Positive Chill Units - July

Certain biennial plants which have a dormant (i.e. rest) season during winter may require a period of accumulated minimum temperatures below a threshold in order to stimulate growth, develop leaves, flower or set fruit. Deciduous trees, for example, need a certain period of winter chilling for completion of their seasonal dormancy. Insufficient winter chilling may result in delayed foliation, reduced fruit set (including fruits remaining small and being misshapen as they ripen) and reduced fruit quality (including reduced firmness, and/or greener fruit coloration). The required amount of chilling for completion of the rest period varies between species, cultivars and different locations. Many chill accumulation models have been formulated, most of them requiring hourly temperatures to have either been observed or estimated. The Linsley-Noakes PCU model was used to produce maps of chill units in this Atlas. The techniques by which PCUs were computed and then mapped are described in the shaded sections which follow.

Derivation of hourly values of temperature for the determination of PCUs hourly values of temperature were determined from the 50 year time series of daily maximum and minimum temperatures generated at each of the 429 700 raster points covering South Africa, using the techniques described by Schulze and Maharaj (2004). For each daytime and night-time cycle the sine-log equations derived by Linsley-Noakes et al. (1995) were employed. In these equations the estimation of hourly temperatures is based on the observation that the daytime solar cycle follows a sine curve from sunrise to sunset, whereas night-time cooling may be described by a logarithmic equation.

部分越冬休眠（即休憩期）的两年生植物，需经历一段累积低于某一阈值的低温时长，方可刺激生长、展叶、开花或坐果。以落叶乔木为例，其需经历一定时长的冬季低温积累，方能完成季节性休眠。冬季低温积累不足会导致展叶延迟、坐果率下降（包括果实成熟时体积偏小、外形畸形）以及果实品质降低（包括硬度下降、果皮色泽偏绿）。完成休眠期所需的低温积温需求量因物种、品种及种植区域而异。目前已开发出多种低温积温模型，其中多数需要实测或估算的逐小时气温数据。本图集采用Linsley-Noakes PCU（Chill Unit，低温单位）模型生成低温单位分布图，该模型的计算与成图方法将在后续的阴影框章节中详述。
用于PCU计算的逐小时气温数据，通过覆盖南非的429700个栅格点各自生成的50年逐日最高、最低气温时间序列，结合Schulze与Maharaj（2004）提出的方法推导得到。针对每个昼夜周期，均采用Linsley-Noakes等人（1995）推导的正弦-对数方程进行计算。该类方程的逐小时气温估算基于以下观测结论：日间太阳辐射循环自日出至日落呈正弦曲线变化，而夜间降温过程则可通过对数方程描述。