Climate change in drylands has caused alterations in the seasonal
distribution of rainfall including increased heavy-rainfall events, longer
dry spells, and a shifted timing of the wet season. Yet the aboveground net
primary productivity (ANPP) in drylands is usually explained by
annual-rainfall sums, disregarding the influence of the seasonal distribution
of rainfall. This study tested the importance of rainfall metrics in the wet
season (onset and cessation of the wet season, number of rainy days, rainfall
intensity, number of consecutive dry days, and heavy-rainfall events) for
growing season ANPP. We focused on the Sahel and northern Sudanian region
(100–800 mm yr−1) and applied daily satellite-based rainfall
estimates (CHIRPS v2.0) and growing-season-integrated normalized difference
vegetation index (NDVI; MODIS) as a proxy for ANPP over the study period:
2001–2015. Growing season ANPP in the arid zone
(100–300 mm yr−1) was found to be rather insensitive to
variations in the seasonal-rainfall metrics, whereas vegetation in the
semi-arid zone (300–700 mm yr−1) was significantly impacted by
most metrics, especially by the number of rainy days and timing (onset and
cessation) of the wet season. We analysed critical breakpoints for all
metrics to test if vegetation response to changes in a given rainfall metric
surpasses a threshold beyond which vegetation functioning is significantly
altered. It was shown that growing season ANPP was particularly negatively
impacted after > 14 consecutive dry days and that a rainfall intensity of
∼ 13 mm day−1 was detected for optimum growing season ANPP.
We conclude that the number of rainy days and the timing of the wet season
are seasonal-rainfall metrics that are decisive for favourable vegetation
growth in the semi-arid Sahel and need to be considered when modelling
primary productivity from rainfall in the drylands of the Sahel and
elsewhere