Ground Surface Moisture Flux Boundary Conditions - Unsaturated Soil Mechanics in Engineering Practice - page 277

Table 6.1 Criteria for Climate Classification a moisture flux at the ground surface can be quite differ-

ent at various times of the year, and as a result there is
a more meaningful way to partition each year. In the north-
ern areas of Canada, for example, it is reasonable to select
the autumn freeze-up date as the beginning of a new year
(Stianson et al., 2010). This is the time of the year when the
“near-ground-surface” water content conditions are essen-
tially “locked in” for several months. Other regions of the
world might have other times that should be used for starting
the analysis of moisture flux at the ground surface.
A common freeze-up date is November 1 in northern
Saskatchewan, Canada (no. 1 in Fig. 6.5). The winter period
is referred to as the inactive season and has been assumed
to extend to April 18 of the following year (nos. 2 and 3 in
Fig. 6.5). Most of the precipitation is in the form of snow
during the inactive season. The snow accumulated during the
inactive season can be converted to an equivalent amount
of rain at the weather station.
The period from April 19 to October 31 is referred to as
the active season (no. 2 to no. 1 in Fig. 6.5). The terms inac-
tive and active are designated in Fig. 6.5 and are used in a
relative magnitude sense since there can be some moisture
movement activity all year round at the ground surface. The
snow and rain from the time of freeze-up accumulates over
winter and then infiltrates the ground surface in the spring
of the year (no. 3 to no. 4 in Fig. 6.5). There is also con-
siderable runoff which can occur during the springtime and
throughout the summer months.

I m Category of Climate

> 100 Perhumid

20-100 Humid

0-20 Moist humid

− 33-0 Dry subhumid

− 67 to − 33 Semiarid

− 100 to − 67 Arid

a Moisture index I m as defined in 1955. (i.e., Eq. 6.1).

are shown in Table 6.1. The Thornthwaite classification is
an empirical equation that was developed from limited cli-
matic data collected in the United States. However, it forms
an adequate basis for establishing the climatic type and vari-
ations in climate.
Climatic classification and the likely variation in climate
should be established prior to considering a particular cover
system solution for the site under consideration. It is impor-
tant to determine whether a particular engineered system
will be suitable for the site under consideration. It is the
ratio of the total annual precipitation P to the total annual
PE, which provides a guide to the suitability of a particular
type of cover system. Figure 6.4 shows contours of average
potential evaporation for the world.

6.2.1 Partitioning of Each Year
Normally a year is viewed as starting on January 1 and
ending on December 31. However, the evaluation of net

6.2.2 Analysis of 28-Year Weather Data Set
The analysis of a 28-year weather data set from a site in
eastern Canada is shown in Table 6.2. The site was at a

Figure 6.4 Average potential evaporation contours for the world. From The Physics of Climate ,
by J. P. Peixoto and by A.H. Oort © 1992, used with permission of American Institute of Physics.