Moisture Growing Season Mean End Of Season Based On Fao (1978) Method

In order to ascertain when, on average, there is enough water in the soil for sustained plant growth to take place, not only does a certain amount of precipitation have to have occurred, but simultaneously the precipitation must exceed some lower threshold of plant evaporative losses for sustained growth to continue. An average moisture growing season over South Africa was determined by adapting a simple water budgeting approach of the FAO (1978), developed originally for agro-ecological zone mapping of Africa. Using this adapted method, the start of the moisture growing season was first determined followed by the determination of the end of the moisture growing season. From these, the duration of the moisture growing season could then be calculated. The adapted FAO approach to determining moisture growing season is assumed that during the period when P ≥ 0.3Er sustained plant growth can take place, where P is median monthly precipitation (mm) and means monthly Eapan is taken as the reference potential evaporation, Er. Average daily magnitudes of P and Eapan were derived by interpolative techniques from their respective 1` x 1` grid values, and the above assumption of P ≥ 0.3Er was tested for. Note that the use of long-term climate averages yield only a simple and very smoothed general moisture growing season, in not accounting for any differences in soils nor for the probability of individual daily rainfall events occurring; however, the averaging does eliminate possible `false starts` growth early in the season when isolated rains may fall.