Soil–Water–Vine Relationships and Water Management - Soils for Fine Wines - page 155

Precipitation P

Canopy

interception

Transpiration

and evaporation

Stem flow

Soil evaporation

Cover crop
interception and evaporation

Direct

rainfall

Surface
runoff

Root uptake

Infiltration and
soil storage

Deep drainage

The hydrologic cycle in a vineyard. Figure 6.6

ward. However, as the top 1-2 cm of soil dries, the hydraulic conductivity falls
to a low value so that further movement of liquid water to the surface is very slow.
The soil surface becomes air-dry . The water content of an air-dry sandy soil might
be only 2-4%. This natural process of surface drying conserves water deeper in
the soil profile and is called a self-mulching effect . The effect is enhanced by shal-
low cultivation of the soil to break the continuity of pores that can conduct wa-
ter to the surface.
Evaporation from leaves or transpiration . Vines in full leaf provide a much
larger surface than the soil for evaporation of water. Whether in the plant or soil,
the energy required to convert liquid water to water vapor is provided from the
atmosphere. Evaporation takes place from the wet cell walls inside a leaf, with wa-
ter vapor escaping through the stomata. As a result of this evaporation, the water
potential in the leaves falls as low as 1.0 to 1.5 MPa during the day. A gra-
dient in

is created between the leaves and the roots, which causes water to be
sucked into the roots. Water flows into the water-depleted zones around absorb-
ing roots to maintain the transpiration stream in the vine. As long as energy is
supplied to the canopy and a significant fraction of the roots is in moist soil, the
vine transpires at approximately the potential rate of evaporation E p . However, when
the water supply becomes limiting—through depletion of soil reserves or simply