Seasonal predictions of agro-meteorological drought indicators for the Limpopo basin

F Wetterhall,E Pappenberger,E Dutra,M Werner,H C Winsemius

doi:10.5194/hess-19-2577-2015

Abstract

Abstract. The rainfall in southern Africa has a large inter-annual variability, which can cause rain-fed agriculture to fail. The staple crop maize is especially sensitive to dry spells during the early growing season. An early prediction of the probability of dry spells and below normal precipitation can potentially mitigate damages through water management. This paper investigates how well ECMWF's seasonal forecasts predict dry spells over the Limpopo basin during the rainy season December–February (DJF) with lead times from 0 to 4 months. The seasonal forecasts were evaluated against ERA-Interim reanalysis data, which in turn were corrected with GPCP (EGPCP) to match monthly precipitation totals. The seasonal forecasts were also bias-corrected with the EGPCP using quantile mapping as well as post-processed using a precipitation threshold to define a dry day. The results indicate that the forecasts show skill in predicting dry spells in comparison with a climatological ensemble based on previous years. Quantile mapping in combination with a precipitation threshold improved the skill of the forecast. The skill in prediction of dry spells was largest over the most drought-sensitive region. Seasonal forecasts have the potential to be used in a probabilistic forecast system for drought-sensitive crops, though these should be used with caution given the large uncertainties.

Highlights

Southern Africa is largely a semi-arid region, which experiences substantial inter- and intra-annual rainfall variability (Barron et al, 2003; Nyakudya and Stroosnijder, 2011)
While a lower total amount of rainfall over the crop growing season will influence the crop yield, it is often the poor temporal distribution of rainfall resulting in dry spells and wet spells that is the cause of reduced crop yields (Rockstrom, 2000; Ingram et al, 2002; Ochola and Kerkides, 2003; Barron, 2004; Usman and Reason, 2004; Barron and Okwach, 2005)
The results from the hindcast period were evaluated in terms of the effect of the precipitation threshold and bias correction

Summary

Introduction

Southern Africa is largely a semi-arid region, which experiences substantial inter- and intra-annual rainfall variability (Barron et al, 2003; Nyakudya and Stroosnijder, 2011). A staple crop that is grown widely in southeastern Africa is maize, the yields of which are sensitive to the occurrence of dry spells, depending on when these occur. Barron et al (2003) discuss the sensitivity of maize to the occurrence of dry spells in different stages of the growing season, being high in the first 50 days after sowing, and again during the grain filling stage (70–90 days after sowing). The onset of the rains is important, with planting only done after initial rains exceeding 30–40 mm on eight consecutive days in areas studied in Tanzania and Kenya (Barron et al, 2003) and 25 mm on 7 consecutive days or 40 mm on 4 in northern Zimbabwe (Nyakudya and Stroosnijder, 2011).

Methods

Results

Discussion

Conclusion