Toward a new parameterization of hydraulic conductivity in climate models: Simulation of rapid groundwater fluctuations in Northern California

Abstract

Preferential flow through weathered bedrock leads to rapid rise of the water table after the first rainstorms and significant water storage (also known as ''rock moisture'') in the fractures. We present a new parameterization of hydraulic conductivity that captures the preferential flow and is easy to implement in global climate models. To mimic the naturally varying heterogeneity with depth in the subsurface, the model represents the hydraulic conductivity as a product of the effective saturation and a background hydraulic conductivity K bkg , drawn from a lognormal distribution. The mean of the background K bkg decreases monotonically with depth, while its variance reduces with the effective saturation. Model parameters are derived by assimilating into Richards' equation 6 years of 30 min observations of precipitation (mm) and water table depths (m), from seven wells along a steep hillslope in the Eel River watershed in Northern California. The results show that the observed rapid penetration of precipitation and the fast rise of the water table from the well locations, after the first winter rains, are well captured with the new stochastic approach in contrast to the standard van Genuchten model of hydraulic conductivity, which requires significantly higher levels of saturated soils to produce the same results. ''Rock moisture,'' the moisture between the soil mantle and the water table, comprises 30% of the moisture because of the great depth of the weathered bedrock layer and could be a potential source of moisture to sustain trees through extended dry periods. Furthermore, storage of moisture in the soil mantle is smaller, implying less surface runoff and less evaporation, with the proposed new model.

Sensitivity of transpiration to subsurface properties: Exploration with a 1‐D model

“…Following the work of Vrettas and Fung [], this article considers a 1‐D (vertical) column comprising a thin (0.5 m deep) soil mantle on top of a saprolite (1.5 m deep) and a weathered bedrock layer, overlying the continuously saturated bedrock with minimal permeability located at a depth of 20 m.…”

“…This work advances a model for exploring the sensitivity of transpiration to subsurface water storage, rooting depth, and root functioning. It builds on Vrettas and Fung [], which presents a new stochastic parameterization for hydraulic conductivity that captures the rapid fluctuations of the water table at seven wells over a 6 years period at “Rivendell.” This rapid rise and slow decline of the water table is not unique to the specific research site, but is observed in multiple wells across the US, and cannot be captured by the existing parameterization in, for example, the Community Land Model CLM [ Oleson et al ., ]. In a series of sensitivity experiments, we specify ET demands by different tree species by their annual magnitude and seasonality, and we ask what subsurface properties are needed to meet the ET demands.…”

“…The organization of the rest of the paper is as follows: Section 2 describes the 1‐D model that is used to simulate the water flow in the vadose zone. A more detailed presentation of this model is given in Vrettas and Fung [], since here we extend this approach by including other processes such as evapotranspiration and hydraulic redistribution, which are described in section 3. The setup of the experimental part is given in section 4, with section 5 providing our findings. The paper concludes with a summary of this work, including a discussion, in section 6.…”

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

“…Researchers have likewise developed a energy-balance snowmelt model that is now being used with remotely sensed data for water supply forecasting (Painter et al, 2016). In other integrative efforts, researchers are modeling how hydraulic conductivity, root water uptake efficiency, and hydraulic redistribution by plants sustain evapotranspiration through dry seasons (Quijano et al, 2012(Quijano et al, , 2013Vrettas and Fung, 2015). Work at the Luquillo CZO has supported interpretations of the controls on bed load grain size and channel dimensions for rivers (Phillips and Jerolmack, 2016).…”

“…At first, much of the synthesis crossed only two disciplines at a time: for example, several papers emphasized how geomorphological concepts related to erosion must be incorporated to understand chemical weathering, and vice versa (Rempe and Dietrich, 2014;Riebe et al, 2016). Likewise, researchers have related tree roots to water cycling (Vrettas and Fung, 2015). Now, researchers are targeting multidisciplinary aspects of CZ entities.…”