Percent Soil Moisture (corrected with SMOS
imagery)
Percent soil moisture is the available water (AW) for
the plant divided by the total water holding capacity (WHC) of the soil
profile. It is useful for determining if the soil profile has enough water
for crop development. The percent
soil moisture product is best
used to monitor an established crop because it estimates the percent soil
moisture within the plant’s root zone, assumed to be one meter or less (with
plant roots less than one meter during
early growth stages or due to shallow impermeable soil layers that prevent root
depth of one meter). In general,
optimum crop development occurs when percent soil moisture is between 50-80
percent and water stress occurs when percent soil moisture values are less
than 40 percent. Figure 1 (from
AgRISTARS, 1981) illustrates percent soil moisture stresses during different
corn growth stages.
Figure 1. Soil moisture stress is
40 percent or less for most corn growth stages Percent
Soil Moisture corrected with SMOS imagery and assimilated by the Ensemble Kalman Filter (EnKF) The percent soil moisture product is
corrected by integrating satellite-derived Soil Moisture Ocean Salinity
(SMOS) mission surface soil moisture retrievals into the modified Palmer two-layer
soil moisture model. The SMOS imagery helps
to correct the modified Palmer soil moisture model by using an Ensemble
Kalman Filter (EnKF) data assimilation approach. The assimilation of SMOS surface
soil moisture estimates is designed specifically to correct the modified
Palmer two-layer soil moisture predictions for the deleterious impact of
rainfall forcing errors - particularly in regions of the world lacking
extensive rain-gauge instrumentation (Bolten, et al, 2010). The European Space
Agency (ESA) Soil Moisture Ocean Salinity (SMOS) mission is a satellite-based,
passive L-band radiometer instrument launched in November 2009. L-band microwave
emission from the land surface can be inverted to provide an estimate of
surface soil moisture conditions at a spatial resolution near 50-km. SMOS Level 2 soil
moisture products provided by ESA are gridded into daily composites by NOAA NESDIS
Soil Moisture Operational Products System. These retrievals are then assimilated into
the modified Palmer two-layer soil moisture model by using an Ensemble Kalman
filter approach which dynamically updates all model-based soil moisture
predictions to reflect information contained in the SMOS imagery.
In addition, SMOS imagery updates are
made to both the (observable) surface soil layer and the (non-directly observable)
subsurface soil layer via the use of error covariance information sampled
from an ensemble of Monte Carlo model forecasts. The assimilation of SMOS surface soil moisture
estimates is designed specifically to correct model-based soil moisture
predictions for the deleterious impact of rainfall forcing errors -
particularly in regions of the world lacking extensive rain-gauge
instrumentation. Palmer Two-layer
Soil Moisture Model in General
The Palmer (1965) two-layer soil moisture model
is a bookkeeping method that accounts for the water gained or lost in the
soil profile by recording the amount of water withdrawn by evapotranspiration
and replenished by precipitation. The final aim of the soil moisture model is
to estimate if soil moisture storage between dry spells was adequate for
maximum plant growth. The soil moisture within
two soil layers is calculated in daily time increments (mm/day of
precipitation or evapotranspiration). The top-layer soil moisture is assumed
to hold a maximum of one inch (or 25-mm) of available water, and the
sub-layer soil moisture may hold 0-275 mm/m of water depending on the soil’s
water-holding capacity (based on soil texture and soil depth) for the LIS
(Land Information System) grid cell. The soil moisture model
assumes precipitation enters the two soil layers by first filling the surface
soil layer and then filling the lower soil layer. Moisture is extracted from
the two soil layers by evapotranspiration, whereby water is first depleted
from the top layer and then extracted from the sub-surface layer. When the
water-holding capacity of both soil layers is reached, excess precipitation
is lost from the model and treated as runoff or deep percolation. Daily evapotranspiration
for the two-layer soil moisture model is calculated by the FAO 56 Penman-Monteith equation (Allen, et al, 1998) and daily precipitation
is estimated from both surface observations and satellite data. The
water-holding capacities for both soil layers were derived from the FAO (1996) Digital Soil Map of the World. Modified Palmer Two-Layer Soil Moisture
Model
The modified Palmer two layer soil model by
FAS/IPAD is similar to the Palmer’s (1965) two-layer soil moisture model, but
Palmer’s two-layer soil moisture model was modified by FAS/IPAD to:
Both the original Palmer and modified-Palmer
models assume the top first inch of available water is held in the top layer,
and remaining soil water is held in the lower layer. Precipitation enters the
model by first completely filling the surface layer and then filling the
lower layer. When the soil water holding capacity of both layers is reached,
excess precipitation is treated as runoff and is lost from the model. The original Palmer model assumed moisture was
removed from the surface layer at rate equal to the potential
evapotranspiration calculated by the Thornthwaite (1948) method, and moisture
was removed from the lower layer at fraction of the potential rate. It also
assumed that moisture could not be removed from the lower layer until the
surface layer was completely dry, but FAS/IPAD later found these assumptions
did not adequately describe water extraction by plants. Therefore, FAS/IPAD slightly
modified the extraction function to allow gradual and more realistic
depletion in the surface layer and to allow moisture to be depleted from the
lower layer before the surface is completely dry. The modified extraction function allows moisture
to be depletion from the surface at the potential evapotranspiration rate to
75 percent of the surface capacity (or 75% of 1 inch of water). When the
surface layer is below 75 percent capacity, moisture is extracted from the
surface at a reduced rate with the lower layer making up the remaining
requirement. Moisture is extracted from the lower layer at a fraction of the
potential, where this fraction is calculated as a ratio of actual water held
to the total water-holding capacity. Total
Water Holding Capacity (WHC) Derived from the Modified FAO Digital Soil Map
of the World (DSMW) The total water holding capacity (WHC) of a soil
is defined as the difference between the soil’s field capacity less the
permanent wilting point, with the total soil water holding capacity dependent
on soil texture and soil depth. The
global spatial distribution of soil texture and soil depth is defined by the
FAO Digital Soil Map of the World (DSMW), and the DSMW was modified to estimate
the total water holding capacity for each LIS (Land Information System, 2006)
grid cell by assuming a maximum soil depth of 1-meter or less (Reynolds, et
al, 2000). From the 1-meter or less
soil depth assumption, the soil water holding capacity within each LIS grid
cell normally ranges from 5 to 8 inches/meter of water depending on soil
texture (ranging from sand to clay) and soil depth (ranging from one meter or
less). The daily available water (AW) within the plant’s
one meter (or less) root zone is calculated by the modified Palmer two-layer
soil moisture model which accounts for the daily amount of water withdrawn by
evapotranspiration and replenished by precipitation. The available water is expressed in
millimeters per day, with percent soil moisture calculated as the daily
available water (AW) divided by the total soil water holding capacity (WHC)
for each LIS grid cell. Refer to "Data Sources" for additional Crop Explorer metadata. References: AgRISTARS, 1981. AgRISTARS- Agriculture and Resources Inventory Surveys through Aerospace Remote Sensing, Fiscal Year1980 Annual Report, June 1981. http://www.nass.usda.gov/Education_and_Outreach/Reports,_Presentations_and_Conferences/GIS_Reports/AgRISTARS%20Annual%20Report%20%28FY%201980%29.pdf Allen, R. G., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop Evapotranspiration; Guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56, Rome. Bolten,
J.D., W.T. Crow, T.J. Jackson, X. Zhan and C.A. Reynolds, Evaluating the
utility of remotely-sensed soil moisture retrievals for operational
agricultural drought monitoring, IEEE Journal of Selected Topics
in Applied Earth Observations and Remote Sensing, 3, 57-66, 10.1109/JSTARS.2009.2037163,
2010. FAO. 1996. The Digitized Soil Map of the World Including Derived Soil Properties, CD-ROM, Food and Agriculture Organization, Rome. LIS (Land Information System) Documentation, 2006, NASA-GSFC (Goddard Space Flight Center), from Kumar, S. V., C. D. Peters-Lidard, Y. Tian, P. R. Houser, J. Geiger, S. Olden, L. Lighty, J. L. Eastman, B. Doty, P. Dirmeyer, J. Adams, K. Mitchell, E. F. Wood and J. Sheffield, 2006. Land Information System - An Interoperable Framework for High Resolution Land Surface Modeling. Environmental Modelling & Software, Vol. 21, 1402-1415. http://lis.gsfc.nasa.gov/LIS_documentation.php Palmer, W.C. 1965. Meteorological Drought. U.S. Weather Bureau Research Paper 45, 58 p. Reynolds, C.A., T.J. Jackson, and W.J. Rawls. 2000. Estimating Soil Water-Holding Capacities by Linking the FAO Soil Map of the World with Global Pedon Databases and Continuous Pedotransfer Functions. Water Resources Research, December, Vol. 36, No. 12, pp. 3653-3662. Reynolds, C.A. 2001. Input Data Sources, Climate Normals, Crop Models, and Data Extraction Routines Utilized by OGA/IPAD, Third International Conference on Geospatial Information in Agriculture and Forestry, Denver, Colorado, November 5-7, 2001, URL: http://www.pecad.fas.usda.gov/cropexplorer/datasources.aspx Thornthwaite, C.W. 1948. An Approach Toward a Rational Classification of Climate. Geograph. Rev., 38:55-94. |
