Lewis C. Linker

Enhanced HSPF Model Structure for Chesapeake Bay Watershed Simulation

AbstractFor more than two decades, an HSPF-based watershed model has been used to simulate nutrient and sediment load delivery to the Chesapeake Bay. Over time, the watershed model has increased in complexity commensurate with the management challenges in Chesapeake Bay restoration. The increased complexity poses challenges to the standard application of HSPF for efficient operation of the model in a large-scale watershed, as well as difficulties in incorporating changes in best management practices (BMPs) and land uses over time. In response, the U.S. Environmental Protection Agency’s Chesapeake Bay Program Office developed a software solution that enhances the existing HSPF model structure. The software system, consisting of preprocessors, an external transfer module, and postprocessors, was devised to conveniently generate and update parameter files essential to operations of a large and complex watershed-modeling system and to implement land-use and non-point-source-pollution management changes on any...

A correction of DIN uptake simulation by Michaelis-Menton saturation kinetics in HSPF watershed model to improve DIN export simulation

Plant uptake of dissolved inorganic nitrogen (DIN) has a major effect on the watershed export of DIN. Plant uptake depends on soil moisture. The literature describes plant uptake under two conditions. One condition, when moisture is deficient, increasing moisture may increase nitrogen (N) uptake (in mass per unit time). The other condition, when moisture is sufficient, i.e., at or above the field capacity, increasing it may dilute the solution and decrease N uptake. The two different observed relationships of moisture and uptake cannot be simply simulated by the Michaelis-Menton saturation kinetics under the current setting in the Hydrological Simulation Program - Fortran (HSPF) software. This paper first compares two methods of simulated plant N uptake. The current HSPF (Version 11) uses concentration (per unit time) as the unit of uptake rate for the entire range of moisture conditions, which is inappropriate for moisture-sufficient conditions and results in higher uptake and lower DIN export during higher flow days. An alternative method uses mass (per unit area per unit time) as the unit of uptake rate, resulting in a better DIN load-flow relationship. However, it overestimates uptake in moisture-deficient conditions. This paper presents an integral method, which simply combines the above two mechanisms to simulate plant uptake in different moisture saturation conditions to improve load-flow relationships. However, it is not optimal in the operation of HSPF. Ultimately, a synthetic method, which is operational through HSPF code modification, is introduced. The synthetic method results in a better relationship between moisture and uptake, and provides reliable exports of DIN under a range of hydrology conditions.

Assessment of Impact of Storm on Point Source Pollutant Transport in Estuary by Dissolved Tracer Modeling

A continuously discharged dissolved conservative tracer was simulated with the Chesapeake Bay Estuary Model Package to study pollutant transport in the estuary in response to point source loads and the impact of the November, 1985 storm. A visualization technology is applied to show 3-dimensional concentration variations in a continuous daily time sequence. The differential responses of daily net transport during storms versus inter-storm periods can be observed from an MPEG movie. It may take 2–3 months for a tracer to travel from the fall-line to the mouth of a river during relatively dry seasons, only 2 weeks in some medium storms, and less than 5 days in a big storm. Plots of daily concentrations from eleven selected locations in the estuary provide quantitative information on the response of tracer concentration to flows. The magnitude and time of tracer peaks related with different weather events in these locations reflect the combined effects of flows from various directions to these locations. The lower tributaries (which are closer to the Bay mouth) are affected more than the upper tributaries by a source discharged at a mid-tributary. A storm can transport materials more effectively to the Bay and affect adjacent tributaries more severely.

Influence of Reservoir Infill on Coastal Deep Water Hypoxia

Ecological restoration of the Chesapeake through the Chesapeake Bay total maximum daily load (TMDL) requires the reduction of nitrogen, phosphorus, and sediment loads in the Chesapeake watershed because of the tidal water quality impairments and damage to living resources they cause. Within the Chesapeake watershed, the Conowingo Reservoir has been filling in with sediment for almost a century and is now in a state of near-full capacity called . The development of the Chesapeake TMDL in 2010 was with the assumption that the Conowingo Reservoir was still effectively trapping sediment and nutrients. This is now known not to be the case. In a TMDL, pollutant loads beyond the TMDL allocation, which are brought about by growth or other conditions, must be offset. Using the analysis tools of the Chesapeake TMDL for assessing the degree of water quality standard attainment, the estimated nutrient and sediment loads from a simulated dynamic equilibrium infill condition of the Conowingo Reservoir were determined. The influence on Chesapeake water quality by a large storm and scour event of January 1996 on the Susquehanna River was estimated, and the same storm and scour events were also evaluated in the more critical living resource period of June. An analysis was also made on the estimated influence of more moderate high flow events. The infill of the Conowingo reservoir had estimated impairments of water quality, primarily on deep-water and deep-channel dissolved oxygen, because of increased discharge and transport of organic and particulate inorganic nutrients from the Conowingo Reservoir.

Using Geographically Isolated Loading Scenarios to Analyze Nitrogen and Phosphorus Exchanges and Explore Tailored Nutrient Control Strategies for Efficient Management

A set of geographically isolated differential nitrogen (N) and phosphorus (P) load model scenarios from major Chesapeake basins provides information on the relative impact of nutrient loads on primary production and dissolved oxygen in the Chesapeake Bay. Model results show the relationships of deep water dissolved oxygen with nutrient limitation-related algal blooms, organic carbon loads from the watershed, estuarine circulation, nutrient cycling, and nutrient diagenesis. The combined effect of changes in load from multiple basins is additive for changes in both chlorophyll-a and deep water dissolved oxygen concentrations. Management of both N and P are required in the Chesapeake watershed and tidal waters to achieve water quality standards, but overall efficiencies could be gained with strategies that place greater emphasis on P control in the upper Bay and greater emphasis on N control in the lower Bay. The areas of the Bay with the highest degree of dissolved oxygen degradation that generally drive management decisions are mostly P-limited and are significantly influenced by the load from the upper Bay’s basins. Reducing P from the upper Bay’s basins will intensify P limitation and would allow an increase in N of about six times the weight of P reduction. Combining the relative nutrient reduction effectiveness with the relative control cost information could improve management efficiency and provide benefits at a lower cost. This article describes initial steps that can be taken to examine the benefits from N-P exchanges.

Dynamic parameterization to simulate DIN export due to gypsy moth defoliation

A module of dynamic parameterization is added into the HSPF watershed software for simulation of dissolved inorganic nitrogen (DIN) export from forest associated with gypsy moth defoliation. It simulates a changing ecosystem following the breakout of defoliation, such as increasing mineralization and nitrification rates and soil temperature, and decreasing interception of precipitation, plant nitrogen uptake rate and evapotranspiration. These parameter values vary with the stages of a defoliation event, such as the progressive period, the peak period, and the recovery period, the simulated DIN export from a multi-occurrence defoliation area in Shenandoah National Park in Virginia, USA, is comparable with the observed data.

Recast of the outputs of a deterministic model to get a better estimate of water quality for decision makings

The outputs of a deterministic water quality model are retreated to recast the model simulation. A multi-variance regression method is used for daily model outputs versus observed data to assess the systematic errors in model simulation of individual model cell. The model outputs are re-adjusted to better represent the actual values, and to yield a better model calibration. Such a recast is important to model application for water quality analysis, especially for TMDL (Total Maximum Daily Load) which requires accurate simulation to exam criteria attainment. This paper addresses the recast method, its prerequisites, and how the results are extended for the scenarios with various load reductions.