USAF 557th
WW Percent Soil Moisture
Percent
soil moisture is the available water for the plant divided by the total water
holding capacity of the soil profile. It is useful for determining if the
soil profile has enough water for crop development. Available
water 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which accounts for the daily amount of water withdrawn by evapotranspiration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 total water holding capacity for each
grid cell was derived from the FAO Digital Soil Map of the World and it is
dependent on soil texture and depth of the soil profile. For grid cells with
soil depths greater than 1-meter, a maximum soil depth of 1-meter was assumed
to approximate the maximum root depth for most plants. Refer
to "Data Sources"
for additional Crop Explorer metadata. Modified Palmer
Two-Layer Soil Moisture Model
PECAD’s
two layer soil model is similar to the Palmer’s (1965) two-layer soil
moisture model, but Palmer’s two-layer soil moisture model was modified by:
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 PECAD later found these assumptions did not
adequately describe water extraction by plants. Therefore, PECAD 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. Soil
water holding capacity is dependent on soil type and soil depth, and these
parameters were derived from FAO’s DSMW (Reynolds, et al, 2000). A maximum
root depth of one meter or less was assumed, and dependent on impermeable
soil layers. From these assumptions, water holding capacity for both layers
normally range from 5 to 8 inches/meter of water depending on soil texture
(ranging from sand to clay) and soil depth (one meter or less). References: 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. FAO. 1996. The Digitized Soil Map of the World Including Derived Soil Properties, CD-ROM, Food and Agriculture Organization, Rome. 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. Thornthwaite, C.W. 1948. An Approach Toward a Rational Classification of Climate. Geograph. Rev., 38:55-94. |