Decoupling factor, aerodynamic and canopy conductances of a hedgerow olive orchard under Mediterranean climate

Abstract

The degree of coupling between canopy and atmosphere, through the decoupling factor Ω, well describes the behaviour of a crop concerning its water use and carbon dioxide exchange. Super high-density hedgerow olive orchard system is in great expansion all over the world and, since it has a complex field structure in rows of adjacent trees, investigations are necessary to assess the Ω patterns, as well as aerodynamic (g_a) and canopy (g_c) conductances in different water conditions. In this study, in a hedgerow olive orchard (cv. “Arbosana”) submitted to full (FI) and regulated deficit irrigation (RDI), cropped under a Mediterranean semi-arid climate (southern Italy), Ω has been determined using g_c, as deduced by inverting the Penman-Monteith equation, and g_a, by upscaling the wind speed measured in a close station to the canopy; the transpiration has been measured by sap flow thermal dissipation method. The results showed that this olive orchard results very well coupled to the atmosphere, in any soil water conditions; Ω is generally very low, being during daytime equal in mean to 0.021±0.003 ms^-1 and 0.018±0.004 ms^-1 for RDI and FI, respectively. This condition is linked to g_a and g_c values; in fact, canopy conductance is much smaller than the aerodynamic one in any water and climatic conditions, except when all canopy surfaces are saturated in water. In this latter case, the g_c assumes the highest values due to the contribution of the part of conductance attributable to the structure of the orchard.

Appendix I

The potential crop evaporation E_p is defined (Katerji and Rana 2011, among many others) as the evaporation of a crop having all evaporative surfaces (leaves, stems, soil) saturated or covered in water. Thus (Eq. 1), no biological control is exerted for the water losses (the crop stomatal conductance G_s → ∞, as well as g_c):

$$\lambda {E}_p=\frac{\varepsilon A+\rho {c}_pD{g}_a/\gamma }{\varepsilon +1}$$

(I.1)

Yet, the crop itself shows a resistance to water vapour transfer, r₀, due to its structure (Perrier 1975a, 1975b; Rana et al. 1994; Daudet et al. 1999; Katerji and Rana 2011). Even if this variable could be thought as a theoretical concept, however it has practical impacts, and it is found in nature, although only during relatively short periods, when free water evaporates from leaves just after a rain, a strong dew or an accurate irrigation by aspersion. Under these conditions it is possible to determine the resistance r₀; in fact, in this case the canopy conductance to the diffusion of water vapour can be written as:

$$\frac{1}{g_{cd}}={r}_0+{r}_{min}$$

(I.2)

where r_min is the minimum canopy resistance of the “big leaf” to the maximum potential transpiration. Combining Eqs. (1), (I.1) and (I.2), the expression of the structure resistance become:

$${r}_0=\frac{\varepsilon +1}{g_a}\left(\frac{\lambda {E}_p}{\lambda T}-1\right)-{r}_{min}$$

(I.3)

here r_min is set equal to 67.9 s m^-1 (Villalobos et al. 2000).