Page 194 - Biosystems Engineering

GIS-Based W atershed Modeling Systems 173

R = the amount of precipitation on day i (mm H O)
day 2
Q = the amount of surface runoff on day i (mm H O)
surf 2
E = the amount of evapotranspiration on day i (mm H O)
a 2
w = the amount of water entering the vadose zone from the
seep
soil profile on day i (mm H O)
2
Q = the amount of return flow on day i (mm H O)
gw 2
To simulate watershed hydrology, a hydrologic cycle is usually
divided into the land phase (large dashed box in Fig. 5.1) and the
routing phase (small dashed box, same figure). The land phase of
the hydrologic cycle transports water, sediment, nutrient, and pesti-
cide loads from a land surface to a stream, whereas the routing phase
transports them through stream channels of the watershed to the out-
let. Figure 5.1 is a schematic of hydrologic processes/water transport
pathways simulated in one of the most widely used watershed mod-
els, the SWAT model. These individual processes have been described
in more detail in Chap. 3.
To account for areal variations in watershed characteristics (e.g.,
soils, land use, slope, and rainfall), a distributed parameter model
subdivides the watershed into subwatersheds, grid cells, or hydro-
logic response units (HRUs). Runoff is predicted separately in each
smaller unit and routed to obtain total runoff for a watershed. The
subdivision of a watershed increases accuracy and gives a more
accurate physical description of the hydrologic processes (Neitsch
et al. 2005).
As discussed above, water balance is the driving force behind
accurate prediction of movement of sediment, nutrients, and pesti-
cides. For example, in many of the watershed-scale models, runoff
volume and peak runoff rate are used to simulate erosion and sedi-
ment yield. Watershed models also track transport and transforma-
tion of various forms of nutrients [nitrogen (N) and phosphorus (P)]
in a watershed. In the soil, transformation of N is governed by the N
cycle, whereas transformation of P is governed by the P cycle. To sim-
ulate point sources, nutrients can be added to the main channel and
transported downstream through the stream flow. Inorganic and
organic forms of nutrients applied can be taken by plants, adsorbed
by soils, move to streams and lakes/reservoirs through surface run-
off or lateral subsurface flows, or percolate to deeper groundwater.
Pesticide movement is controlled by its solubility, degradation half-
life, and soil organic carbon adsorption coefficient.
The loadings of water, sediment, nutrients, and pesticides from
the landscape are routed through a watershed’s stream network.
While keeping track of the mass flow of pollutants in the stream
channel, most watershed models also account for transformation of
pollutants in the stream. Many sophisticated watershed models also
simulate movement and transformation of pollutants in lakes and
reservoirs, although not as comprehensively as the reservoir models.