Subsurface water dominates Sierra Nevada seasonal hydrologic storage

TitleSubsurface water dominates Sierra Nevada seasonal hydrologic storage
Publication TypeJournal Article
Year of Publication2019
AuthorsEnzminger T.L, Small E.E, Borsa A.A
Date Published2019/11
Type of ArticleArticle; Early Access
ISBN Number0094-8276
Accession NumberWOS:000494061300001
Keywordscalifornia; displacements; elevation; equivalent; Geology; GPS; precipitation; runoff; snow; temperature; western united-states

Vertical displacements (dz) in permanent Global Positioning System (GPS) station positions enable estimation of water storage changes (Delta S), which historically have been impossible to measure directly. We use dz from 924 GPS stations in the western United States to estimate daily Delta S in California's Sierra Nevada and compare it to seasonal snow accumulation and melt over water years 2008-2017. Seasonal variations in GPS-based Delta S are 1,000 mm. Typically, only 30% of Delta S is attributable to snow water equivalent (SWE). Delta S lags the snow cycle, peaking after maximum SWE and remaining positive when all snow has melted (SWE = 0). We conclude that seasonal Delta S fluctuations are not primarily driven by SWE but by rainfall and snowmelt stored in the shallow subsurface (as soil moisture and/or groundwater) and released predominantly through evapotranspiration. Seasonal peak GPS Delta S is larger than accumulated precipitation from the Parameter-elevation Relationships on Independent Slopes Model and North American Land Data Assimilation System, indicating that these standard inputs underestimate mountain precipitation. Plain Language Summary When a large amount of water is added to an area of the Earth's surface-such as through a storm in the mountains-its weight compresses the solid Earth, causing the land surface to move downward. When the water evaporates or runs off, the land surface rebounds instantly. In the western United States, a network of permanent Global Positioning System (GPS) stations measures these vertical motions of the land surface at high precision. We use GPS data to estimate the amount and distribution of changes in water mass in the Sierra Nevada of California. Water stored underground (as soil moisture and groundwater) is very difficult to measure compared to snow, which can be estimated reasonably accurately with satellite remote sensing. Using GPS data, we calculate the amount of underground storage: the total mass minus independent estimates of the mass of snow. We find that underground storage makes up the majority (70%) of total stored water. Furthermore, the amount of water estimated by GPS is larger than independent estimates of total precipitation (rain + snow). This suggests that more precipitation falls in the Sierra Nevada than has previously been thought, and this precipitation is mainly stored underground.

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