waterMDPIArticleAssessing the Water Footprint of Wheat and Maize inHaihe River Basin,Northern China(1956-2015)Yuping Han 1,2,3,Dongdong Jia 1,La Zhuo 4,5,Sabine Sauvage 5,Jose-Miguel Sinchez-Perez6,Huiping Huang1 and Chunying Wang 12,31North China University of Water Resources and Electric Power,Zhengzhou 450046,China;han0118@163.com (Y.H.);13526882916@163.com (H.H);wangchunying1987@yahoo.com (C.W.)2Collaborative Innovation Center of Water Resources Efficient Utilization and Support Engineering,Zhengzhou 450046,China3Henan Key Laboratory of Water Environment Simulation and Treatment,Zhengzhou 450046,ChinaInstitute of Soil and Water Conservation,Northwest A F University,Yangling 712100,China;zhuola@nwafu.edu.cn5Institute of Soil and Water Conservation,CAS MWR,Yangling 712100,China6EcoLab,Universite de Toulouse,CNRS,INPT,UPS,Toulouse,France,Avenue de I'Agrobiopole,31326 Castanet Tolosan CEDEX,France;sabine.sauvage@univ-tlse3.fr S.S.),jose-miguel.sanchez-perez@univ-tlse3.fr (J.-M.S.-P.)Correspondence:dongdongjia06@163.com;Tel:+86-185-0085-4530Received:17 May 2018;Accepted:21 June 2018;Published:29 June 2018Abstract:Assessing the water footprint(WF)of crops is key to understanding the agricultural waterconsumption and improving water use efficiency.This study assessed the WF of wheat and maizein the Haihe River Basin (HRB)of Northern China over the period1956-2015,including rain-fed,sufficient,and insufficient irrigation conditions by different irrigation intensity to understand theagricultural water use status.The major findings are as follows:(1)The annual average total WFof wheat and maize production is 20.1 (52%green,29%blue,and 19%grey)and 15.1(73%green,3%blue,and 24%grey)billion m3 year-,respectively.The proportion of grey WF is much larger thanthe world average;(2)Wheat has larger unit WF(1580 m3t-1)than maize(1275 m3 t-1).The unit WFof both wheat and maize shows exponentially decreasing trends,indicating that water use efficiencyhas been improved.The unit WF is heterogeneous in space,which is larger in Tianjin and Huanghuaand smaller in the Southern HRB;(3)Rain-fed crops have the largest unit WE,followed by cropsunder insufficient and sufficient irrigation conditions for both wheat and maize.To improve thesustainability of water resources,the application of fertilizer must be reduced,and irrigation isan effective way to improve water use efficiency in water-abundant areas.Keywords:water footprint;irrigation intensity;wheat;maize;Haihe River Basin1.IntroductionWater scarcity has been a growing concern worldwide [1-3].Agriculture consumes 70%ofthe global freshwater withdrawal [4].With growing populations and expanding irrigated acreage,the water demand of agriculture continues to increase.Meanwhile,extensive application offertilizer has caused severe,diffuse agricultural water pollution,which increases the competitionfor freshwater[5].In some river basins,due to limited water supply facilities and high water prices,crops are irrigated with inadequate water supply under field conditions.A comprehensive andaccurate assessment of the volume and structure of agricultural water consumption under thoseconditions is key to improving water use efficiency and effectively managing water resources.Nater2018,10,867;doi:10.3390/w10070867www.mdpi.com/journal/waterNater2018,10,8672of18The water resources can be divided into green and blue water resources during water resourceplanning and management [6,7].The concept of the "water footprint(WF)"was introduced byHoekstra [8]and it provides a tool to assist with water resource management and deals with waterscarcity,such as changing consumption patterns or improving the water efficiency of production [9-12].The WF of a product refers to the sum of the water volume consumed to produce the product [13].The blue WF refers to the volume of surface and groundwater consumed(evaporated)as a result of theproduction of a good.The green water footprint refers to the rainwater consumed.For crops,this refersto the portion of rainfall that infiltrates the soil and is accessible by plants to generate vapor flow insupport of biomass growth [9].The grey WF of a product refers to the volume of freshwater that isrequired to assimilate the load of pollutants based on existing ambient water quality standards [13].The WF of unit production,which is also recognized as the virtual water content [14,15]when assessingvirtual water flows among regions,reflects the regional water productivity or water use efficiencyWithin the agricultural sector,WF has been intensively studied from global levels to regionallevels.Mekonnen and Hoekstra [16,17]estimated the green,blue,and grey WF of global wheatand quantified the green,blue,and grey WF of global crop production for the period 1996-2005.Siebert and Doll [18]quantified the green and blue WF in global crop production,as well as potentialproduction losses without irrigation.At the national level,Zhuo et al.[19,20]set up benchmark levelsof consumptive WF of winter wheat and assessed the green and blue WF and virtual water trade inChina under alternative future scenarios.Cao et al.[21]assessed the blue and green water utilizationin wheat production of China.Zoumides et al.[22]employed a supply utilization approach alongwith two indicators,economic productivity of crop use and the blue water scarcity index,to assessthe WF for the semi-arid island of Cyprus.Schyns and Hoekstra [23]demonstrated the added valueof the detailed analysis of the human water footprint within Morocco and thoroughly assessed thevirtual water flows.At the regional level,Bulsink et al.[24]analyzed the WF of an Indonesian provincerelated to the consumption of crop products.Duan et al.[25]explored the spatial variations of the WFand their relationships with agricultural inputs in Northeast China.Gobin et al.[26]calibrated cropyield for a water balance model,"Aquacrop"at the field level and analyzed variability in the WF ofarable crop production across European regions.At the river basin level,Aldaya and Llamas [27]analyzed the WF and virtual water in the semiaridGuadiana Basin.Yin et al.[28]calculated the total WF and the net external WF of consumption inthe Yellow River Basin of China.Zeng et al.[29]quantified the WF in the Heihe River Basin of Chinaduring 2004-2006.Zang et al.[30]reported on spatial and temporal patterns of both green and bluewater flows,also in the Heihe River Basin.Zhuo et al.[31]estimated the inter-and intra-annual WF ofcrop production in the Yellow River Basin for the period 1961-2009.Assessing WF at the river basinlevel is an important step to understanding how human activities influence the water cycle and isa basis for integrated water resource management and sustainable water uses within the basin [29].Prior studies analyzed or assessed the WF of crops by dividing them into pure rain-fed crops andirrigated crops with sufficient water.However,because of a lack of detailed long-term irrigation data,few studies assessed the WF with insufficient water supply restricted by water volume,water cost,and water supply facilities.Assessing the WF under those conditions can effectively improve ourunderstanding regarding the agricultural water use status to improve agricultural water use efficiency.Additionally,few studies have investigated the spatial and temporal characteristics within the basinunder the influence of many factors,such as climate,geography,soil property,and managementpractice (e.g.,irrigation,fertilizer application).In order to effectively understand the agriculturalwater use status and reasonably allocate water resources within the basin,it is necessary to assess thespatial and temporal WF by dividing the basin into small regions according to administrative divisionswhich have their own record,climate,and geographical conditions.Among the above influenceson spatial and temporal variations of the WF,irrigation is a key factor controlling the accuracy ofWF assessment,especially in river basins facing water scarcity [22,23,32,33].The irrigation quota isrecommended by the local government to guide the farmers'irrigation practice.Furthermore,in the