This is a repository copy of Index decomposition analysis of urban crop water footprint.

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1 Ths s a repostory copy of Index decomposton analyss of urban crop water footprnt. Whte Rose Research Onlne URL for ths paper: Verson: Accepted Verson Artcle: Zhao, X, Tllotson, MR, Lu, YW et al. (3 more authors) (2017) Index decomposton analyss of urban crop water footprnt. Ecologcal Modellng, 348. pp ISSN Elsever B.V. Ths manuscrpt verson s made avalable under the CC-BY-NC-ND 4.0 lcense Reuse Unless ndcated otherwse, fulltext tems are protected by copyrght wth all rghts reserved. The copyrght excepton n secton 29 of the Copyrght, Desgns and Patents Act 1988 allows the makng of a sngle copy solely for the purpose of non-commercal research or prvate study wthn the lmts of far dealng. The publsher or other rghts-holder may allow further reproducton and re-use of ths verson - refer to the Whte Rose Research Onlne record for ths tem. Where records dentfy the publsher as the copyrght holder, users can verfy any specfc terms of use on the publsher s webste. Takedown If you consder content n Whte Rose Research Onlne to be n breach of UK law, please notfy us by emalng eprnts@whterose.ac.uk ncludng the URL of the record and the reason for the wthdrawal request. eprnts@whterose.ac.uk

2 Index decomposton analyss of urban crop water footprnt X. Zhao 1*, M. R. Tllotson 2, Y.W. Lu 3, W. Guo 4, A.H. Yang 5, Y.F. L 1 1 Key Laboratory of Integrated Regulaton and Resource Development on Shallow Lak es, Mnstry of Educaton, College of Envronment, Hoha Unversty, Nanjng , Chna 2 water@leeds, School of Cvl Engneerng, Unversty of Leeds, Leeds LS2 9JT, Unted Kngdom; 3 School of Nature Conservaton, Bejng Forestry Unversty, Bejng , Chna 4 School of Envronmental and Chemcal Engneerng, North Chna Electrc Power Unversty, Bejng , Chna 5 Water Stewardshp of Lvng Yangtze Programme Bejng Offce, World Wde Fund for Nature, Bejng P.R. Chna Abstract Rapd urbanzaton has resulted n often unplanned ncreases n populaton, and food demand n ctes. Hstorcally, hnterlands to these ctes have acted as breadbaskets producng food to the urban resdents. Accordngly, a large amount of avalable freshwater has been needed to support these croplands. However, the rapd expanson of ctes n developng countres has sgnfcantly changed both the croplands around ctes and the water demand. It s thus mportant to quanttatvely nvestgate the water-food nexus of ctes related to the changng hnterland agrculture. Water footprnt s an ndcator reflectng the human mpact on water. In ths study, we quantfed both the blue and green water footprnt of major crop products n Suzhou cty, Chna usng a bottom-up accountng method. A novel decomposton analyss was carred out wth a Logarthmc Mean Dvsa Index (LMDI) method to study the drvng forces that changed the water footprnt durng the perod The drvers were desgned to * Correspondng author. Tel.: E-mal address: xuzhao@hhu.edu.cn; xu.zhao.water@hotmal.com (X. Zhao).

3 reflect the factors related to farmland, such as yeld and crop area. Ths s dfferent from prevous decomposton analyses, whch focused on economc factors such as GDP. The results show that the crop water footprnt of Suzhou cty has seen a general decreasng trend between 2001 and The decomposton analyss showed that the declne of crop area was the man drver that decreased the crop water footprnt, followed by the vrtual water content (water consumpton per unt of producton). In contrast the changes of crop combnaton and yeld contrbuted to an ncrease n the crop water footprnt. Although the shrnk of urban croplands decreased the water footprnt of crop products. Ctes ncreasng demand for food wll ncrease the crop water footprnt of consumpton. Ths wll ncrease the dependence of ctes on external water footprnt of crop products (water emboded n mported crops), whch may mpact upon food securty n ctes n the long term. Keywords: hnterland agrculture, crop water footprnt, decomposton analyss, LMDI Hghlghts Redesgn of drvng forces for crop water footprnt changes Analyss of nteracton between hnterland agrculture and water demand Dscusson of farmland mpact upon urban water demand

4 Introducton Ctes consttute prmary agglomeratons of people. In % of the world s populaton lved n ctes, and ths s forecast to rse to 66% by 2050 (Unted Natons, 2014). One of the key challenges faced by ctes around the world s to meet food demand for resdents (Barthel and Isendahl, 2013; Lynch et al., 2013). Tradtonally, farmlands around ctes, also known as hnterlands, have supported ths food demand (Zezza and Tascott, 2010). Such urban agrculture has hstorcally been crtcal to achevng food securty n ctes (Lynch et al., 2013). However, the huge water demand assocated wth agrcultural producton conflcts wth the ncreasng water demand due to urban populaton growth. Current trends of rapd expanson of ctes, especally n developng countres, has sgnfcantly changed both the croplands around ctes and the assocated water demand. To the best of our knowledge, few studes have focused on the nteractons between hnterland agrculture and the water demand assocated wth urbanzaton. The water-food nexus of ctes related to changng hnterland agrculture can be evaluated usng the water footprnt (WF) concept. The WF s defned as the volume of freshwater used durng the producton process (Hoekstra et al., 2011). It has been wdely used n quantfyng and assessng freshwater consumpton n crop producton (e.g. Chapagan and Hoekstra, 2011; Mekonnen and Hoekstra, 2011; Vanham et al., 2013). Freshwater refers to both green water and blue water. Green water s the precptaton on land whch does not run-off or recharge groundwater but s stored n the sol or remans on the surface of the sol or vegetaton. The accountng of green water footprnt s closely related to crop growth. The blue water for crop growth can be substtuted by green water, so a complete pcture can be obtaned only by accountng for both (Hoekstra et al., 2011). A bottom-up method s wdely appled to accountng for the crop WF, whch starts from the smallest unt feasble n assessng the WF and aggregates each unt to the desred scale and perod (Yang et al., 2013). The changes n the WF of crops can be related to crop producton and changes n

5 hnterland usage, wth drvers such as water productvty, yeld, agrcultural area etc., to understand the nteractons between hnterland agrculture changes and freshwater consumpton. In recent years, decomposton analyses has been appled to study the drvng forces or determnants that underle changes to the WF (Feng et al., 2015). For example, Zhang (2012) decomposed the effects of contrbutng factors to Bejng s WF changes durng The contrbutng factors were technologcal, economc system effcency, scale, and structural effects. Zhao et al. (2014) nvestgated the mpact of populaton, affluence, urbanzaton level, and det factors on the WF of agrcultural products n Chna based on an extended STIRPAT model. The above decomposton analyses, however, were not desgned to reflect the factors related to changng farmland, such as crop yeld or area, and thus were unable to dentfy the nterrelatonshps between hnterland agrcultural changes and assocated water consumpton. In addton, green water was excluded from most decomposton analyses of WF changes. In the context of ncreasng urbanzaton n developng countres, ths study has quanttatvely nvestgated the water-food nexus n Suzhou cty, Chna by performng a novel decomposton analyss wth a Logarthmc Mean Dvsa Index (LMDI) model. The am was to study the contrbutng factors to urban crop WF changes, ncludng vrtual water content (recprocal of water productvty), yeld, crop structure, and crop area. To best of our knowledge the drvng forces related to crop producton that changes both green and blue WF has been rarely reported. The drvng forces and the mplcatons to water-food securty at urban scale are also dscussed.

6 93 2 Water endowment and water stress n Suzhou cty Fg. 1. Locaton of Suzhou cty, Chna Suzhou cty s located n the Tahu Lake Basn, whch s a subtropcal humd area of plentful ranfall. The annual avalable water resource n Suzhou s 2.98 bllon m 3 (n 2010). The total admnstratve area of Suzhou s 8,488 km 2, wth 3,609 km 2 covered by water (Suzhou Water Resources Bureau, 2010). Lake Tahu, a large shallow freshwater lake n the lower Yangtze Delta, s close to Suzhou (Fg. 1), and s the man water resource for Suzhou. Sgnfcant nutrent polluton from wastewater dscharges, along wth agrcultural run-off from the northwestern shores flows nto Lake Tahu. Nutrent concentratons decrease wth the current towards the eastern and southern

7 reaches of the lake whch, as a result, have better water qualty.e. the reaches close to Suzhou cty, despte extensve blue-green algae problems n the northwestern part of the lake (Hu et al., 2010). Suzhou s an deal case for llustratng how hnterland agrculture can be changed through urbanzaton and ndustralzaton. Although n contemporary Chna, Suzhou s known as an ndustralzed cty wth many hgh-tech ndustres, t was untl the 1980 s on of Chna s gran producton center. The Tahu Lake Basn has long been known as the land of rce and fsh n Chna. Agrculture n the Tahu Lake Basn sustaned hgh productvty for more than nne centures (Ells and Wang, 1997). As such, Suzhou has hstorcally had a large amount of hnterland agrculture dedcated to producng rce and other gran products for both local consumpton and export to other regons n Chna. After the foundaton of the People s Republc of Chna n 1949, Suzhou was establshed as a gran producton base (Wang et al., 2015). In 1984, the sown area was about 5000 km 2 wth gran producton peakng at 3.1 mllon tons (Suzhou Statstcs Bureau, 2011). Snce then Suzhou has accelerated ts ndustralzaton transformaton process by creatng a seres of ndustral park and development zones to stmulate ndustral development and attract Foregn Drect Investment (Wang et al., 2015). Today, Suzhou has become one of the wealthest ndustral ctes n Chna. In 2010, GDP n Suzhou ranked 5 th among Chna s 337 ctes, followng the mega-ctes of Bejng, Shangha, Guangzhou, and Shenzhen. Per capta GDP was about 87,607 CNY (about 12,800 US dollars) (Suzhou Statstcs Bureau, 2011). Urbanzaton n Suzhou, as wth other Chnese ctes, has experenced land grab and populaton growth, whch have substantal mpacts on hnterland agrculture. Despte ts locaton n a subtropcal and humd area, Suzhou as a developed cty n Chna faces water stress. We evaluated water stress n Suzhou durng wth two well-known water scarcty ndces. The Falkenmark Index evaluates water stress through the total annual renewable water resource per capta (Falkenmark et al., 1989), and the Crtcalty rato evaluates water stress usng the rato of total annual

8 wthdrawals to renewable water resources (Alcamo et al., 2000). The classfcaton of both ndces was adjusted accordng to Zeng (2013) and Zhao (2016) followng Chna s water endowment. As a result, four classfcatons were generated wth C as the Crtcalty rato and F as the Falkenmark Index: Absolute Scarcty (C >1 or F< 500 m 3 /capta); Scarcty (1> C > 0.4 or 1000 m 3 /capta > F > 500 m 3 /capta); Stress (0.4 > C > 0.2 or 1700 m 3 /capta > F > 1000 m 3 /capta); and No Stress (C < 0.2 or F > 1700 m 3 /capta). The results for the Crtcalty rato show the hghest level of water stress n Suzhou (Table 1), whle the results for the Falkenmark Index show the second hghest level of water stress durng , and the hghest level n These results suggest that ntensve water use and hgh populaton densty are the man causes of Suzhou s water stress. Table 1 Results of water scarcty ndces n Suzhou cty Year Annual renewable water Populaton Water wthdrawal Falkenmark Index Crtcalty Rato resources(bllon m 3 ) (bllon m 3 ) (F) (C) Method and data The WF of crop products n ths study refers to the WF of crop growth. The ndrect water requrement for crop producton,.e. the water requred n producton of upstream products only takes a small share of the total crop WF (Zhao et al., 2009), thus s gnored n ths study. A bottom-up method to quantfy the WF of crop products can be expressed as follows: (1) WF WF WF [ CWR A ] [ CWR A ] tot g b g. b. 152 Where WF tot, WF g and WFb refer to the total, green and blue water footprnt of crops, 153 s the type of crops planted, A s the plant area of crop, CWRg. and CWRb. are

9 annual green and blue crop water requrements per hectare of crop. Crop water requrement can be calculated usng the CROPWAT model developed by the Food and 156 Agrculture Organzaton (FAO) (avalable at databases cropwat.html). The CROPWAT model takes nto account both ranfed and rrgated condtons. So n the CROPWAT model, the green crop water requrement s obtaned through quantfyng effectve ranfall, whle the blue crop water requrement s obtaned through quantfyng rrgaton We used the LMDI model to decompose the WF of crops n Suzhou cty. The LMDI method was ntally developed by Ang and Lu (2001), and has been wdely used n analyzng the drvng forces of carbon doxde (CO2) emssons or energy effcency (e.g. Ang, 2004; Da and Gao, 2016; Fernández González et al., 2014; Lu et al., 2012), and a small number of applcatons n analyzng WF changes (Xu et al., 2015; Zhao and Chen, 2014). The method has the advantages of expressng n a smple form wth no resdual errors, so has been recommended for general use (Ang, 2004). In ths study, we redesgned the drvng forces of the crop WF to reflect the nterrelatonshps between urban agrculture and assocated freshwater consumpton. The total WF of crops n Suzhou was decomposed nto four drvng forces: vrtual water content, yeld, crop structure, and crop area. Vrtual water content (VWC) s the amount of water consumed to produce a unt of each crop, whch s also the recprocal of water productvty. Yeld s producton volume per unt area. Crop structure s the proporton of specfc crop area to total area of all crops, and crop area s the total plantng area for all crops. The total WF of crops can be expressed wth the above four drvng forces as follows: WF(t) P(t) A(t) WF(t) [ V t Y(t) S(t) A(t)] [ A(t)] 2 P(t) A(t) A(t) 179 Where WF(t) s the water footprnt of all crops, V t, Y (t), and S (t) represent 180 VWC, yeld, and crop structure for crop n year t respectvely. A (t) s the total crop

10 181 area n year t. (t) P s the producton volume of crop n year t, and (t) A s the crop area of crop n year t. Accordng to LMDI, the varaton of WF ( WF ) from year 0 to year t can be decomposed nto four parts: the varaton of WF caused by change n VWC ( the varaton of WF related to change n yeld ( WF y due to change n the proporton of a crop area n the total area ( WF v ), ), the varaton of WF whch s WF s ), and the varaton of WF caused by changes to the total area ( WFa ). The decomposton form s shown n Eq. (3): WF WF t WF WF WF WF WF 3 () (0) v s y a The four drvng forces n Eq. (3) can be quantfed as: V (t) WFv [ [ WF(t), WF(0)]ln ] 4 V (0) S (t) WF [ [ WF (t), WF (0)]ln ] 5 S (0) s Y (t) WF [ [ WF (t), WF (0)]ln ] 6 Y (0) y A(t) WF [ [ WF (t), WF (0)]ln ] 7 A(0) a where functon [ WF (t), WF (0)] s the logarthmc average of two postve numbers WF (t) and WF (0) whch are gven by: 197 WF(t) WF(0), WF(t) WF(0) [ WF (t), (0)] ln (t) ln (0) WF WF WF WF(0), WF(t) WF(0) In ths study, the four man crops n Suzhou were consdered: wheat, rce, cotton,

11 and rapeseed. The total cultvated area of the four crops accounted for about 70% of the total cultvated area n Suzhou (Suzhou Statstcs Bureau, 2011). The nput data of the CROPWAT model ncluded clmatc, crop and sol parameters. Clmatc nputs ncluded average maxmum and mnmum ar temperature, precptaton, relatve humdty, sunlght duraton, radaton and wnd speed, whch were obtaned from the Chna Meteorologcal Data Sharng Servce System ( Crop and sol parameters were taken from the default values n the CROPWAT software provded by FAO. The data for crop producton and crop areas were obtaned from the Suzhou Statstcal Yearbook (Suzhou Statstcs Bureau, 2011) Results and Dscusson Fg. 2. Changes of blue and green water footprnt of crop products n Suzhou ( ) The average crop WF n Suzhou was mllon m 3 durng , ncorporatng m 3 of blue WF and m 3 of green WF. Green WF domnated

12 the WF for wheat, cotton, and rapeseed, accountng for 73%-79% of the total WF on average for these three crops. Blue WF domnated the WF of rce, accountng for 62% of the total WF on average. The average WF for rce took the greatest share amongst the WF of the four crops studed, accountng for 63% of the total, followed by the WF of wheat whch accounted for 29% of the total. As shown n Fg. 2, the crop WF experenced a general decreasng trend from 2001 to Specfcally, the crop WF experenced contnued reducton between 2001 and 2007, before fluctuatng slghtly from 2007 to Blue WF decreased from mllon m 3 n 2001 to mllon m 3 n 2007, before ncreasng to mllon m 3 n Green WF fluctuated between mllon m 3 to mllon m 3 durng the 10 study years. The bggest share of the blue WF to total WF of crop products was 59.64% n 2001, and the smallest share was 31.32% n ! Fg. 3. Drvng forces of crop WF durng the study perod of As shown n Fg. 3, the total crop WF decreased by 526 mllon m 3 durng 2001-

13 , wth total area contrbutng the most to ths reducton. The decrease of total area would have decreased the crop WF by a total of 608 mllon m 3, f other factors (VWC, crop structure, and crop yeld) had remaned statc at 2001 levels. In the study perod ( ) area changes were the man drvng forces n crop WF reducton. Accordng to Fg. 3, area changes sharply reduced crop WF from 2001 to 2004, contnued to cause a gentle reducton between 2004 and 2007, fnally havng less mpact between 2007 and The mpact of VWC fluctuated over the study perod, but decreased by a total of 115 mllon m 3 of the crop WF durng For example, the VWC decreased the crop WF by mllon m 3 durng , but ncreased by mllon m 3 of the crop WF durng The change n crop yeld and crop structure would have ncreased the crop WF by 120 and 75 mllon m 3 respectvely. It s obvous that the crop WF growth due to the effects of crop yeld and crop structure could not offset the crop WF reducton owng to the effects of crop area and VWC.

14 Vrtual water content effect Fg. 4. Vrtual water content changes for four major crops Fg. 3 shows that the pattern of crop WF change s hghly related to the change of the VWC whch fluctuated over the study perod. Such fluctuaton can be lnked to changes n clmatc factors such as temperature, sunlght, and precptaton etc. Clmatc factors can change both crop output and assocated water consumpton, and as a result, have mpact on the VWC. For example, Bocchola and Soncn (2013) found that crop yeld decreased and the WF ncreased wth ncreasng temperature and decreasng precptaton. Kang et al. (2009) found that clmate change led to changes n sol evaporaton and plant transpraton and consequently the crop growth perod may be changed havng nfluence on crop water productvty,.e. the recprocal of the VWC. Some prevous studes assumed the same VWC for dfferent years e.g. (Lu et al., 2007), whch gnored the mpact of the changng VWC on the crop WF. However, our results show that gnorng the dfference of the VWC wll lead to notceable bas for crop WF accountng.

15 Crop Fg. 5. Contrbuton of crop area to crop WF changes Durng the study perod ( ) area changes were the man drvng force n crop WF reducton. Accordng to Fg. 3, area changes sharply reduced crop WF between 2001 and 2004, contnued to cause a gentle reducton between 2004 and 2007, and had less mpact from 2007 to The above changes correlated wth crop area changes n Suzhou. The crop area reducton per annum was km 2 from 2001 to 2004, 76 km 2 from 2004 to 2007, and 31.6 km 2 from 2007 to The man cause of crop area reducton s attrbuted to rapd urbanzaton n Chna, along wth urban land expanson. In urban expanson t s common to see crop land occuped by newly bult urban nfrastructure, such as dwellngs and factores. Consstent wth Chna s natonal urbanzaton trend, the urban land take around Suzhou n the early 21st century has also undergone rapd expanson (Wang et al., 2015). As a result, agrcultural land around Suzhou has shrunk rapdly snce Such reductons have attracted the attenton of government and scentsts: a major concern s that rapd urban expanson wll threaten food securty n Chna. In 2006, 0.12 bllon ha of arable land area was set as a cap,.e.

16 a redlne to lmt arable land reducton (State Councl of the People's Republc of Chna, 2006). Consequently, the downward trend of crop area has slowed snce Crop structure Fg. 6. Contrbuton of crop structure to WF change The overall mpact of crop structure on crop WF was small but resulted n a steady ncrease n crop WF durng the study perod. As shown n Fg. 6, the proporton of wheat area amongst the major crop areas ncreased between 2004 and 2010, whle the area proporton for the other crops decreased. The ncreasng crop area proporton of wheat possbly reflects a decreased labour force for Suzhou s crop producton. In 2010, labour costs for wheat producton n Suzhou was the lowest of the four crops at about 1950 CNY/ha (285 dollars/ha), whlst labour costs for rapeseed was hghest, at about 4440 CNY/ha (650 dollars/ha) (Natonal Development and Reform Commsson, 2010). Snce the populaton workng on Suzhou s hnterland agrculture decreased by 218 thousand from 2001 to 2010, t s reasonable to nfer that more farmers chose to cultvate wheat to overcome labour shortage. Snce wheat had the second largest VWC

17 among the four crops studed, and the largest plant area, the ncreased proporton of wheat also ncreased the total WF. 297 eld Fg. 7. Contrbuton of yeld to WF changes From 2001 to 2010 yeld generally ncreased crop WF for Suzhou. As shown n Fg. 7 the yeld of wheat contrbuted most to crop WF ncrease, rsng from 3,136 ton/thousand ha to 4,727 ton/thousand ha, about a 51% ncrease durng the study perod. It can be nferred that the ncrease n wheat yeld also stmulated the ncreased proporton of wheat plantng area to the total areas of the four studed crops. The yeld for rce decreased n 2005 and 2007, and then ncreased to reflect the general trend n subsequent years. Crop yeld s hghly related to clmatc factors such as radaton, temperature, and precptaton etc. (Lu et al., 2016), and wll thus be consdered n the decomposton analyss of crop WF n future work.

18 Conclusons Chna has long advocated food self-suffcency; even developed ctes take a sgnfcant stake n governng agrcultural land use. Such efforts provde an effectve way of supportng food securty, but also ncrease the pressure on urban water supples. Ths study has proposed a new set of parameters ncludng vrtual water content, yeld, crop structure, and crop area to reflect the nteractons of hnterland agrculture and water demand on a developed Chnese cty. Our attempt hghlghts the mportance of ncorporatng drvers related to agrcultural land changes nto urban land and water management, thus can support decson makng n balancng the trade-offs between local food demand and water resource allocaton. Decreased WF of crop products mtgate urban water stress to an extent, but ncrease the relance of the cty on external water supples whch can be acqured through both physcal and vrtual forms. Research and modellng on the sustanablty and equty for large ctes relyng on external water supply s an urgent ssue. Acknowledgement The research was supported by the Fundamental Research Funds for the Central Unverstes (2016B13814), Chnese Natonal Scence Foundaton ( , , , ), Jangsu Provnce Natonal Scence Foundaton (BK ), and Natonal Scence Funds for Creatve Research Groups of Chna (No ); the program of Dual Innovatve Talents Plan and Innovatve Research Team n Jangsu Provnce, and the Specal Fund of State Key Laboratory of Hydrology- Water Resources and Hydraulc Engneerng. Reference Alcamo, J., Henrchs, T., Rösch, T., World water n 2025: Global modelng and scenaro analyss for the world commsson on water for the 21st century. Center for Envronmental Systems Research, Unversty of Kassel. Ang, B.W., Decomposton analyss for polcymakng n energy. Energ Polcy 32,

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