*Note: Factors included in suitability model.
from 1971-2000 from 86 Illinois
weather stations data interpolated to provide statewide coverage. The data were interpolated to include statewide coverage.
Precipitation – Annual average
distribution statistic was used to calculate temperature probability
0.25 and 0.75 probability levels were used to insure that critical values
would meet crop temperature requirements 3 out of 4 years.
Illinois climate was estimated using 30-year air temperature and precipitation data from 86 stations distributed throughout the
state (see figure). The 0.25 and 0.75 probability levels were used for daily
temperatures (minimum and maximum) values to reduce the expected risk to one
year in four. Had the average (0.50 probability level) been used, the expected
risk of exceeding critical levels would be one year in two.
Temperature and growing days scores were combined to describe the number of days that a crop’s
temperature requirements are met. Together these two traits are more meaningful
than when either is discussed alone. Growth habit is also of interest. The
growing season length for a winter annual crop often includes the winter
dormancy period. Perennial crops live several years, but typically their
reported growing seasons do not include overwintering or resting periods. For
these reasons growing days, temperature, and growth habit were considered in
calculating growing days and temperature scores.
Suitability maps for all four-climate requirements were constructed by assigning scores to the 86
weather stations and interpolating to provide statewide coverage.
Crop precipitation requirements are often reported as a yearly range. Therefore station precipitation
data were summarized as an annual average. Scores were assigned by comparing
the crop’s minimum and maximum requirements to an area’s annual precipitation.
It was assumed that irrigation of up to 350 millimeters (mm, 14 inches) per
year was available. Suitability scores for crop areas requiring irrigation were
lower than for areas with sufficient natural rainfall to meet a crop’s
Growing days suitability maps were based on the reported minimum and maximum number of required days
that temperature requirements were met. Stations were assigned growing days
scores based on the number of days that crop temperature requirements were met.
Growing days suitability scores were constructed for crops with unknown
temperature requirements, using the frost-free period. However, temperature
suitability maps and overall suitability maps were not created for
Daily Air Temperature
Absolute and optimum (minimum and maximum) temperature requirements were compared to weather station
minimum and maximum temperatures. Temperature suitability maps were created for
each unique temperature, growing days, growth habit combination. Temperatures
associated with the 0.25 (minimum temperature) and 0.75 (maximum temperature) probability
levels were used for comparison so that favorable temperatures would be
expected in three out of four years.
Each day in the growing season was assigned a daily suitability value by
comparing estimated hourly temperatures to the absolute and optimum temperature
requirements. Daily suitability values were averaged to estimate the overall
suitability scores for the growing season.
Winter Minimum Temperature or Cold Tolerance
Stations were assigned scores for overwintering crops (perennials, biennials,
and winter annuals). Weather station minimum temperatures (0.25 probability
level) were compared to crop requirements. Crops with minimum temperature
thresholds equal to or above an area’s winter minimum temperature were
considered suited to the area. Areas were assigned one of five suitability
scores, from unsuitable (0) to highly suitable (4). Areas with minimum annual
temperatures below the crop’s requirement were considered unsuitable. Areas
with higher winter minimum temperature requirements were assigned progressively
higher suitability scores in increasing 1.8 degree Fahrenheit (1 degree
Celsius) gradations. Non over wintering crops were considered to be highly suited to winter minimum temperature
throughout the state.
Illinois soil information was obtained from the
Natural Resources Conservation Service STATSGO database. Soil suitability
scores were developed using individual crop requirements. Soil suitability was
mapped by relating suitability scores to map units defined by the STATSGO
The STATSGO Natural Resource Conservation Service Soil database includes 82 geographic map units containing up to 21 unique soil series:
||Soil drainage (Weighted average for each map unit)
||Soil texture (Surface layer weighted average)
||Soil pH (Surface layer weighted average)
Soil map units (see figure) are the most detailed Illinois soil descriptions available for
geographic information systems. These soil map units are composed of up to 21
soil types. Because the STATSGO database provides soil characteristics at the
map unit level, soil component data were averaged to estimate mean map unit
soil characteristics. Soil pH, texture, and drainage average values were
calculated by weighting individual component values by the fraction of the map
unit occupied by the soil. Surface layer values were used to characterize pH
and texture. These values were then compared to crop requirements.
Crops were assigned to one of 22 soil texture classifications based on
the comparison of crop requirement and the weighted average percentage of sand
and clay in each map unit. Disturbed soils, urban soils, extremely high organic
matter soils, and soils containing a large amount (>8.7%) of coarse
fragments were not included in the map unit average as these soils are not
Crops were assigned to soil drainage classifications using the NRCS
drainage classes. Suitability classifications allowed for improvement by
mechanical drainage (tiling).
Crops were assigned to one of 30 possible pH classifications. The mean
and range of the crop’s reported pH requirement were considered.
Scores and Maps
Component suitability scores for daily temperatures, soil drainage,
soil texture, soil pH, winter minimum temperature tolerance (cold tolerance),
growing days, and precipitation requirements were assigned based on crop
requirements and the climate and soils of the area. A mathematical model
combined component suitability value into an overall suitability score. If
winter minimum temperature, growing days, or precipitation were unsuitable for
an area, the area was considered to be unsuited, regardless of the other component suitability scores.
Darker colors indicate greater suitability; lighter colors
lesser suitability. In the equation depicted in the figure below the 4 in the denominator is
used to compute the average of the daily temperature, soil drainage, soil
texture, and soil pH scores resulting in a value with a range from 0 to 4. The
64 in the denominator of the second component on the right hand side of the
equation scales the product of cold tolerance, growing days, and precipitation
requirements to a range of 0 to 1, and the exponent (0.3) reduces the effect of
the law of the minimum on the overall suitability unless one of the three
variables is equal to zero. The law of the minimum states that the lowest
suitability score limits the overall suitability. The effects of the law of the minimum is shown
below in the computations of the overall suitability for winter barley. The winter minimum temperature
makes the northern one-half of the state unsuitable for winter barley.