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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 landsurface
change impacts are consistent in space and time with independent forest damage data and thus confirm the validity of the separation approach.