An estimation of global virtual water flow and sources of water withdrawal for major crops and livestock products using a global hydrological model

SummaryThe concept of virtual water, which is the volume of water consumption required to produce commodities traded to an importing or exporting nation (or any region, company, individual, etc.), is a useful complement to water resource analyses of water availability and use by region. Identifying the source of virtual water, such as precipitation (green water) and irrigation water (blue water), further enhances this concept because each differs in the level of sustainability and opportunity cost. Recent improvements in global hydrological models consisting of both physically based hydrological and anthropogenic activity modules enabled us to simulate the virtual water content of major crops consistent with their global hydrological simulation. Enhancing one of these models, called H08, we were able to assess two major sources of virtual water flow or content simultaneously: green water and blue water. Blue water was further subdivided into three subcategories (i.e., streamflow, medium-size reservoirs, and nonrenewable and nonlocal blue water). We conducted a global hydrological simulation for 15 years from 1985 to 1999 at a spatial resolution of 0.5° × 0.5° (longitude and latitude). Total precipitation on land was 113,900 km3 yr−1, with 72,080 km3 yr−1 on average evaporating in the period 1985–1999. Green water evapotranspiration from rainfed and irrigated cropland and blue water evapotranspiration from irrigated cropland was estimated at 7820, 1720, and 1530 km3 yr−1, respectively. Next, using global trade data for 2000 and the simulated virtual water content of major crops, the virtual water flow was estimated globally. Our results indicated that the global virtual water export (i.e., the volume of water that an exporting nation consumes to produce the commodities that it trades abroad) of five crops (barley, maize, rice, soybean, and wheat) and three livestock products (beef, pork, and chicken) is 545 km3 yr−1. Of the total virtual water exports, 61 km3 yr−1 (11%) are blue water (i.e., irrigation water) and 26 km3 yr−1 (5%) are nonrenewable and nonlocal blue water.