Abstract.To assess global water availability and use at a subannual timescale, an integrated global water resources model was developed consisting of six modules: land surface hydrology, river routing, crop growth, reservoir operation, environmental flow requirement estimation, and anthropogenic water withdrawal. The model simulates both natural and anthropogenic water flow globally (excluding Antarctica) on a daily basis at a spatial resolution of 1 • ×1 • (longitude and latitude). This first part of the two-feature report describes the six modules and the input meteorological forcing. The input meteorological forcing was provided by the second Global Soil Wetness Project (GSWP2), an international land surface modeling project. Several reported shortcomings of the forcing component were improved. The land surface hydrology module was developed based on a bucket type model that simulates energy and water balance on land surfaces. The crop growth module is a relatively simple model based on concepts of heat unit theory, potential biomass, and a harvest index. In the reservoir operation module, 452 major reservoirs with >1 km 3 each of storage capacity store and release water according to their own rules of operation. Operating rules were determined for each reservoir by an algorithm that used currently available global data such as reservoir storage capacity, intended purposes, simulated inflow, and water demand in the lower reaches. The environmental flow requirement module was newly developed based on case studies from around the world. SimulatedCorrespondence to: N. Hanasaki (hanasaki@nies.go.jp) runoff was compared and validated with observation-based global runoff data sets and observed streamflow records at 32 major river gauging stations around the world. Mean annual runoff agreed well with earlier studies at global and continental scales, and in individual basins, the mean bias was less than ±20% in 14 of the 32 river basins and less than ±50% in 24 basins. The error in the peak was less than ±1 mo in 19 of the 27 basins and less than ±2 mo in 25 basins. The performance was similar to the best available precedent studies with closure of energy and water. The input meteorological forcing component and the integrated model provide a framework with which to assess global water resources, with the potential application to investigate the subannual variability in water resources.

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[1] Decreases in pan evaporation (E p ) over the last decades have been reported in many regions of the world. In this study, we investigated E p dynamics in the hyper-arid region of China during the period 1958-2010 using a generic physical model based on long-term meteorological data collected at 81 ground-based meteorological stations. We also quantified the contribution of climatic factors to the E p change using partial derivatives. We found that E p in the region exhibited an obvious decreasing trend until early 1990s (1993), at a rate of À6.0 mm yr À2 . However, the downward trend reversed in 1993, and the rate of increase after that was 10.7 mm yr À2. We also assessed the sensitivity of rates of evaporative demand to changes in aerodynamic and radiative components, and found that pan evaporation could be mostly attributed to changes in the aerodynamic component, with some regional contributions from solar irradiance. Observed near-surface wind speed is the primary contributor to the decline of pan evaporation during 1958-1993, while wind speed (WS) and vapor pressure deficit (VPD) were both major contributors to the increase of pan evaporation after 1993.

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