EPICphase, a version of the EPIC model simulating the effects of water and nitrogen stress on biomass and yield, taking account of developmental stages: validation on maize, sunflower, sorghum, soybean and winter wheat

The plant module of the EPIC model (Erosion Productivity Impact Calculator) has been modified to simulate the effects of water and nitrogen (N) stress on biomass production and grain yield, taking account of the sensitivity of the crops to water and N stress during the course of their developmental cycle. This French version of the model, EPICphase, was validated with maize (Zea mays L.), sunflower (Helianthus annuus L.), sorghum (Sorghum bicolor L. Moench), soybean (Glycine max L.), and winter wheat (Triticum aestivum L.) over 9 years, using experimental data from a long-term cropping system experiment carried out at three levels of cropping intensity. The results have been compared with those from a normal version of EPIC. The two versions use the same input files. The results show that EPIC overestimates crop production by comparison with the measured data, notably under conditions of severe moisture stress. The additional crop parameters introduced into EPICphase concern the water extraction capacities peculiar to each crop, the division of the growth period into four phases, with adjustments to the conversion efficiency of intercepted radiation into biomass, and the drought adaptation of sunflower and soybean. Finally, the sensitivity of the harvest index to water and N stress has been introduced for each phase of growth. These refinements have led to simulations very close to the measured values. Comparison of results from the two versions was done by means of a statistical study of mean biomass production and grain yield, standard deviations, root mean square error, regression lines, and R² values.