3 Results
3.1 The global picture
The global water footprint of crop production in the period 1996–2005 was 7404Gm3 yr−1 (78% green, 12% blue, and 10% grey). Wheat takes the largest share in this total volume; it consumed 1087Gm3 yr−1 (70% green, 19% blue, 11% grey). The other crops with a large total water footprint are rice (992Gm3 yr−1) and maize (770Gm3 yr−1). The contribution of the major crops to the global water footprint related to crop production is presented in Fig. 1. The global average green water footprint related to crop production was 5771Gm3 yr−1, of which rain-fed crops use 4701Gm3 yr−1 and irrigated crops use 1070Gm3 yr−1. For most of the crops, the contribution of green water footprint toward the total consumptive water footprint (green and blue) is more than 80 %. Among the major crops, the contribution of green water toward the total consumptive water footprint is lowest for date palm (43 %) and cotton (64 %). The global average blue water footprint related to crop production was 899Gm3 yr−1. Wheat (204Gm3 yr−1) and rice (202Gm3 yr−1) have large blue water footprint together accounting for 45% of the global blue water footprint. The grey water footprint related to the use of nitrogen fertilizer in crops cultivation was 733Gm3 yr−1. Wheat (123Gm3 yr−1), maize (122Gm3 yr−1) and rice (111Gm3 yr−1) have large grey water footprint together accounting for about 56% of the global grey water footprint.