In 2018, a group of students from the Universities of Innsbruck, Austria, and Hamburg, Germany, were on a research excursion close to the village of Llupa in the Rio Santa valley in the Peruvian Andes. While they were busy installing a weather station in preparation for their research project, they were surprised by unexpected rainfall-a brief yet notable shower. To the students, this seemed highly unusual. It was August, months into the Andean dry season.
The Rio Santa valley, located at about 3000 meters, lies at the foothills of the impressive snow-capped peaks of the Cordillera Blanca mountain range that attracts thousands of hikers, climbers and other tourists every year. The valley is home to many small-scale farmers that grow crops like wheat, maize and potatoes, which they depend on for their self-subsistence and for trade on the markets of Huaraz, the region’s capital. Traditional farming here relies on the experience and memories gained by generations of farmers that are deeply rooted in the valley.
Research teams from the University of Innsbruck and Hamburg have studied the regions’ water cycle for decades. In the beginning, they used weather station data and models to understand the state of the rapidly retreating glaciers. These studies were essential to understand the underlying physical processes that accelerate the melting process, as tropical glaciers behave very differently to those in the European Alps.
Following the meltwater downstream, researchers examined how the water was used by local communities. Conversations with local farmers highlighted the importance of seasonal rainfall for farming, as few farms have direct access to irrigation systems with year-round water availability. These discussions prompted the researchers to shift their focus from glacier dynamics to atmospheric processes and their impact on vegetation and farming.
This is how the research project " AgroClim Huaraz " came into being. Formed by an international collaboration of researchers from institutions in Peru, Austria, the UK, France, and Switzerland, and funded by the Austrian Academy of Sciences (ÖAW), the project set the new goal to better understand precipitation patterns in the region, how they affect rain-fed agriculture, and how they changed in the past and might change in the future.
Unreliable rainBut let’s go back to the rainfall that surprised the students in 2018. Intrigued by this event, the researchers set out to understand more. Talking to local farmers, they soon found out that these rains were referred to as "Pushpa" and are of crucial importance to farmers in the Rio Santa Valley. Pushpa rains mark the end of the dry period and the beginning of the rainy season and typically occur between August and September. They moisten the soil for the first time after the dry season, and farmers await them eagerly to begin with the sowing of their crops.
"Small-scale farmers in Huaraz are highly dependent on the rain, because for most it is the only source of water", says Wolfgang Gurgiser from the University of Innsbruck’s research area " Mountain Regions ", who was involved in several of the project’s studies. "The water coming from the glaciers is contaminated by naturally occurring heavy metals, which makes it unusable for consumption or irrigation in some watersheds."
Hence, predicting rain is highly important. Which makes it all the more concerning that the Pushpa rains are very unreliable.
From interviews with farmers, scientists were made aware of several perceived changes and threats: according to the farmers’ observations, Pushpa rainfall had become increasingly delayed over the last years, had occurred less regularly and become more intense, which locals believe may be a consequence of climate change. Farmers reported the experienced changes were threatening their traditional practices and livelihoods.
Rolando Cruz Encarnación, a local expert from the National Water Authority in Peru, adds: "The light rainfalls are supposed to announce the beginning of the rainy season, but if farmers decide to sow and Pushpa is followed by a dry period, this has negative implications for the entire season."
Understanding Pushpa"Until then, Pushpa was undocumented in the scientific literature. Hearing about these rains moved our focus into a completely new and interesting direction", says Cornelia Klein, a former postdoc at the Department of Atmospheric and Cryospheric Sciences of the University of Innsbruck and now at the UK Centre for Ecology and Hydrology in Wallingford. "How is Pushpa rainfall different from the rainfall in the core of the wet season? And how did it change in the past?" These new questions motivated her and Cornelia Zauner, a former Masters student at the University of Innsbruck.
The resulting study, published in the journal Environmental Research Communications , is the first to describe the mechanisms behind the Pushpa rains in the Rio Santa valley under a scientific scope. Surprisingly, Pushpa rains are found to be connected to dry winds coming from the Pacific in the west, which are typical for August and usually bring dry conditions - or, as now discovered, light rain. After the dry season, the winds turn and bring the towering heavy rain clouds from the Amazon basin in the east. Pushpa rains are indeed of a different nature than the intense rain in the wet season.
By looking at model rainfall data representing the past 40 years, Dr. Klein and her colleagues also found that the Pushpa rains are highly variable, alternating between abundant and scarce years. This is also supported by an observational study conducted by Lorenz Hänchen, PhD candidate at the Department of Ecology of the University of Innsbruck, and published in the journal Earth System Dynamics : "By looking at years of satellite data of plant greenness, we found that the start of the growing season can vary by up to two months from one year to another", says Hänchen. "This is certainly a very difficult situation for agricultural planning".