Intra- and inter-annual monitoring of the Atmospheric Boundary Layer at a semiarid site in the Eastern Ebro basin during the current long-term drought

The site of Els Plans is located since June 2021 in a semiarid area protected for steppe-land birds in the Eastern Ebro Basin, in the term of Preixana (Catalonia), close to a wide irrigated area. Its initial installation was as a reference site in the non-irrigated area of the 2021 LIAISE campaign (Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment), mainly intended to study the effect of missing irrigation in numerical models. After the end of the campaign in September 2021, some of the teams involved in Els Plans decided to leave the instruments there for some more time. In particular, the UKMO deployment, which included a 50m-high meteorological tower, stayed until July 2022. The resulting 13-month series includes the start of a drought period after a very moist winter in 2021. It allows to inspect the intra-annual evolution of some ABL characteristics at the semi-arid site, including evaporation, dew and frost, soil moisture, local circulations, fog events or mesoscale circulations.

Since July 2022 a Surface Energy Budget station and a weighable lysimeter continue operation allowing to perform the inter-annual monitoring of the aforementioned quantities, including the exceptional drought event in Catalonia that started during 2021 and is still active at the time of writing. The previous long-term drought was in 2008. This information allows to perform an interannual analysis using the three-year period available. The Canal d'Urgell bringing water from the dams in the Pre-Pyrenees was closed for the first time in history in 2023 with a huge impact in the local agriculture and economy. Some substantial rain events have happened since fall 2023, allowing for some greening in the area and the progress of the winter cereals. Long-term observations are necessary to better understand prolonged drought and its impact on terrestrial ecosystem and its feedback mechanism between the different compartments (soil, water, atmosphere, and vegetation).