A worldwide analysis of trends in water-balance evapotranspiration

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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. © Author(s) 2013.

Figures

  • Fig. 1. Map of the study basins, classified into wet and dry according to aridity index (Sect. 2.1). The basins indicated are those used as examples in Fig. 4.
  • Fig. 2. The mean prop rtion of a nual runoff (in %) due t consumption in wet and dry basins.
  • Table 1. The proportion of interannual variability explained by all predictor variables and precipitation only (expressed as R2), and the unique effect of meteorological and land surface variables as inferred from variance partitioning (expressed as adj. R2) across wet and dry basins using CRU- and GPCC-based ET and precipitation, respectively.
  • Fig. 3. The proportion of interannual variability in water balance ET explained by all predictor variables in each basin (A); and the unique effect of meteorological (B) and land surface (C) variables in controlling ET variability, determined by variance partitioning. All values shown are based on CRU precipitation.
  • Fig. 4. Examples of interannual variability in water balance ET and its relationship with precipitation, solar radiation, stomatal conductance, foliage cover, air temperature and wind speed. Three-year block averages were used to smooth time series. Correlation coefficients for ET against precipitation (r1) and the second plot variable (r2) are shown. The location of each basin is indicated in Fig. 1.
  • Fig. 5. Trends in observed CRU TS 3.1 precipitation, runoff, CRU-based water balance ET, solar radiation, wind speed, air temperature and land cover, and modelled stomatal conductance and foliage cover. Basins with significant trends are shown shaded.
  • Table 2. Attribution of water balance ET trends (in mm yr−2) in wet basins based on coefficients and p values from a multiple regression analysis, using CRU- and GPCC-based ET and precipitation, respectively. Significant p values (≤ 0.05) and coefficients are in italics.
  • Table 3. The proportion of ET trends explained by all predictor variables and precipitation only (expressed as R2) across wet and dry basins, using CRU- and GPCC-based ET and precipitation, respectively.

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CITATION STYLE

APA

Ukkola, A. M., & Prentice, I. C. (2013). A worldwide analysis of trends in water-balance evapotranspiration. Hydrology and Earth System Sciences, 17(10), 4177–4187. https://doi.org/10.5194/hess-17-4177-2013

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