Developing a drought-monitoring index for the contiguous US using SMAP

Abstract

Since April 2015, NASA's Soil Moisture Active Passive (SMAP) mission has monitored near-surface soil moisture, mapping the globe (between 85.044 • N/S) using an L-band (1.4 GHz) microwave radiometer in 2-3 days depending on location. 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. However, the short SMAP record length poses a statistical challenge for meaningful assessment of its indices. This study presents initial insights about using SMAP for monitoring drought and pluvial regions with a first application over the contiguous United States (CONUS). SMAP soil moisture data from April 2015 to December 2017 at both near-surface (5 cm) SPL3SMP, or Level 3, at ∼ 36 km resolution, and root-zone SPL4SMAU, or Level 4, at ∼ 9 km resolution, were fitted to beta distributions and were used to construct probability distributions for warm (May-October) and cold (November-April) seasons. To assess the data adequacy and have confidence in using short-term SMAP for a drought index estimate, we analyzed individual grids by defining two filters and a combination of them, which could separate the 5815 grids covering CONUS into passed and failed grids. The two filters were (1) the Kolmogorov-Smirnov (KS) test for beta-fitted long-term and the short-term variable infiltration capacity (VIC) land surface model (LSM) with 95 % confidence and (2) good correlation (≥ 0.4) between beta-fitted VIC and beta-fitted SPL3SMP. To evaluate which filter is the best, we defined a mean distance (MD) metric, assuming a VIC index at 36 km resolution as the ground truth. For both warm and cold seasons, the union of the filters -which also gives the best coverage of the grids throughout CONUS -was chosen to be the most reliable filter. We visually compared our SMAP-based drought index maps with metrics such as the U.S. Drought Monitor (from D0-D4), 1-month Standard Precipitation Index (SPI) and near-surface VIC from Princeton University. The root-zone drought index maps were shown to be similar to those produced by the root-zone VIC, 3month SPI, and the Gravity Recovery and Climate Experiment (GRACE). 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.

Multicomponent Satellite Assessment of Drought Severity in the Contiguous United States From 2002 to 2017 Using AMSR‐E and AMSR2

“…The strong sensitivity of AMSR observations to land surface wetness and relatively favorable spatial and temporal coverage over the CONUS domain have promoted the use of AMSR‐derived land surface products for regional drought assessment (A et al, ; Yin et al, ). Lower frequency (L‐band) satellite microwave sensors, such as SMAP and SMOS, have coarser sensor footprints than AMSR but similar temporal fidelity, and with potentially enhanced sensitivity to soil moisture that may provide additional synergistic information for drought detection gained through overlapping multifrequency satellite microwave observations (Du, Kimball, Galantowicz et al, ; Sadri et al, ).…”

“…In particular, soil moisture products derived from AMSR‐E (Advanced Microwave Scanning Radiometer for the Earth Observing System) and other spaceborne microwave sensors have been applied for operational drought assessment and crop growth monitoring (Bolten et al, ; Yin et al, ). Recent satellites designed for global soil moisture monitoring include the European Space Agency SMOS (Soil Moisture Ocean Salinity) and National Aeronautics and Space Administration SMAP (Soil Moisture Active Passive) missions, which provide lower frequency (L‐band) microwave observations that have shown promising results in tracking agricultural drought (Martínez‐Fernández et al, ; Sadri et al, ). For hydrological drought, satellite gravimetric measurements of total water storage (TWS) changes from GRACE (Gravity Recovery And Climate Experiment) and the associated drought indices developed from these data have been successful in evaluating persistent drought and pluvial events (Thomas et al, ; A et al, ; Zhao et al, , ).…”

Monitoring Soil Moisture Drought over Northern High Latitudes from Space

“…Even though the L-band penetration depth is deeper during very dry conditions a limitation of the satellite derived drought index will be the sensing depth, we are still unable to quantify the amount of water in the root-zone. However, the STBI can provide valuable information on surface water availability when plants develop [15] and for monitoring preconditions of wildfire.…”

“…Although the Nordic region is projected to get wetter conditions on average under climate change [12], aridity is expected to increase during the boreal summer months [10], and thus a way of monitoring and mapping droughts over the northern regions is much needed. One way of doing this is by satellite remote sensing [13][14][15], as satellites could provide near-real-time observations covering large regions within a relative short time-period. Satellite retrieval of surface soil moisture (SSM) over northern latitudes is difficult because of snow cover, high open water fraction, steep topography and dense boreal vegetation that affect the microwave emissions from the soil [16].…”

“…Due to its close relation to SSM we compare and validate the STBI index against two SSM indices and a one month precipitation index. The one month precipitation and SSM indices have been found to closely describe agricultural drought [15,19,20].…”