Climate change, re-/afforestation, and urbanisation impacts on evapotranspiration and streamflow in Europe

Abstract

Abstract. Since the 1950s, Europe has seen large shifts in climate and land cover. Previous assessments of past and future changes in evapotranspiration or streamflow have either focussed on land use/cover or climate contributions, or have focussed on individual catchments under specific climate conditions. Here, we aim to understand how decadal changes in climate (e.g., precipitation, temperature) and land use (e.g., de-/afforestation, urbanization) have impacted the amount and distribution of water resources availability across Europe since the 1950s. To this end, we simulate the distribution of green and blue water fluxes at high-resolution (1 × 1 km) by combining (a) a steady-state Budyko model for water balance partitioning constrained by long-term (lysimeter) observations across different land-use types, (b) a novel decadal high-resolution historical land use reconstruction, and (c) gridded observations of key meteorological variables. The continental-scale patterns in the simulations agree well with coarser-scale observation-based estimates of evapotranspiration, and also with observed changes in streamflow from small basins across Europe. We find that strong shifts in the continental-scale patterns of evapotranspiration and streamflow have occured from 1950 to 2010. In Sweden, for example, increased precipitation dominates effects of large scale re- and afforestation leading to increases in both streamflow and evapotranspiration. In most of the Mediterrenean, decreased precipitation combines with increased forest cover and potential evapotranspiration to reduce streamflow. In spite of local and regional scale complexity, the Europe-wide net contribution of land use, precipitation and potential evapotranspiration changes to changes in ET is similar with around ~ 40 km3/y, equivalent to the discharge of a large river. For streamflow, changes in precipitation dominate land use and potential evapotranspiration contributions with ~ 90 km3/y compared to ~ 45 km3/y. Locally, increased forest cover and urbanisation have lead to significant decreases and increases of available streamflow.

Global drought trends and future projections

“…In north-eastern Brazil, increased hydrological drought severity is mostly related to the rapid increase in the surface area of irrigated land, which grew by more than 200% between 1996 and 2017 [83]. Also, in the Mediterranean region, most of the increase in the frequency and severity of hydrological droughts can be traced to changes in the land in the headwaters and a higher demand for irrigation [58,[84][85][86].…”

Unraveling the influence of atmospheric evaporative demand on drought and its response to climate change

“…This would have consequences on hydrological drought conditions, since independently of the water use efficiency of vegetation, the projected increase in the vegetation green fraction in non‐water‐limited regions (Mahowald et al, ) may increase ET, reducing streamflow and affecting the severity of hydrological droughts (Mankin, Smerdon, Cook, Williams, & Seager, ; Mankin, Seager, Smerdon, Cook, & Williams, ). This phenomenon was observed in the northern part of the Mediterranean region when strong increase in forest coverage—given natural succession after land abandonment in mountain slopes—caused strong reduction of surface runoff as a consequence of higher water consumption by vegetation favored by an increased AED (Beguería, López‐Moreno, Lorente, Seeger, & García‐Ruiz, ; García‐Ruiz et al, ; López‐Moreno et al, ; Teuling et al, ; Vicente‐Serrano, Azorin‐Molina, Sanchez‐Lorenzo, Revuelto, López‐Moreno, et al, ; Vicente‐Serrano, Azorin‐Molina, Sanchez‐Lorenzo, Revuelto, Morán‐Tejeda, et al, ; Vicente‐Serrano, Lopez‐Moreno, et al, ; Vicente‐Serrano, Peña‐Gallardo, et al, ). Nevertheless, despite large uncertainties in the climate model projections for future scenarios, it should be always considered that climate models themselves are fully coupled atmosphere–land (and ocean) models, which are internally consistent (yet imperfect) representations of the climate system (Milly & Dunne, ; Y. Yang et al, ).…”

Afforestation impacts on terrestrial hydrology insignificant compared to climate change in Great Britain

“…LULC alters streamflow by changing hydrological processes (e.g., subsurface flow) over multiple spatial and temporal scales. One example of LULC is afforestation, which can lower catchment water tables (Kellner and Hubbart, 2018;Shuttleworth et al, 2019;Peskett et al, 2020), increase precipitation downwind (Teuling et al, 2019;Meier et al, 2021;Xu et al, 2022) and alter transpiration rates over time (Newson and Calder, 1989;Hudson et al, 1997;Marc and Robinson, 2007). Many studies suggest afforestation can reduce overall streamflow, however, land management practices 30 (e.g., artificial ditching and road cutting) may also increase peak streamflow (Beschta et al, 2000;Bathurst et al, 2018).…”