Historical land-use-induced evapotranspiration changes estimated from present-day observations and reconstructed land-cover maps

摘要

Recent results from the LUCID model intercomparison project have revealedlarge discrepancies in the simulated evapotranspiration (ET) response to thehistorical land-use change. Distinct land-surface parameterizations arebehind those discrepancies, but understanding those differences rely onevaluations using still very limited measurements. Model benchmarking studieswith observed ET are required in order to reduce the current uncertainties inthe impacts of land use in terrestrial water flows. Here we present newestimates of historical land-use-induced ET changes based on threeobservation-driven products of ET. These products are used to deriveempirical models of ET as a function of land-cover properties andenvironmental variables. An ensemble of reconstructions of past ET changesare derived with the same set of land-cover maps used in LUCID, with which weobtain an average decrease in global terrestrial ET of1260 ± 850 km3 yr?1 between the preindustrial period and thepresent-day. This estimate is larger in magnitude than the mean ET changesimulated within LUCID with process-based models, and substantially weakerthan other estimates based on observations. Although decreases in annual ETdominate in deforested regions, large summertime increases in ET arediagnosed over areas of large cropland expansion. The multiple ETreconstructions carried out here show a large spread that we attributeprincipally to the different land-cover maps adopted and to the crops' ETrates deduced from the various products assessed. We therefore conclude thatthe current uncertainties of past ET changes could be reduced efficientlywith improved historical land-cover reconstructions and better estimates ofcropland ET.