201-6 Modeling Land Subsidence: So Many Choices, So Little Data!

Abstract:

Land subsidence due to groundwater withdrawals poses a major risk to infrastructure and aquifer storage capacity in many of the world’s unconsolidated aquifer systems. In most aquifer systems that experience significant consolidation, the surface subsidence is caused by a combination of rapid consolidation of coarse-grained layers or thin fine-grained layers, and delayed, long-term consolidation of thick clay lenses that can take decades to equilibrate to the water level, or head, of the primary aquifer that is measured in monitoring wells. In order to accurately characterize land subsidence, many models have been developed to simulate head and consolidation within clays. One great challenge of developing these models lies in choosing a conceptual model and model forcings that capture the key information needed to simulate subsidence as accurately as possible. Two critical pieces of information, that are often not well known, are the number and thickness of clay layers in the subsurface, and the head in the aquifer, which is treated as a boundary condition to simulate the heads within clay layers. For example, many regions with high subsidence are limited to one measurement of head per year, but confined aquifers in heavily pumped regions often experience 30 m or more of seasonal head change. Pulling from recent modeling work in the western US, this presentation will focus first on the impact of modeling choices relating to clay layers and head in the aquifer. Next, the presentation will explore methods to improve estimates of model forcings, with case studies in Utah and California.

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