drainage network indicates that the model captures the essence of the surface‐subsurface hydraulic characteristics of the watershed. The calibrated model is then subjected to two time series of input rainfall data and the calculated discharge hydrographs are compared to the observed rainfall‐runoff responses. The calculated and observed rainfall‐runoff responses were shown to agree moderately well for both rainfall data series that were utilized. Additionally, the spatial and temporal responses of the watershed with respect to the overland flow areas contributing to streamflow and the surface‐subsurface exchange fluxes across the land surface during rainfall inundation and subsequent drainage phases demonstrate the dynamic nature of the interaction occurring between the surface and subsurface hydrologic regimes. Overall, it is concluded that it is feasible to apply a fully‐integrated, surface/variably‐saturated subsurface flow model at the watershed scale and possibly larger scales.

An enhanced technique in construction of the discrete drainage network from low-resolution spatial database

A digital elevation model (DEM) of a watershed can be used to acquire various parameters such as basin-wide information about overland flow direction, flow accumulation and area contributing flow to any point. The resolution and quality of a DEM are important to achieve a significant level of accuracy in derived parameters. Inadequate elevation information exacerbated by applied interpolation methods, reduce DEM accuracy, resulting in pits and flat areas and makes flow tracing a difficult task. These types of problems are more prominent in cases of residual undulating terrains. In the present paper, an attempt has been made to review and suggest an improved method for the generation of a DEM from raster contour data. Further, a criteria-based region growing method (CBRGM) is presented for the extraction of discrete drainage network (cell size: 20×20 m) of the watershed. Here, the flat area removal algorithm, with a variable increment, is used to generate the DEM. This induces a gradual slope even in the case of a large contour interval (20 m) extended over larger area, as is commonly available from a topographic map at a scale of 1:50,000. Further, in order to capture topographic information in flow tracing, the CBRGM is followed. The rasterised stream network from the same topographic sheet is used as ancillary data to make the concentrated flow lines to follow the channel. The methodology has been tested over Gandheshwari subwatershed under the lower part of Chhotanagpur Plateau in Eastern India. The DEM generated using this method gives a better representation of the terrain, which shows good agreement with the terrain information delineated by using the contour and channel information available in the topographic sheet. The drainage network derived shows additional extra-concentrated flow lines, many of which match the drainage network obtained from satellite imagery (cell size: 23.5×23.5 m). The algorithm thus shows superiority over other available methods for the extraction of drainage networks.

Overland flow systems for treatment of landfill leachates—Potential nitrification and structure of the ammonia-oxidising bacterial community during a growing season

Overland flow systems are useful for treating landfill leachates, because they provide favourable conditions for nitrification and they are easy to maintain. However, little is known about the microbial communities in such systems or the nitrification capacity of those microorganisms. In this study, seasonal variations in potential nitrification and in community composition of nitrifying bacteria were investigated in two overland flow areas receiving leachate from landfills at Korslöt and Hagby, Sweden. Samples were collected in the settling ponds sediment and at two depths in the overland flow areas (the macrophyte litter layer and the rhizosphere) in May, August and November 2003. A short-term incubation method was used to measure potential oxidation of ammonia and nitrite (designated PAO and PNO). The ammonia-oxidising bacterial (AOB) community was investigated using a 16S rRNA gene approach that included PCR amplification and analysis of PCR products by denaturing gradient gel electrophoresis (DGGE), followed by nucleotide sequencing and phylogenetic analysis. PAO was determined in the range 5–2700 (NO 2 −+NO 3 −)-N g −1 dw d −1 and PNO in the range 60–2000 μg NO 2 −-N g −1 dw d −1. At Korslöt, PAO and PNO showed similar temporal variation in the different ecosystems, whereas no such relationship was noticed at Hagby. Considering both sites, there was no obvious change in the composition of the AOB community over the growing season. However, the composition did differ between the ecosystems: Nitrosomonas-like sequences were more common in the ponds, and in the litter layers they were found as often as Nitrosospira-like sequences, whereas Nitrosospira-like sequences were more common in the rhizospheres. Altogether, we found nine different AOB sequences, five Nitrosomonas-like and four Nitrosospira-like, which belonged to clusters 0, 2, 3b, 6a, 6b and 7. There was no apparent relationship between the number of AOB populations and the PAO in different soil layers and sediments.

Effectiveness of constructed overland flow areas in decreasing diffuse pollution from forest drainages.

Forestry is the largest scale human impact affecting catchments in Finland and a prominent source of diffuse pollution in many water courses. Among the forestry activities, draining of wetlands had the most pronounced impacts on sediment, nutrient, and metal loading in the past. At present, renovation of old ditches and fertilization of peatlands constitute the major risk of forestry-induced diffuse pollution. Contemporary forestry aims at decreasing this risk with various riparian buffer strip designs. Among such designs, creation of overland flow areas by plugging the outlet ditches is increasingly used. Our objectives were to evaluate the potential of constructed overland flow areas to function as riparian buffers and estimate the quality and quantity of diffuse pollution from old versus recent forest drainages. We studied retention and release of pollutants from 20 constructed, 2- to 10-m-wide overland flow areas receiving drainage water from forested peatlands. Drainage waters were sampled above and below the plugged ditches three times per year from 1998 to 1999. Chemical oxygen demand and nutrient and metal loads and concentrations varied strongly between seasons, years, and drainage areas. Areas subjected to recent ditch renovations and fertilizations had clearly elevated seasonal loads and concentrations of total phosphorus (TP), PO4, Fe, and Al in comparison to old treatment areas. Especially TP loads were high above the national average values measured for forestry-induced diffuse pollution. In general, water quality above and below the buffer strips did not differ significantly. Our results indicate that plugged outlet ditches and associated narrow overland flow areas do not function as proper buffers in peatland areas. We suggest that wider buffers with extensive overland flow areas are needed in order to control diffuse pollution from forested and drained peatlands.

Parameter Identifiability for Three Sediment Entrainment Equations

A process-based erosion model is used to study parameterization problems of sediment entrainment equations in overland flow areas. One of the equations for entrainment by flow is developed based on a theory of excess stream power, while the other two relate to excess hydraulic shear. The investigation is conducted in two steps. The first step examines parameter optimization for simulated data sets where the parameter values are known. In the second step, parameter optimization for the most robust equation is examined using experimental data from rainfall simulator plots. Results demonstrate that although the model is capable of estimating total sediment yields with relatively small errors in parameter estimates, the converse is true when the optimization is performed for sediment concentrations. Although sediment yields calculated from simulated sediment concentrations match well with observed data, the parameter estimates generally underestimate sediment concentrations on the rising limb of the sediment graphs, and they overestimate them on the falling limb. This difficulty might be related to structural problems in the model, and unique solutions for parameter estimates cannot be obtained.

GRISTORM: GRID?BASED VARIABLE SOURCE AREA STORM RUNOFF MODEL

A gridbased storm runoff model in a variable source area is described. This model predicts temporal variationsand spatial distributions of subsurface flowsaturation overland flow with various shallow soil depths. The model usesASCIIformatted map data supported by the GRASS Geographic Information Systems (GIS) and generates distributed resultssuch as discharge, flow depth, and soil moisture in overland flow areas. The model uses a single overland flowpath algorithmand simulates surface and/or subsurface water depth at each grid element for a given time increment. A combinedsurfacesubsurface kinematic modeling approach was used to simulate surface and subsurface flow. The model was appliedto a 170 ha watershed located in New York State. Predicted flows from six summer storm events in 1994 were compared withobserved flows at the watershed outlet. The initial soil moisture conditions for the storm events were based on calibratedvalues from Frankenbergers Soil Moisture Routing (SMR) model and were adjusted to include soil moisture variations withina day. The average NashSutcliffe efficiency for predicting runoff at the watershed outlet using calibrated parameters forthree storm events was 0.72. The initial soil moisture distribution proved to be the most sensitive parameter affecting streamflow at the watershed outlet and required calibration to obtain reasonable results. The second most sensitive parameter wasMannings roughness coefficient for stream and overland areas. It affected the time and magnitude of peak stream flow. Resultsshowing temporal variations and spatial distributions of overland flow areas were presented using GRASS. According to theresults showing the spatial effect with differing grid element sizes, the model behavior was also sensitive to DEM gridresolution.

GRIEROM: GRID–BASED VARIABLE SOURCE AREA SOIL–WATER EROSION AND DEPOSITION MODEL

Erosion in a variable source area and the sediment in surface runoff have a major impact on soil conservationand water quality. It is necessary to locate the soil erosion areas and the paths of sediment transport efficiently to alleviatepotential soil loss problems and protect water quality. The objective of this study was to predict temporal variations and spatialdistributions of sediment transport in a variable source area for storm events. A GRIdbased soilwater EROsion anddeposition Model (GRIEROM) is described. This model uses ASCIIformatted map data supported by the GRASS GeographicInformation Systems (GIS) and generates distributed results such as sediment concentration and flux in overland flow areas.GRISTORM was adopted for the hydrological processes. A processbased soil erosion concept was adopted to simulatesoilwater erosion and deposition. The model deals separately with rill and sheet flow and handles vegetation in terms of soilcover. The model was applied to a 170 ha watershed located in the Northern Catskill region of New York. The initial soilmoisture conditions for storm events were based on calibrated values from a Soil Moisture Routing (SMR) model and wereadjusted to include soil moisture variations within a day. Predicted flows from four storm events in 1993 were compared withobserved flows at the watershed outlet for hydrologic calibration. The average NashSutcliffe efficiency for predicting flowat the outlet using calibrated data was 0.847. Parameters related to sheet/rill flow on bare soil were more sensitive than thoseon vegetated soil. Sediment depositability (
) proved to be the most sensitive parameter, followed by the low value of soilerodibility (). The NashSutcliffe efficiencies for predicting sediment yield at the outlet for the two storms were negativebecause of additive prediction and time error of sediment concentration, respectively. The average NashSutcliffe efficiencyfor another two storms was 0.476. The temporal variations and spatial distributions of overland flow and sediment areas werepresented using GRASS. These presentations suggest that most sediment was transported to the stream from the areas thathad high overland flow velocities.

Performance Evaluation of Overland Flow Wastewater Treatment System under Winter and Summer Conditions

A study was conducted to evaluate the effects of seasonal variations, summer and winter conditions, on the performance of an existing and successfully operated overland flow wastewater treatment system in the Midwest. The effect of design parameters, such as hydraulic and organic loading rates, and the seasonal temperature effects on performance were investigated. The overland flow system studied consists of an aerated lagoon, two conventional lagoons, and an overland flow area. The overland flow area is divided into six groups of terraces and the terraces are constructed at a slope of 2% with slope length of 341.3 m (1,120 ft). The influent and effluent samples were analyzed for biological oxygen demand (BOD 5), suspended solids (SS), pH, and temperature. The effluent samples were also analyzed for ammonia nitrogen. The results of the study indicate that the overland flow system provided excellent treatment of municipal wastewater. The average removal for BOD 5 and SS were 89% and 85%, respectively, in warm weather. During cold weather the percent removals for these parameters were 81% and 69%, respectively. The effluent ammonia-nitrogen was 1.2 and 2.9 mg/L, respectively, for summer and winter, which indicates significant ammonia nitrification, especially in summer months. Regardless of seasonal conditions, the effluent BOD 5 and SS for the system met the National Pollutant Discharge Elimination System effluent permit requirements throughout the year.

Nutrient retention in the vegetation of an overland flow treatment system in northern Finland

The importance of vegetation for nutrient retention was estimated in an overland flow area (OFA) in northern Finland, where peat mining water is purified by conducting it across a virgin mire. In the OFA the phytomass of herbs, sedges, graminoids and below-ground parts of plants was greater, and that of shrubs and bryophytes smaller, than in a reference area (RA). Overland flow treatment favoured plant species, such as Menyanthes trifoliata and Carex lasiocarpa, and discriminated against Sphagnum spp. in particular. The differences in the nutrient contents of plants in the OFA resulting from nutrient enrichment by peat mining water were compared with the actual decrease in amounts of nutrients in the area. Total phytomass in the OFA increased by 20%, and its N concentration increased from 1.00 to 1.24% of dry weight, but the P concentration decreased from 0.26 to 0.22% of dry weight. These changes in total phytomass and nutrient concentrations resulted in a 40% increase in the N bound by plants in the area, but a 5% decrease in P. The increase in N bound by the total phytomass in the OFA during the 6 years of usage accounted for only about 4% of the observed total retention of inorganic N. In contrast, the vegetation in the area served as a source of P rather than a sink, with the decrease in P accounting for nearly 20% of the measured PO 4-P retention. The average total decreases in both P and N in the OFA in 1987–1991 were 55%. Thus, the net retention of N and P from the peat mining water was mainly the result of other processes rather than retention in the plant cover.

Retention of organic Fe-P-colloids from peat mining water in an overland flow wetland treatment system in northern Finland

The importance of dissolved organic matter (DOM) for the organic, Fe and P reductions achieved in an overland flow area (OFA) was estimated and the changes in the nature of DOM studied by gel filtration at a site in northern Finland. In the OFAs peat mining water is purified conducting it across a natural mire of a given size. DOM plays an important role in the transport and reduction of organic matter, Fe and P in OFAs. The retention of Fe in filtrates, dissolved organic P and to some extent also DOM in the OFA under low flow conditions in summer is probably promoted by the high Fe content of the highest apparent molecular (HAMW) weight DOM in the peat mining water. On the other hand, the originally low Fe content of the DOM in the HAMW fraction may be one reason for the generally low reductions in these water quality parameters achieved during flood periods.