Abstract
Abstract. To assess global water resources from the perspective of subannual variation in water availability and water use, an integrated water resources model was developed. In a companion report, we presented the global meteorological forcing input used to drive the model and six modules, namely, the land surface hydrology module, the river routing module, the crop growth module, the reservoir operation module, the environmental flow requirement module, and the anthropogenic withdrawal module. Here, we present the results of the model application and global water resources assessments. First, the timing and volume of simulated agriculture water use were examined because agricultural use composes approximately 85% of total consumptive water withdrawal in the world. The estimated crop calendar showed good agreement with earlier reports for wheat, maize, and rice in major countries of production. In major countries, the error in the planting date was ±1 mo, but there were some exceptional cases. The estimated irrigation water withdrawal also showed fair agreement with country statistics, but tended to be underestimated in countries in the Asian monsoon region. The results indicate the validity of the model and the input meteorological forcing because site-specific parameter tuning was not used in the series of simulations. Finally, global water resources were assessed on a subannual basis using a newly devised index. This index located water-stressed regions that were undetected in earlier studies. These regions, which are indicated by a gap in the subannual distribution of water availability and water use, include the Sahel, the Asian monsoon region, and southern Africa. The simulation results show that the reservoir operations of major reservoirs (>1 km3) and the allocation of environmental flow requirements can alter the population under high water stress by approximately −11% to +5% globally. The integrated model is applicable to assessments of various global environmental projections such as climate change.
Highlights
Previous assessments of global water resources have projected current and future global water stress, focusing mainly on the spatial, rather than temporal, distribution of water availability and water use
We addressed the following key question: Given that earlier global water resources assessments were on annual basis, does the assessment of water resources on a subannual basis reveal any water-stressed regions that were previously overlooked by annual assessments?
An integrated water resources model was developed that consists of six modules to simulate both natural and anthropogenic water flows at daily intervals
Summary
Previous assessments of global water resources have projected current and future global water stress, focusing mainly on the spatial, rather than temporal, distribution of water availability and water use. A typical approach is to display the global distribution of per capita annual water availability (Arnell, 1999; Arnell, 2004) or the withdrawal to availability ratio on an annual basis (Vorosmarty et al, 2000; Oki et al, 2001; Alcamo et al, 2003a; Alcamo et al, 2003b) Seasonality in both water availability and water use occurs in some parts of the world. We developed an integrated global water resources model with six modules: land surface hydrology, river routing, crop growth, reservoir operation, environmental flow requirements, and anthropogenic water withdrawal The model simulates both natural and anthropogenic water flow globally (excluding Antarctica) at a spatial resolution of 1◦×1◦ (longitude and latitude). Hall et al, 2006 (ISLSP2) Ramankutty and Foley, 1998 Doll and Siebert, 2000 Leff et al, 2004 Oki and Sud, 1998 (TRIP) Globally uniform (bulk transfer coefficient: 0.003) Globally uniform (depth: 1 m, field capacity: 0.30 m3/m3, wilting point: 0.15 m3/m3)
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