A study has shown how evapotranspiration, i.e., water used by vegetation or evaporated from the soil, drives the water balance during multi-year droughts. Vegetation water consumption during droughts is still a poorly understood topic, and these results will provide a starting point to improve our knowledge and consequently our predictive models
It is intuitive to think that water resources will be abundant during periods with high precipitation, while periods with low precipitation will translate into water shortages. However, the proportion between precipitation and the consequent water supply is not always as expected. In fact, scientists have already observed that, during multi-year droughts, water availability will generally be lower than one would expect. But why? According to a study recently published in Hydrology and Earth System Sciences and coauthored by one of the researchers of CIMA Research Foundation, this is because of plants, which require some extra time to adapt to droughts.
An unbalanced balance
“In areas with a Mediterranean climate, i.e., those in which precipitation is larger in winter and lower in summer, multi-year droughts generally lead to a drop in available water resources. This decrease is even 30+ percent larger than you would expect,” Francesco Avanzi, researcher in the Hydrology and Hydraulics Department of CIMA Research Foundation and first author of the paper, explains. “This process is rarely reproduced by hydrological models. More precisely, these predictive models often overestimate the amount of water that will be available during a long period of drought. In this research, we investigated the mechanisms underlying this shift in the water balance, in order both to better understand the possible effects of drought and to improve the modeling.”
The study focused on a particular area of California, a US state with Mediterranean climate that has been affected by four major and long droughts since the 1970s. “The last one was in 2012 and lasted until 2016-17. It has had very serious effects on crops, vegetation, ecosystems, and even water supply for civil consumption. Vegetation across the Sierra Nevada is still suffering from the adverse effects of this drought”, Avanzi continues.
The first step of their researchers was to quantify the shift observed during the four major Californian droughts; in other words, they calculated how much less water there was compared to expected. Then, using past droughts as testbed, they investigated the predictive accuracy of the PRMS model, a tool that is widely used in operational hydrology across the US. Finally, they pinpointed the factors that most influenced the drop in predictive accuracy during droughts. Comparing observed data with those produced by the model, they realized that the factor that was most affecting the results was evapotranspiration, i.e. water used by vegetation and that evaporates from the soil.
Plants with little elasticity
“For vegetation, adapting to changing environmental conditions is not immediate. On the contrary, in periods of drought, plants can initially consume even more water than usual. This depends on several factors, not least the fact that the recent drought has been associated with comparatively high temperatures”, Avanzi explains. “Only after a certain period of time, a series of adaptation processes take place: some plants reduce their water consumption, others die, giving way to species that are more able to live in drought, others are consumed by fires.”
Avanzi and coauthors called “climate elasticity of evapotranspiration” the capability of vegetation-water use to adapt to changes in environmental conditions. This factor is important in order to assess how much vegetation can affect water availability during droughts – including water needed by our societies.
“This study helps us better understand the impact of multi-year droughts on the water balance, an important field of study, especially considering that climate change may affect the timing and frequency of droughts. It also allows us to work on improving forecasting models in a more targeted way, so that these models will be able to simulate all components of the water balance and not “only” those that have been traditionally important for our disciplines, such as streamflow. In other words, these studies will allow us to get the right answer for the right reason: this is what we are now doing with the Continuum model, developed by CIMA Research Foundation”, the researcher concludes.