Watershed Model Research Articles

Water availability and response of Tarbela Reservoir under the changing climate in the Upper Indus Basin, Pakistan

Agriculture is one of the major contributors to the Gross Domestic Product (GDP) of Pakistan which relies on the availability of water. Hydropower contributes approximately 35% to the national electricity gid of Pakistan. Indus River is the main river of the Indus River System (IRS) which provides water for agriculture, hydropower and other purposes. The outputs of the Conformal-Cubic Atmospheric Model (CCAM) are used to force the University of British Columbia Watershed Model (UBCWM) in the Upper Indus Basin (UIB), to investigate future water availability under the two IPCC emission scenarios (RCP4.5 and RCP8.5). Tarbela Reservoir which is the outlet of UIB is used as a measurement tool to assess water availability and response of the reservoir to climate change. The results show that maximum and minimum temperature are increasing in the future in comparison to the reference period. The largest increases in maximum temperature are projected for MAM (March–April–May) and JJA (June–July–August), with increases up to 2 °C in MAM and increases up to 6.4 °C in JJA under the RCP4.5 and RCP8.5, respectively, in the future. Minimum temperature has maximum increase (6.7 °C) in DJF (December–January–February) during 2071–2100 under RCP8.5. Precipitation shows a 5.1% decrease in DJF during 2011–2040 under RCP4.5. The statistics about water availability suggest that there is consistent increase in most of the months in the future, however, under the RCP4.5, there is decline in the river flow during 2071–2100 as compared to the 2041–2070. The findings of this study show that most of the time there will be more water available but in some months, there may be water scarcity under the RCP4.5, however, proper management and optimal utilization can reduce the water scarcity.

E-DBCM: A dynamically coupled upland and in-stream water quality model for watershed water quality simulation

A dynamic bidirectional coupled modeling framework for water environment simulation (E-DBCM), including an upland watershed model (UWSM) and a two-dimensional (2D) downstream waterbody model (DWBM), is proposed. The UWSM is implemented to describe the rainfall-runoff and determine the pollutant load to downstream waterbodies, whereas the DWBM is used to simulate the pollutant transport and flood processes on downstream waterbodies. The UWSM and DWBM are spatially connected through a moving boundary, which can ensure the mass and momentum conservation. The proposed E-DBCM is verified using three case studies and the results indicate that the E-DBCM has satisfactory numerical accuracy, which can effectively reproduce the pollutant transport process and achieve satisfactory results. The water environment in Yanqi River Basin is assessed based on the proposed model. The simulated results are consistent with the measured data, indicating that the E-DBCM is reliable and the prediction accuracy can meet the requirements of engineering practices. Water is seriously polluted in this watershed, especially during peak tourist season when many pollutants are produced. Various measures should be taken to protect the water environment in this basin.

Downscaling approaches of climate change projections for watershed modeling: Review of theoretical and practical considerations

Water resources managers must increasingly consider climate change implications of, whether the concern is floods, droughts, reservoir management, or reliably supplying consumers. Hydrologic and water quality modeling of future climate scenarios requires understanding global climate models (GCMs), emission scenarios and downscaling GCM output, since GCMs generate climate predictions at a resolution too coarse for watershed modeling. Here we present theoretical considerations needed to understand the various downscaling methods. Since most watershed modelers will not be performing independent downscaling, given the resource and time requirements needed, we also present a practical workflow for selecting downscaled datasets. Even given the availability of a number of downscaled datasets, a number of decisions are needed regarding downscaling approach (statistical vs. dynamic), GCMs to consider, options, climate statistics to consider for the selection of model(s) that best predict the historical period, and the relative importance of different climate statistics. Available dynamically-downscaled datasets are more limited in GCMs and time periods considered, but the watershed modeler should consider the approach that best matches the historical observations. We critically assess the existing downscaling approaches and then provide practical considerations (which scenarios and GCMs have been downscaled? What are some of the limitations of these databases? What are the steps to selecting a downscaling approach?) Many of these practical questions have not been addressed in previous reviews. While there is no “best approach” that will work for every watershed, having a systematic approach for selecting the multiple options can serve to make an informed and supportable decision.

Assessment of groundwater discharge pathways in a till-dominated coastal aquifer

Study regionMabou Harbour located in Cape Breton Island, Nova Scotia, Canada, is representative of the many natural harbours throughout the Maritime region of Canada as the surrounding landscape is overlain by glacial deposits, predominantly composed of glacial till. Study focusUnderstanding the pathways facilitating groundwater flow and associated solute transport to the ocean is key for developing conceptual hydrogeologic models for coastal watersheds and for informing coastal zone management. Most local-scale field and modelling submarine groundwater discharge (SGD) studies have been conducted in high-permeability formations given the likely importance of SGD in these coastal settings. This study investigates direct (SGD) and indirect (seaward stream baseflow) groundwater discharge from a till dominated coastal aquifer using a combination of field measurements (river flow and baseflow separation, seepage meter measurements, and water temperature analysis) and a calibrated 3D numerical groundwater flow model. New hydrological insights for the regionResults show that seaward stream baseflow greatly exceeds direct SGD to the ocean on the scale of the full harbour watershed (96.1% vs. 3.9 of total groundwater discharge). Particle tracking shows that the vast majority of SGD originates in the subcatchment immediately surrounding the harbour with limited intermediate or regional flow. The shorter flow paths and residence times for direct SGD results in less opportunity for natural attenuation processes and may have implications for groundwater-borne contamination to this harbour.

Hydrological Forecasting under Climate Variability Using Modeling and Earth Observations in the Naryn River Basin, Kyrgyzstan

The availability of water resources in Central Asia depends greatly on snow accumulation in the mountains of Tien-Shan and Pamir. It is important to precisely forecast water availability as it is shared by several countries and has a transboundary context. The impact of climate change in this region requires improving the quality of hydrological forecasts in the Naryn river basin. This is especially true for the growing season due to the unpredictable climate behavior. A real-time monitoring and forecasting system based on hydrological watershed models is widely used for forecast monitoring. The study’s main objective is to simulate hydrological forecasts for three different hydrological stations (Uch-Terek, Naryn, and Big-Naryn) located in the Naryn river basin, the main water formation area of the Syrdarya River. We used the MODSNOW model to generate statistical forecast models. The model simulates the hydrological cycle using standard meteorological data, discharge data, and remote sensing data based on the MODIS snow cover area. As for the forecast at the monthly scale, the model considers the snow cover conditions at separate elevation zones. The operation of a watershed model includes the effects of climate change on river dynamics, especially snowfall and its melting processes in different altitude zones of the Naryn river basin. The linear regression models were produced for monthly and yearly hydrological forecasts. The linear regression shows R2 values of 0.81, 0.75, and 0.77 (Uch-Terek, Naryn, and Big-Naryn, respectively). The correlation between discharge and snow cover at various elevation zones was used to examine the relationship between snow cover and the elevation of the study. The best correlation was in May, June, and July for the elevation ranging from 1000–1500 m in station Uch-Terek, and 1500–3500 m in stations Naryn and Big-Naryn. The best correlation was in June: 0.87; 0.76; 0.84, and May for the elevation ranging from 1000–3500 m in station Uch-Terek, and 2000–3000 m in stations Naryn and Big-Naryn. Hydrological forecast modeling in this study aims to provide helpful information to improve our under-standing that the snow cover is the central aspect of water accumulation.

Upscaling Hydrological Processes for Land Surface Models With a Two‐Hydrologic‐Variable Model: Application to the Little Washita Watershed

AbstractLand Surface Models (LSMs) are key components of Earth System Models, which the Intergovernmental Panel on Climate Change relies on in many of their studies. However, these models either neglect or oversimplify basin‐scale hydrological processes that produce the land surface water balance. Bringing the dominant physical processes from the local scale up to the LSMs presents a significant challenge for improving the models. This article presents an upscaling (or bottom‐up) approach to identify the basin‐scale driving variables that need to be exported into LSMs. The approach is developed on the Little Washita Watershed (OK, USA) using 20‐year hydrology (1993–2013). A physically based 3D model built with HydroGeoSphere first produces a reference simulation. An equivalent hillslope model is then able to capture both 3D simulated water balance and local water table dynamics with reasonable accuracy. The physical analysis of the water balance in the different hillslope compartments leads to the identification of two driving variables: seepage face extension and water table slope. The two variables are then implemented in a conceptual model. Results show a good capacity of this model to capture the water balance of hillslopes having different lengths and slopes. Moreover, the model is able to capture the water balance of the reference simulation with reasonable accuracy. The proposed approach thus reduces the 3D watershed model to a two‐variable conceptual model that constitutes a basis for developing an improved LSM hydrology.

Spatiotemporal variations in runoff and runoff components in response to climate change in a glacierized subbasin of the Upper Indus Basin, Pakistan

Change in seasonal snowfall and glaciers ablation control year-to-year variations in streamflows of the Upper Indus Basin (UIB) and hence ultimately impacts the water availability in downstream areas of UIB. This situation calls for an urgent response to study the long-term variations in runoff components in response to climate change. The current study investigates the spatiotemporal variations in runoff and runoff components in response to climate change to the streamflows of the Gilgit River from 1981 to 2020 by using the University of British Columbia Watershed Model (UBC WM). Three statistical indices such as the Nash–Sutcliffe efficiency (NSE), the coefficient of determination (R2), and the correlation coefficient (CC) were used to evaluate the performance of UBC WM in simulating the streamflows against observed streamflows. According to statistical indices, the UBC WM performed fairly well during both calibration (1981–2000: R2 = 0.90, NSE = 0.87, and CC = 0.95) and validation periods (2001–2015: R2 = 0.86, NSE = 0.83, and CC = 0.92). Trend analysis revealed a significant increase in all runoff components with large interannual variations in their relative contributions to streamflows from 1981 to 2020. From 1981 to 2020, the average relative contribution of snowmelt, glacier melt, rainfall-runoff, and baseflow was estimated to be 25%, 46%, 5%, and 24%, respectively to the streamflows of the Gilgit River. Seasonal analysis showed that about 86% of total runoff was contributed to the Gilgit River during the summer season (April–September) while only 14% in the winter season (October–March). Further analysis of runoff at a spatial scale revealed that approximately 76% of the total runoff of Gilgit River is generated between elevations from 3680 to 5348 m while 19% of total runoff is generated at an elevation <3680 m and only 5% at an elevation >5348 m. Moreover, it was observed that groundwater contribution from soil lower zone (i.e., 76%) to streamflows was found greater than soil upper zone (i.e., 24%). The outcomes of this study will help the water resource managers and hydrologists to manage the water resources in downstream areas of the UIB for local consumption, industrial use, and agriculture.

Flood hazard and susceptibility assessment in a semi-arid environment: A case study of Seyad basin, south of Morocco

Flood, is a constant phenomenon, especially in semi-arid and flood plain regions are one of the most destructive natural hazards endangering a people's life, property, and physical and economic well-being. This paper focuses on hydraulic modelling using the HEC-RAS model in combination with the Watershed Modeling System (WMS) tools and compares the results to the Flood Hazard Index (FHI) method using GIS in the Seyad Basin situated in the southwestern region of Morocco with an area of 1512.85 km2. The goal of the study is to evaluate flood hazards in the Seyad Basin in which the cities of Taghjijt, Aday, Amtoudi, and Tagriante are located. The HEC-RAS approach combines the hydraulic surface water flow model and the digital terrain model data. This combination allows the mapping of the flood zones by using the WMS tools. This approach predicts flood occurrence probability for different times and determines the intensity of the flood (depth and velocity of floodwater) using the existing hydrological data.On the other hand, the Flood Hazard Index method (FHI) presents a multi-criteria index to assess areas prone to flood hazards, using six physical parameters: permeability, slope, distance from rivers, land use, drainage density, and flow accumulation. A weight is calculated from the analytic hierarchy process method and applied to each parameter. HEC-RAS method allows the mapping of a flood with a flood water surface profile that shows the flood depth for Annual Exceedance Probability (AEP). At the same time, FHI permits establishing flood hazard levels without showing the water depth. In both approaches, six simulations were performed with the return periods of 10, 20, 50, 100, 200, and 500 years. The simulation revealed that the most susceptible areas to flooding are the areas along the Wadi Seyad.

The Response Mechanisms of Topographic Changes in Small Loess Watershed under Rainstorm

This paper uses a small watershed entity model to simulate the rainfall experiment and combines traditional water and sediment observation and terrain three-dimensional laser scanning technology to reveal the characteristics of erosion and sediment yield in small watersheds and the law of channel sediment transport and quantitatively describe the response mechanism of terrain changes in small watersheds to the layout of silt dams and rainfall intensity. Rainfall intensity with three types (30, 60, and 120 mm/h) under three soil conservation measure types (none dam, single dam, and double dams) was simulated, and a small watershed entity model was adopted. The changes in topography were recorded by a Focus 3D laser scanner (Faro) for each experiment. The main results were as follows: (1) Soil erosion under the effect of rainfall occurs on the slope of the watershed and in the gully, while deposition usually occurs on the gentle slope of the gully or in low-lying areas. (2) When the runoff volume is small, deposition occurs easily in the gully, and vice versa. (3) The increase in the number of silt dams deployed has a small effect on the rate of runoff yield on the small watershed, but the limitation on the rate of sand production is especially obvious. Silt dam measures have a good flood and sand reduction effect on small- and medium-intensity rain, but for high-intensity rain, their runoff and sand reduction effect will be reduced, so rainfall is the dominant factor in the formation of soil erosion. Our results provide the scientific basis for identifying key parts of soil erosion and for the rational arrangement of soil and water conservation measures in loess areas.

Improvement of Integrated Watershed Management in Indonesia for Mitigation and Adaptation to Climate Change: A Review

Climate change is a major challenge for Indonesia due to its impact on food, water, energy sustainability, and environmental health. Almost all Indonesian regions are exposed to floods, landslides, soil erosion, drought, and heavy rains. In response to these challenges, the Government of Indonesia has determined integrated watershed management (IWM) to be one of the key programs to reduce greenhouse gas (GHG) emissions, as stated in the updated Indonesian nationally determined contribution (NDC). This paper intends to review Indonesia′s efforts in mitigating and adapting to climate change through an IWM approach, and its attempts to realize a decent life and environment for all communities. Improvement of the IWM can be conducted by strengthening the synergy between the responsible institutions for watershed management and the responsible institutions for handling mitigation and adaptation of climate change impacts. In addition, it is important to prioritize coordination, participation, and collaboration not only at the national government level but also at the international level, since numerous problems may exist in the transboundary between countries, and finding solutions should involve planning, implementation, monitoring, and evaluation. Implementing the micro watershed model (MWM), supported by culture, local wisdom, and traditional knowledge in communities, can be used to improve the current IWM.

Examining model performances and parameter uncertainty for streamflow and suspended sediment regime simulation: Comparison of three calibration methods

Spatially distributed watershed models are commonly utilized to address a wide range of water-related issues. However, setting up a reliable watershed model is a difficult task involving several essential decisions making. Choice of calibration method is one of the most important decisions that has been sparsely investigated in semi-distributed watershed models. In this study, therefore, we used the Soil and Water Assessment Tool (SWAT) model to investigate the impact of three calibration methods: sequential (SQN), simultaneous (SML) and sequential-simultaneous (SQN_SML) on model performance and parameter uncertainty in the Kantamal catchment of the Mahanadi basin, India. The findings across the calibration methods; evaluated fit scores of streamflow for respective calibration and validation period; showed that SQN_SML calibration has the least amount of bias (PBAIS = 1.7, −4.2), the highest NSE (0.91, 0.92), KGE (0.95, 0.94) and R2 (0.91, 0.92). Furthermore, SQN_SML outperformed the other two methods in all three streamflow regimes (low, medium and high) of flow duration curve analysis. Suspended sediment load (SSL) analyses of partitioned sediment duration curve showed the best performance of SQN_SML for mid and low SSL regimes while all three calibration methods performed similarly in the high SSL regime. SML calibration approach showed the least parameter uncertainty followed by SQN_SML and SQN. The P-factor for sediment simulation was better for the SQN_SML approach, indicating the minimal model error for sediment simulation. The SQN_SML produced the least equifinal solution, while the SQN approach produced the highest equifinal solution. Overall, the findings of this study may help the watershed modelling communities for selecting suitable calibration strategies when dealing with integrated water resources management.

The impact of overgrazing on water fluxes in a semi-arid watershed – The suitability of watershed scale modeling in a data scarce area

Study regionSouth Africa. Study focusThis study focuses on the impact of overgrazing on the water resources in a semi-arid catchment in South Africa. We have applied the SWAT+ model to a data-scarce area. During the modeling process, hydropedological principles were applied in model parameterization and analysis of simulation results to identify areas which are sensitive to overgrazing. The scenario analysis indicates that overgrazing might not necessarily have a large impact on the water balance of the whole watershed, but may have negative impacts at the local subbasin scale. These impacts are increased surface runoff and evaporation which are the consequence of compaction of topsoils. New hydrological insights for the regionThe application of hydropedological principles in the analysis of simulation results help to identify areas sensitive to overgrazing. Shallow soils, soils with low surface infiltration capacities and hydromorphic soils are vulnerable to adverse effects from overgrazing.

Comparison of Two Hydrological Models, HEC-HMS and SWAT in Runoff Estimation: Application to Huai Bang Sai Tropical Watershed, Thailand

In the present study, the streamflow simulation capacities between the Soil and Water Assessment Tool (SWAT) and the Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) were compared for the Huai Bang Sai (HBS) watershed in northeastern Thailand. During calibration (2007–2010) and validation (2011–2014), the SWAT model demonstrated a Coefficient of Determination (R2) and a Nash Sutcliffe Efficiency (NSE) of 0.83 and 0.82, and 0.78 and 0.77, respectively. During the same periods, the HEC-HMS model demonstrated values of 0.80 and 0.79, and 0.84 and 0.82. The exceedance probabilities at 10%, 40%, and 90% were 144.5, 14.5, and 0.9 mm in the flow duration curves (FDCs) obtained for observed flow. From the HEC-HMS and SWAT models, these indices yielded 109.0, 15.0, and 0.02 mm, and 123.5, 16.95, and 0.02 mm. These results inferred those high flows were captured well by the SWAT model, while medium flows were captured well by the HEC-HMS model. It is noteworthy that the low flows were accurately simulated by both models. Furthermore, dry and wet seasonal flows were simulated reasonably well by the SWAT model with slight under-predictions of 2.12% and 13.52% compared to the observed values. The HEC-HMS model under-predicted the dry and wet seasonal flows by 10.76% and 18.54% compared to observed flows. The results of the present study will provide valuable recommendations for the stakeholders of the HBS watershed to improve water usage policies. In addition, the present study will be helpful to select the most appropriate hydrologic model for humid tropical watersheds in Thailand and elsewhere in the world.

Implementing landscape connectivity with topographic filtering model: A simulation of suspended sediment delivery in an agricultural watershed.

The widespread availability of high-fidelity topography combined with advances in geospatial analysis offer the opportunity to reimagine approaches to the difficult problem of predicting sediment delivery from watersheds. Here we present a model that uses high-resolution topography to filter sediment sources to quantify sediment delivery to the watershed outlet. It is a reduced-complexity, top-down model that defines transfer functions-topographic filters-between spatially distributed sediment sources and spatially integrated sediment delivery. The goal of the model is to forecast changes in watershed suspended sediment delivery in response to spatially distributed changes in sediment source magnitude or delivery, whether a result of watershed drivers or intentional management actions. Such an application requires the context of a watershed model that accounts for all sediment sources, enforces sediment mass balance throughout the spatial domain, and accommodates sediment storage and delivery over time. The model is developed for a HUC-8 watershed with a flat upland dominated by corn-soybean agriculture and deeply incised valleys near the watershed outlet with large sediment contributions from near-channel sources. Topofilter computes delivery and storage of field-derived sediment according to its spatial and structural connectivity to the stream channel network; subsequently, delivery of both field- and near-channel-derived sediment along with floodplain storage are computed in the stream channel network to the watershed outlet. The model outputs provide a spatially rich representation of sediment delivery and storage on field and along the stream that is consistent with available independent information on sediment accumulations and fluxes. Rather than a single best-calibrated solution, Topofilter uses the Generalized Likelihood Uncertainty Estimate (GLUE) approach to develop many possible solutions with sediment delivery rates expressed as probability distributions across the watershed. The ensemble of simulation outputs provides a useful basis for estimating uncertainty in sediment delivery and the effectiveness of different landscape management allocation across a watershed.

Watershed‐Geochemical Model to Simulate Dissolved and Particulate 137Cs Discharge From a Forested Catchment

AbstractMeasurements in forest catchments in Fukushima Prefecture indicate that cesium‐137 (137Cs) leaching from forest litter affects dissolved 137Cs levels in river water. Existing simulations have been unsuccessful in reproducing this phenomenon due to mechanistic limitations in the models connecting the forest ecosystem and the river system. This paper introduces a new coupled watershed‐geochemical model to address this. We connected a forest ecosystem compartment model to the 3D watershed model General‐purpose Terrestrial Fluid‐Flow Simulator to describe 137Cs transfer between forests and the river system. The compartment model included nonleachable and leachable 137Cs sites in the organic layer. The latter sites model how leachable 137Cs stocks increase with ambient temperature owing to decomposing organic matter. The model was tested by simulating dissolved and particulate 137Cs discharges from a forested catchment upstream of the Ohta River, Fukushima Prefecture. The results for dissolved 137Cs concentrations in river water for 1 January 2014, to 31 December 2015, correlated well with catchment measurements. The simulations reproduced dissolved 137Cs concentration peaks that occurred during three typhoon events and the seasonal variations under baseflow conditions. The results support the theory that leaching from the organic layer in forests is a primary factor affecting river water dissolved 137Cs concentrations. The wider contribution of this study is it introduces a coupled watershed‐geochemical model, applicable not only for evaluating the dynamics of 137Cs, but also those of other substances that adsorb onto sediments and soil particles, and leach from forest organic matter.

A process-based mesh-distributed watershed model for water runoff and soil erosion simulation

ABSTRACT Runoff and sediment movement induced by storms have serious economic, environmental, and social impacts around the world. Hydrological models have become useful decision support tools for watershed management. The development of process-based distributed models is complex as it involves a range of disciplines and spans multiple spatial and temporal scales. In this study, a process-based and mesh-distributed model, suitable for both event-based and continuous simulations, is developed. The watershed is conceptualized in three distinct zones: a surface overland region, an unsaturated subsurface zone, and a groundwater zone. Overland flow is governed by the 2D diffusive wave equation with an optional 1D channel network solver; water in the unsaturated zone is modelled through mass conservation and vertical dominance of the processes; and saturated groundwater flow is governed by the 2D Dupuit equation. Soil erosion and transport are governed by the multi-size, non-equilibrium equations incorporating soil entrainment, transport, and deposition. The model is driven by meteorological input, land use and soil type properties. The numerical approach is a generalization of existing models to overcome some current modelling shortcomings. New contributions of the model are highlighted and model verification and validation are reported against benchmark cases. The model is then applied to the Cache Creek watershed in California, USA. Satisfactory agreement with the measured data is obtained with both the runoff flow rate and sediment load.

A numerical modeling framework for simulating the key in-stream fate processes of PAH decay in Muskeg River Watershed, Alberta, Canada

Most previous water quality studies oversimplified in-stream processes for modeling the fate and transport of critical organic contaminants, such as Polycyclic Aromatic Hydrocarbons (PAHs). Taking four selected PAHs as representative organic contaminants, we developed a numerical modeling framework using a Water Quality Analysis Simulation Program 8 (WASP8) and a well-established watershed model, i.e., Soil and Water Assessment Tool (SWAT) to: (1) address the influence of in-stream processes, including direct photolysis, volatilization, partitioning of PAHs to suspended solids, and DOC complexation processes on PAH concentrations; and (2) establish relationships between spatiotemporal distribution of environmental factors (e.g., ice coverage, water temperature, wind, and light attenuation), in-stream processes, and PAH concentrations at a watershed scale. Using calibrated SWAT and WASP8 models, we evaluated the impacts of seasonal changes in environmental factors on in-stream processes in the Muskeg River watershed, which is part of the Athabasca Oil Sands Region (AOSR), the third-largest crude oil reserves of the world in western Canada. Among four selected PAHs, simulation results suggest that Naphthalene primarily decay in the water through volatilization or direct photolysis. For Phenanthrene, Pyrene, and Chrysene, DOC complexation, volatilization, and direct photolysis all contribute to their decay in the water, with a strong dependence on seasonality. Model simulations indicated that direct photolysis and volatilization rates are meager in cold seasons, mainly due to low river temperature and ice coverage. However, these processes gradually resume when entering the warm season. In summary, the model simulation results suggest that critical in-stream processes such as direct photolysis, volatilization, and partitioning and their relationship with environmental factors should be considered when simulating the fate and transport of organic contaminants in the river systems. Our results also reveal that the relationship between environmental factors and fate processes affecting PAH concentrations can vary across a watershed and in different seasons.

Coupling mountain and lowland watershed models to characterize nutrient loading: An eight-year investigation in Lake Chaohu Basin

Quantifying nitrogen (N) and phosphorus (P) sources and their contributions at a watershed scale is a primary step to design best management practices. This task is challenging for a mixed watershed with both mountain and lowland areas, because most existing watershed models were developed for mountain areas with clear water flow directions. To solve this challenge, we coupled the Soil and Water Assessment Tool (SWAT) for mountain watersheds, Phosphorus Dynamic model for lowland Polder systems (PDP) and Nitrogen Dynamic model for lowland Polder systems (NDP) for lowland artificial watersheds (i.e., polders) to characterize N and P loading for a typical mountain-lowland mixed watershed, Lake Chaohu Basin in China. The coupled model was calibrated based on eight-year (2011–2018) meteorological, hydrological and water quality datasets, and had a satisfactory performance for both hydrological and water quality variables (R2 > 0.6, NS > 0.6). Our eight-year investigation in the five watersheds of Lake Chaohu Basin revealed that 1) the estimated N and P loading had an annual averaged amount of 10,832 and 781 t/yr, and an intensity of 15.72 and 1.33 kg/ha/yr, respectively. Three watersheds (Nanfei, Pai and Baishitian River Watersheds) had relatively high loading intensity for N (35.16, 18.34 and 13.34 kg/ha/yr) and P (2.49, 1.25 and 2.72 kg/ha/yr), and contributed 67.6% (N) and 68.5% (P) to the loading of the five investigated watersheds. 2) Polders in Lake Chaohu Basin can be N and P sources or sinks for their surrounding rivers, depending on the polder-river interaction. Within Nanfei, Pai and Baishitian River Watersheds, N and P retention ability was higher in polders than that in mountain watersheds. 3) Population density, precipitation intensity and soil erosion were critical drivers of N and P loss and showed positive correlation with N and P loss intensity. This study demonstrated the high value model coupling to characterize N and P loading in a mountain-lowland mixed watershed system, and thus to improve our understanding on its N and P cycling.

Advances on water quality modeling in burned areas: A review

Wildfires are a recurring hazard in forested catchments representing a major threat to water security worldwide. Wildfires impacts on water quality have been thoroughly addressed by the scientific community through field studies, laboratory experiments, and, to a lesser extent, the use of hydrological models. Nonetheless, models are important tools to assess on-site and off-site wildfires impacts and provide the basis for post-fire land management decisions. This study aims to describe the current state of the art of post-fire model adaptation, understanding how wildfires impacts are simulated and the options taken by the modelers in selecting parameters. For this purpose, 42 publications on modeling wildfire impacts on the hydrologic cycle and water quality were retrieved from the SCOPUS database. Most studies simulated post-fire hydrological and erosion response in the first year after the fire, while few assessed nutrients changes and long-term impacts. In addition, most simulations ended at the watershed outlet without considering the fate of pollutants in downstream waterbodies. Ash transport was identified as a major research gap, given the difficulties of its incorporation in the current models’ structure and the high complexity in predicting the heterogeneous ash layer. Including such layer would improve models’ ability to simulate water quality in post-fire conditions, being ash a source of nutrients and contaminants. Model complexity and data limitations influenced the spatial and temporal scale chosen for simulations. Post-fire model adaptations to simulate on-site soil erosion are well established, mainly using empirical equations extensively calibrated in the literature. At the watershed level, however, physical and process-based models are preferred for their ability to simulate more complex burned area characteristics. Future research should focus on the simulation of the ash transport and the development of integrated modelling frameworks, combining watershed and aquatic ecosystem models to link the on and off-site impacts of fires.

Effects of return flows on stream water quality and availability in the Upper Colorado, Delaware, and Illinois River Basins

Understanding effects of human water use and subsequent return flows on the availability and suitability of water for downstream uses is critical to efficient and effective watershed management. We compared spatially detailed estimates of stream chemistry within three watersheds in diverse settings to available standards to isolate effects of wastewater and irrigation return flows on the suitability of downstream waters for maintaining healthy aquatic ecosystems and for selected human uses. Mean-annual flow-weighted total and source-specific concentrations of nitrogen and phosphorus in individual stream reaches within the Upper Colorado, Delaware, and Illinois River Basins and of total dissolved solids within stream reaches of the Upper Colorado River Basin were estimated from previously calibrated regional watershed models. Estimated concentrations of both nitrogen and phosphorus in most stream reaches in all three watersheds (at least 78%, by length) exceed recommended standards for the protection of aquatic ecosystems, although concentrations in relatively few streams exceed such standards due to contributions from wastewater return flows, alone. Consequently, efforts to reduce wastewater nutrient effluent may provide important local downstream benefits but would likely have minimal impact on regional ecological conditions. Similarly, estimated mean-annual flow-weighted total dissolved solids concentrations in the Upper Colorado River Basin exceed standards for agricultural water use and (or) the secondary maximum contaminant level (SMCL) for drinking water in 52% of streams (by length), but rarely due to effects of irrigation return flows, alone. Dissolved solids in most tributaries of the Upper Colorado River are attributable primarily to natural sources.