National Ecological Framework (16 of 23)

Temperature and precipitation

The 1961-1990 data for temperature and precipitation included only stations with averages based on more than 19 years of data (Appendix 1). Data from additional stations which had temperature and precipitation normals for the 1951-1980 period were also used to provide maximum station density, but these normals were first adjusted to the 1961-1990 period by comparison with nearby stations.

TMAX, TMIN, TMEAN, RAIN, SNOW and TOTALP were interpolated using the Thiessen polygon method. The Thiessen polygons were overlayed with Ecodistrict polygons, and an area-weighted value generated for each Ecodistrict Area-weighted polygon to polygon overlays were done using ARCINFO GIS based methods developed by AAFC and Pole Star Geomatics called PARS. Data from stations which were more than 350 metres above the lowest elevation of each Ecodistrict were eliminated from the weighting procedure to avoid using stations at high elevations in mountainous terrain (e.g. in British Columbia) which were not considered to be representative of valleys and plateaus where agricultural activities are present. Separate Thiessen coverages were developed for temperature and for precipitation for each month to make use of all available station data.

Vapour pressure, wind speed, sunshine, solar radiation, dew point

For the observed climate variables wind, solar radiation, vapour pressure, sunshine and dew point, monthly values were extracted for available years and then averaged (Appendix 1). Only station averages that included 8 years or more of data were used for these variables (period of record was compromised to achieve adequate station density in these cases).

The observed climate variables VP, WI, SH, SR and DP as defined in the table above were interpolated using gridded surface interpolation methods, since the density of climate stations was generally inadequate for using the Thiessen approach. A grid with 1.5 minute Latitude and Longitude spacing was generated using the inverse distance method to weight the four stations nearest to each grid cell. Inverse Distance Weighting was done using the GRASS GIS R.SURF.IDW2 module (Appendix 3). For the variables extrapolated using this method, each monthly variable has a maximum, minimum and mean value determined for each Ecodistrict (i.e. the maximum is the highest, minimum is the lowest and mean is the average of all grid point values found within the district )

Potential evapotranspiration and water deficit

Average monthly and annual potential evapotranspiration (PE) were estimated from monthly climatic normals for each Ecodistrict (Appendix 1) using the Penman and the Thornthwaite methods. The Penman procedure was similar to that used in the WOFOST Crop Simulation Model (van Diepen et al. 1988), with some modifications. Daily normal values of climate variables required as input into the Penman equations were generated from monthly normals using the Brooks (1943) sine wave interpolation procedure. Wind speed was converted from the 10 metre height to 2 metres using the power law (Jensen 1973): U1 = U2 * (h1/h2)**0.2 where U1 and U2 are wind speeds at height h1 and h2 respectively. Daylength values were computed based on a computer subroutine called SOLARR (De Jong, personal communication). The Penman PE calculations were made on a daily basis assuming a grass cover with an albedo of 0.25 when average mean daily air temperatures were above 0 degrees Celsius. When temperatures were below freezing, an albedo of 0.75 for snow cover was assumed, similar to the procedure used in the Penman PE calculated for the Land Potential Data Base (Kirkwood et al. 1989). Negative daily PE values which could occur in winter were set to zero. Daily normal PE values were summed to obtain monthly and annual normal values for Penman PE.

Average monthly and annual Thornthwaite Potential Evapotranspiration (PE) values and Water Deficits (WD) were computed using methods described by Thornthwaite and Mather (1957). WD values were estimated for soils with 100, 150, 200 and 250 mm available water-holding capacity using both the Penman and the Thornthwaite PE estimates. A precipitation surplus/deficit was computed by subtracting the PE from Total Precipitation (i.e. TOTALP-PE) using both the Penman and the Thornthwaite PE calculations.

Growing degree days

Annual growing degree-days (GDD) above base temperatures of 0, 5, 10 and 15 degrees Celsius (GDD0, GDD5, GDD10 and GDD15) were computed from the monthly mean air temperature data (Appendix 1). Brooks (1943) interpolation procedure was used to generate daily mean air temperatures from monthly values and daily growing degree-days were calculated by subtracting the base temperature from the mean daily temperature (negative values were set to zero). Daily values were summed to obtain the annual total. Calculating GDD from mean daily air temperatures involves some error near the start and end of the accumulation period, since the temperature averages include days when the temperature was below the base value. However, this procedure has been commonly accepted as being of sufficient accuracy (Chapman and Brown 1966).

Growing season start, end date, and length

The date of the growing season start (GSS) and end (GSE) were determined by the first and last day of the year when the mean daily air temperature equals or exceeds 5 degrees Celsius. This is generally considered to coincide with the growing period for perennial forage crops (Chapman and Brown 1966). Growing season length (GSL) was computed as GLS=GSE-GSS+1, where GSE and GSS are calendar (Julian) days.

Effective growing degree-days (EGDD)

Effective growing degree-days (EGDD) are growing degree-days (GDD) above 5 °C adjusted for growing season and day length, and are used in the rating the suitability of land for spring-seeded small grains in Canada ( Pettapiece, 1995).

EGDD were calculated from monthly temperature normals (Appendix 1) using the procedures outlined by Pettapiece (1995), with the following modifications:

See Appendix 4 for a more complete description of procedures.