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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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
Review status
This discussion paper is a preprint. It is a manuscript 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 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.
Citation: Sadri, S., Wood, E. F., and Pan, M.: A SMAP-Based Drought Monitoring Index for the United States, Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2018-182, in review, 2018.
Sara Sadri et al.
Sara Sadri et al.
Sara Sadri et al.

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Short summary
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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