Water Flow in Soil
Therefore, 8 days will be required for the transpiration rate to be reduced
from 5 to 1 mdday. Note how quickly the plant runs out of water at the
end of 7 days. This is a consequence of the steepness of the curve in Fig.
9.8. Obviously the plant will virtually stop transpiring on the ninth day.
9.6 The Water Balance
We have discussed infiltration, redistribution, evaporation, and transpiration as if they are isolated processes. Of course, they are not, and most go
on simultaneously. At any particular time conservation of mass requires
that the rate of change in water content in a depth of soil L equal the sum
of the inputs and losses. This mass balance equation can be written as
where the terms on the left represent the rate of storage and the terms on
the right are for infiltration (Jwi), deep percolation below depth L (Jwd),
evaporation from the soil surface (E,), and plant transpiration (E,). It
is not difficult to solve Eq. (9.23) and obtain a record of water content
changes in the soil over time, but the solution is only practical using
numerical methods on a computer. The methods are covered in detail in
Campbell (1985). The analyses presented here are intended to give insight
into the processes and how they operate, but do not lead to a full solution
of the water balance equation.
References
Campbell, G. S. (1985) Soil Physics with BASIC: Transport Models for
Soil-Plant Systems. New York: Elsevier.
Green, W. H. and G. A. Ampt. (191 1.) Studies in soil physics: the flow
of air and water through soils. J. Agric. Sci. 4:l-24.
Rawls, W.J., L.R. Ahuja, and D.L. Brakensiek (1992) Estimating soil hydraulic properties from soil data. In Indirect Methods for Estimating
Hydraulic Properties of Unsaturated Soils. M. th. Van Genucthen,
F.J. Leij, and L.J. Lund (eds.) U.C. Riverside Press, Riverside, CA.
Problems
9.1. For a sandy loam soil, estimate the -33 J kg-' and -1500 J kg-'
water contents. Assume 8, = 0.45 m
3 m-3.
a. If the rooting depth is 100 cm, estimate the maximum available water
to the plant.
b. If 50 mrn of rain falls on this soil in one hour, estimate the runoff if
the initial volumetric water content of the soil is 0.1 . Use Eq. (9.8) to
estimate the total infiltration in an hour. Compute the water potential
at the wetting front from Eq. (9.10), and assume that the average hydraulic conductivity of the transmission zone is the geometric mean
Therefore, 8 days will be required for the transpiration rate to be reduced
from 5 to 1 mdday. Note how quickly the plant runs out of water at the
end of 7 days. This is a consequence of the steepness of the curve in Fig.
9.8. Obviously the plant will virtually stop transpiring on the ninth day.
9.6 The Water Balance
We have discussed infiltration, redistribution, evaporation, and transpiration as if they are isolated processes. Of course, they are not, and most go
on simultaneously. At any particular time conservation of mass requires
that the rate of change in water content in a depth of soil L equal the sum
of the inputs and losses. This mass balance equation can be written as
where the terms on the left represent the rate of storage and the terms on
the right are for infiltration (Jwi), deep percolation below depth L (Jwd),
evaporation from the soil surface (E,), and plant transpiration (E,). It
is not difficult to solve Eq. (9.23) and obtain a record of water content
changes in the soil over time, but the solution is only practical using
numerical methods on a computer. The methods are covered in detail in
Campbell (1985). The analyses presented here are intended to give insight
into the processes and how they operate, but do not lead to a full solution
of the water balance equation.
References
Campbell, G. S. (1985) Soil Physics with BASIC: Transport Models for
Soil-Plant Systems. New York: Elsevier.
Green, W. H. and G. A. Ampt. (191 1.) Studies in soil physics: the flow
of air and water through soils. J. Agric. Sci. 4:l-24.
Rawls, W.J., L.R. Ahuja, and D.L. Brakensiek (1992) Estimating soil hydraulic properties from soil data. In Indirect Methods for Estimating
Hydraulic Properties of Unsaturated Soils. M. th. Van Genucthen,
F.J. Leij, and L.J. Lund (eds.) U.C. Riverside Press, Riverside, CA.
Problems
9.1. For a sandy loam soil, estimate the -33 J kg-' and -1500 J kg-'
water contents. Assume 8, = 0.45 m
3 m-3.
a. If the rooting depth is 100 cm, estimate the maximum available water
to the plant.
b. If 50 mrn of rain falls on this soil in one hour, estimate the runoff if
the initial volumetric water content of the soil is 0.1 . Use Eq. (9.8) to
estimate the total infiltration in an hour. Compute the water potential
at the wetting front from Eq. (9.10), and assume that the average hydraulic conductivity of the transmission zone is the geometric mean