Geoscience Reference
In-Depth Information
daily mean temperature fell above 5°C [23].
*
This is a useful
temperature parameter as it gives an indication of the quality of
the growing season over a defined period ([23], p. 17).
†,‡
Given
that the Norwegian climate restricts the growing season for
most crops from April to September, it was decided to exclude
recorded GDD from months outside this period.
§
Annual precipitation, measured in millimeters, is the second
weather variable. Precipitation accumulated outside the grow-
ing season was included for two reasons. First, it is likely that
a significant part of the precipitation falling outside this period
would be retained as moisture in the soil, and thereby eventu-
ally affecting crop growth when the growing season begins.
Second, as a large proportion of precipitation commonly falls
in the form of snow during the Norwegian winter, when the
onset of spring causes it to melt, a large share of it is likely
to serve as a water supply, potentially feeding both soil and
crops, before and during the growing season. As temperature
increases some of the effect of increased precipitation will dis-
appear due to increased evaporation [15].
As the Norwegian Meteorological Institute (DNMI) col-
lects data from weather stations that are located on the basis
of meteorological interest rather than along county bound-
ary lines, it was necessary to make some decisions regarding
which stations to use and how to aggregate station data to the
county level. This process was made more precise with the use
of GIS mapping. A digital land use map of Norway, identifying
areas of agricultural activity, was obtained from the Norwegian
Institute of Land and Forest Mapping (NIJOS), and geographi-
cal coordinates of weather station locations were provided by
*
To give a simple example, if a month contained just 2 days where the average
temperature rose above 5°C, and the average temperature was 7°C on the
first day and 9°C on the second, then GDD for that month would be 6°C (i.e.
2°C + 4°C).
†
See http:/ / www. smhi. se/ hfa_ coord/ nordklim/ report06_ 2001. pdf.
‡
An alternative temperature parameter is effective growing degree days
(EGDD), employed by Bootsma et al. [1]. The authors justify their use of
GDD, explaining that GDD 'are designed to represent the growth period
for perennial forage crops, while EGDD are specifically designed to be
more applicable to the growth period for spring-seeded small grains cere-
als'. EGDD is defined as the sum of GDD from 10 days after the start of the
growing season until the day preceding the average date of the first frost.
They find a negative correlation between yield and EGDD, and suggest
that this might be due to a higher development rate of crops under warmer
temperatures.
§
In Norway, the length of the growing season is defined as the annual sum of
days in which the mean temperature exceeds 5°C. The growing season can
also be understood as the actual time period (e.g. April-September).
