Eco-hydrological feedbacks between surface sealing and woody vegetation in dry climates

Physical sealing of the soil is a widespread natural process in the bare soil patches between shrubs, occurring frequently in dry environments. The seal layer has significantly lower hydraulic conductivity than the underlying undisturbed soil and thus, it affects significantly hydrological fluxes. Recent studies highlighted the significant role of the seal layer in controlling woody vegetation water availability through the allocation of overland flow. The seal layer however, induces another feedback by suppressing evaporation fluxes from the soil. This feedback is usually disregarded but it could have a broad impact on vegetation root water uptake (RWU). Up to date, no attempt has been made to quantify how the modulation of soil water content by the presence of a seal layer affects woody vegetation RWU. We approach this research gap using a model-based analysis of long term local climatic records (36y). The study site is a hillslope (0.075 KM2), located at the LTER Lehavim site in the Negev desert at the south of Israel (31020' N, 34045' E). Annual rainfall is 290 mm and the soils are brown lithosols and arid brown loess, prone to surface sealing. The vegetation is characterized by the dwarf shrub ( Sarcopoterium spinosum). The Feddes RWU parameters of the shrubs were acquired using a high resolution lysimeter experiment, enabling the simulation of vegetation RWU using The Hydrus2D model accounting explicitly for topographic and soil hydraulic parameters. Hydraulic properties of the seal layer at the soil surface were modeled following Mualem and Assouline (1989). The results indicate that the seal layer has a strong positive effect on vegetation RWU, with higher RWU in shrubs surrounded a sealed soil layer. This effect was found to be augmented during dry rainfall seasons, and is tightly coupled with within-season climatic variability (e.g. the timing and intensity of rainfall events). Assuming dry land vegetation go into a period of 'stress' when the soil potential drops below 4 MPa, the seal layer was found to be an efficient mechanism which reduces the duration in which vegetation is in stress. This study highlights the important role of surface sealing in the eco-hydrological feedbacks of dry environments.