Creation of the WATCH Forcing Data and Its Use to Assess Global and Regional Reference Crop Evaporation over Land during the Twentieth Century

TLDR

The WATCH project evaluates the terrestrial water cycle by using land surface and hydrological models to assess evaporation, soil moisture, and runoff. This paper creates the WATCH Forcing Data (1958–2001 from ERA‑40 and 1901–57 from reordered reanalysis) to supply meteorological inputs for those models. It also analyzes model‑independent estimates of reference crop evaporation. Global average annual reference crop evaporation, a standard potential evapotranspiration metric, shows no significant trend from 1979 to 2001 despite rising vapor pressure deficit and falling net radiation and wind speed, yet regional patterns reveal declines in some areas such as the Murray–Darling basin and increases elsewhere.

Abstract

Abstract The Water and Global Change (WATCH) project evaluation of the terrestrial water cycle involves using land surface models and general hydrological models to assess hydrologically important variables including evaporation, soil moisture, and runoff. Such models require meteorological forcing data, and this paper describes the creation of the WATCH Forcing Data for 1958–2001 based on the 40-yr ECMWF Re-Analysis (ERA-40) and for 1901–57 based on reordered reanalysis data. It also discusses and analyses model-independent estimates of reference crop evaporation. Global average annual cumulative reference crop evaporation was selected as a widely adopted measure of potential evapotranspiration. It exhibits no significant trend from 1979 to 2001 although there are significant long-term increases in global average vapor pressure deficit and concurrent significant decreases in global average net radiation and wind speed. The near-constant global average of annual reference crop evaporation in the late twentieth century masks significant decreases in some regions (e.g., the Murray–Darling basin) with significant increases in others.