Hydrology

Background

The ability to weigh mass change at Earth's surface with continuous GNSS observations of time-variable vertical motions provides a methodology to evaluate available water resources as input to water policy considerations.  The solid Earth responds elastically to changes in mass surface load, which allows us to infer changes in total water at Earth's surface as a function of location and time. The spatial resolution is only limited by the spacing of the GNSS stations.  A spacing of 10 km would be sufficient to determine mass change in individual watersheds and ice basins. See project's Algorithm Theory Basis Document ATBD for more information.

(Green curve) Vertical motion in elastic response to unloading of a disk with a radius of 14 km and a water thickness of 1 m. This disk has the same area as a pixel at 36°N for which we estimate water thickness for (1/4° latitude by 1/4° longitude). (Blue curve) Vertical motion in elastic response to unloading of a disk with a radius of 7 km and a water thickness of 4 m.  (Pink curve) Vertical motion that would be inferred by GRACE is approximated by a Gaussian distribution with a halfwidth of 200 km. “Gt” is gigatons (1012 kg).

(Above, left) Vertical land displacement observed with GNSS is inverted to infer (above, right) change in total water during harsh drought conditions from Oct 2011 to Oct 2015.  At left, the vertical displacement field is determined using  GNSS stations recording solid Earth's elastic response to water change; Stations in the Central Valley that subside in porous response to groundwater loss are excluded.  At right, groundwater loss in Central Valley is assumed to be to 34 km³ in the inversion; groundwater loss at 1/4o pixels (small gray letters) are set equal to –1.64 m (Z's), –1.23 m (Y's), –0.82 m (X's), and ­–0.41 m (A's).  Because snow accumulation in California is insignificant in October, we infer all water change from Oct to Oct to be in the ground.  During the four years of drought, the Sierra Nevada mountains lost an average of 0.66 m of water in the ground, for a total loss of 45 km³.  The Sierra Nevada, Klamath mountains, and Coast Ranges loss an average of 0.51 m of water in the ground, for a total loss of 97 km³, far exceeding the 18 km³ of water lost in a composite hydrology model.  From Argus et al. (2017).

Products

Changes in components of water storage are provided every six months in seven text files and corresponding grids in pdf format, with a latency of one month. The first two columns in each file are latitude and longitude. ReadMe file.

Data files and grids (temporary location): ftp://garner.ucsd.edu/pub/projects/eseses.2019/

Change in total water storage inferred from GPS – “water.gps”

column 3- equivalent water thickness (mm); column 4- uncertainty in equivalent water thickness (mm)

Change in equivalent water thickness – “atmosphere”

column 3- change in equivalent water thickness (mm)

Change in snow water equivalent (SWE) – “snow”

column 3- change in snow water equivalent (mm)

Snow depth and snow water equivalent (SWE) data are available from NOAA’s National Weather Service's National Operational Hydrologic Remote Sensing Center (NOHRSC) SNOw Data Assimilation System (SNODAS).

Change in soil moisture content (SMC) – “soil”

column 3- change in soil moisture (mm)

NLDAS_NOAH is a monthly climatology data set contains a series of land surface parameters, including Soil Moisture Content (SMC), simulated from the Noah land-surface model (LSM) for Phase 2 of the North American Land Data Assimilation System (NLDAS-2).

Change in artificial reservoir surface water – “reservoir”

column 3- change in artificial reservoir surface water (mm)

These hydrological data are from the California Data Exchange Center (CDEC) Weather Gauging Stations, including automatic snow reporting gages for the Cooperative Snow Surveys Program and precipitation and river stage sensors for flood forecasting.

Change in total mass – “mass”

Total mass = water.gps + reservoir + atmosphere

column 3- change in soil moisture (mm); column 4-

Change in water in the ground not in hydrology models – “ground”

Inferred to be water.gps - snow - soil moisture

column 3- change in water in the ground not in hydrology models (mm); column 4-