Are existing large-scale simulations of water dynamics wrong?

Soils are complicated porous media that are highly relevant for the sustainable use of water resources. Not only the essential basis for agriculture, soils also act as a filter for clean drinking water, and, depending on soil properties, they dampen or intensify surface runoff and thus susceptibility to floods. Moreover, the interaction of soil water with the atmosphere and the related energy flux is an important part of modern weather and climate models.

An accurate modeling of soil water dynamics thus has been an important research challenge for decades, but the prediction of water movement, especially at large spatial scales, is complicated by the heterogeneity of soils and the sometimes complicated topography.

Simulation models are typically based on Richards' equation, a nonlinear partial differential equation, which can be solved using numerical solution methods. A prerequisite of most solution algorithms is the partitioning of the simulated region into discrete grid cells. For any fixed region, such as a soil profile, a hill slope, or an entire watershed, the grid resolution is usually limited by the available computer power. But how does this grid resolution affect the quality of the solution"

This problem was explored by Hans-Joerg Vogel from the UFZ - Helmholtz Center of Environmental Research in Leipzig, Germany and Olaf Ippisch from the Institute for Parallel and Distributed Systems of the University of Stuttgart, Germany. The results are published in the article "Estimation of a Critical Spatial Discretization Limit for Solving Richards' Equation at Large Scales," Vadose Zone J. Vol. 7, p. 112-114, in the February 2008 issue of Vadose Zone Journal.

Vogel and Ippisch found that the critical limit for the spatial resolution can be estimated based on more easily available soil properties: the soil water retention characteristic. Most importantly, this limit came out to be on the order of decimeters for loamy soils, and is even lower, on the order of millimeters, for sandy soils. This is much smaller than the resolution used in many practical applications.

This study implies that large-scale simulations of water dynamics in soil may be imprecise to completely wrong. But, it also opens new options for a specific refinement of simulation techniques using locally adaptive grids. The derived critical limit could serve as an indicator that shows where a refinement is necessary. These findings should be transferable to applications such as the simulation of oil reservoirs or models for soil remediation techniques.

Source: Soil Science Society of America

Related stories:

Old growth giants limited by water-pulling ability
The Douglas-fir, state tree of Oregon, towering king of old-growth forests and one of the tallest tree species on Earth, finally stops growing taller because it just can't pull water any higher, a new study concludes.
Search for salt tolerant grasses aims to improve roadside plantings
Standing in a greenhouse at the University of Rhode Island, Rebecca Brown was smiling even though it appeared that something had gone terribly wrong. Almost all of the 16 species of grass she planted last February in hundreds of small pots were dead.
New Analysis Shows Important Slowdown in Lake Tahoe Clarity Loss
For the first time since researchers began continuously measuring Lake Tahoe's famed water clarity 40 years ago, UC Davis scientists reported today that the historical rate of decline in the lake's clarity has slowed considerably in recent years.
Herbicide-tolerant crops can improve water quality
The residual herbicides commonly used in the production of corn and soybean are frequently detected in rivers, streams, and reservoirs at concentrations that exceed drinking water standards in areas where these crops are extensively grown.
Fueling ethanol production while protecting water quality
Grain-based ethanol production has increased dramatically in recent years as the cost and instability of oil has increased. New U.S. government policies require major increases in ethanol production. While future plans call for a viable cellulosic ethanol industry, expanded grain ethanol production will lead to further growth of corn acres in the near term, with unintended negative water quality impacts.
Smithsonian study: Sediment prediction tools off the mark
A recent study led by Smithsonian ecologist Kathy Boomer suggests it is time for a change in at least one area of watershed management. Boomer has been examining the tools scientists and managers use to predict how much sediment runs into the Chesapeake Bay, and by her account, they are way off the mark. The study, co-authored by SERC ecological modeler Donald Weller and ecologist Thomas Jordan, appears in the January/February issue of the Journal of Environmental Quality.
Researchers alarmed by levels of mercury and arsenic in Chinese freshwater ecosystem
A team of researchers, led by biologists at Dartmouth, has found potentially dangerous levels of mercury and arsenic in Lake Baiyangdian, the largest lake in the North China Plain and a source of both food and drinking water for the people who live around it.
Overgrazing accelerating soil erosion in northern Mexico
Every year in the world an estimated 20 million hectares of arable land are rendered infertile simply owing to water-induced erosion. It is therefore crucial to understand how these processes arise in order better to protect the layer of a few tens of metres of fertile soil essential for plant growth and therefore for sustaining agriculture. In the North of Mexico, about ten years ago IRD teams studied the erosion phenomenon which affects this region where pastoral activities and tree felling aggravate the process.

News discussion:

Space & Earth science news