A Model for the Assessment of the Water Footprint …
77
(K), is a constant, so it is assumed that infiltration is constant throughout the storm,
and is expressed in terms of speed (cm/s). Darcy (1856) defined this parameter for
sands and showed that it was equally valid for other soils. The methods for both
field and laboratory determination are standardized by the American Society for
Testing and Materials (ASTM). From Darcy’s law, the expression that relates the
flow of water through a sample to its permeability taking into account the pressure
differential (Fig. 6, Eq. 11) is derived.
When measuring infiltration values, Bárcena and Hurtado [7], mention that the
range of permeability coefficient K values is very wide and extends from 102 cm/s
for very coarse gravel, to a negligible value in the case of clays, as presented in Table
3.
A portion of the precipitation that falls on the earth infiltrates into the soil and
becomes part of the groundwater. The volume of water filtered through a surface is
related to the effective rainfall, the permeability of the soil through the infiltration
flow and the permeable or infiltration section or surface (Fig. 6, Eq. 12). Once on the
ground, some of this water moves close to the earth’s surface and quickly emerges
to be discharged into the beds of the water currents, but due to gravity, a large part
of it continues to move towards deeper areas.
The movement of water below the surface depends on the permeability and
porosity of the subsurface rock. If the rock allows water to move relatively freely
within it, water can move significant distances in a short period of time. But water
can also move to deeper aquifers, from where it will take years to become part of the
environment again.
Water demand
For the quantification of the real evapotranspiration (water demand) of crops and
green areas, the procedure proposed by FAO is used, which introduces the concepts:
reference evapotranspiration and the crop coefficient (Fig. 6, Eq. 13). Reference
evapotranspiration is calculated with a class A cube for each area, as this is a valid
low-cost method for estimating high diffusion and evapotranspiration for a large
number of production plots [21].
In the formulation of the water demand calculation, a correction coefficient is
introduced (crop coefficient (Kc)) that contemplates the biophysical variations of the
plant, that is, how the height of the plant and soil cover vary in the growth cycle, since
this influences the evapotranspiration. Which means that this coefficient varies over
time. Depending on the phase in which the plant is, germination, growth, flowering
and ripening (from sowing to harvesting), the amount of water consumed by the crop
varies. This variation is also reflected in the productivity of the crop, depending on
the sizes and maximum quantities.
Evapotranspiration of a reference crop
In order to calculate the water demand of a crop, it is necessary to know the effect
of the climate at a certain point to obtain an ideal crop. This data is called reference
evapotranspiration (ETo), expressed in millimetres of height of evapotranspired water
per day (mm/day). As its name suggests, it is used as a reference indicator and is
77
(K), is a constant, so it is assumed that infiltration is constant throughout the storm,
and is expressed in terms of speed (cm/s). Darcy (1856) defined this parameter for
sands and showed that it was equally valid for other soils. The methods for both
field and laboratory determination are standardized by the American Society for
Testing and Materials (ASTM). From Darcy’s law, the expression that relates the
flow of water through a sample to its permeability taking into account the pressure
differential (Fig. 6, Eq. 11) is derived.
When measuring infiltration values, Bárcena and Hurtado [7], mention that the
range of permeability coefficient K values is very wide and extends from 102 cm/s
for very coarse gravel, to a negligible value in the case of clays, as presented in Table
3.
A portion of the precipitation that falls on the earth infiltrates into the soil and
becomes part of the groundwater. The volume of water filtered through a surface is
related to the effective rainfall, the permeability of the soil through the infiltration
flow and the permeable or infiltration section or surface (Fig. 6, Eq. 12). Once on the
ground, some of this water moves close to the earth’s surface and quickly emerges
to be discharged into the beds of the water currents, but due to gravity, a large part
of it continues to move towards deeper areas.
The movement of water below the surface depends on the permeability and
porosity of the subsurface rock. If the rock allows water to move relatively freely
within it, water can move significant distances in a short period of time. But water
can also move to deeper aquifers, from where it will take years to become part of the
environment again.
Water demand
For the quantification of the real evapotranspiration (water demand) of crops and
green areas, the procedure proposed by FAO is used, which introduces the concepts:
reference evapotranspiration and the crop coefficient (Fig. 6, Eq. 13). Reference
evapotranspiration is calculated with a class A cube for each area, as this is a valid
low-cost method for estimating high diffusion and evapotranspiration for a large
number of production plots [21].
In the formulation of the water demand calculation, a correction coefficient is
introduced (crop coefficient (Kc)) that contemplates the biophysical variations of the
plant, that is, how the height of the plant and soil cover vary in the growth cycle, since
this influences the evapotranspiration. Which means that this coefficient varies over
time. Depending on the phase in which the plant is, germination, growth, flowering
and ripening (from sowing to harvesting), the amount of water consumed by the crop
varies. This variation is also reflected in the productivity of the crop, depending on
the sizes and maximum quantities.
Evapotranspiration of a reference crop
In order to calculate the water demand of a crop, it is necessary to know the effect
of the climate at a certain point to obtain an ideal crop. This data is called reference
evapotranspiration (ETo), expressed in millimetres of height of evapotranspired water
per day (mm/day). As its name suggests, it is used as a reference indicator and is