Abstract
Abstract. Land evapotranspiration (ET) estimates are available from several global data sets. Here, monthly global land ET synthesis products, merged from these individual data sets over the time periods 1989–1995 (7 yr) and 1989–2005 (17 yr), are presented. The merged synthesis products over the shorter period are based on a total of 40 distinct data sets while those over the longer period are based on a total of 14 data sets. In the individual data sets, ET is derived from satellite and/or in situ observations (diagnostic data sets) or calculated via land-surface models (LSMs) driven with observations-based forcing or output from atmospheric reanalyses. Statistics for four merged synthesis products are provided, one including all data sets and three including only data sets from one category each (diagnostic, LSMs, and reanalyses). The multi-annual variations of ET in the merged synthesis products display realistic responses. They are also consistent with previous findings of a global increase in ET between 1989 and 1997 (0.13 mm yr−2 in our merged product) followed by a significant decrease in this trend (−0.18 mm yr−2), although these trends are relatively small compared to the uncertainty of absolute ET values. The global mean ET from the merged synthesis products (based on all data sets) is 493 mm yr−1 (1.35 mm d−1) for both the 1989–1995 and 1989–2005 products, which is relatively low compared to previously published estimates. We estimate global runoff (precipitation minus ET) to 263 mm yr−1 (34 406 km3 yr−1) for a total land area of 130 922 000 km2. Precipitation, being an important driving factor and input to most simulated ET data sets, presents uncertainties between single data sets as large as those in the ET estimates. In order to reduce uncertainties in current ET products, improving the accuracy of the input variables, especially precipitation, as well as the parameterizations of ET, are crucial.
Highlights
In recent years, several global multi-year evapotranspiration data sets based on in situ observations or satellite retrievals of different indirect variables have been derived
The different merged synthesis products created from single categories only and from all categories coincide to a large extent in their global land mean ET (Fig. 1), with highest values in the merged product based on reanalyses only (563 mm yr−1) and lowest in that based on land-surface models (LSMs) only (423 mm yr−1)
The largest difference in the list of data sets included in the short and long merged synthesis products is the inclusion of 28 LSMs versus only 5 LSMs
Summary
Several global multi-year evapotranspiration data sets based on in situ observations or satellite retrievals of different indirect variables have been derived. The decline in global land ET trend after 1998 was attributed to a decrease in moisture availability in Southern Hemisphere supply (i.e. water)- limited evaporative regimes, which might indicate that a limit to the temperature-driven acceleration of the hydrological cycle was reached during the 1998–2008 time period. It is important to note that uncertainties in forcing data sets used to derive such ET trends are large and may entail spurious features linked to the use of reanalyses products assimilating non-homogeneous satellite products or variations in the density of stations considered in gridded precipitation products Besides the evaluation of the temporal variability of the benchmark products and the underlying single data sets, the present study compares these to precipitation, which is one of the most important drivers of ET, especially in soil-moisture-limited regions (see, e. g., Teuling et al, 2009; Seneviratne et al, 2010)
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