Abstract
Abstract. Climate change is expected to alter the global hydrological cycle, with inevitable consequences for freshwater availability to people and ecosystems. But the attribution of recent trends in the terrestrial water balance remains disputed. This study attempts to account statistically for both trends and interannual variability in water-balance evapotranspiration (ET), estimated from the annual observed streamflow in 109 river basins during "water years" 1961–1999 and two gridded precipitation data sets. The basins were chosen based on the availability of streamflow time-series data in the Dai et al. (2009) synthesis. They were divided into water-limited "dry" and energy-limited "wet" basins following the Budyko framework. We investigated the potential roles of precipitation, aerosol-corrected solar radiation, land use change, wind speed, air temperature, and atmospheric CO2. Both trends and variability in ET show strong control by precipitation. There is some additional control of ET trends by vegetation processes, but little evidence for control by other factors. Interannual variability in ET was overwhelmingly dominated by precipitation, which accounted on average for 54–55% of the variation in wet basins (ranging from 0 to 100%) and 94–95% in dry basins (ranging from 69 to 100%). Precipitation accounted for 45–46% of ET trends in wet basins and 80–84% in dry basins. Net atmospheric CO2 effects on transpiration, estimated using the Land-surface Processes and eXchanges (LPX) model, did not contribute to observed trends in ET because declining stomatal conductance was counteracted by slightly but significantly increasing foliage cover.
Highlights
Climate change is expected to alter the global hydrological cycle (Huntington, 2006), shifting the timing and distribution of freshwater resources (Kundzewicz et al, 2008) and changing the balance between precipitation, runoff and evapotranspiration (Zhang et al, 2012)
Climatic effects may be compounded by changes in vegetation, whether due to land use/land cover change or vegetation physiological, compositional and structural responses caused by climate change or increasing atmospheric CO2
Interannual variability in ET in both wet and dry basins was found to be strongly controlled by variability in precipitation
Summary
Climate change is expected to alter the global hydrological cycle (Huntington, 2006), shifting the timing and distribution of freshwater resources (Kundzewicz et al, 2008) and changing the balance between precipitation, runoff and evapotranspiration (Zhang et al, 2012). Climatic effects may be compounded by changes in vegetation, whether due to land use/land cover change or vegetation physiological, compositional and structural responses caused by climate change or increasing atmospheric CO2 Together these effects have the potential to change the amount of water available to the biosphere and for human use. Evapotranspiration (ET) is the sum of evaporation from soil and open water, interception loss and plant transpiration. It is a key ecosystem variable linking hydrological, energy and carbon cycles and amounts to up to 60 % of global land precipitation (Oki and Kanae, 2006; Teuling et al, 2009).
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