Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 140

Keywords Watershed model Hydrology Sediment transport Nonpoint source
Nutrient transformations
8.1 Introduction
Nutrient pollution is a leading cause of water quality impairment in lakes and
estuaries and is also a significant issue in rivers (USEPA 2007 ). Non-point source
(NPS) pollution, especially from nitrogen and phosphorus, has consistently ranked
as one of the top causes of degradation in some U.S. waters. Nutrient problems can
exhibit themselves locally or much further downstream leading to degraded
estuaries, lakes and reservoirs, and to hypoxic zones where fish and aquatic life can
no longer survive. The growing concern about the environmental impact of NPSs
has enhanced a ''watershed approach'' to reduce NPS pollution and coordinate the
management of water resources. The concept of this watershed approach is based
on multi-purpose, multi-objective management, and in examining all water needs
in the watershed and receiving water bodies. A watershed scale flow, sediment and
water quality modeling system has been developed at U.S. Army Engineer
Research and Development Center (ERDC) in support of the U.S. Army Corps of
Engineers (USACE)' watershed approach.
This chapter describes the on-going watershed water quality modeling devel-
opment and integration with the Gridded Surface Subsurface Hydrologic Analysis
(GSSHA) model. The major chemical and physical processes influencing sedi-
ments and nutrients in the soil, overland flow and stream have been accounted for
in the GSSHA. The hydrological variables required to drive the sediment and
nutrient simulation were provided using the existing GSSHA model. Integrated
physically based hydrologic models with sediment and nutrient transport across
the landscape give more realistic descriptions of the sediment and nutrient
dynamics in watersheds. This is especially important for agricultural watersheds
where the sediment and nutrient play important roles and their occurrence are
highly variable both in time and space. Hence, hot-spots with high contaminant
loading sources can be more accurately identified and watershed management
practices to reduce sediment and nutrient transport can be made more confidently.
8.2 Watershed Scale Water Flow and Sediment Model
The U.S. Army Corps of Engineer's Gridded Surface Subsurface Hydrologic
Analysis (GSSHA) is a physically-based, distributed-parameter, structured grid,
hydrologic model that simulates the hydrologic response and sediment transport of
a watershed subject to given hydrometeorological inputs. The watershed is divided
into grid cells that comprise a uniform finite difference grid. GSSHA is a