Geoscience Reference
In-Depth Information
dry beans, and sugar beets. Over the years, primarily agricultural and urban runoff,
improper manure management, CSOs, and industrial pollution have led to high
sediment and nutrient loadings, eutrophication in the bay, toxic contamination of
fish, restrictions on fish consumption, loss of fish and wildlife habitat, and beach
closures in the basin (He et al.
1993
; He and Croley
2006
,
2008
; Michigan
Department of Natural Resources
1988
). To address the eutrophication problem,
the Great Lakes Water Quality Agreement between the United States and Canada
has established a target Total Phosphorus (TP) load of 440 metric tons/year for
Saginaw Bay (Tao et al.
2010
). Achievement of this goal requires estimation of
spatial and temporal distribution of nutrients from both point and nonpoint sources.
This paper applies the DLBRM to the Saginaw Bay Basin to help ecological
researchers and resource managers better understand the dynamics of nutrients
for comprehensively managing the pollution problems on a regional scale (He
and DeMarchi
2010
).
6.3 Watershed Model
The watershed quality model under development evolves from GLERL's DLBRM
(Croley and He
2005
,
2006
; He and Croley
2007a
). The DLBRM subdivides
a watershed into a 1-km
2
grid network and simulates hydrologic processes for the
entire watershed sequentially. Each 1-km
2
“cell” of the watershed is composed of
moisture storages of upper soil zone, lower soil zone, groundwater zone, and
surface, which are arranged as a serial and parallel cascade of “tanks” to coincide
with the perceived basin storage structure. Water enters the snow pack, which
supplies the basin surface (degree-day snowmelt) (Fig.
6.2
). Infiltration is propor-
tional to this supply and to saturation of the upper soil zone (partial-area infiltra-
tion). Excess supply is surface runoff. Flows from all tanks are proportional to their
amounts (linear-reservoir flows). Mass conservation applies for the snow pack and
tanks; energy conservation applies to evapotranspiration.
The model computes potential evapotranspiration from a heat balance, indexed
by daily air temperature, and calculates actual evapotranspiration as proportional
to both the potential and storage. It allows surface and subsurface flows to inter-
act both with each other and with adjacent-cell surface and subsurface storages. The
model has been applied extensively to nearly 40 watersheds draining into the
Laurentian Great Lakes for use in both simulation and forecasting (Croley and
He
2005
,
2006
,
2008
; Croley et al.
2005
; He and Croley
2006
,
2007a
). The unique
features of the DLBRM include: (1) it uses readily available climatological,
topographical, hydrologic, soil and land use databases; (2) it is applicable to large
watersheds; (3) mass continuity equations are used to govern the hydrologic
processes and solved analytically, thus, making model solution analytically trac-
table (Croley and He
2005
,
2006
). Currently, the model is being modified to add
materials runoff through each of the storage tanks routing from upstream to
downstream. The movement of pollutants through storages in a watershed is
