The Yellow River basin (YRB), the second largest river basin of China, is currently producing 13% of national grain production with only 2% of national water resources. Agriculture accounts 77% of total blue water consumption within the YRB, 91% of which for irrigation in 2009. In the last half century, agriculture within the YRB has grown by 1.5-fold-expansion of irrigated cropland and more than 4-time-increase of crop production from 1960s to 2000s. Meanwhile, climate change in the same period has resulted in decrease in precipitation and increase in temperature. However, there is less information available for current and future water managers of the YRB on the accompanying historical development of actual water consumption on croplands as well as its impacts on the sustainability of water resource within the basin. In order to fill the gap, our study aims at (i) investigating the long-term variability of green (from rainfall), blue (from irrigation) and grey (water needed for pollution assimilation) water footprints of major crop production within the YRB for 1961-2009, as well as (ii) the monthly variation of blue water scarcity of the YRB, taking blue water footprints by agriculture, industry and households into consideration, for 1978-2009.
The study was carried out at a spatial resolution of 5 by 5 arc-minute (~7.4 km×9.3km within the YRB). We have considered 17 crops that account for 93% of YRB's total crop production (in tonne) (2009). By following the calculation scheme of Hoekstra et al. (2011), the yearly green and blue crop water footprints for the study period were estimated through the crop water productivity model AquaCrop plug-in (Steduto et al., 2009; Raes et al., 2009; Hsiao et al., 2009). The yearly grey water footprints related to nitrogen (N) and phosphors (P) were also estimated. The monthly blue water scarcity of the YRB was evaluated as the ratio of the total blue water footprint to the maximum sustainable blue water footprint (Hoekstra et al., 2012).
Results show that (i) for the period of 1961-2009, annual total green-blue water footprint of crop production in the YRB was 48.8 billion m3y-1 (~25% in blue). The annual grey water footprint was 86.7 billion m3y-1 related to N and 37.8 billion m3y-1 related to P; (ii) all three colours of total crop water footprints in the YRB varied among years but showed over-all increasing trends during the decades, by 13%, 37%, 24 times, and 36 times in green, blue, grey related to P, and grey related to N, respectively from 1960s to 2000s. The long-term increase of the total blue water footprint occurred mainly in the upper reach while the increase of the green water footprint was mainly from the upper and lower reaches; (iii) The green-blue water footprint per tonne of a crop reduced significantly (e.g. 6542 m3t-1 of a cereal crop in 1960s to 1571 m3t-1 in 2000s) due to improved crop yield, while the grey water footprint increased because of a significant increase in fertilizer use (e.g. 1765 m3t-1 of soybean in 1960s to 12093 m3t-1 in 2000s); (iv) The total annual blue water footprint was 19~52% of the natural runoff in the YRB. The distribution of the monthly blue water footprint differs from the available water resources both temporally and spatially. The peak of the monthly blue water footprint was one or two months earlier than the flood season for the whole basin. Moreover, the severe blue water scarcity always happened at the east of the upper reach, the north of the middle reach, and the lower reach with higher ET0 and lower natural runoff. Therefore even in the flood season in a wet year, more than half of the area of the basin, especially the north of the YRB still suffered severe blue water scarcity. Apparently, the YRB is blue water scarce. But in a long future, the YRB will keep facing the water use risk with its crucial role in agriculture and economy of China. To meet the improving requirement of crop production, the area of irrigated cropland within the basin is planned to be expanded by 12% of current till 2030 by the government (YRCC, 2013). Reducing the water footprint, particularly the blue water footprint of crop production should be one of the primary goals for the basin's sustainability water use. Based on the current results, we can conclude that an optimized cropping scheme i.e., planting more high-water--productive crops such as more maize but less of wheat should be the very first step. 1. Hoekstra, A.Y., Chapagain,A.K., Aldaya, M.M. and Mekonnen, M.M. (2011) The water footprint assessment manual: Setting the global standard. Earthscan, 224 p. 2. Hoekstra, A.Y., Mekonnen, M.M., Chapagain, A.K., Mathews, R.E. and Richter, B.D. (2012) Global monthly water scarcity: Blue water footprints versus blue water availability, PLoS ONE 7(2), e32688. 3. Hsiao, T.C., Heng, L., Steduto, P., Rojas-Lara, B., Raes, D., Fereres, E. (2009) AquaCrop—The FAO crop model to simulate yield response to water. III. Parameterization and testing for maize. Agronomy J. 101 (3), 448–459. 4. Raes, D., Steduto, P., Hsiao, T.C., Fereres, E. (2009) AquaCrop—The FAO crop model to simulate yield response to water. II. Main algorithms and software description. Agronomy J. 101 (3), 438–477. 5. Steduto, P., Hsiao, T.C., Raes, D., Fereres, E. (2009) AquaCrop—the FAO crop model to simulate yield response to water. I. Concepts and underlying principles. Agron.J. 101, 426–437. 6. YRCC (Yellow River Conservancy Commission) (2013) Comprehensive planning of the Yellow River basin for 2012-2030 (Summary). http://www.yellowriver.gov.cn/zwzc/lygh/zhgh/201303/t20130321_129411.html. (In Chinese) Last access: May, 2014.