National Ecological Framework (21 of 23)

Effective Growing Degree Days: Procedure

1. Input Data Requirements.

For each Ecodistrict, the following data are required as input into the calculations:

monthly values of average daily maximum air temperature (TMAX).
monthly values of average daily minimum air temperature (TMIN).
monthly values of average daily mean air temperature (TMEAN).
Latitude (LAT, decimal degrees) of the Ecodistrict centroid and mean elevation (ELEV_MEAN, metres).

These variables are all available from the Ecodistrict climate normals database.

2. Using procedures as in the Penman PET calculations for Ecodistricts, generate the following:

daily values of TMAX, TMIN and TMEAN (procedures from subroutine DAILY were followed).
daily values of daylength (N) based on the latitude of the Ecodistrict (procedures from subroutine SOLARR were followed).

3. Compute daily values of GDD > 5 C and accumulate these between start and stop dates (see below on how to determine these).

Daily GDD = TMEAN - 5.0

3.1 Starting date.

Start accumulating Daily GDD 10 days after TMEAN 5 C in spring or on April 1 (whichever is later).

3.2 Stopping date.

Stop accumulating Daily GDD the day before the average date of the first fall frost (FF) or on October 31, whichever, is earlier.

Since FF is not in the Ecodistrict database, it needs to be estimated by using the procedure developed by Sly et al. (1971). The following variables need to be calculated:

DF42 = the first date in fall when TMEAN 5.56 C (or 42 F).

X1 = [TMIN on day DF42 - TMIN on day (DF42-30)] × 1.8

X2 = [TMIN for July - TMIN for January] × 1.8

X3 = ELEV_MEAN × 3.2808 (to convert elevation to feet)

X4 = [(24 - N)2 × (TMAX - TMIN)] × 1.8

where N = day length on day DF42 and TMAX and TMIN are also for day DF42
Calculate the LAG as follows:

LAG = -32.7 + 0.769(X1) + 0.341(X2) - 0.00484(X3) + 0.00928(X4)

Average date of fall frost FF = DF42 - LAG

(Note: if the LAG is a positive value, then the frost date is before day DF42).

4. Adjust GDDSUM for the day length factor (DLF) based on the Ecodistrict latitude as follows:


where DF is based on Fig. A.1 in the LSRS report by Pettapiece (1995), and can be estimated as follows:

I. if LAT 49 N, then DLF = 1.0

II. if 49.0 < LAT < 61.0 then

DLF = -19.3257 + 1.158643(LAT) - 0.022107689(LAT)2 +0.0001413685(LAT)3

III. if LAT 61.0 N, then DLF = 1.180