Heat Units (Standard Deviation) - September

The concept of the heat unit (or degree day), known since the mid-18th century already, revolves around the development of a plants or organism`s being dependent upon the accumulated heat to which it was subjected during its lifetime, or else during a certain developmental stage. This measure of accumulated heat is known as physiological time. In general it holds that the lower the temperature, the slower the rate of growth and development of plants and invertebrate animals and the amount of heat required to complete a given organism`s development does not vary. Physiological time is usually expressed and approximated in units of degree days, also popularly called heat units, where these are an accumulation of mean temperatures above a certain lower threshold value and below an upper developmental threshold (above which growth is considered to remain static or even decline), over a period of time. The degree day concept is not without its limitations. These include that, a linear relationship is assumed between growth and temperature, threshold temperatures may change during the life cycle of a crop or Pest, temperatures exceeding the upper threshold may, in fact, have a detrimental/negative effect on development. Derivation over South Africa of heat units were done by calculating degree days for the daily temperature fluctuations that occur in nature. These range from degree hours, sine and triangular methods based on hourly temperatures, as well as so-called cut-off methods when the upper threshold of temperature is reached. To estimate heat units (HUs) over South Africa, the 50 year time series of daily maximum and minimum temperatures (Tmxd, Tmnd), generated at each of the 429 700 raster points covering the area at 1` x 1` latitude/longitude, was used (Schulze and Maharaj, 2004). The generation of this time series is described in Section 2a. HUs was computed for each day for a threshold temperature of 10 degree Celsius, i.e. HU = [(Tmxd + Tmnd) / 2 - 10] for HU ≥ 0.0 This method is a considerable advance on the computation of HUs in the previous version of this Atlas (Schulze, 1997), in which HUs were calculated from regression derived monthly means of Tmx and Tmn. From these daily values, monthly as well as seasonal and annual HUs and their standard deviations, could be calculated and mapped.

热单位（heat unit，亦称度日（degree day））的概念早在18世纪中期就已被提出，其核心逻辑为：植物或生物体的生长发育依赖于其生命周期内或特定发育阶段所累积接收的热量。这种累积热量的度量方式被称为生理时间（physiological time）。一般而言，温度越低，植物与无脊椎动物的生长发育速率越慢，而完成某一生物体发育所需的总热量则为恒定值。生理时间通常以度日为单位进行表示与近似计算，度日亦常被称为热单位，其定义为一段时间内，介于某一下限阈值与上限发育阈值之间的平均气温的累积值——当气温高于上限发育阈值时，生物体的生长会趋于停滞甚至衰退。度日概念并非没有局限性：其一，其假设生长与温度之间呈线性关系；其二，阈值温度可能会随作物或害虫的生命周期发生变化；其三，超过上限阈值的温度实际上可能会对发育产生有害影响。针对南非地区的热单位推导工作，通过计算自然环境下的日气温波动所对应的度日值完成，计算方法涵盖基于逐时气温的逐时积温法（degree hours）、正弦法与三角法，以及当气温达到上限阈值时的截断法。为估算南非地区的热单位（heat units，简称HUs），研究使用了覆盖该区域的429700个1′×1′经纬度栅格点（raster points）所生成的50年日最高气温（Tmx_d）与日最低气温（Tmn_d）时间序列数据（Schulze与Maharaj，2004）。该时间序列的生成方式详见第2a节。研究以10℃为阈值温度，逐日计算热单位：当HU≥0.0时，HU = [(Tmx_d + Tmn_d)/2 - 10]。相较于该图集1997年版中基于月平均最高、最低气温的回归计算方法，本次的计算方法取得了显著进步——1997年版的热单位计算依托回归得到的月平均最高、最低气温完成。基于逐日热单位值，可进一步计算得到月、季、年尺度的热单位及其标准差，并完成空间制图。