Trees as Sensors: Signatures of Belowground Moisture through Aboveground Water Use

Abstract:

The unsaturated weathered bedrock on a hillslope serves as a significant modulator of streamflow, water chemistry, and plant-accessible moisture availability. However, the amount of moisture stored within the unsaturated zone fluctuates in response to seasonal rainfall and individual recharge events, and is difficult to directly measure via conventional methods. Here we propose a model inversion approach that uses plant transpiration as a constraint on deep water availability. We couple a simple plant hydraulics model with a belowground model that captures the moisture dynamics within the soil, the unsaturated, and the saturated zones (see figure), including fast preferential flow within the fractured bedrock and slow saturation and release within the saprolite matrix. This model is then calibrated with meteorological, isotopic, hydrological (in wells and a stream outlet), and plant physiological and sapflow data from the Eel River CZO. We adopt two representative species with contrasting plant water strategies —the Douglas fir and the Pacific madrone—to illustrate the differential contribution of this deep water source for the persistence of each species. Different sensitivities of transpiration to increasing vapor pressure deficit and declining water potentials notwithstanding, reliance on deep moisture allows Douglas fir to transpire maximally during the spring and the Pacific madrone to continue transpiring during the dry summer season. Temporal variations in moisture levels are buffered by root redistribution of water between the upper soil layer and the unsaturated zone. Our approach presents a promising way of inferring the timing and amount of plant-available water in deep reservoirs via more readily available knowledge of aboveground processes and complementing other geophysical methods for inferring subsurface structure and properties.