Fundamentals of Biometeorology: The Biological Environment, Vol. 2; William P. Lowry, Porter P. Lowry II; Peavine Publications/Missouri Botanical Garden Press, ISBN 0-882002-04-0

This paper presents a method for evaluating criteria used to advise farmers on planting dates at the onset of the rainy season. The criteria were evaluated using an analysis of the relative transpiration rate (the ratio of the actual transpiration to the potential transpiration) over a 30-day period following sowing (initial growth stage) by means of a soil water balance model (BUDGET). Crop failure or a false start to the season was indicated by a relative transpiration rate of less than 35% over the 30 days. The first recommended planting dates have been identified for a number of stations in the six agro-ecological zones of Zimbabwe and for the last 30 years, using three different criteria. The criteria included two, which are currently utilized in Zimbabwe: the AREX criterion of the Agricultural Research Extension (25 mm rainfall in 7 days) and the MET criterion of the Department of Meteorological Services (40 mm in 15 days). A third criterion, DEPTH (40 mm in 4 days), based on farmers practices, is introduced in this paper. In general, for all agro-ecological zones, the earliest planting dates were found with the AREX criterion. Those generated by the MET criterion are typically 7–10 days later. The planting dates generated by the DEPTH criterion are some 2–3 weeks later than the AREX dates. The evaluation revealed that in non-failure years, the relative transpiration rate over the 30 days is about 70% for all criteria and for all zones. However, the risk a farmer runs of a false start when applying the AREX criterion is unacceptably high (1 year out of 2). Applying the MET criterion reduces the failures to 2 years out of 5 and when the DEPTH criterion is applied, the probability of failure drops to 1 year out of 4.