Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
the context of site climate and soil type. In many respects, it is more useful to state the TAWHC
of a site rather than the rooting depth. The TAWHC is the total amount of moisture that a soil
can store for plant growth when fully charged with water.
Table 5-1 shows how the TAWHC is calculated. Using the same format and equations, a similar
spreadsheet can be created by copying the equations into the “Results” column. In this example,
there is 1 foot of topsoil and 2 feet of subsoil over a highly fractured basalt. The topsoil has a
loam texture and available water-holding capacity of 2.0 inches (estimated from Figure 5-12 or
obtained from lab results) but, because of the rock fragments, it is reduced by approximately
0.4 inch. The subsoil has a high water-holding capacity because of high clay content, but the
available water-holding capacity is reduced by half due to rock. Highly fractured basalt is
encountered at a depth of 2 feet, and it is estimated from the road cut that approximately 30
percent is actually fractured. Within these weathered fractures is a gravelly clay loam textured
material storing approximately 0.5 inch of water. The TAWHC for this site would be the sum
of all sections of soil (approximately 3.2 inches).
TAWHC is useful for comparing water relationships between revegetation units and reference
sites. For example, the TAWHC for a post-construction soil is 3.6 inches compared to an adjacent
reference site, which is 6 inches. If the desired future condition of the post-construction soil is
to be similar to the adjacent reference site, then the TAWHC of 3.6 inches must be increased
upward toward 6 inches for the site to be capable of supporting the vegetative community
occurring on the reference site.
Table 5.1 | Calculating total available water-holding capacity
The total available water-holding capacity (TAWHC) is the total amount of moisture that a soil can store for plant growth when fully charged with
water. TAWHC values for each revegetation unit are helpful for determining which species will perform well and in developing the mitigating
measures necessary to increase water-holding capacity or rooting depth. TAWHC is calculated by determining the texture, rock fragment content,
and depth of each soil layer, and calculating how much water each layer will optimally store. The water- holding capacity of each soil layer is added
together to obtain the TAWHC for the soil profile.
Soil
Strata
0 to 1'
1 to 2'
2 to 5'
Soil Characteristics Data Results Equations
AWHC of texture (inches / foot): 2 (A) 2 From Figure 5.11 or lab results
Small Rock (%): 20 (B) 0.264 = A * (B / 100) * 0.66
Large Rock (%):
Thickness (ft):
5
1
(C)
(D)
0.1
1
= A * (C / 100)
(the thickness of soil section)
Available water by section:
1.6 (E) = (A - B - C) * D
AWHC of texture (inches / foot): 2.2 (F) 2.2 From Figure 5.11 or lab results
Small Rock (%)
Large Rock (%):
35
25
(G)
(H)
0.5082
0.55
= F * (G / 100) * 0.66
= F * (H / 100)
Thickness (ft): 1 (I) 1 (the thickness of soil section)
Available water by section:
1.1 (J) = (F - G - H) * I
AWHC of texture (inches / foot): 2.2 (K) 2.2 From Figure 5.11 or lab results
Small Rock (%)
Large Rock (%):
35
70
(L)
(M)
0.5082
1.54
= K * (L / 100) * 0.66
= K * (M / 100)
Thickness (ft): 3 (N) 3 (the thickness of soil section)
Available water by section:
0.5 (O) = (K - L - M) * N
Total Available Water Holding Capacity (inches): 3.2
= Sum of (E, J, O)
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
74