Groundwater Storage vs. Surface Water Storage – Why Sustainability Requires a Different Management Framework

Abstract:

Storing water in times of excess for use in times of shortage is an essential water-management tool, especially in climates typified by precipitation in one season and demand in another. The three primary water storage mechanisms in the Western US, and much of the world in fact, are: seasonal snow pack, surface water reservoirs, and groundwater aquifers. In California, nearly every major river has one or more large dam and reservoir and current focus has shifted toward off-stream storage. In addition to California’s surface reservoirs, groundwater aquifers provide huge volumes of water storage that are heavily utilized during times of drought. With California’s new Sustainable Groundwater Management Act (SGMA) substantial attention is presently focused on developing strategies for using groundwater storage more effectively in conjunction with surface-storage reservoirs. However, compared to surface water storage, we need to think differently and develop new frameworks if we want to manage groundwater storage sustainably. Despite its immense capacity, groundwater storage is harder to manage because there are physical constraints to how fast water can be put into and withdrawn from aquifers, its boundaries are not as well defined as those of a surface reservoir, and it is part of a dynamic, porous media flow system where the Theis concepts of capture govern. Therefore, groundwater does not behave as a level pool like surface water reservoirs, which has several implications for effective management: 1) extraction/injection locations can have substantial impacts on the system, 2) interactions with the surface water systems can be nonlinear and complex and 3) hydraulic effects can continue long after pumping/injection has stopped. These nonlinear spatial and temporal responses, coupled with long time scales, makes management of groundwater storage much different than surface water storage. Furthermore, failure to fully understand these issues can lead to mismanagement of groundwater storage with long-term implications on surface water/groundwater interactions. This work uses numerical models of groundwater flow to quantify capture from pumping and demonstrate issues of sustainable groundwater storage while using Lake Tahoe for a comparative analogy.