How heavy rainfalls challenge the sustainability of irrigated agriculture

On October 6, we were in Valencia to present our research on the sustainability of irrigation methods in citrus orchards. It was one of these rare days in the year at which Valencia experiences one of its few, but heavy rainfall events. More than 100 mm fell in one day - this makes up almost 20% of its total rainfall in an entire year! For us, the heavy rain meant that we arrived at the conference with wet feet. For the farmers, it meant that much of the fertilizer they have applied to their citrus orchards in the previous weeks was about to be washed down to the groundwater, compromising groundwater quality and challenging the health of nearby groundwater-related ecosystems. Interestingly, farmers practicing the technologically advanced drip irrigation were likely losing more of their fertilizer than farmers practicing traditional flood irrigation. Why is that? And what does that mean for the sustainability of citrus production?

Why and when can advanced irrigation systems be less efficient than traditional irrigation practices?

The efficiency of irrigation systems is typically quantified by the percentage of irrigation water and fertilizer lost to groundwater. Since water is the “transport vehicle” of fertilizer, it is important to understand when groundwater recharge is occurring. Our computer simulations for the citrus orchards in Valencia show that the amount of groundwater recharge is a result from the combined effect of irrigation and rainfall.

Picture from: Pool, S., Francés, F., Garcia-Prats, A., Puertes, C., Pulido-Velazquez, M., Sanchis-Ibor, C., Schirmer, M., Yang, H. and Jiménez-Martínez, J., 2022. Impact of a transformation from flood to drip irrigation on groundwater recharge and nitrogen leaching under variable climatic conditions. Science of The Total Environment, 825, p.153805.

Flood irrigation is conducted once a month during the dry spring and summer months, and recharge is occurring due to excess irrigation. Instead, in drip irrigation water is applied several days a week and also during months with rainfall. On very rainy days, such as October 6, soils in drip irrigation are already wet from the recent irrigation. Soils in drip-irrigated orchards have less capacity to store rainwater than the drier soils in flood-irrigated orchards, therefore leading to enhanced groundwater recharge and loss of fertilizer. Nevertheless, drip irrigation is more efficient than flood irrigation on long-term, because the number of heavy rainfalls, which are the main driver of these unexpected fertilizer losses, varies largely between years.

We also found that heavy rainfalls play a crucial role not just for the day they happen, but also for the coming year. For example, in a relatively dry year, less fertilizer is flushed away than in a relatively wet year, leading to an increased potential for fertilizer loss in the following year. This effect is also known as the memory effect.

Citrus orchard in the research area during a field trip (Picture: Sandra Pool)

How can the sustainability in citrus production be improved?

The introduction of drip-irrigation is an important measure to reduce fertilizer losses to the groundwater. However, understanding when and why fertilizer is lost, helps to further improve water and nitrogen management – and there seems to be quite a lot of potential to become more sustainable. Our studies show that (i) the timing of fertilizer application, such as avoiding inputs before the onset of the rainy months, is key for further reducing the nitrogen leaching risk, and (ii) considering the year-to-year memory of soil nitrogen storage and its dependence on annual precipitation can be used to implement a simple adaptive inter-annual fertilizer management.

Clearly, heavy rainfall events challenge the current and future sustainability of irrigated agriculture in Mediterranean regions. Knowing, understanding, and adapting to this challenge can substantially contribute to a more sustainable agriculture and increased groundwater protection.

ABOUT THE AUTHOR

Sandra Pool is a scientist in the group of Subsurface Environmental Processes at EAWAG. She is trained as a hydrologist with a focus on modelling hydrological systems at the catchment scale using large-sample datasets.

Joaquín Jiménez-Martínez is a scientist (Group Leader of the Subsurface Environmental Processes Group) at EAWAG and Lecturer and Research Associate at the Department of Civil, Environmental and Geomatic Engineering-ETH Zurich as part of the Chair of Groundwater and Hydromechanics. His research focuses on the study of transport and reaction phenomena in porous and fractured media, particularly in multiphase systems.