True water constraint under a rainfall interception experiment in a Mediterranean shrubland (Northern Tunisia): confronting discrete measurements with a plant-soil water budget model

Increased drought length and intensity is expected in the Mediterranean basin under anthropogenic increase in atmospheric CO2, leading to extreme events not yet encountered in the present climate variability. Understanding ecosystems responses and capturing peculiar ecophysiological processes related to these events have been investigated in the field by rainfall manipulation experiments. Quantifying the actual drought faced by the ecosystem under control and dry plots, or among experiments remain a key challenge for explaining functional impacts on plant growth. Full-profile soil water content can be tricky to assess in rocky soils, and time-consuming plant water potential measurements remain a discrete information unable to capture short rainfall pulses. We propose here to fully investigate the water budget of a total rainfall interception manipulation on a Mediterranean shrubland, coupled with a plant-soil water balance model. We could accurately simulate the seasonal course of plant water status, including small rainfall pulses. We then derived yearly estimates of water stress integral for each water treatment, leading to an estimate of 66-86 % increase of drought intensity for the dry treatment compared to the control. Comparing actual and expected plant water budget from simulations in the dry plots allowed to identify and quantify the impact of methodological issues related to rainfall interception experiments as side effects for intrusive rain drops and subsurface lateral water flow.