Technical Note: Reducing the spin-up time of integrated surface water–groundwater models

H Ajami,S Stisen,J P Evans,M F Mccabe

doi:10.5194/hess-18-5169-2014

Abstract

Abstract. One of the main challenges in the application of coupled or integrated hydrologic models is specifying a catchment's initial conditions in terms of soil moisture and depth-to-water table (DTWT) distributions. One approach to reducing uncertainty in model initialization is to run the model recursively using either a single year or multiple years of forcing data until the system equilibrates with respect to state and diagnostic variables. However, such "spin-up" approaches often require many years of simulations, making them computationally intensive. In this study, a new hybrid approach was developed to reduce the computational burden of the spin-up procedure by using a combination of model simulations and an empirical DTWT function. The methodology is examined across two distinct catchments located in a temperate region of Denmark and a semi-arid region of Australia. Our results illustrate that the hybrid approach reduced the spin-up period required for an integrated groundwater–surface water–land surface model (ParFlow.CLM) by up to 50%. To generalize results to different climate and catchment conditions, we outline a methodology that is applicable to other coupled or integrated modeling frameworks when initialization from an equilibrium state is required.

Highlights

The issue of model initialization is important for hydrologic simulation and prediction, as the initial state has a major impact on a catchment’s modeled response (Berthet et al, 2009)
The challenge lies in designing methodologies to reduce the spin-up period in computationally intensive integrated hydrologic models such as ParFlow.CLM (Ashby and Falgout, 1996; Jones and Woodward, 2001; Kollet and Maxwell, 2006) when initialization from equilibrium states is required for transient simulations
We considered two approaches to defining pressure head distribution above the water table: (1) implementing the commonly used hydrostatic equilibrium assumption, where the pressure head at the water table was linearly decreased as a function of elevation head towards the land surface; and (2) adjusting the pressure head distribution of the unsaturated zone from the last day of the sixth cycle of ParFlow.CLM spin-up simulations based on new depth-to-water table (DTWT) values from the DTWT function

Summary

Introduction

The issue of model initialization is important for hydrologic simulation and prediction, as the initial state has a major impact on a catchment’s modeled response (Berthet et al, 2009). In coupled or integrated surface– subsurface models, uncertainty in a catchment antecedent condition is of particular importance, as both the soil moisture distribution and depth-to-water table (DTWT) need to be specified at the start of a simulation (Ivanov et al, 2004; Noto et al, 2008). Sivapalan et al (1987) used a topography– soil index to map the spatial distribution of initial DTWT. Since information on the spatial pattern of water table and soil moisture distributions is generally unavailable, various approaches have been developed to determine the initial DTWT variation. In another approach, Troch et al (1993) used recession flow analysis to estimate the effective water table height of a catchment. Regardless of the choice of initial DTWT, the uncertainty involved is such that a period of spin-up is always required (Cloke et al, 2003), as the applied atmospheric forcing is often inconsistent with the hydrodynamic initialization of the catchment inferred from limited observations (Ajami et al, 2014)

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Results

Conclusion