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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-182
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Apr 2018

Research article | 16 Apr 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).

A SMAP-Based Drought Monitoring Index for the United States

Sara Sadri1, Eric F. Wood2, and Ming Pan2 Sara Sadri et al.
  • 1University of California, Los Angeles (UCLA), Department of Geography, 1255 Bunche Hall, Los Angeles, CA 90095, USA
  • 2Princeton University, Department of Civil and Environmental Engineering, 59 Olden St, Princeton, NJ 08540, USA

Abstract. Since April 2015, NASA's Soil Moisture Active Passive (SMAP) mission has monitored near-surface soil moisture, mapping the globe between the latitude bands of 85.044°N/S in 2–3 days depending on location. SMAP Level 3 passive radiometer product (SPL3SMP) measures the amount of water in the top 5 cm of soil except for regions of heavy vegetation (vegetation water content >4.5 kg/m2) and frozen or snow covered locations. SPL3SMP retrievals are spatially and temporally discontinuous, so the 33 months offers a short SMAP record length and poses a statistical challenge for meaningful assessment of its indices. The SMAP SPL4SMAU data product provides global surface and root zone soil moisture at 9-km resolution based on assimilating the SPL3SMP product into the NASA Catchment land surface model. Of particular interest to SMAP-based agricultural applications is a monitoring product that assesses the SMAP near-surface soil moisture in terms of probability percentiles for dry and wet conditions. We describe here SMAP-based indices over the continental United States (CONUS) based on both near-surface and root zone soil moisture percentiles. The percentiles are based on fitting a Beta distribution to the retrieved moisture values. To assess the data adequacy, a statistical comparison is made between fitting the distribution to VIC soil moisture values for the days when SPL3SMP are available, versus fitting to a 1979–2017 VIC data record. For the cold season (November–April), 57% of grids were deemed to be consistent between the periods, and 68% in the warm season (May–October), based on a Kolmogorov–Smirnov statistical test. It is assumed that if grids passed the consistency test using VIC data, then the grid had sufficient SMAP data. Our near-surface and root zone drought index on maps are shown to be similar to those produced by the U.S. Drought Monitor (from D0-D4) and GRACE. In a similar manner, we extend the index to include pluvial conditions using indices W0-W4. This study is a step forward towards building a national and international soil moisture monitoring system, without which, quantitative measures of drought and pluvial conditions will remain difficult to judge.

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This paper provides a near real-time SMAP-based drought monitor that publishes maps of drought in the U.S. at near surface and root zone online every 24 hours. This is a tool that can be used toward both drought and flood early detection and planning. Hence it is of policy and stakeholders interest. Such a system would be an important step towards building an international SMAP-based soil moisture monitoring system, without which, quantitative measures of drought will remain difficult to judge.
This paper provides a near real-time SMAP-based drought monitor that publishes maps of drought...
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