Agricultural Impacts of Regional Variability of the West African monsoon

1. Agricultural Impacts of Regional Variability of the West African monsoon ETP 15N 22.5N sowing Crop coefficient 15N 10N 10N 5N 5N Reservoir First rain 10W 10E 5N Root front Humectation front Rain Rain Crop coefficient A water balance model at the local scale 5N 15N 10N Vegetative stage Reproducing stage Maturation I Maturation II Flowering Grain ripening A biomass dynamics model B. SULTAN*, C. BARON**, M. DINGKUHN** and S. JANICOT** IRD –LODYC / IPSL (France) benjamin.sultan@lodyc.jussieu.fr** CIRAD – Amis / Agronomie / Ecotrop (France) michael.dingkuhn@cirad.fr SARRAH : a generic model to simulate potential yield Stability of attainable yield around the sowing date Introduction Two main aspects of the rainy season at regional scale : An ensemble of simulations while varying the sowing date between the 50 days before and the 30 days after the date of the regional installation of monsoon : A composite series around the regional onset date An abrupt shift of the Intertropical convergence zone (ITCZ) linked to the onset of the rainy season The evidence of enhanced and weakened monsoon phases at intra-seasonal time-scale Regional onset date 1st objective : To compare the traditional and local view of the onset of the rainy season, taking in account in agricultural strategy for the choice of the sowing date, to our dynamic definition of the onset at regional scale 2nd objective : To point out the agricultural impact of the occurrence of dry spells during the rainy season Use a regional information on monsoon dynamics at the local scale of the agricultural field A high stability of yield during nearly one month around the onset date A sensitivity experiment on a field in Niamey (Niger) An abrupt shift of the ITCZ « abrupt » shift of the ITCZ A different impact of the intra-seasonal dry sequences relative to the crop dynamics Daily rainfall values averaged from 10W to 10E for the year 1978 19 years of daily observations of rainfall and ETP over the 1968-1990 period Regional onset date Simulated yield Yield in 1990 Break Impact Experiment I : THE TRADITIONNAL METHOD The yield is simulated by using a traditionnal method to choose the sowing date : a local rainfall threshold not followed by significant dry spell • To define the date of the installation of the monsoon : • A decrease of the 5N rainfall index • An increase of the positive slope of the 10N rainfall index Break Impact Yield in 1968 Experiment II : THE REGIONAL ONSET OF THE MONSOON We have fixed for sowing date the onset date of the monsoon defined at regional scale : a regional information at the local scale of the field Sowing date (t0-50  t0+30) « abrupt shift » of the ITCZ Phenological stages Sahelian rainfall Control experiment : THE IDEAL SOWING DATE We sart the sowing each day of the year and we retain the date where the yield is maximum : the ideal sowing date at local scale The onset of the monsoon : 17 june 1978 Rainfall in 1990 Flowering and grain ripening : the critical stages during the crop development Rainfall in 1968 The results : sowing dates and potential yield Ideal sowing date Mean : 22 June Sdv : 11 days Over the 1968-2002 period, the mean shift date : 24 June Break Break Monsoon onset Mean : 24 june Sdv : 8 days Time (t0-90  t0+140) Evidence of enhanced and weakened monsoon phases A strong coherence between the results of the model forced by rainfall at the plot level and regional rainfall dynamics on the whole Sahel through a different impact of intra-seasonal variability relative to the crop dynamics Traditionnal method Mean : 02 June Sdv : 32 days An example of rainfall seasonal cycle : A Sahelian rainfall index averaged from 2.5W to 2.5E and between 12.5N and 15N for the year 1968. The mean ideal sowing date and its interannual variability are very similar to the ones of the regional onset dates Conclusions Wavelet Modulus (mm²) The study of the West African monsoon dynamics has highlighted two important points : The onset stage of the monsoon over West Africa is linked to an abrupt shift of the ITCZ with a mean date of 24 June and a standard deviation of 8 days Intra-seasonal signal between 30 and 60 days The evidence of an intra-seasonal variability located in two different spectral windows : 10-25 and 30-60 days Intra-seasonal signal between 10 and 25 days Sensitivity experiments on the mil crop in Niamey with the crop model SARRAH have shown a strong coherence between this large scale of the monsoon dynamics and the plot level : An increase of yield by choosing the regional onset of the summer monsoon as the sowing date Synoptic variability A good knowledge of the seasonal cycle and its onset in West Africa can be useful to improve the yield through a better choice of the sowing date Sahelian Rainfall (mm) Regional monsoon onset Traditionnal method Average 75% 56% A different impact of dry spells at low frequency relative to the crop dynamics Std.Dev 26% 36% Var.Coeff 34% 64% Sultan B, C Baron, M Dingkuhn, B Sarr and S Janicot (2004) Agricultural impacts of large-scale variability of the West African monsoon, Agr. For. Meteorol., in press. A stronger interannual variability with the traditionnal method 1 May 31 Oct 1968