Agriculture Reference
In-Depth Information
Average Pore Water Velocities and Wetting Depths in Soils
of Different Texture and Structure During Infiltration
Table 6.2
Average
Effective Water
Content
Rainfall or
Irrigation
Intensity
(mm/hr)
Depth z of
Wetting Front
in 5 hr
(cm)
During
Infiltration
(cm 3 /cm 3 )
Average Pore
Water Velocity
Soil Type
(mm/hr)
Sandy soil
Clay soil with a
massive structure b
Well-structured
clay soil with
macroporosity
10
10
0.25 a
0.45 a
40
22
20
11
10
0.1 c
100
50
a Representative values of
at field capacity
b The infiltration rate of this soil may fall below the rainfall intensity so that ponding could occur. The
depth of wetting would then be less.
c The macropores create a preferred pathway for water movement, and much of the soil matrix is
bypassed.
The Hydrologic Cycle in a Vineyard
6.4
Precipitation
Rainfall, infiltration, runoff, and drainage are all part of the cycling of water among
the atmosphere, soil, plants, and water bodies—the hydrologic cycle . The full range of
processes in the hydrologic cycle for a vineyard in leaf is illustrated in figure 6.6.
Because rain may turn to hail or snow, the input from the atmosphere is of-
ten referred to as precipitation P . Much of the rain that falls is intercepted by the
vine canopy, which may cover up to 80% of the soil surface, depending on the
trellis system and vine vigor. Rain that falls in the inter-row area may be inter-
cepted by a cover crop or fall directly on the soil. Particularly near the sea and in
river valleys, the canopy can also be wetted by mist and fog—this is called inter-
ception . The canopy storage capacity amounts to 1-2 mm of water, depending on
the foliage density and architecture of the canopy. Any excess water drips from
the canopy or runs down the trunks to the soil. Water retained by the canopy is
lost by evaporation and is referred to as interception loss ; this can be a significant
fraction of the total rainfall (up to one-quarter) if frequent light showers fall dur-
ing the summer. Canopy drip, stem flow, and direct rainfall on the soil together
comprise the net precipitation or net rainfall .
6.4.1
Evaporation
Of the rain falling on the soil, some infiltrates and some may run off, as discussed
in section 6.3.1. Infiltrated water replenishes the soil water. The main loss of wa-
ter from soil is by evaporation , which refers to loss of water in the vapor phase.
Evaporation can take place from the soil surface or from leaves.
Evaporation from the soil surface . As water evaporates, a gradient in
6.4.2
m is es-
tablished between the surface and the wetter soil below, which draws water up-
 
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