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An Explicit Representation of River Networks in a Continental-Scale Catchment-based Land Surface Model

Abstract

Surface water bodies (rivers, lakes, reservoirs, wetlands, floodplains, and other inland waters) are key components of the terrestrial hydrological cycle with an important role in regulating climate both regionally and globally. As population and the associated demand for water continue to grow, the need for integrated continental- and global-scale surface water dynamics simulation systems is becoming more apparent. Without a comprehensive understanding of both the seasonal and long-term variability and spatial distribution of surface water, decision makers will be challenged to manage water resources or to adapt to the changing global waterscape sustainably.

In this study, we present a continental-scale implementation of the Catchment-based Hydrological And Routing Modeling System (CHARMS) that includes an explicit representation of river networks to estimate river discharge, river depth and the corresponding inundation extent. The river networks and contributing catchment boundaries of the contiguous U. S. are upscaled from the National Hydrography Dataset Plus (NHDPlus) dataset. The average upscaled catchment size is 2773 km2, and provides the template for Community Land Model (CLM) version 3.5 implementation. Runoff generated by the land surface model within each catchment enters a unique main river channel, each consisting of several river reaches of average length 1.6 km. Nineteen sets of empirical relationships between channel dimension (bankfull depth and width) and drainage area are derived, based on USGS gauge observations, to describe river geometry for the 19 water resource regions of the NHDPlus representation of the contiguous United States. These channel dimensions are used to separate main river channel and floodplain. Modeled daily streamflow values show reasonable agreement with gauge observations and demonstrate that basins with fewer anthropogenic modifications are more accurately simulated. Modeled daily river depth and floodplain extent associated with each river reach are also explicitly estimated over the contiguous U. S., although such simulations are more challenging to validate.

Following the introduction and objectives described in Chapter 1, Chapter 2 next provides a literature review on the runoff generation by land surface model, flow routing algorithms, the current state of both grid and catchment-based river routing models, and empirical relationships between discharge and river geometry (river depth and width) with both in situ and remote sensing observations. Chapter 3 presents the continental-scale, explicit representation of river dynamics within a catchment-based land surface modeling framework. Chapter 4 displays a further exploration of hydraulic geometry relationships that are needed by large-scale river routing models. Summary and proposed future works are presented in Chapter 5.

This study has implications for capturing the seasonal-to-interannual dynamics of surface water in climate models. Such a continental-scale modeling framework development would, by design, facilitate the use of existing in situ observations and be suitable for integrating the upcoming NASA Surface Water and Ocean Topography (SWOT) mission measurements for a range of studies in climate, hydrology and water management. It will also allow for simulation of sediment and nutrient transport and trace gas exchange along rivers. Ultimately, such an assimilation-ready continental-scale model template will enable significant improvement in predictive understanding of surface water dynamics.

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