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Seasonal Drought Index

An alternative approach

The Seasonal Drought Index (SDI)

The Seasonal Drought Index (SDI) is a compromise. It uses the well-tried tools of MarkSim and the failed seasons algorithm to build in a flexible approach to an overall crop drought susceptibility. Plant breeders are interested in where drought occurs not only geographically, but also in the sequence of crop development. Choice of cultivars with differing phenology makes the overall envelope of the phenology of a crop difficult to model 
ab initio. We therefore decided to present a set of maps that can be interpreted for different crops in different places without restricting ourselves to an overarching phenology.

To be of use for more than one crop type the key factor is that they incorporate the crucial cutoff Ea/Et values for the crop. These are as yet unknown for many crops but can be inferred by analogy. 
Phaseolus beans are rather less drought tolerant than maize, cowpeas are one of the more drought tolerant crops, probably more so that sorghum but maybe less than millet. We can therefore construct a list of crops ranked by their perceived drought hardiness and, using the two values already obtained in this study, infer the level at which Ea/Et starts to severely impede yield.

The baseline planting date was taken from the algorithm that produced Map 1. Derived from multiple runs of the failed seasons algorithm, it calculated the modal planting date from 30 runs of MarkSim data using a count of season starts into variable bins and arithmetic modulo 2π. There are four sets of maps one set corresponding to each of the following Ea/Et cutoff points; 0.30, 0.35, 0.40 and 0.45. Within each set there are ten maps corresponding to the 10 consecutive 20-day periods from planting to 200 days. The maps show the probable proportion of days with each 20-day growing period with Ea/Et less than the critical value calculated from a water balance on 30 years of MarkSim output.

To use the maps first choose the Ea/Et value corresponding to the crop in question. We must use some guesswork here but if we rank the crops in their order of drought susceptibility we might get something like the following table



Scientific name


Sweet potato





Ipomoea batatas

Phaseolus vulgaris

Arachis hypogea

Oryza sativa

Solanum tuberosum







Zea mays

 Cicer arietinum

Triticum aestivum




Sorghum spp




Vigna unguiculata



(Cassava and banana cannot be conveniently placed in this context because they have growing periods throughout the year)

Next choose the phenology appropriate for the crop. This depends heavily on the cultivar in question and the environmental context and so cannot be readily generalized. However a little knowledge of the crop is often enough to suffice; for example, (1) dry beans in the lowland tropics with bush type cultivars have about 80 to 90 days to harvest with flowering after about 35 days, but (2) climbing beans in the uplands have about 140 days to harvest with flowering after about 50 to 60 days.

In all cases the map to consult for early season drought stress is map 1 from each Ea/Et set. For stress during and close after flowering, consult maps 2 or 3 for case (1) and maps 3 or 4 for case (2). For late grain filling consult map 4 or 5 for case (1) and maps 6 or 7 for case (2).

Note that the maps refer only to crops sown as first season crop and will not apply for beans in relay after maize.