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How to make a GRACE-derived Drought Indicator Data Rods query

Hydrology variables generated by gravity mapping mission

How to make a GRACE-derived Drought Indicator Data Rods query

Precise gravity measurements made by the GRACE mission can be used for hydrological research.

How to make a GRACE-derived Drought Indicator Data Rods query


Three new hydrology data variables have been made available as “data rods,” accessible via a Web service at the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC). These data variables are provided in a drought indicator product derived from the Gravity Recovery and Climate Experiment (GRACE) mission data. The three data variables are

  • Groundwater Percentile (gws_inst)
  • Root Zone Soil Moisture Percentile (rtzsm_inst)
  • Surface Soil Moisture Percentile (sfsm_inst)

The GRACE mission was launched on March 17, 2002 and became operational in early 2003, so this drought indicator product currently spans the period January 6, 2003 – April 27, 2015. The GRACE mission consists of two satellites which orbit in tandem, using the Global Positioning System and microwave ranging to precisely measure the distance between the satellites. Changes in this distance are caused by variations in Earth’s gravity field, so GRACE data can very precisely map such variations.  Because the amount of groundwater and soil moisture affects the gravity field, GRACE provides information on the hydrology of the surface and subsurface.



The GRACE mission in orbit (artist's rendering).


 An earlier GES DISC news article, entitled "Data Rods from the GES DISC" , described the concept of data rods and demonstrated how to create a Web service query for North American and Global Land Data Assimilation System (NLDAS and GLDAS) data products.

The following example uses the GRACE drought indicator variables to examine extended drought conditions occurring in northern New Mexico, for the period January 2010 through December 2014.   During this period, much of the U.S. Southwest experienced drought conditions. The area examined is the Valles Caldera, near Los Alamos National Laboratory and Santa Fe. The Valles Caldera, a large ancient volcanic caldera, is an important hydrological catchment in the northwestern mountains of New Mexico, providing water to the Rio Grande River.

The latitude-longitude coordinates used for the Valles Caldera are 106.625 W, 35.875 N. For the data rod query URL, west longitude and south latitude are expressed as negative numbers, so 106.625 W in the query URL becomes -106.625.

Here is the full data rod query URL for the Groundwater Percentile data variable:, 35.875)&type=plot&keep=1

The components of the URL are shown below.

Data product and data variable designation:

Start Date and End Date:


Geographical Location:

&location=GEOM:POINT(-106.625, 35.875)

Plot Type: 


“asc2” can be substituted for “plot” to provide a two-column ASCII text output of the time increment and the corresponding data variable value or time-value pair (TVP).

Below is the resulting data rod time-series plot.  Click on the image to see it full-size. 


data rods result for groundwater storage percentile

This time-series plot shows that groundwater storage reached low levels in the summer of 2011, and, after a period of partial groundwater storage increase, the region experienced a prolonged period of low groundwater storage levels, beginning in late summer 2012 and extending to autumn of the following year.

Shown below are plots for the same period of time for the Root Zone Soil Moisture Percentile (rtzsm_inst) at top and the Surface Soil Moisture Percentile (sfsm_inst) on the bottom:

root zone soil moisture query result
surface soil moisture query result

Questions or comments? Email the NASA GES DISC Help Desk:
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Last updated: Apr 11, 2016 04:31 PM ET