Assessing drought vulnerability of maize production in the Po Valley

Drought affects a wide range of economic activities, with agriculture as the worst affected sector by the consequences of such an extreme in many regions of the world. Past studies showed that droughts and heat waves are the weather extremes that significantly reduce cereal production at global level, while there is no evidence on the influence of floods and extreme cold on cereal yields. The projected increase in the severity and frequency of droughts can lead to water scarcity situations in regions that are already water-stressed and to overexploitation of available water resources in other areas.

The way regulators and farmers manage water resources during droughts has effects on agricultural resilience and the increased frequency of drought and water scarcity will require more collaborative partnership-based approaches to water resources and drought management in the next future. The development of quantitative models to establish relationships between water scarcity and crop yield losses can help in understanding in which situations farmers need access to water to avoid high losses.

This study develops crop specific vulnerability curves that establish a relationship between water deficit and yield losses during various crop growth stages (vegetative, flowering and yield formation) and can thus provide useful indication on how to allocate water resources to avoid irreparable yield losses. The case study region is the Po river basin (Northern Italy).

The Po river basin is the largest Italian agricultural area and accounts for 35% of the country’s agricultural production. The basin is characterized by the presence of big cities and wide rural zones. Over the past years the it has been hit by multiple droughts. Ten cities were considered in the analysis, based on maize yield data provided by the Italian National Institute for Statistics (ISTAT).

At first the Agricultural Production System sIMulator (APSIM) crop model was used to simulate maize growth. The model was calibrated and validated over the ten provinces based on ISTAT data. An R² of 0.75 was found for both the steps.

The yield in the absence of any water stress during the entire growing season was computed as the reference yield. Then, the reduced yield for the same season was derived introducing a water stress in a single growth stage by progressively reducing the precipitation amount during that growth stage. The yield reduction was expressed as one minus the ratio between the reduced yield and the reference yield.

The water deficit for each season and each growth stage was derived from APSIM. The relationship between yield reduction and water deficit was plotted to derive the vulnerability curves and data points were fitted to appropriate functions.

In the case of maize, flowering was found to be the most sensitive stage to water deficit, followed by yield formation and vegetative. During the establishment phase the crop never went under water stress in the considered area. Soil texture has also proven to play a role on the response of the crop to the water deficit, particularly in the flowering and yield formation stages.