Scaling-up the AFRCWHEAT2 model to assess phenological development for wheat in Europe

A method was developed for scaling-up the AFRCWHEAT2 model of phenological development from the site to the continental scale. Four issues were addres sed in this methodology: (i) the estimation of daily climatic data from mon thly values, (ii) the estimation of spatially variable sowing dates, (iii) the simulation of multiple cultivars, and (iv) the validation of broad-scal e models. Three methods for estimating daily minimum and maximum temperatur es from monthly values were compared using AFRCWHEAT2: a sine curve interpo lation, a sine curve interpolation with random daily variability, and two s tochastic weather generators (WGEN and LARS-WG). The sine curve interpolati on was selected for the continental scale application of AFRCWHEAT2 because computational time was short and errors were acceptably small. The average root mean square errors (RMSEs) for the dates of double ridges, anthesis a nd maturity were 6.4, 2.2 and 2.1 days, respectively. The spatial variabili ty of European sowing dates was reproduced using a simple climatic criterio n derived from the AFRCWHEAT2 vernalization curve. The use of several culti var calibrations enabled the broad-scale model to capture current responses and compare responses to future climate change. Results from the continent al scale model were validated using a geographically-referenced database of observed phenological dates, output from other site-based models and sensi tivity analysis. The spatial model was able to emulate a similar spatial an d temporal variability in phenological dates to these sources under the pre sent climate. The predominant effect of an increase in mean temperature was a reduction in the emergence to double ridges phase. The shift in the timi ng of subsequent development stages to earlier in the season meant that cha nges in their duration were relatively minor. Changes in inter-annual tempe rature variability resulted in only small changes in the mean date of devel opment stages, but their standard deviation altered significantly. (C) 2000 Elsevier Science B.V. All rights reserved.