Application of the Soil and Water Assessment Tool for six watersheds of Lake Erie: Model parameterization and calibration

The Soil and Water Assessment Tool (SWAT), a physically-based watershed-scale model, holds promise as a means to predict tributary sediment and nutrient loads to the Laurentian Great Lakes. In the present study, model performance is compared across six watersheds draining into Lake Erie to determine the applicability of SWAT to watersheds of differing characteristics. After initial model parameterization, the Huron, Raisin, Maumee, Sandusky, Cuyahoga, and Grand SWAT models were calibrated (1998-2001) and confirmed, or validated (2002-2005), individually for stream water discharge, sediment loads, and nutrient loads (total P, soluble reactive P, total N, and nitrate) based on available datasets. SWAT effectively predicted hydrology and sediments across a range of watershed characteristics. SWAT estimation of nutrient loads was weaker although still satisfactory at least two-thirds of the time across all nutrient parameters and watersheds. SWAT model performance was most satisfactory in agricultural and forested watersheds, and was less so in urbanized settings. Model performance was influenced by the availability of observational data with high sampling frequency and long duration for calibration and confirmation evaluation. In some instances, it appeared that parameter adjustments that improved calibration of hydrology negatively affected subsequent sediment and nutrient calibration, suggesting trade-offs in calibrating for hydrologic vs. water quality model performance. Despite these considerations, SWAT accurately predicted average stream discharge, sediment loads, and nutrient loads for the Raisin, Maumee, Sandusky, and Grand watersheds such that future use of these SWAT models for various scenario testing is reasonable and warranted.     

Temperature,redox, and amendments alter wetland soil inorganic phosphorus retention dynamics in a Laurentian Great Lakes priority watershed

Surface water phosphorus loading must be reduced to improve water quality and decrease harmful algal blooms. Many wetlands have a natural capacity to retain inorganic reactive PO₄^3? via soil sorption. However, soil PO₄^3? retention capacity is finite and may be limited by soil legacy phosphorus effects in agricultural and urban areas. This study evaluated soil PO₄^3? retention in soils from a wetland constructed on former agricultural land in the Lake Erie, Maumee River watershed targeted for nutrient load reduction. Soil PO₄^3? sorption isotherms were evaluated under cool (10 °C), warm (22°), aerobic, and anaerobic treatments to determine changes in PO₄^3? retention due to environmental conditions and estimate seasonal changes in PO₄^3? sorption. The soils displayed a strong capacity for PO₄^3? retention by sorption. However, results indicate that cooler temperatures and anaerobic conditions decreased PO₄^3? sorption and lowered retention rates at PO₄^3? concentrations observed in the region. Soil amendment experiments investigated opportunities to increase PO₄^3? retention because many soils display elevated phosphorus concentrations due to historic land use, limiting their ability to adsorb additional PO₄^3?. Amendments increased PO₄^3? retention capacity compared to unamended soils in the presence of high PO₄^3? concentrations, suggesting soil PO₄^3? retention can be improved in areas where natural storage capacity has been exhausted. Results from this study can inform natural resources managers in the Laurentian Great Lakes and elsewhere when identifying potential nutrient reduction wetland locations and assist with developing operational guidelines to optimize PO₄^3? retention and water quality improvements using wetlands.     

Mitigating lake eutrophication through stakeholder-driven hydrologic modeling of agricultural conservation practices: A case study of Lake Macatawa,Michigan

Lake Macatawa is a hypereutrophic water body that connects with Lake Michigan via a navigation channel. Excess phosphorus (P) concentrations have resulted in a Total Maximum Daily Load (TMDL) for total phosphorus (TP) in the lake, which has not been met. To guide land management and water pollution control in the Macatawa watershed, a Soil and Water Assessment Tool (SWAT) model and scenarios of agricultural best management practices (BMPs) were developed in consultation with stakeholders. Modelling emphasized incorporating practices representative of local agricultural conditions. Approaches to initializing high legacy soil P levels in SWAT were tested. The validated model was used to evaluate the influence of BMPs on lake water quality and identify which practices are necessary for meeting the TMDL. The model showed that eliminating manure applications would have small effect on curbing TP loading, but continuous no-till and high residue combined with already used subsurface manure application would yield notable TP reductions. Achieving TMDL-mandated TP reduction of 72% is possible through a widespread adoption of multiple BMPs (continuous no-till with high residue, cover crops, filter strips, and conversion of some marginal croplands to perennial grasses) across all the watershed’s row croplands. The study highlights how guidance from a local community interested in watershed improvement was integrated with modeling towards addressing eutrophication with informed watershed management. The Lake Macatawa case study presents a tractable system from which management solutions could be transferred to similar small agricultural tile-drained watersheds with high legacy soil P levels in the Great Lakes basin.     

Watershed and lake influences on the energetic base of coastal wetland food webs across the Great Lakes Basin

We examined factors that influence the energy base of Great Lakes coastal wetland food webs across a basin-wide gradient of landscape disturbance. Wetland nutrient concentrations were positively correlated with a principal components-based metric of agricultural practices. Hydraulic residence time influenced the energy base of wetland food webs, with high residence-time systems based mostly on plankton and low residence-time systems based mostly upon benthos. In systems with plankton, the importance of planktonic carbon to the resident fish community generally increased with residence time. A stronger relationship was apparent with an index of nutrient loading that combined residence time and nutrient concentration as the predictor (R² = 0.289, p = 0.026). Shifts toward plankton-based food webs occurred at relatively low levels of loading. In riverine wetlands without plankton, contributions of detrital carbon to fish communities decreased significantly in response to watershed disturbance that reflected nutrient loading. In a third class of wetlands the wetland-resident fish communities were not entirely supported by within-wetland carbon sources and were significantly subsidized by nearshore habitats, which provided 35 (± 22) to 73 (± 9) % of fish community carbon. When lake-run migrant fish were included in the analyses, nearshore subsidies to all 30 wetland food webs ranged from 3 (± 2) to 79 (± 12) %. We obtained similar ranges when examining nearshore contributions to a single wetland species, northern pike. These results illustrate the spatial scale and the degree to which the energetics of coastal wetland food webs are influenced by interactions with their watersheds and Great Lakes.     

Assessing an Enhanced Version of SWAT on Water Quantity and Quality Simulation in Regions with Seasonal Snow Cover

Winter soil-temperature simulations with soil and water assessment tool (SWAT) are generally inaccurate in snow-dominated areas of the word, because the existing empirically-based soil-temperature module in SWAT does not account for snow-insulation effects. This problem, in turn, leads to all subsequent projections including water flow and nutrient loading biased. To address this issue, a physically-based soil-temperature module was developed and incorporated into SWAT as an alternative to the empirical formulation, which was found generate better overall estimates of winter soil temperature in a previous study. In the current study, we continue to examine the modified version of SWAT performance on watershed modeling by making pairwise comparisons between the outputs from the original and modified versions of SWAT against related field measurements in a testing watershed. Results show that the physically-based soil-temperature formulation helps to dramatically improve SWAT estimations of base flow discharge and NO₃-N loading for the watershed as a result of improvement in modeled winter soil temperatures. Compared with the original version of SWAT, the new version of SWAT predicted overall lower surface runoff and soil moisture content, as well as higher percolation and lateral flow in winter, resulting in clear differences in modeled flow paths and fate of chemical pollutants. Adding a physically-based treatment of soil temperature in SWAT is an important generalization that has potential of making the model more relevant to snow-dominated areas of the world.     

Temporal Dynamics of Nutrients (N and P) and Hydrology in a Lake Superior Coastal Wetland

Coastal wetlands on Lake Superior are hydrologically complex ecosystems situated at the interface of upland catchments and the oligotrophic lake. Little is known about nutrient dynamics within coastal wetlands or their role in modifying or contributing to nutrient fluxes from watersheds to Lake Superior. We conducted an intensive study of Lost Creek Wetland (LCW) near Cornucopia, WI, with the objective of determining influences of temporal variability in hydrology on dynamics and retention of N and P. We measured hydrologic inputs and distributions of inorganic and organic forms of nitrogen and phosphorus within LCW under hydrologic conditions ranging from summer base flow to spring snow melt. Our study confirms that the interrelationship between hydrologic connections to lake and tributary and seasonal variations in hydrology can regulate internal nutrient dynamics of coastal wetlands. The strength of hydrologic linkage of LCW to Lake Superior and tributary varied greatly among seasons, resulting in shifts in the relative importance of these nutrient sources and influencing spatial distribution of nutrients within the wetland. Ratios of inorganic nitrogen and phosphorus in the wetland were consistently low (< 16) indicating a potential for nitrogen limitation. Retention of inorganic nitrogen ranged from 11% to 94% and was positively related to hydraulic residence time which ranged from < 1 day during snow melt to 26 days in summer. Retention of total and soluble reactive phosphorus was generally lower than retention of inorganic nitrogen and was not related to hydraulic residence time.     

Pesticide retention in the watershed and in a small constructed wetland treating diffuse pollution.

Loss of pesticides is likely from watersheds where pesticides are used. The herbicides propachlor, linuron and metamitron, and the fungicides propiconazole, fenpropimorph and metribuzin and metalaxyl, were applied on an arable soil plot. A mass balance study showed that approximately 96% of the applied pesticides disappeared within the watershed. Three pesticides remained as residuals in the soil profile one year after the application. The 4% of the pesticides that were lost from the watershed gave peak concentrations, appearing immediately after spraying, reaching levels that can be hazardous to aquatic life. The constructed wetland situated in the first-order stream generally managed to lower the peak concentrations significantly. For the summer season, retention varied from 12 to 67% the first year. The second year, we observed both loss and retention. Increasing the wetland surface from 0.2% to 0.4% of the watershed area increased the average retention with 21% units the first year and 9% units the second year. Chemical properties of the pesticides could explain some of the behaviour in the watershed and in the wetland.     

Optimizing Structural Best Management Practices Using SWAT and Genetic Algorithm to Improve Water Quality Goals

A genetic algorithm (GA), an evolutionary optimization technique, is coupled with a semi-distributed hydrologic model, Soil and Water Assessment Tool (SWAT) to find an optimum combination of structural Best Management Practices (BMPs) that meets the treatment goals at a watershed scale. The structural BMPs considered in the study are detention ponds, parallel terraces, filter strips, grassed waterways, and grade stabilization structures which are all applicable in agricultural watersheds. The decision variables in the optimization model are the type, size, and location of BMPs which minimize the construction cost and simultaneously reduce sediment and nutrients to target levels at the watershed outlet. The model is demonstrated on the Silver Creek, a sub-watershed of the Lower Kaskaskia watershed in Illinois. The model is used to compare three different sediment and nutrient reduction cases (i.e. 20%, 40%, and, 60%) at the watershed outlet.     

A Diagnostic Decision Support System for BMP Selection in Small Urban Watershed

Best Management Practices (BMPs) have become the most effective way to mitigate non-point source pollution (NPS) issues. Much attention has been paid to NPS in rural areas, where agricultural activities increase nutrients, toxics, and sediments in surface water. Stormwater from urban areas is also a major contributor to NPS pollution. For watersheds bearing various soil types and land uses, a single type of BMP cannot be the panacea to all stormwater problems. To solve these problems, a Diagnostic Decision Support System (DDSS) was developed in this research. The DDSS can identify and locate the most critical NPS areas (hotspots) within a watershed in high spatial resolution. The DDSS can provide a series of spatially distributed small-scale BMPs which are effective in treating the NPS and are suitable for the physical environment. The BMPs, varying in types and locations, are recommended at HRU (Hydrologic Response Unit) level. The DDSS was tested in Watts Branch, a small urban watershed of the Anacostia River in metropolitan Washington D.C., USA. The process-based hydrologic model, Soil and Water Assessment Tool (SWAT), was used to simulate watershed responses. The simulation results were then used by the DDSS for BMP recommendation. Hotspots of different NPS were successfully located and prescribed with spatially distributed BMPs. The DDSS serves as a useful tool to better understand urban watersheds and to make proper stormwater management plans.     

Pesticide retention in an experimental wetland treating non-point source pollution from agriculture runoff.

Pesticide losses to the environment are unwanted due to possible environmental and health hazards. An experimental wetland is established to study the efficiency with respect to retention of sediments, nutrients and pesticides. Pesticides were applied on an arable soil plot in the watershed. Statistical analyses were carried out on three selected pesticides: propachlor, metalaxyl and chlorfenvinfoss. All pesticides were found in the experimental wetland, with peak concenttrations shortly after spraying. In 2003 pesticide retention varied from 11% to 42% and in 2004 retention varied from 19% to 56%. Comparing eight different wetland filters, we found that L6 and L8, with flagstones and straw, respectively, had a higher total pesticide retention than a standard Norwegian wetland (L4). When the compounds were treated separately, however, the picture was different. Statistical analyses showed that the treatments were signficantly different from zero in six of the wetlands for remowal of propachlor, for removal of metalaxyl none were significantly different, and for removal of chlorfenvinphos four treatments were significantly different. For the three compounds none of the relative treatments were significantly different from L4. Chemical properties of the pesticides could explain some of the behaviour in the watershed and in the wetland.     

SWAT Setup with Long-Term Detailed Landuse and Management Records and Modification for a Micro-Watershed Influenced by Freeze-Thaw Cycles

In the widely used soil and water assessment tool (SWAT), the standard hydrological response units (HRUs) delineation method has low spatial resolution with respect to model inputs and outputs and renders difficulties in using long-term detailed landuse and management records. In addition, the modified universal soil loss equation (MUSLE) uses a constant K-factor which cannot address seasonal variation in soil erodibility caused by freeze-thaw cycles in cold regions. The current study presents a simple method to incorporate detailed landuse and management inputs in SWAT. The method delineates HRUs based on field boundaries and associates each HRU with a particular field. As a result, long-term detailed records can be incorporated into the SWAT management files. In addition, the existing MUSLE in SWAT was modified by introducing a variable K-factor to address effects of freeze-thaw cycles on soil erosion for cold regions. This modified version of SWAT was calibrated and validated for an agricultural micro-watershed, i.e., Black Brook Watershed in New Brunswick, Canada. The results showed that, compared with the standard HRU-delineation method, field-based HRU-delineation method was able to improve landuse and management practice input accuracy for SWAT and save time and effort for long-term simulation, and provide high resolution outputs in the watershed. As a result, the field-based HRU-delineation method can facilitate decision making not only at the subbasin scale but also at the field scale. In addition, results showed that sediment loading simulation accuracy was improved with the modified-MUSLE compared with the original-MUSLE.     

Assessment of site-specific agricultural Best Management Practices in the Upper East River watershed,Wisconsin, using a field-scale SWAT model

The Great Lakes “Priority Watershed” effort targeted the Upper East River watershed, a 116.5-km² tributary watershed to Wisconsin's Green Bay, to reduce its sediment and nutrients loads from agricultural sources. A Soil and Water Assessment Tool (SWAT) model was created to determine the effectiveness of agricultural Best Management Practices (BMPs) funded through the Great Lakes Restoration Initiative. The model was calibrated at the monthly time-step for flow, sediment, dissolved reactive phosphorus (DRP), total phosphorus (TP), nitrate, and total nitrogen (TN). Field- and watershed-scale sediment and nutrient reductions were calculated due to the implementation of 74 BMP combinations on dairy and cash grain rotations. Modeling results indicated that when multiple BMPs were placed on a field, especially those including filter strips and grassed waterways, sediment and nutrient loads generally were reduced more than single BMP implementation. The most effective in-field practice at reducing DRP and TP on dairy fields was a combination of 5 different BMPs: cover crops, crop rotation, nutrient management plan, reduced tillage, and a filter strip. Conservation cover was the single most effective practice at reducing sediment and nutrient yields. Sediment and nutrient loads decreased at the watershed scale as the quantity and coverage of BMPs increased. When all contracted BMPs were simulated at the watershed scale, sediment loads were reduced 2%, while TP, DRP, TN and nitrate loads were reduced 20%, 9%, 24%, and 17%, respectively. Modeling scenarios also indicated that over-winter manure storage was important to keep soluble nutrients out of waterways.     

A reflection on restoration progress in the Saginaw Bay watershed

The Saginaw Bay watershed is unique and remains one of the most diverse watersheds in Michigan, containing the largest contiguous freshwater coastal wetland system in the United States. The watershed and Saginaw Bay support a wide variety of flora and fauna, agriculture and recreation opportunities. However, the rapid industrialization and population growth of the watershed in the 20th century strained the region's natural resources. Excessive nutrient loading, elevated bacteria levels, aquatic habitat loss, and chemical contamination all altered the watershed's ecosystem. These stressors contributed to declining fish and wildlife populations, loss of coastal wetlands, water quality concerns, beach closings, and the buildup of contaminants in the food web. Over the past four decades, extensive federal, state, and regional priority-based assessments and planning have positioned the Saginaw Bay watershed for significant restoration. There is a continued commitment by federal, state, and regional partners to advance restoration efforts. This paper reflects upon those activities and provides additional actions that would aid restoration work in the Saginaw Bay watershed and in the Saginaw Bay, a region of the Great Lakes that still must address significant environmental challenges to reach its full potential.     

An Integrated Approach for Targeting Critical Source Areas to Control Nonpoint Source Pollution in Watersheds

This study presents an integrated approach for targeting critical source areas (CSAs) to control nonpoint source pollution in watersheds. CSAs are the intersections between hydrologically sensitive areas (HSAs) and high pollution producing areas of watersheds. HSAs are the areas with high hydrological sensitivity and potential for generating runoff. They were based on a soil topographic index in consistence of a saturation excess runoff process. High pollution producing areas are the areas that have a high potential for generating pollutants. Such areas were based on simulated pollution loads to streams by the Soil and Water Assessment Tool. The integrated approach is applied to the Neshanic River watershed, a suburban watershed with mixed land uses in New Jersey in the U.S. Results show that several land uses result in water pollution: agricultural land causes sediment, nitrogen and phosphorus pollution; wetlands cause sediment and phosphorus pollution; and urban lands cause nitrogen and phosphorus pollution. The primary CSAs are agricultural lands for all three pollutants, urban lands for nitrogen and phosphorus, and wetlands for sediment and phosphorus. Some pollution producing areas were not classified into CSAs because they are not located in HSAs and the pollutants generated in those areas are less likely to be transported by runoff into streams. The integrated approach identifies CSAs at a very fine scale, which is useful for targeting the implementation of best management practices for water quality improvement, and can be applied broadly in different watersheds to improve the economic efficiency of controlling nonpoint source pollution.     

A Watershed Integrity Definition and Assessment Approach to Support Strategic Management of Watersheds

Watersheds are spatially explicit landscape units that contain a range of interacting physical, ecological and social attributes. They are social–ecological systems that provide a range of ecosystem services valued by the society. Their ability to provide these services depends, in part, on the degree to which they are impaired by human‐related activity. An array of indicators is used by natural resource managers, both private and government, to assess watersheds and their sub‐components. Often these assessments are performed in comparison with a reference condition. However, assessments can be hampered because natural settings of many systems differ from those sites used to characterize reference conditions. Additionally, given the ubiquity of human‐related alterations across landscapes (e.g. atmospheric deposition of anthropogenically derived nitrogen), truly unaltered conditions for most, if not all, watersheds cannot be described. Definitions of ‘integrity’ have been developed for river ecosystems, but mainly at the reach or site scale and usually for particular species, such as fish or macroinvertebrates. These scales are inappropriate for defining integrity at the watershed scale. In addition, current assessments of endpoints do not indicate the source of impairment. Our definition of watershed ‘integrity’ is the capacity of a watershed to support and maintain the full range of ecological processes and functions essential to the sustainability of biodiversity and of the watershed resources and services provided to society. To operationalize this definition as an assessment tool, we identify key functions of unimpaired watersheds. This approach can then be used to model and map watershed integrity by incorporating risk factors (human‐related alterations or stressors) that have been explicitly shown to interfere with and degrade key functions in watersheds. An advantage of this approach is that the index can be readily deconstructed to identify factors influencing index scores, thereby directly supporting the strategic adaptive management of individual components that contribute to watershed integrity. Moreover, the approach can be iteratively applied and improved as new data and information become available. © 2015 The Authors. River Research and Applications published by John Wiley & Sons Ltd.     

Estimation of Sediment Yield and Areas of Soil Erosion and Deposition for Watershed Prioritization using GIS and Remote Sensing

A Geographical Information System (GIS) based method is proposed and demonstrated for the identification of sediment source and sink areas and the prediction of sediment yield from watersheds. Data from the Haharo sub-catchment having an area of 565 km² in the Upper Damodar Valley in Jharkhand State in India was taken up for the present study due to availability of gauged data at multiple locations within watershed area. The watershed was discretized into hydrologically homogeneous grid cells to capture the watershed heterogeneity. The cells thus formed were then differentiated into cells of overland flow regions and cells of channel flow regions based on the magnitude of their flow accumulation areas. The gross soil erosion in each cell was calculated using the Universal Soil Loss Equation (USLE). The parameters of the USLE were evaluated using digital elevation model, soil and landuse information on cell basis. The concept of transport limited sediment delivery (TLSD) was formulated and used in ArcGIS for generating the transport capacity maps. An empirical relation is proposed and demonstrated for its usefulness for computation of land vegetation dependent transport capacity factor used in TLSD approach by linking it with normalized difference vegetation index (NDVI) derived from satellite data. Using these maps, the gross soil erosion was routed to the watershed outlet using hydrological drainage paths, for derivation of transport capacity limited sediment outflow maps. These maps depict the amount of sediment rate from a particular grid in spatial domain and the pixel value of the outlet grid indicates the sediment yield at the outlet of the watershed. Up on testing, the proposed method simulated the annual sediment yield with less than ±40% error.     

The Contribution of Conservation Practices in Reducing Runoff, Soil Loss, and Transport of Nutrients at the Watershed Level

The lack of land use planning and the absence of conservation practices in a watershed can contribute to increased runoff, soil loss, and nutrient transport, which compromise the environmental quality in a watershed, especially the water resources. The objective of this study was to assess the contribution of conservation practices in reducing runoff and soil and nutrient losses using the Soil and Water Assessment Tool (SWAT) in the S?o Bartolomeu Stream Watershed, which is a significant watershed in Brazil. The modeling allowed us to identify critical areas regarding sediment yield, runoff, and nutrient loss. After that, conservation practices aimed at reducing the impacts of such processes were simulated. We also identified the most sensitive model parameters to simulate changes in management practices. Simulation results showed an average annual runoff (R) of 35?mm, average annual sediment yield (SY) of 51?t ha^-1?year^-1, 3.6?t ha^-1?year^-1 of total nitrogen (TN), and 1.6?t ha^-1?year^-1 of total phosphorus (TP). When considering the adoption of conservation practices, results showed an increase in water infiltration in the watershed and reductions of 18?%, 66?%, 25?%, and 30?% for R, SY, TN, and TP, respectively. Interventions which prioritize adequate management practices can be highly efficient and avoid changes in consolidated land uses.     

不同管理措施对密云水库流域水量水质变化的影响

在密云水库上游控制流域建立SWAT(Soil and Water Assessment Tool)水文模型,根据实测水量-水质数据对模型的参数进行优化。识别流域土壤侵蚀和污染物关键区域,并对不同管理措施对流域水量-水质的影响进行定量研究。研究结果表明:密云水库流域土壤侵蚀强度较大的集中在流域中下游临近河道的区域;潮河流域非点源污染状况较为严重,潮河和白河总氮流失超高风险区分别占总面积的62.62%和43.09%,白河流域总磷均为低流失风险区,潮河流域总磷高流失风险区占17.81%;等高耕作和梯田种植对于产沙量和污染物都有较好的去除效果,其中等高耕作对于产沙量和总氮、总磷负荷的削减率,潮河分别为25.16%、10.79%和32.89%,白河为47.60%、34.92%和53.49%;通过对比退耕还林和退耕还草措施得知,退耕还林的效果更优。研究结果可为密云水库流域水土保持和水环境治理提供决策依据。     

若尔盖湿地土壤特性空间变化研究

若尔盖湿地是青藏高原高寒湿地生态系统的典型代表,近年来湿地急剧萎缩和退化。根据若尔盖湿地分布特点,采用野外调查和室内试验分析相结合的方式,对在6个样区采取的349个土壤样品进行测试分析,研究若尔盖湿地土壤质地、土壤水分、养分空间特性变化,探讨湿地生态系统演化对土壤状况的影响。研究结果表明:若尔盖湿地土壤质地变化在空间和垂向上差异较小,花湖地区主要为壤土,纳若桥、达水曲、排水沟、哈丘湖为砂壤土,喜马拉也地区则为壤砂土。土壤pH值的变异系数最小,空间变异性相对较弱,有机质的变异系数相对较大,其次为速效磷和速效钾;有机质从表层到80 cm处呈降低趋势,有机质含量花湖纳若桥排水沟哈丘湖达水曲喜马拉也。不同区域的土壤水分特征曲线Van Genuchten模型参数差异显著,喜马拉也和达水曲区域的A和n值明显高于其余区域,土壤水分持水性能较弱。研究结合不同区域的土壤性质、水分及养分特性变化,对若尔盖未来湿地保护和管理的重点方向和区域进行指导和建议。     

Water level fluctuation and sediment–water nutrient exchange in Great Lakes coastal wetlands

The influence of water level fluctuation on sediment–water nutrient exchange in coastal wetlands of Lakes Michigan and Huron was investigated using controlled, laboratory experiments. At each wetland, sediment cores were collected from 5 locations along a transect perpendicular to the shoreline, desiccated for 8 weeks, and then re-wetted with original site water for 24 h to simulate water level fluctuation. Soluble reactive phosphorus release declined exponentially along transects, with highest release rates from sediments collected at the ordinary high water mark (OHWM), and lowest rates from sediments underlying water > 0.25 m in depth. Nitrate exchange showed no obvious pattern in the Lake Michigan wetlands but nitrate was lost at all locations in the Lake Huron wetlands, suggesting denitrification. Ammonium was released at all sites, but with no obvious pattern along transects. Sulfate release was low at the OHWM locations and increased in a lakeward direction, plateauing by the 0.25 m water depth.