Some wineyard characteristics:

Around 98% of the « Côte de Bourg » appelation are red wines. They are commonly made of Merlot, Cabernet Franc, Cabernet Sauvignon and Malbec cépages. The spatial (vertical) relation between the substratum, the soil composition and the soil thickness is crucial to understand in order to assess the hydrologic response to grapevine roots in such wineyards.

Sedimentology of the bedrock:

The bedrock is made of calcarenites and/or calcirudites (limestones, CaCO₃).  The sediments have been deposited around 30 millions years ago (Stampian) on a carbonate platform under the sporadic influence of oceanic currents, probably nearby major river mouths. These rivers had also carbonate-rich waters. Polypary corals, melobesia and shells are the common fossil markers in these palaeo-reef facies.  Some cross-bedding stratifications can locally be recognized in such rocks.

How could we use this trench interpretation?

Soil/bedrock structure:

In the near-surface, the available space (soil) for the grapevine roots appears restricted to 20/50 cm, before to reach the limestone interface. Underneath the soil, the 1 meter-thick upper part of the limestone is highly fractured  and could correspond to the upper part of a karstic* system (epikarst) that has partly been remobilized by the grapewine anthropic establishment. The hanging saturated zone, corresponding to the bottom of the epikarst where the limestone seems almost intact (bottom of the sketch), can sometimes be water-saturated, in particular from October to June.

*Karst are ruiniform relief due to dissolution of soluble rocks (Radulovic, 2013), in particular by meteoric waters in this study case. The term Karst or carst has been firstly used in the work of Cvijic, (1893).

Such soil/bedrock structure probably implies a non-linear hydraulic response to the roots, in particular during summer times. 

Feel free to constructively comment this open discussion.

Scientific references:

Cvijić, J. (1893). DAS Karstphänomen. Versuch einer morphologischen Monographie-Geographische Abhandlungen. Bd. V. Heft. Wien, 3, 1-114.

Doerfliger, N., Jeannin, P. Y., & Zwahlen, F. (1999). Water vulnerability assessment in karst environments: a new method of defining protection areas using a multi-attribute approach and GIS tools (EPIK method). Environmental geology, 39(2), 165-176.

Radulović, M. M. (2013). A new view on karst genesis. Carbonates and evaporites, 28(4), 383-397.