Abstract
Current regional and global climate models generally do not represent groundwater flow between grid cells as a component of the water budget. We estimate the magnitude of between-cell groundwater flow as a function of grid cell size by aggregating results from a numerical model of equilibrium groundwater flow run and validated globally. We find that over a broad range of cell sizes spanning that of state-of-the-art regional and global climate models, mean between-cell groundwater flow magnitudes scale with the reciprocal of grid cell length. We also derive this scaling a priori from a simple statistical model of a flow network. We offer operational definitions of ‘significant’ groundwater flow contributions to the grid cell water budget in both relative and absolute terms (between-cell flow magnitude exceeding 10% of local recharge or 10 mm y−1, respectively). Groundwater flow is a significant part of the water budget, as measured by a combined test requiring both relative and absolute significance, over 42% of the land area at 0.1° grid cell size (typical of regional and mesoscale models), decreasing to 1.5% at 1° (typical of global models). Based on these findings, we suggest that between-cell groundwater flow should be represented in regional and mesoscale climate models to ensure realistic water budgets, but will have small effects on water exchanges in current global models. As well, parameterization of subgrid moisture heterogeneity should include the effects of within-cell groundwater flow.
Highlights
Groundwater comprises Earth’s main store of liquid freshwater
The significance of lateral groundwater flow depends on climate, topography and geology—e.g., more recharge, steeper terrain, and thicker aquifers favor greater lateral groundwater convergence [21]— here we focus on the effect of spatial scale, in the hope of deriving a simple scaling relationship that can directly inform the development of the land component of climate and Earth system models
We find that the global mean lateral flow at the model grid resolution and its scaling with aggregation is reasonably well described by a simple statistical framework for groundwater flow
Summary
Groundwater comprises Earth’s main store of liquid freshwater. Groundwater has emerged as a critical source of water for irrigation and other uses. Groundwater quantities and flows are usually difficult to observe [1]. Progress has recently been made in quantifying steady-state global groundwater flow patterns [2] and human withdrawals [3,4,5]. Local and regional studies have emphasized the role of groundwater in sustaining ecosystems ranging from the Amazon rainforest [6, 7] to desert oases [8, 9], a role vulner-.
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