Generalized Watershed Loading Function Research Articles

Variations in source apportionments of nutrient load among seasons and hydrological years in a semi-arid watershed: GWLF model results

Quantifying source apportionments of nutrient load and their variations among seasons and hydrological years can provide useful information for watershed nutrient load reduction programs. There are large seasonal and inter-annual variations in nutrient loads and their sources in semi-arid watersheds that have a monsoon climate. The Generalized Watershed Loading Function model was used to simulate monthly nutrient loads from 2004 to 2011 in the Liu River watershed, Northern China. Model results were used to investigate nutrient load contributions from different sources, temporal variations of source apportionments and the differences in the behavior of total nitrogen (TN) and total phosphorus (TP). Examination of source apportionments for different seasons showed that point sources were the main source of TN and TP in the non-flood season, whereas contributions from diffuse sources, such as rural runoff, soil erosion, and urban areas, were much higher in the flood season. Furthermore, results for three typical hydrological years showed that the contribution ratios of nutrient loads from point sources increased as streamflow decreased, while contribution ratios from rural runoff and urban area increased as streamflow increased. Further, there were significant differences between TN and TP sources on different time scales. Our findings suggest that priority actions and management measures should be changed for different time periods and hydrological conditions, and that different strategies should be used to reduce loads of nitrogen and phosphorus effectively.

Modeling nutrient loads to the northern Adriatic

Summary The northern Adriatic (NA) is one of the most productive areas of the Mediterranean Sea with a watershed area spread over four neighboring countries, Italy, Slovenia, Switzerland and Croatia. Up to date research include mainly the nutrient load estimations from the biggest nutrients sources, like the Po River watershed (Italian part). This research aims to estimate the loads from nearly the entire contributing area, including the Italian, the Slovenian and the Croatian part. The ArcView Generalized Watershed Loading Function (AVGWLF) model was applied to simulate nutrient loads from the NA watershed. The model can simulate runoff, sediment and nutrients (nitrogen and phosphorus) loads from the watershed, by taking dispersed and point sources into account. Calibration and validation of the model were performed on the data from 1999 to 2002 and 2003 to 2007, respectively. Comparison between the simulated and measured loads in the Po River watershed shows a good match and thus high reliability of the model. As expected, the Po River dominated the nutrient loads to the NA. However, the results also point to other contributing areas, e.g. smaller watersheds with high intensity of nutrient loads, causing significant local impacts (e.g. Lagoon of Venice). The model also shows that the major sources of nutrients are not only agricultural but also urban areas without proper wastewater treatment (secondary at present). Proper management of these areas, such as introducing suitable (tertiary) wastewater treatment, or other nutrient management measures shall reduce the nutrient loadings to the NA. Further work is focused to use the model for optimizing the control and management of the activities in the contributing areas, smaller as well as bigger sub-watersheds.

Identifying critical source areas of nonpoint source pollution with SWAT and GWLF

Identification of critical source areas (CSAs) (areas contributing most of the pollutants in a watershed) is important for cost-effective implementation of best management practices. Identification of such areas is often done through watershed modeling. Various watershed models are available for this purpose, however it is not clear if the choice (and complexity) of a model would lead to differences in locations of CSAs. The objective of this study was to use two models of different complexity for identifying CSAs. The relatively complex Soil and Water Assessment Tool (SWAT) and the simpler Generalized Watershed Loading Function (GWLF) were used to identify CSAs of sediment and nutrients in the Saugahatchee Creek watershed in east central Alabama. Models were calibrated and validated for streamflow, sediment, total nitrogen (TN) and total phosphorus (TP) at a monthly time scale. While both models performed well for streamflow, SWAT performed slightly better than GWLF for sediment, TN and TP. Sub-watersheds dominated by urban land use were among those producing the highest amount of sediment, TN and TP loads, and thus identified as CSAs. Sub-watersheds with some amount of agricultural crops were also identified as CSAs of TP and TN. A few hay/pasture dominated sub-watersheds were identified as CSAs of TN. The identified land use source areas were also supported by field collected water quality data. A combined index was used to identify the sub-watersheds (CSAs) that need to be targeted for overall reduction of sediment, TN and TP. While many CSAs identified by SWAT and GWLF were the same, some CSAs were different. Therefore, this study concludes that model choice will affect the location of some CSAs.

Comparison of approaches for snowpack estimation in New York City watersheds

AbstractSnow is a substantial component of historical annual precipitation in New York City (NYC) water supply watersheds in the Catskill Mountains, and the pattern of snow accumulation and snowmelt has important implications for the management of the reservoirs and watersheds that are part of the NYC water supply. NYC currently estimates reservoir basin‐scale snowpack throughout the snow season by extrapolation from biweekly snow survey data. These estimates are complemented by the NOAA Snow Data Assimilation System (SNODAS) product. Snowpack models are used in short‐term projections to support reservoir operations and long‐term simulations to evaluate the potential effects of climate change, land use change, and watershed management on the water supply. We tested three snowpack estimation approaches compared with snow survey data: the lumped‐parameter temperature index approach from the Generalized Watershed Loading Function (GWLF) watershed model, a spatially distributed temperature index (SDTI) model, and the spatially distributed NOAA SNODAS product. Of the spatially distributed approaches, SNODAS estimated the spatial variability of snow water equivalent (SWE) among snow survey sites within a basin better than the SDTI model. All three snowpack estimation approaches, including the lumped‐parameter GWLF model, performed well in estimating basin‐wide SWE for most of the basins studied. Copyright © 2013 John Wiley & Sons, Ltd.

Effect of projected changes in winter streamflow on stream turbidity, Esopus Creek watershed in New York, USA

AbstractThis study focuses on the impact of changes in winter streamflow on in‐stream turbidity in the Esopus Creek watershed, one of the New York City water supply watersheds. Projected changes in daily precipitation and air temperature from a suite of five global climate models and three emission scenarios for future periods 2046–2065 and 2081–2100 were downscaled for the study region. The simulated climate scenarios were used to project future streamflows using the Generalized Watershed Loading Functions – Variable Source Area watershed model. Seasonal turbidity rating curves based on measured historical streamflow and stream turbidity were used in combination with the simulated streamflow for generating future stream turbidity scenarios. Results indicate an increase in future ambient stream turbidity from November to March and a decrease during April. These results are the effects of increased winter rainfall, reduced snowfall, and a shift to early timing of spring snowmelt runoff, causing an increase in streamflow during early winter. It also suggests a reduction in the traditional peak streamflow around April that is expected to occur in this region. As a result, our models simulate a consistent increase in the low to medium percentile range of turbidity values associated with low to medium range of streamflows and no apparent change in high‐percentile turbidity values associated with high streamflows. Our results may be applicable in regions where snowmelt runoff is an important process and turbidity caused by the suspension of fine clay particles is a water quality concern. Copyright © 2013 John Wiley & Sons, Ltd.

Application of the ReNuMa model in the Sha He river watershed: Tools for watershed environmental management

Models and related analytical methods are critical tools for use in modern watershed management. A modeling approach for quantifying the source apportionment of dissolved nitrogen (DN) and associated tools for examining the sensitivity and uncertainty of the model estimates were assessed for the Sha He River (SHR) watershed in China. The Regional Nutrient Management model (ReNuMa) was used to infer the primary sources of DN in the SHR watershed. This model is based on the Generalized Watershed Loading Functions (GWLF) and the Net Anthropogenic Nutrient Input (NANI) framework, modified to improve the characterization of subsurface hydrology and septic system loads. Hydrochemical processes of the SHR watershed, including streamflow, DN load fluxes, and corresponding DN concentration responses, were simulated following calibrations against observations of streamflow and DN fluxes. Uncertainty analyses were conducted with a Monte Carlo analysis to vary model parameters for assessing the associated variations in model outputs. The model performed accurately at the watershed scale and provided estimates of monthly streamflows and nutrient loads as well as DN source apportionments. The simulations identified the dominant contribution of agricultural land use and significant monthly variations. These results provide valuable support for science-based watershed management decisions and indicate the utility of ReNuMa for such applications.

Modelling catchment hydrological responses in a Himalayan Lake as a function of changing land use and land cover

In this paper, we evaluate the impact of changing land use/land cover (LULC) on the hydrological processes in Dal lake catchment of Kashmir Himalayas by integrating remote sensing, simulation modelling and extensive field observations. Over the years, various anthropogenic pressures in the lake catchment have significantly altered the land system, impairing, inter-alia, sustained biotic communities and water quality of the lake. The primary objective of this paper was to help a better understanding of the LULC change, its driving forces and the overall impact on the hydrological response patterns. Multi-sensor and multi-temporal satellite data for 1992 and 2005 was used for determining the spatio-temporal dynamics of the lake catchment. Geographic Information System (GIS) based simulation model namely Generalized Watershed Loading Function (GWLF) was used to model the hydrological processes under the LULC conditions. We discuss spatio-temporal variations in LULC and identify factors contributing to these variations and analyze the corresponding impacts of the change on the hydrological processes like runoff, erosion and sedimentation. The simulated results on the hydrological responses reveal that depletion of the vegetation cover in the study area and increase in impervious and bare surface cover due to anthropogenic interventions are the primary reasons for the increased runoff, erosion and sediment discharges in the Dal lake catchment. This study concludes that LULC change in the catchment is a major concern that has disrupted the ecological stability and functioning of the Dal lake ecosystem.

Adaptation of Land-Use Demands to the Impact of Climate Change on the Hydrological Processes of an Urbanized Watershed

The adaptation of land-use patterns is an essential aspect of minimizing the inevitable impact of climate change at regional and local scales; for example, adapting watershed land-use patterns to mitigate the impact of climate change on a region’s hydrology. The objective of this study is to simulate and assess a region’s ability to adapt to hydrological changes by modifying land-use patterns in the Wu-Du watershed in northern Taiwan. A hydrological GWLF (Generalized Watershed Loading Functions) model is used to simulate three hydrological components, namely, runoff, groundwater and streamflow, based on various land-use scenarios under six global climate models. The land-use allocations are simulated by the CLUE-s model for the various development scenarios. The simulation results show that runoff and streamflow are strongly related to the precipitation levels predicted by different global climate models for the wet and dry seasons, but groundwater cycles are more related to land-use. The effects of climate change on groundwater and runoff can be mitigated by modifying current land-use patterns; and slowing the rate of urbanization would also reduce the impact of climate change on hydrological components. Thus, land-use adaptation on a local/regional scale provides an alternative way to reduce the impacts of global climate change on local hydrology.

Investigating the impact of climate change on New York City’s primary water supply

Future climate scenarios projected by three different General Circulation Models and a delta-change methodology are used as input to the Generalized Watershed Loading Functions – Variable Source Area (GWLF-VSA) watershed model to simulate future inflows to reservoirs that are part of the New York City water supply system (NYCWSS). These inflows are in turn used as part of the NYC OASIS model designed to simulate operations for the NYCWSS. In this study future demands and operation rules are assumed stationary and future climate variability is based on historical data to which change factors were applied in order to develop the future scenarios. Our results for the West of Hudson portion of the NYCWSS suggest that future climate change will impact regional hydrology on a seasonal basis. The combined effect of projected increases in winter air temperatures, increased winter rain, and earlier snowmelt results in more runoff occurring during winter and slightly less runoff in early spring, increased spring and summer evapotranspiration, and reduction in number of days the system is under drought conditions. At subsystem level reservoir storages, water releases and spills appear to be higher and less variable during the winter months and are slightly reduced during summer. Under the projected future climate and assumptions in this study the NYC reservoir system continues to show high resilience, high annual reliability and relatively low vulnerability.

Artificial Neural Network Models of Watershed Nutrient Loading

This paper illustrates the use of artificial neural networks (ANNs) as predictors of the nutrient load from a watershed. Accurate prediction of pollutant loadings has been recognized as important for determining effective water management strategies. This study compares Haith’s Generalized Watershed Loading Function (GWLF) and Arnold’s Soil and Water Assessment Tool (SWAT) to multilayer artificial neural networks for monthly watershed load modeling. The modeling results indicate that calibrated feed-forward ANN models provide prediction which are always essentially as accurate as those obtained with GWLF and the SWAT, and some times much more accurate. With its flexibility and computation efficiency, the ANN should be a useful tool to obtain a quick simulation assessment of nutrient loading for various management strategies.

Economic Analysis of Best Management Practices to Reduce Watershed Phosphorus Losses

In phosphorus-limited freshwater systems, small increases in phosphorus (P) concentrations can lead to eutrophication. To reduce P inputs to these systems, various environmental and agricultural agencies provide producers with incentives to implement best management practices (BMPs). In this study, we examine both the water quality and economic consequences of systematically protecting saturated, runoff-generating areas from active agriculture with selected BMPs. We also examine the joint water quality/economic impacts of these BMPs-specifically BMPs focusing on barnyards and buffer areas. Using the Variable Source Loading Function model (a modified Generalized Watershed Loading Function model) and net present value analysis (NPV), the results indicate that converting runoff-prone agricultural land to buffers and installing barnyard BMPs are both highly effective in decreasing dissolved P loss from a single-farm watershed, but are also costly for the producer. On average, including barnyard BMPs decreases the nutrient loading by about 5.5% compared with only implementing buffers. The annualized NPV for installing both buffers on only the wettest areas of the landscape and implementing barnyard BMPs becomes positive only if the BMPs lifetime exceeds 15 yr. The spatial location of the BMPs in relation to runoff producing areas, the time frame over which the BMPs are implemented, and the marginal costs of increasing buffer size were found to be the most critical considerations for water quality and profitability. The framework presented here incorporates estimations of nutrient loading reductions in the economic analysis, and is applicable to farms facing BMP adoption decisions.

Impact of climate change on paddy field irrigation in southern Taiwan

Climate change can have a serious impact on water resources. The main agricultural product in southern Taiwan is rice, the planting of which consumes far more water than other crops. This makes agriculture in Taiwan especially vulnerable to climate change. In this study, we used the generalized watershed loading functions (GWLF) hydrological model to simulate the discharge of the Kaoping River under climate change scenarios A2 and B2 as released by the Intergovernmental Panel on Climate Change. We discussed the potential impact of climate change on water resources based on the results of GWLF simulations carried out using rainfall and temperature data from five general circulation models (GCMs). The simulation results indicate that river discharge in the wet season increases significantly, and decreases in the dry season. The discharge variations from using the various GCMs as inputs fall within the range of −26 to +15 % for the dry season and −10 to +82 % for the wet season. The variation in available water will seriously impact the first period rice farming (the period between the beginning of January and the end of May) in southern Taiwan. Consequently, effective reduction in conveyance loss in the irrigation canal systems and proper fallowing of paddy fields will be the main challenges to Taiwan’s agricultural sector for alleviating the impact of climate change. For further decision making, we show the effects of adapting to climate change by various degrees of the following two methods: fallowing paddy fields to various degrees and reducing conveyance loss in irrigation canal systems.

Using models to bridge the gap between land use and algal blooms: An example from the Loweswater catchment, UK

The goods and services that lakes provide result from complex interactions between meteorology, hydrology, nutrient loads and in-lake processes. Hydrology and nutrient loads are, in turn, influenced by socio-economic factors such as human habitation, water abstraction and land-management, within their catchments. Models provide a means of linking these different domains and also of forecasting and evaluating the effects of different management scenarios on lakes. This paper describes the application of such models to Loweswater, a well-studied lake with water quality problems in the English Lake District, where a community-based approach to catchment management is being undertaken.Three models were linked. Firstly, PLANET (Planning Land Applications of Nutrients for Efficiency and the environmenT), an ‘off the shelf’ farm nutrient budgeting model, was supplemented by local information on septic tanks and used to produce an annual nutrient load to the lake. Secondly, GWLF (Generalized Watershed Loading Function), a generic nutrient runoff model, was used to generate daily nutrient runoff values using input from PLANET plus additional information on land-cover, air temperature and rainfall within the catchment. Thirdly PROTECH (Phytoplankton RespOnses To Environmental CHange), driven by input from GWLF and locally measured meteorology, was used to forecast the abundance of different algal types within the lake. The linked models were used to describe the current impact of catchment management on lake water quality, validated by in situ measurements, and to explore the potential impact of a number of alternative catchment management scenarios. Issues surrounding the use of generic modelling applications for catchment management and relevance for stakeholders living in and/or managing land within the catchment are discussed.

Effects of changes in snow pattern and the timing of runoff on NYC water supply system

AbstractThis study focuses on the effect of projected changes in rainfall, snow accumulation and snowmelt, and consequent changes in the timing of runoff on NYC water supply system storage and operation as simulated by the NYC reservoir system Operational Analysis and Simulation of Integrated Systems (OASIS) model. The Generalized Watershed Loading Functions—Variable Source Area (GWLF‐VSA)—watershed model is used with future climate scenarios derived from different General Circulation Models (GCMs) to simulate future inflows to reservoirs that are part of the New York City Water Supply System (NYCWSS). Future scenarios that use current system operation rules and demands, but changed reservoir inflows, suggest that changes in precipitation and snowmelt will affect regional water availability on a seasonal basis. The combined effect of projected increases in winter air temperatures, increased winter rain, and earlier snowmelt may result in more runoff during winter. This will cause reservoir storage levels, water releases and spills to increase during the winter and earlier reservoir refill in the spring. An overall increase in precipitation will result in a reduction in number of days the system is under drought conditions, despite increased evapotranspiration later in the year. Copyright © 2011 John Wiley & Sons, Ltd.

A Hybrid Approach to Modeling Sediment for Total Maximum Daily Load Development

Total maximum daily loads (TMDLs) have been developed for sediment in more than 50 watersheds throughout Virginia. The majority of these sediment TMDLs have been developed using the Generalized Watershed Loading Function (GWLF) model and a reference watershed approach. In this approach, the TMDL is set based on area-adjusted, GWLF-modeled sediment loads in a similar unimpaired watershed. While this approach has been used extensively, it possesses significant shortcomings. The approach is extremely sensitive to reference watershed selection, and modeled sediment loads are rarely calibrated or referenced to any measured sediment load. To avoid these limitations and accentuate the strengths of the GWLF model, researchers in Virginia developed a hybrid modeling approach for sediment TMDLs that combines GWLF modeling with load duration modeling. This hybrid modeling approach utilizes the strengths of each modeling tool while avoiding some of the limitations. This approach has been used successfully for sediment TMDLs developed in the Rivanna River, South River, and Spout Run, Virginia.

Fluxes of water and nutrients from river runoff to the Mediterranean Sea using GIS and a watershed model

The Mediterranean Sea is an important regional European Union (EU) sea, often used to assess the global change of the environment because of its practically enclosed character. As most of the northern part is bordered by the EU, it is also of interest in the evaluation of the environmental and economic impact of the implementation of the EU Water Framework Directive and related legislation. Within this framework, nutrient loads from the adjacent land surfaces need to be assessed. For this purpose, the geographic information system (GIS)–interfaced hydrologic/water quality model ArcView Generalized Watershed Loading Function Model (AVGWLF) was applied to all coastal‐adjacent catchments of the Mediterranean Sea. Results of this exercise have shown that the modeled average streamflow and nutrient loads fall within the ranges reported in the literature. Therefore, successful application of this model is feasible. However, for improved representation of the actual Mediterranean Sea streamflow and nutrient status, more accurate input data are required, particularly for the Asian and North African regions.

AVGWLF‐Based Estimation of Nonpoint Source Nitrogen Loads Generated Within Long Island Sound Subwatersheds1

Abstract: The Generalized Watershed Loading Functions (GWLF) model and its ArcView interface (AVGWLF) were used to estimate and examine the components of the total nitrogen (TN) nonpoint source (NPS) load generated within New York and Connecticut (CT) watersheds surrounding Long Island Sound (LIS, the Sound). The majority of data used as model inputs were generally available from online sources, and the work involved an overall calibration to streamflow and TN data in accordance with generic guidelines recommended in the GWLF manual. The GWLF model performance for three calibration and two validation watersheds in CT was compared with results of a detailed model, Hydrological Simulation Program in Fortran, developed in a previous study. The results of the application illustrate the usefulness of the relatively simpler, less parameter‐intensive GWLF model in performing exploratory loading analysis in preparation for adaptive nutrient management in the LIS watersheds. The presented methodology is valuable for identification of priority watersheds for NPS pollution reduction and also for planning‐level evaluation of best management practices to achieve the desired reductions. It is estimated that ground‐water base flow may be the largest pathway for NPS TN to the Sound, contributing about 54% of the total NPS TN load, a finding with significant implications for LIS total maximum daily load reduction scenarios. In addition to ground water, septic systems are estimated to contribute about 17% of the total load, with the remaining TN load being mostly runoff from urban (17%), agricultural (5%), and low impact (e.g., forest) areas (6%).

Three approaches to estimate inorganic nitrogen loading under varying climatic conditions from a headwater catchment in Finland

Inorganic nitrogen loading was simulated using two dynamic catchment scale models, Integrated Nutrients in Catchments–Nitrogen (INCA-N) and the Generalized Watershed Loading Functions (GWLF). The simulated N loading was compared to a standard method to calculate annual loading using measured discharge and discharge-weighted concentrations. The main aim of the study was to compare these three estimation approaches with regards to their performance in hydrologically variable years in a small headwater catchment in southern Finland. Inter-annual variability of INCA-N and GWLF was compared with measured inorganic N concentrations at the catchment outlet. In years where snow melt dominates the annual discharge pattern all methods gave concurrent annual loading estimates. However, the loading estimates differ between the studied methods in years where large rainfall events in late summer or autumn dominate the annual discharge pattern, or when the model was not able to reproduce the spring discharge maximum properly. The results suggest that both models can be useful tools in estimating dissolved inorganic nitrogen loading from a catchment under changing climate conditions, providing that the key influencing driver, hydrology, is well captured.

Importance of diffuse pollution control in the Patzcuaro Lake Basin in Mexico

In the catchment area of the Lake Patzcuaro in Central Mexico (933 km2) the apportionments of erosion, sediment, nutrients and pathogen coming from thirteen micro basins were estimated with the purpose of identifying critical areas in which best management practices need to be implemented in order to reduce their contribution to the lake pollution and eutrophication. The ArcView Generalized Watershed Loading Functions model (AV-GWLF) was applied to estimate the loads and sources of nutrients. The main results show that the total annual contribution of nitrogen from point sources were 491 tons and from diffuse pollution 2,065 tons, whereas phosphorus loads where 116 and 236 tons, respectively during a thirty year simulation period. Micro basins with predominant agricultural and animal farm land use (56% of the total area) accounts for a high percentage of nitrogen load 33% and phosphorus 52%. On the other hand, Patzcuaro and Quiroga micro basins which comprise approximately 10% of the total catchment area and are the most populated and visited towns by tourist 686,000 people every year, both contributes with 10.1% of the total nitrogen load and 3.2% of phosphorus. In terms of point sources of nitrogen and phosphorus the last towns contribute with 23.5% and 26.6% respectively. Under this situation the adoption of best management practices are an imperative task since the sedimentation and pollution in the lake has increased dramatically in the last twenty years.

Simulation of Soil Loss and Available Water Content to Assess the Sustainability of Selected Farm Practices

ABSTRACT One consequence of unsustainable farming practices is considered to be the deterioration of soil quality; however, finding a measure of soil quality in relation to sustainability has proven elusive. Defining soil quality in terms of productivity is problematic because of the numerous factors affecting productivity, including use of managed inputs that can mask soil deterioration. This paper explores, as one measure of soil quality, the consistency of the soil water content available for crop growth. By comparing soil water content under different cropping practices with that for a known sustainable system, a mature forested soil, we show how variation in water content can be used as an indicator of sustainability. The model CENTURY was used to simulate changes in soil organic matter content over time, and the Generalized Watershed Loading Function was used to simulate soil erosion and fluctuations in water content. Effects of soil type, initial soil organic matter content, slope, cropping system, tillage, and manure application were considered. Generally, only small differences were predicted in the mean available water content among these systems, but the coefficient of variation in available water content was higher in the cropped systems than in the mature system. Systems with higher rates of soil erosion also exhibited greater variability in available water content. This finding suggests that variability in available water, as compared with that of a mature system, is an indicator of the sustainability of farm practices.