Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 260

instantaneously during steady water flow. The salt solution moves through the soil
column with the moving water and causes changes in the concentration of solute
in the soil column. The changing concentrations are monitored over time until the
whole solution moves out of the column. The solute concentration is monitored
using TDR or automated technique.
In the step-type experiment, salt solution is continuously added to the soil column
at a constant rate. The concentration of salt in the soil column increases gradually
and reaches a constant value when the whole initial soil-water phase is replaced
by the flowing solution. The increasing concentrations are monitored over time to
construct the BTCs.
The pulse-type experiment gives a better insight into the physical processes of the
transport system from the falling segment of BTCs. It also needs small quantities of
salt solution. But for unsaturated condition, a pulse experiment is time consuming.
On the other hand, the step-type experiment needs several steps of application of
solute to characterize the soil properly. This needs large quantity of salt solution. It
is time consuming to attain a constant concentration of salt in the experimental soil
under field condition.

7.5.2.3 Analysis of Breakthrough Curve

Breakthrough curves (BTCs) are analyzed to determine the parameters governing
the transport of solute involved in the transport process. A number of mathematical
models predict the transport of solutes through the vadose zone. The methods used
to analyze the breakthrough curve are as follows:

• analytical method

• numerical method

• quasi-analytical method

• method of moment

• curve-fitting method

The most widely used method to determine the solute transport parameters is
to fit the transport models to breakthrough curves measured in the laboratory or
field. Minimizing the squared deviations (least-square technique) is an accurate and
reliable method for fitting the observed BTCs to the transport models. Different
models are usually based on the analytical solutions of the one-dimensional
convection-dispersion equation, usually for steady flow of water through soil. The
method of time moment is applied to determine the solute transport parameters
directly from the measured breakthrough data. It is applicable to linear processes
only.
In least-square technique, the BTCs measured are compared with those prec-
dicted by the analytical solutions based on assumed values of parameters. If the
measured and estimated BTCs agree well, the values of the parameters used for the
prediction are regarded as reliable.