The global distribution and dynamics of surface soil moisture

Journal name:
Nature Geoscience
Volume:
10,
Pages:
100–104
Year published:
DOI:
doi:10.1038/ngeo2868
Received
Accepted
Published online

Abstract

Surface soil moisture has a direct impact on food security, human health and ecosystem function. It also plays a key role in the climate system, and the development and persistence of extreme weather events such as droughts, floods and heatwaves. However, sparse and uneven observations have made it difficult to quantify the global distribution and dynamics of surface soil moisture. Here we introduce a metric of soil moisture memory and use a full year of global observations from NASA’s Soil Moisture Active Passive mission to show that surface soil moisture—a storage believed to make up less than 0.001% of the global freshwater budget by volume, and equivalent to an, on average, 8-mm thin layer of water covering all land surfaces—plays a significant role in the water cycle. Specifically, we find that surface soil moisture retains a median 14% of precipitation falling on land after three days. Furthermore, the retained fraction of the surface soil moisture storage after three days is highest over arid regions, and in regions where drainage to groundwater storage is lowest. We conclude that lower groundwater storage in these regions is due not only to lower precipitation, but also to the complex partitioning of the water cycle by the surface soil moisture storage layer at the land surface.

At a glance

Figures

  1. The stored precipitation fraction.
    Figure 1: The stored precipitation fraction.

    Top left and right: the stored precipitation fraction FP(f) is a dimensionless measure of the degree to which a soil layer (of depth Δz) retains precipitation inputs (P) over a given timescale 1/f, given losses due to evapotranspiration (E), drainage (D) and runoff (R). Bottom left and right: two example soil moisture time series from in situ observations30, sampled at two different sampling frequencies — f = 12 d−1 (black, crosses) and f = 1/3 d−1 (red, circles). Inset: the sum of positive increments in the soil water time series, for the two different sampling frequencies.

  2. Global distribution and memory of surface soil moisture.
    Figure 2: Global distribution and memory of surface soil moisture.

    a, Global map of annual mean SSM (1 April 2015–31 March 2016), with PDF (inset) and zonal mean (right panel). Marker sizes in all zonal plots are proportional to zonal land area; shaded region shows ±1 standard deviation. White regions in map are missing or masked (see Methods). b, The same as in a, except for FP(1/3), and the marker colours in the zonal plot indicate the zonal mean SSM. c, The same as in a, for mean daily precipitation.

  3. Global relations between stored precipitation fraction and soil moisture content and texture.
    Figure 3: Global relations between stored precipitation fraction and soil moisture content and texture.

    a, Global relation between FP(1/3) and annual mean SSM, estimated using one year of observations (1 April 2015–31 March 2016). Boxplots show the median (red horizontal line), 25th and 75th percentiles (top and bottom of the grey shaded box, respectively), and maximum and minimum observed values (edges of the top and bottom whiskers, respectively). b, Global relation between FP(1/3) and sand fraction. c, Global relation between FP(1/3) and clay fraction.

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Author information

Affiliations

  1. Department of Civil and Environmental Engineering, MIT, Cambridge, Massachusetts 02139, USA

    • Kaighin A. McColl,
    • Seyed Hamed Alemohammad,
    • Ruzbeh Akbar,
    • Alexandra G. Konings &
    • Dara Entekhabi
  2. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

    • Kaighin A. McColl
  3. Department of Earth System Science, Stanford University, Stanford, California 94305, USA

    • Alexandra G. Konings
  4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • Simon Yueh
  5. Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, Massachusetts 02139, USA

    • Dara Entekhabi

Contributions

K.A.M. wrote the manuscript. R.A., K.A.M. and S.H.A. conducted analyses and produced figures. D.E. conceived and led the project, and developed the ‘stored precipitation fraction’ in discussions with K.A.M., S.H.A. and A.G.K. S.Y. contributed to interpretation of the results. All authors discussed and edited drafts of the manuscript.

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The authors declare no competing financial interests.

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