Abstract
Abstract. Understanding and quantifying the impact of changes in climate and land use/land cover on water availability is a prerequisite to adapt water management; yet, it can be difficult to separate the effects of these different impacts. In this paper we illustrate a separation and attribution method based on a Budyko framework. We assume that evapotranspiration (ET) is limited by the climatic forcing of precipitation (P) and evaporative demand (E0), but modified by land-surface properties. Impacts of changes in climate (i.e., E0/P) or land-surface changes on ET alter the two dimensionless measures describing relative water (ET/P) and energy partitioning (ET/E0), which allows us to separate and quantify these impacts. We use the separation method to quantify the role of environmental factors on ET using 68 small to medium range river basins covering the greatest part of the German Federal State of Saxony within the period of 1950–2009. The region can be considered as a typical central European landscape with considerable anthropogenic impacts. In the long term, most basins are found to follow the Budyko curve which we interpret as a result of the strong interactions of climate, soils and vegetation. However, two groups of basins deviate. Agriculturally dominated basins at lower altitudes exceed the Budyko curve while a set of high altitude, forested basins fall well below. When visualizing the decadal dynamics on the relative partitioning of water and energy the impacts of climatic and land-surface changes become apparent. After 1960 higher forested basins experienced large land-surface changes which show that the air pollution driven tree damages have led to a decline of annual ET on the order of 38%. In contrast, lower, agricultural dominated areas show no significant changes during that time. However, since the 1990s effective mitigation measures on industrial pollution have been established and the apparent brightening and regrowth has resulted in a significant increase of ET across most basins. In conclusion, data on both, the water and the energy balance is necessary to understand how long-term climate and land cover control evapotranspiration and thus water availability. Further, the detected land-surface change impacts are consistent in space and time with independent forest damage data and thus confirm the validity of the separation approach.
Highlights
Evapotranspiration (ET) is physically limited by the supply of both, water and energy (Budyko, 1974), while landsurface characteristics strongly modify the accessibility of water and absorbed energy for ET
Looking at the decade 1950–1959 in Fig. 7a, we find the hydro-climatic conditions of the basins to be generally close, not all basins were gaged at that time
There is the control by climatic conditions which is best described by the aridity index
Summary
Evapotranspiration (ET) is physically limited by the supply of both, water and energy (Budyko, 1974), while landsurface characteristics strongly modify the accessibility of water and absorbed energy for ET. The partitioning of water and energy at the land surface through actual evapotranspiration emerges from the interaction of various. M. Renner et al.: Separating the effects of changes in land cover and climate land-surface processes under the atmospheric supply and demand for water. One of the key questions for environmental sciences is how evapotranspiration ET might vary under the pressure of environmental changes. Changes in the water partitioning into ET and runoff are, for example, relevant for the management of water resources, agriculture and forestry, while changes in the energy partitioning may affect regional climates (Milly and Dunne, 2001)
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