River basin salinization as a form of aridity
Significance Combining observations and model results, we show that soil salinization does affect the long term soil water balance of saline river basins, eventually acting as an aridity enhancer. The strength of this control depends on the capability of vegetation to cope with salt stress through specific functional traits i.e. salt tolerance and is key to the understanding of vegetation water relations in arid and transitional watersheds. Our approach can also be applied to infer crucial data about the salinization level and vegetation salt tolerance of river basins. Such information is essential to develop reverse engineering measures to mitigate catchment scale aridity and salinization, based on salt tolerance plant traits. Soil salinization affects, to a different extent, more than one third of terrestrial river basins (estimate based on the Food and Agriculture Organization Harmonized World Soil Database, 2012) Among these, many are endorheic and ephemeral systems already encompassing different degrees of aridity, land degradation, and vulnerability to climate change. The primary effect of salinization is to limit plant water uptake and evapotranspiration, thereby reducing available soil moisture and impairing soil fertility. In this, salinization resembles aridity and similarly to aridity may impose significant controls on hydrological partitioning and the strength of land vegetation atmosphere interactions at the catchment scale. However, the long term impacts of salinization on the terrestrial water balance are still largely unquantified. Here, we introduce a modified Budyko's framework explicitly accounting for catchment scale salinization and species specific plant salt tolerance. The proposed framework is used to interpret the water budget data of 237 Australian catchments 29% of which are already severely salt affected from the Australian Water Availability Project (AWAP) Our results provide theoretical and experimental evidence that salinization does influence the hydrological partitioning of salt affected watersheds, imposing significant constraints on water availability and enhancing aridity. The same approach can be applied to estimate salinization level and vegetation salt tolerance at the basin scale, which would be difficult to assess through classical observational techniques. We also demonstrate that plant salt tolerance has a preeminent role in regulating the feedback of vegetation on the soil water budget of salt affected basins.
Proceedings of the National Academy of Sciences