Monthly patterns of Heat Accumulation can be as important as Annual patterns for a better understanding of microclimates on your farm. This article follows on from the article Microclimates to Measure or to Model? and should be read in conjunction with that article. When the monthly GDD4 patterns are mapped for the Five Rivers Plains Area, a number of important patterns of heat accumulation become evident.
There is little heat accumulation in the area in July, Mid-Winter in this area, except for a small northeast facing sunny slope (shown in mid-blue towards the bottom of the map) where daily temperatures rise above the base temperature, 4 Degrees C, and small amounts of heat are accumulated. By August, there is some heat accumulation occurring and the pattern is markedly influenced by aspect and topography. Warm, north facing slopes accumulate more heat than the valley bottom or south facing slopes.
By September, early spring warming is starting to kick in and most of the valley (midgreen) except the north facing steeper slopes (lighter green) receives between 125 and 150 GDD4. By October, spring has well and truly happened and most of the valley receives between 175 and 200 GDD4. The green areas on the October map represent colder hollows where cold air tends to accumulate, frost would still be occurring and only 150-175 GDD4 of accumulated heat would be occurring.
From November through the summer months, heat is accumulating at a much higher rate between 200 and 300 GDD4 per month. In November, there is still a pattern on the map reflecting the influence of slope, aspect and air drainage but by December, the pattern of accumulated heat is even across the area at between 275 and 300 GDD4. Again, in December, small areas of northeast facing slopes are still slightly warmer than the average figures for the area.
By January, its the height of the summer and the whole area, except for a couple of cooler south facing slopes, receives between 325 and 350 GDD4 for the month. February continues the pattern with even GDD4 levels of between 275 and 300 GDD4 across the area.
March tends to follow the February heat accumulation pattern but in April, the impact of the colder hollows on the plains start to show up (in green) as colder GDD4 accumulation areas.
As Autumn grows into Winter over the May-June period, the pattern changes yet again. In May, the cooler areas of the plains are more evident and widespread. Warmer monthly GDD4 are experienced on areas with better slope and air drainage. By June, there is very little accumulated heat on which plants can grow. Only in a few, favoured, sheltered or NW facing areas is a little accumulated heat experienced. When you view all twelve months of monthly accumulated heat patterns for the Five Rivers District, a number of significant patterns start to emerge.
For instance, note the paired pattern of similar months between spring and autumn July and June, September and May, April and October, December and February. Note also that there are internal patterns of heat in the maps at the shoulder growing parts of the season and also note the evenness of heat patterns on the maps from November until February. This suggests that another driver becomes more critical over those months, most likely water availability or Potential Evapotranspiration (PE). When the same monthly accumulated heat patterns are produced at 10º C Base temperature, the information provides similar paired patterns of months during the shoulder periods of growth but also more pattern than evenness during the December through February period of summer. You could feasibly use this map to identify the favoured parts of the area for clover production, as clover requires a minimum base temperature of 10º C to initiate growth. 10 Degrees is also the trigger point for a number of crops such a grapes or maize, so there is a potential use for the information in determining which parts of the area could successfully grow maize for winter silage, and also for determining the optimal planting dates and harvest dates for different parts of the area.
