Multicriteria analysis on rock moisture and streamflow in a rainfall‐runoff model improves accuracy of model results

Abstract

Although shallow (<1.5 m) soil water storage has been extensively studied, the significance of deeper unsaturated zone water storage to flow generation is poorly documented. However, a limited but growing body of empirical work shows that the weathered bedrock vadose zone, not soil, stores the majority of plant available water in many seasonally dry and semi‐arid landscapes. Moreover, this storage dynamic mediates recharge to hillslope groundwater systems that generate stream discharge and support ecologically significant baseflows. Explicit representations of bedrock vadose zone processes are rarely incorporated into runoff models, due in part to a paucity of observations that can constrain simulations. Here, we develop a simple representation of the weathered bedrock vadose zone that is guided by in situ field observations. We incorporate this representation into a rainfall‐runoff model, and calibrate it on streamflow alone, on rock moisture (i.e., weathered bedrock vadose zone moisture) alone, and on both using the concept of Pareto optimality. We find that the model is capable of accurately simultaneously simulating dynamics in rock moisture and streamflow, in terms of Kling‐Gupta Efficiency, when using Pareto optimal parameter sets. Calibration on streamflow alone, however, is insufficient to accurately simulate rock moisture dynamics. We further find that the posterior distributions of some model parameters are sensitive to choice of calibration scenario. The posterior distribution of high‐performing model parameters resulting from the streamflow only calibration scenario include physically unrealistic values that are not yielded by the rock moisture only or Pareto calibration strategies. These results suggest that the accuracy of some model results can be increased and parameter uncertainty decreased via incorporation of rock moisture data in calibration, without sacrificing streamflow simulation quality. Emerging recognition of the global significance of weathered bedrock water storage in seasonally dry and semi‐arid regions motivates more observations of weathered bedrock moisture and integration of this variable into earth system models.

Streamflow Prediction at the Intersection of Physics and Machine Learning: A Case Study of Two Mediterranean‐Climate Watersheds

“…Alternatively, or in addition, hybrid models may perform better than PB models out‐of‐sample when they effectively perform bias‐correcting for an unclosed water balance (either due to imperfect data or inter‐basin water transfers), which often pose a challenge for PB models (Beven, 2020; Liu et al., 2020). For instance, all three of our PB models assume that exchange of watershed water with rock moisture or deep groundwater is negligible, despite the study watersheds being underlain by weathered bedrock; yet, weathered bedrock was shown in another northern California watershed to contain enough rock moisture to regulate plant‐available water and streamflow (La Follette et al., 2022; Rempe & Dietrich, 2018). Meanwhile, better performance of hybrid models compared to data‐driven models may indicate that PB models contribute useful information about hydrologic processes beyond the learning capabilities of the LSTM.…”

Towards more credible models in catchment hydrology to enhance hydrological process understanding: Preface

“…La Folette et al (2022) study streamflow simulation in the 16.9 km 2 Elder Creek catchment in Northern California, where the geology is characterized by fractured bedrock overlain by a, typically thin (0.5 m), soil layer. This is the first study where unsaturated weathered bedrock water storage is explicitly incorporated in a catchment model and used as a calibration target.…”

Towards more credible models in catchment hydrology to enhance hydrological process understanding: Preface

“…La Folette et al (2022) study streamflow simulation in the 16.9 km 2 Elder Creek catchment in Northern California, where the geology is characterized by fractured bedrock overlain by a, typically thin (0.5 m), soil layer. This is the first study where unsaturated weathered bedrock water storage is explicitly incorporated in a catchment model and used as a calibration target.…”