Positive Chill Units - August

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.

部分越冬存在休眠（即静止）期的二年生植物，需经历一段低于某一阈值的累积低温时段，方可启动生长、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累积，才能完成季节性休眠过程。冬季低温累积不足则可能导致展叶延迟、坐果率下降（包括果实发育迟缓、成熟时形态畸形）以及果实品质降低（包括硬度下降、果皮色泽偏绿等）。完成休眠所需的低温累积量因物种、品种及种植区域的不同而存在差异。目前已构建了多种低温累积模型，其中绝大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模型绘制低温单位（chill units）分布图。PCU的计算与制图方法将在后续的阴影区块中详述。用于PCU计算的逐小时气温数据，通过Schulze与Maharaj（2004）提出的方法，由覆盖南非的429700个栅格点所生成的50年逐日最高、最低气温时间序列推算得到。针对每个昼夜循环，研究团队采用了Linsley-Noakes等人（1995）推导的正弦-对数方程。该类方程的逐小时气温估算基于以下观测规律：日间太阳辐射升温过程遵循日出至日落的正弦曲线变化，而夜间降温过程则可通过对数方程进行描述。