Page 218 - Biosystems Engineering

196 Cha pte r S i x

soil. Proper water management can reduce the buildup of salts within
the root zone. A well-designed water management system can remove
excess salts by applying excess water to wash the salts below the root
zone. The deep percolating water below the root zone is removed by
drainage systems and treated off-site to remove the salts. Biosystems
engineers also design engineered wetlands that contain plants capa-
ble of removing salts from the drainage return flows. An overview of
the design of a sustainable water management system is presented in
this chapter.

6.2 Physical Properties of Soil
In order to understand the soil and water interaction, it is important
to learn about basic soil water retention and movement within the
soil. Soils are made up of different-sized soil particles consisting of
sand, silt, and clay, along with humus, that is formed by the degrada-
tion of organic residues. Plant roots take up water that occupies the
pore space between the soil particles. The water is held within the
pore space of the soil particles by capillary forces. Capillary forces
exist at the interface formed by the pore water, soil particle, and pore
air. Capillary pressure is defined as the difference in pressure between
pore air and pore water across the meniscus (Hillel 1998). Figure 6.1
shows the relationship between the pore size and the relative force
required to remove the water. The smaller the radii of curvature of the
meniscus, the greater is the capillary pressure across it. This requires
greater negative pressure to remove the water from within these

A B C

C B

A
Soil particle Soil particle

FIGURE 6.1 Relationship between capillary pressure and water content of
soil.