4.1 Drainage

The moisture regime of the natural soil is thought to be the most important attribute affecting vineyard establishment and maintenance (Roach and Arnold 1971). Drainage is influenced by many factors including texture, depth to imprevious layer, slope and landscape position. Additional inferences can be made about soil permeability, runoff and seasonably high water tables. The general characteristics of the subsoil should be considered when a site is selected (Jordan et al. 1980).   Well drained soils have bright, uniformly  yellowish brown or brown sub-soil ( Moderately drained soils have some mottling (spots of yellow, gray and orange)  in the subsoil below 15 to 20 inches.  Somewhat poorly drained soils have pale-colored subsoils that are highly mottled below 6 to 15 inches.    Poorly drained soils have gray subsois have grey subsoils which may be mottled.  Tiling may improve soil drainage however the better the soil drainage the better the site is for grape production.   In the STASGO database there are 4 basic hydrologic classes A to D. In the map below the percentage of soils in class A (high infiltration rates) and class B (moderate infiltration rates) are displayed.

Figure. Percentage of soil belonging to hydrologic groups A (well drained) or B (moderately well drained). Data presented at 1km2 resolution.

Hydrologic 

Soil Group

 Criteria for placement in hydrologic soil group
A Saturated hydraulic conductivity is very high or in the upper half of high and the internal free water occurance is very deep.
B  Saturated hydraulic conductivity is in the lower half of high or in the upper half of moderately high and the internal free water occurance is deep or very deep
C Saturated hydraulic conductivity is in the lower half of moderately high or in the upper half of moderately low and the internal free water occurance is deeper than shallow.
D Saturated hydraulic conductivity is below the upper half of moderately low and the internal free water occurance is shallow or very shallow and transitory through permanent.
Methodology used in construction of soil maps

Available water holding capacity

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