Authors: Aniket Gupta, Ali Behrangi – University of Arizona; Ahmad Tavakoly – US Army Engineer Research and Development Center, Vicksburg, MS, USA; Guo-Yue Niu – University of Arizona
Title: Challenges in Simulating the Zero Flows: An Example from the Sacramento and San Joaquin Watershed
Abstract: The coupled surface-subsurface hydrological processes are important for terrestrial water and ecosystem responses to the warming and drying climates, especially the dry fluvial system in the US southwest. The surface-subsurface processes, however, are not well-represented in current Earth System Models (ESMs), e.g., the Unified Forecasting System (UFS) for hydroclimatic predictions and macro-scale hydrological models, e.g., the National Water Model (NWM) for water resource projections and streamflow predictions. One of the major drawbacks includes simulating the zero flows in the ephemeral rivers in the US southwest. In this study, we focus on assessing the capability of the current NWM and Noah-MP to predicting the zero flows in the two selected watersheds in California: Sacramento and San Joaquin watersheds. The Sacramento watershed covers an area of approximately 7,500 square miles, while the San Joaquin watershed covers an area of 15,600 square miles. We have run NWM and Noah-MP in the two watersheds to understand the physical processes responsible for the zero flows. Noah-MP represents surface hydroclimatic exchanges and vertical subsurface flows, while NWM predicts the lateral subsurface flows and overland flows in a fully coupled setup. We compared the zero flow events resulting from the model to those in the observed discharge datasets for the zero flow cases that are classified based on meteorological conditions. The results show that both models fail to capture the zero flow events. The major problem in the model lies in their representations of infiltration, subsurface flow, subgrid lateral hillslope flow as well as uncertainties in the precipitation forcing. Our goal is to fix these identified issues and improve by implementing a dual-permeability flow model and hillslope hydrology (currently being developed in our group) to represent preferential flow into Noah-MP (coupled with UFS) and NWM to improve hydroclimatic predictions.