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Crop-water management to reduce blue water scarcity : a case study for the Yellow River basin

Bao, W. (2020) Crop-water management to reduce blue water scarcity : a case study for the Yellow River basin.

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Abstract:Water scarcity in crop-intensive basins has raised wide attention as it threatens food security to meet the increasing global population demand. The Yellow River basin (YRB) is one of these basins that serve as a major food production basin but face severe blue water scarcity. Agriculture is the primary section for water use in the basin. Researchers have explored the reduction in the blue WF of crop production. But it is not clear how much contribution reducing the blue WF of crop production makes to alleviate the water scarcity in the YRB. This study aims to assess the blue water scarcity in the YRB and its alleviation by crop-water management. The study is carried out in four steps. Firstly, we analyzed the reference blue water scarcity following the 'water footprint assessment' framework. The blue WFs of 17 crops in YRB is calculated in a 5*5 arcmin resolution at the dry (2006), the wet (2007), and the average year (2009). The generation of evapotranspiration (ET) and yield are through AquaCrop plug-in modeling. The blue ET is further separated from the AquaCrop output for blue WF calculation. Adding the blue water use from domestic and industrial sectors to the crop blue WF, the total blue WF is obtained. Then, the total blue WF is compared to the maximum available water in order to evaluate the blue water scarcity. The blue water scarcity is analyzed temporally (yearly and monthly) and spatially (grid cell) to have a comprehensive perspective of the blue water scarcity in the YRB. Secondly, two strategies that can best reduce the crop blue WF are formed. One strategy is to limit the irrigation water while maintaining stable yield by deficit irrigation and mulching. The other is to close the yield gap (the difference between observed yield and attainable yield in the region) by assuming the biophysical factors such as fertilizer, pesticides, and weed control to be optimized. Further, an additional scenario of each strategy is designed to adjust production to the reference level with proportional cropping area change. This additional scenario compensates for the change in total production brought by the two strategies and compares the blue WFs (m3) to the reference at the same level of production. Thus, the four scenarios in this study is formed as: i) Strategy 1, area as the reference (S1). ii) Strategy 1, area adjusted (S1AA). iii) Strategy 2, area as the reference (S2). iv) Strategy 2, area decreased (S2A-). Thirdly, the blue water scarcity of the scenarios are then compared to the reference temporally (yearly and monthly) and spatially (grid cell). The effect of crop-water management on the blue water scarcity in the YRB is then assessed. Results show that the yearly blue water scarcity in YRB is 47%, 47%, and 39% to the maximum available blue water in 2006 (dry year), 2007 (wet year), and 2009 (average year) respectively. It means that the YRB has severe blue water scarcity for 2006 and 2007, and significant blue water scarcity for 2009. The monthly blue water scarcity in YRB is severe from February to June in all three years. There are three months of the phase lag of available water to the total blue WF due to the mismatch of precipitation season and the cropping season. Spatially, half of the basin suffers from severe blue water scarcity throughout the whole year, and 70% of the area experiences different levels of blue water scarcity during the cropping season (from March to June). Winter wheat and maize, which cover 50% of the total blue WF from March to August, is noticeable. After applying scenarios to the crops in YRB, the blue WFs of crops are effectively reduced. In general, an average of 41%-44% of blue water (m3) is saved over three years by applying S1, and 56%-58% of blue water (m3) is saved by S2A-. The potential of water-saving aligns with the precipitation distribution temporally and spatially in scenario S1 and S1AA, and ranges from 0 to 80%. The potential of water-saving in S2A- is between 60% - 80% in most of the middle basin. The annual blue water scarcity is relieved by scenarios but cannot be solved entirely. S1 and S1AA ii can bring down the annual blue water scarcity one level down in all three years, and S2A- can bring down the annual blue water scarcity two levels down in 2006 and 2007, one level down in 2009. Scenarios flatten the peak water demand for cropping from March to June in all three years. Scenarios can also relieve 4-5 months (out of all the months in three years) from the level of water scarcity in which the total blue WF is more than 500% of the maximum available water. However, there are still five months each year that suffer from severe blue water scarcity under any of the scenarios, and these months align with the growing season. Scenarios relieve the water scarcity in the north and middle Inner Mongolia, middle Shaanxi province, and west of Qinghai province. Moreover, the month (October) with the lowest blue water scarcity under these scenarios shows a bimodal distribution. We can deduce that any blue water use can cause tremendous blue water scarcity in some areas due to the blue water's uneven spatial distribution. There are many limitations to the study. For example, the choice of environmental water flow standard varies; the effect of reservoirs is not considered; the monthly blue water use data in industrial and domestic sectors are not available. However, this study is the first to assess the blue water scarcity in a finer resolution by bringing down the crop blue WF in the YRB. The results can be fundamental to understand where the blue water scarcity still needs to be improved and the direction of improvement.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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