Geoscience Reference
In-Depth Information
11.5 Transport of Contaminants by Runoff
11.5.1 Overland Runoff
The fraction of rainfall or irrigation water that flows over a land surface from
higher to lower elevations, known as overland runoff, is an additional pathway for
contaminant transport. Runoff occurs when the amount of rain or irrigation water
is greater than the soil infiltration capacity. The formation of a crust on the soil
surface is a major contributor to runoff formation in arid and semiarid zones,
because it decreases the infiltration capacity. The soil crust is a thin layer
(0-3 mm) with a high density, fine porosity, and low hydraulic conductivity
compared to the underlying soil. This ''skin'' forms as a result of falling raindrops
or sodification of soil clays.
Overland runoff may be expressed as a conservation of mass in a flow domain,
where the excess rainfall or irrigation rate, R
e
, is equal to the difference between
the rainfall and infiltration rates:
R
e
¼
o
h
ot
þr
q
;
ð
11
:
13
Þ
where h is the flow depth and q is the unit discharge. The excess rainfall may be
determined directly, while the infiltration rate may be estimated by empirical or
physical models. Equation
11.13
is adjusted to consider spatial variability and land
hydraulic properties, to reduce errors in estimating runoff infiltration (Wallach
et al.
1997
; Michaelides and Wilson
2007
).
After sediment has been detached by rainfall impact during an erosion event,
the majority of it returns to the soil surface after being carried some distance
downslope. Through the continuous addition of sediment to the water layer, by
impact of raindrops, fine sediment forms a ''suspended load'' with a settling
velocity less than the typical velocity of whirling eddies in flowing water. This
velocity is called the friction velocity (Rose
1993
). The rate of deposition, d
i
,
defined as the mass of sediment reaching the soil surface per unit area per time (kg/
m
2
/s), is given by
d
i
¼ v
i
c
i
;
ð
11
:
14
Þ
where c
i
is the sediment concentration and v
i
is the settling velocity. The rate of
rainfall detachment and redetachment is proportional the rainfall rate, raised to
some power that has been found empirically to be close to unity (Hairsine and
Rose
1991
).
When water flows over a contaminated land surface, pollutants released from
higher elevations are transported, as dissolved solute or adsorbed on suspended
particles, and accumulate at lower elevations. This behavior is reflected in the
