SATELLITE BASED REGIONAL-SCALE EVAPOTRANSPIRATION IN THE HEBEI PLAIN, NORTHEASTERN CHINA

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1 SATELLITE BASED REGIONAL-SCALE EVAPOTRANSPIRATION IN THE HEBEI PLAIN, NORTHEASTERN CHINA Wejig Li (1), Rogier va de Velde (2), Z. Su (2) (1) Istitute of Hydrogeology ad Evirometal Geology, 561, Shijiazhuag, P.R.Chia, l: (2) Iteratioal Istitute for Geo-iformatio Sciece ad Earth Observatio (ITC), 75AA, Eschede, the Netherlads, ABSTRACT Evapotraspiratio is oe of the most sigificat compoets of the hydrologic budget. Satellite sesors that observe the earth from the space give a possibility to estimate evapotraspiratio at a big scale. I this paper, Surface Eergy Balace System (SEBS) has bee utilized to estimate the surface fluxes over Hebei Plai i Northeaster Chia by usig MODIS/TERRA images, i combiatio with meteorological data collected i meteorological statios distributed over the study area. The estimated daily evapotraspiratio by SEBS i cloud free days are first compared with measuremets by the large weighig lysimeter i Luacheg Agro- Ecosystem Statio (LAES). The comparisos show that the estimated evapotraspiratio from SEBS have a good agreemet with the groud truth data. Based o the validatio of the model, the spatial-temporal distributios of actual evapotraspiratio were aalyzed by combiatio of the up-to-date lad cover map i Hebei Plai. 1. INTRODUCTION Apart from precipitatio, the most sigificat compoet of the hydrologic budget is evapotraspiratio. Evapotraspiratio varies regioally ad seasoally accordig to ambiet evirometal coditios, such as climate coditio, lad cover, lad use, soil moisture, ad available radiatio etc. Because of this variability, ad its importace for itegratig water resources modellig, dyamic crop-weather modellig ad drought moitorig, a thorough uderstadig of the evapotraspiratio process ad kowledge about the spatial evapotraspiratio is eeded. I the last few decades the theoretical ad applied aalysis of evapotraspiratio ad its compoets traspiratio ad evaporatio have received much attetio. A physically based equatio for potetial evapotraspiratio (ET ) was derived by Pema by combiig eergy balace equatio with the aerodyamic equatio for vapour trasfer [1]. It was subsequetly modified by Moteith to iclude a caopy resistace for vapour diffusio out of stomata [2]. Apart from above metioed priciples, there are may other methods that have bee proposed for estimatig ET. By comparig 2 differet methods of estimatig ET, Jese et al. showed that the Pema-Moteith equatio provide the best accurate estimate of evaporatio from well-watered grass or alfalfa uder varied climate coditios [3]. However, these covetioal techiques are based o the poit measuremets ad are represetative oly of local scales ad will fail for large scales because of the dyamic ature ad regioal variatio of ET. O the other hads, Pema-Moteith equatio provided oly a estimatio of the potetial ET, which limits its use i practical applicatios. Evaporatio of water requires relatively large amouts of eergy, either i the form of sesible heat or radiat eergy. Therefore the evapotraspiratio process is govered by eergy exchage at the lad surface ad is limited by the amout of eergy available. Because of this limitatio, it is possible to predict the regioal actual evapotraspiratio by applyig the priciple of eergy coservatio. Recetly, remote sesig techiques have developed rapidly. From satellite observatio, oe ca obtai cosistet ad frequet spectral reflectace ad emittace of radiatio of the lad surface i a basi scale, so it is possible to estimate the regioal evapotraspiratio rate by combiig remotely sesig data with the solar radiatio observatio based o surface eergy balace model. I the past decades, cosiderable efforts have bee made to gaiig experiece ad derivig appropriate models to couter this challege [4, 5, ad 6]. Several algorithms were developed ad they all have bee applied ad validated i some regios [7, 8, ad 9]. The Surface Eergy Balace System (SEBS) model was developed by Su [6] to estimate lad surface fluxes usig remotely sesed data ad available meteorological observatios. It has the most importat advatage of its iclusio of a physical model for the estimatio of the roughess height for heat trasfer which is the most Proceedigs of the 1st EPS/MetOp RAO Workshop, May 26, ESRIN, Frascati, Italy (ESA SP-618, August 26)

2 critical parameter i the parameterizatio of the heat fluxes of lad surface. I this paper, SEBS has bee utilized firstly to estimate the surface fluxes over Hebei Plai i Northeaster Chia by usig MODIS/TERRA. Thereafter, spatial distributios of the daily evapotraspiratio i Hebei Plai i combiig with the lad cover maps were aalyzed. 2. METHODOLOGY 2.1. Surface Eergy Balace System The surface eergy balace is commoly writte as R = G + H + λe (1) where R is the et radiatio, G the soil heat flux, H the sesible heat flux ad λ E is the latet heat flux, which ca be expressed as height of water, i.e. evapotraspiratio. The equatio to calculate the et radiatio is give by R = ( 1 α ) R + ε R ε σ T (2) swd where, R swd, R lwd is icomig shortwave ad logwave radiatio respectively, α is the surface albedo, ε the emissivity of the surface, σ the Stefa-Bolzma costat, equals to ad T the surface radiative temperature. The equatio to calculate soil heat flux is parameterized as G lwd [ Γ + ( ) ( Γ Γ )] = R 1 f 4 c c s c (3) where Γ c ad Γ s are empirical coefficiet. For most bare soil coditios a Γ s value of.315 is valid, ad for vegetatio ofte Γ c is assumed to be.5. f c is fractioal vegetatio cover determied from remote sesig data. The surface eergy balace computatio with the SEBS algorithm is based o the determiatio of the relative evaporatio fractio. I order to determie the relative evaporatio fractio, use is made of eergy balace cosideratios at limitig cases. Uder the dry-limit, the latet heat (or the evaporatio) becomes zero due to the limitatio of soil moisture, ad the sesible heat flux is at its maximum value, from Eq. (1), it follows, λe H dry dry = R = R G G H dry, or (4) Uder the -limit, where the evaporatio takes place at potetial rate, λ E (i.e. the evaporatio is oly limited by the available eergy uder the give surface ad atmospheric coditios), the sesible heat flux takes its miimum value, H i.e. λe H = R = R G G H, λe or The relative evaporatio the ca be evaluated as H H (5) Λ r = 1 (6) H dry H The evaporative fractio is fially give by: λe λe Λ r λe Λ = = = H + λe R G R G Eq. 1-7 costitute the basis formulatio of SEBS. Further, i SEBS the actual sesible heat flux H is obtaied by solvig a set of o-liear equatios ad is costraied i the rage set by the sesible heat flux at the limit H, ad the sesible heat flux at the dry limit. H dry (7) By assumig that the daily value of evaporative fractio is approximately equal to the istataeous value, the daily evaporatio ca be determied as, E E daily daily = Λ R λρ (8) where is the actual evaporatio o daily 1 basis ( mm d ), λ the latet heat of vaporizatio _ ( JKg 1 ), ρ 3 ω the desity of water ( Kgm ) ad R is the daily et radiatio flux Parameterizatio of Surface bio-physical characteristics I order to ru SEBS, surface bio-physical parameters should be prepared firstly as model iputs. Those iputs W

3 ca be derived from remote sesig data by applyig related empirical equatios. Vegetatio fractioal cover. Fractioal Vegetatio Cover is a importat parameter that plays a key role i the eergy exchages at the lad surface. A simple procedure to determie fractioal vegetatio cover is proposed by Gutma et al. as [1], f NDVI NDVI max = (9) NDVI max NDVI mi Vegetatio height ad displacemet height. A coversio is performed accordig to Brutsaert to derive vegetatio height ad displacemet height for a give aerodyamic roughess height [12], z h = m (13) d = h (14) 3 where, the ormalized differece vegetatio idex (NDVI) is determied by MODIS chaels 1 ( µm) ad 2 ( µm), NDVI mi is the NDVI for bare soil ad NDVI max for full vegetatio coverage. Leaf area idex (LAI). Leaf Area Idex (LAI) is the leaf area per uit groud area, which reflects the vertical vegetatio amout. The relatioship proposed by Su is used [7], this reads 1+ NDVI LAI = NDVI 1 NDVI + δndvi 1/ 2 (1) This formula is strictly oly good for low vegetatio sice NDVI saturates at higher LAI values. However, because of limited iformatio for the study area to support more sophisticated formulatios, this equatio is adopted i this study. Surface emissivity. By aalysig the relatio betwee surface emmisivity ad NDVI, a experimetal relatioship was obtaied by Va de Gried et al. to determie surface emissivity [11], ε = l( NDVI) (11) Aerodyamic roughess height. Aerodyamic roughess height is a very importat parameter i surface eergy balace model, which ifluece greatly the turbulet characteristics ear the surface where the heat fluxes origiate. Aerodyamic roughess height ca be estimated by a simple relatioship proposed by Su [7], NDVI z m = (12) NDVI max STUDY AREA AND DATASET 3.1. Study area The study area, Hebei Plai, is located i the ortheaster Chia betwee the rage of E E ad 36 N-39 4 N (Fig. 1). It is oe of the largest agricultural areas i Chia ad also oe of the most desely populated regios i the world. Groudwater resource is oe of the most importat atural resources i this area ad more tha 79% of groudwater abstracted from aquifer was used for irrigatio [13]. Due to the traditioal irrigatio patter, a lot of water resources were wasted directly. With the overexploitatio for more tha 3 years, a series of evirometal problems have occurred i the past few decades ad the future sustaiability of water resources i Hebei Plai is at risk [14, 15]. I order to make better use of the groudwater resources i the Hebei Plai, may studies have bee cocetrated o the groudwater et recharge ad agriculture water use [16], which take importat roles i the water resource estimatio ad maagemet to solve the related evirometal problem for the momet. Several approaches have bee developed to quatify groudwater et recharge. I regioal studies, it is commo to use water balace method to estimate areal et recharge [17], which is maily cotrolled by precipitatio, surface ruoff as well as actual evapotraspiratio for a cosiderable log period. Due to the little differece of raifall patters i arid or semiarid area ad commoly available real time ruoff data, mai difficulty comes from the estimatio of areal evapotraspiratio patters, which have large differeces because of lad surface diversity. O the other had, duo to the large proportio of agriculture water use i the Hebei Plai, it is also very importat to determie the spatial ad temporal evapotraspiratio to guide the irrigatio water use. Hece, the mai problem goes ito accurate evapotraspiratio estimatio spatially ad temporally.

4 N lysimeter represetative of the regio, the same agroomic practices were carried out iside ad outside of the lysimeter [2]. Beijig Lad cover 1,25 2,5 km TaiHag Moutai TaiHag Moutai Lagfag Bazhou Tiaji Baodig BoHai Sea Cagzhou Shijiazhuag Hegshui Xigtai Hada Capital city Locatio of city Due to the limited kowledge about the groud truth lad cover over Hebei Plai, the MOD12Q1 Lad Cover Product was selected i this study. Totally, 12 kids of lad cover types were recogized i study area. Amog them, the domiat lad cover type is crop lad, ad more tha % of the area is for agricultural lad use. Urba ad builtup areas are the secod large lad cover class i Hebei Plai, which has more tha 3 percet accordig to the map. Grass also plays a importat role i study area, most of which located i the seashore area of easter Hebei Plai Dataset Figure 1.Locatio map of the study area Satellite observatios Six sets of cloud free satellite images over the Hebei Plai durig August, 24 to March, 25 were used. Use is made of the surface reflectace products ad surface temperature products of MODIS images, MOD 9 ad MOD 11, which ca be ordered ad dowloaded from Earth Observig System Data Gateway. I order to get whole coverage of the study area, two set of images titled h26v5 ad h27v5 were selected. The surface albedo were estimated based o the algorithm proposed by Liag [18]. Meteorological observatios The Chiese Natioal Meteorological Cetre (NMC) operates several meteorological statios i Hebei Plai o a daily basis. The dataset available from these statios iclude relative humidity, wid speed, air temperature at 2m height, actual vapour pressure, raifall, sushie hours ad ope water evaporatio, etc. All of those measuremets have bee collected from 6 meteorological statios equally distributed withi the study area o March 4, 25. Pre-processig of those measuremets was made to derive the variables over satellite passig time as iputs of SEBS. Lysimeter observatios The lysimeter data measured at Luacheg Agro- Ecosystem Statio (LAES) were selected as groud truth evapotraspiratio to validate the remotely sesed actual evapotraspiratio. Note that, to keep the 4. Result aalysis 4.1. Accuracy aalysis Fig. 2 shows the compariso betwee SEBS result ETa at the weather statio pixel ad poit measuremet based evapotraspiratio for cloud free days durig August, 24 to March, 25. It shows a good agreemet with the SEBS modeled ETa ad groud truth ETa measured by lysimeter except the day of 17, August 24. It ca be explaied by combiig the climatic coditio for that certai period over study area ad pixel based remote sesig techiques. Most of the raifall over Hebei Plai comes i the summer seaso, especially i the July ad August, which accout for about 65~75% of yearly total amout raifall. I additio, it is also the hottest seaso over the year. Therefore, a high evapotraspiratio value based o the high humidity ad high available solar radiatio ca be reached. O the other had, by takig crop types ito accout, domiat crops of August over Hebei Plai are cor, which has a high water requiremet i August due to the flowerig seaso with a high leaf area idex (LAI), so the high ETa value measured by lysimeter is possible. The relative lower ETa values estimated by SEBS model ca be explaied by the pixel by pixel based algorithms. The image used i this study is the MODIS product with 1 km resolutio. Due to the high itesity distributio of small villages i Hebei Plai, the lad cover types icluded i oe pixel are ot oly the crop lad, but also the rural area that have small potetial evapotraspiratio, so a lower ETa values that weightig these differet lad covers i oe pixel is reasoable.

5 ETa measured by lysimeter ETa of SEBS results Julia day Figure 2.Compariso of the daily evapotraspiratio estimated by SEBS ad field observed 4.2. Spatial distributio of ETa i Hebei Plai Due to the various irrigatio performaces i differece place ad high spatially distributio of raifall, the soil moisture i the root zoe i differet site is highly differet. Therefore, varies spatial distributio patters of ETa over Hebei Plai ca be observed as Fig. 3 show. Figure 3. Daily evapotraspiratio i Hebei Plai As far as differet lad cover types were cocered, most of the high ETa values are foud i barre or sparely vegetated area ad water body. Whe uder the same evirometal coditios, for example, the same

6 surface wid speeds, radiatio, air temperature etc., highest ETa values i ope water surface with abudat water for evaporate is expected. The lowest ETa values were foud i crop lad (Tab. 1), which is cotradictig to the well irrigatio maagemet status over the farm lad of Hebei Plai. However, due to large coverage of the study area, high spatial variety is possible ot oly for the local climatic coditios but also the irrigatio performace i differet area, so a lower mea ETa values is reasoable ad explaiable. The statistical characteristics ad histograms of ETa over each lad cover are show i Tab. 1 ad Fig.4. Table 1.The statistics over each lad cover classes i study area o 5 March, 25 (mmday -1 ) Barre Croplad Forest Grass lad Shrub lad Urba Water Miimum Maximum Average Media St. dev Percetage (%) water Percetage (%) barre Percetage (%) croplad Percetage (%) forest Percetage (%) grass lad Percetage (%) shrub Percetage (%) urba Figure 4.Histogram of daily ETa over differet lad cover types i Hebei Plai o 5 March, 24 The statistics reflects that the urba ad built-up area also have a certai amout of evapotraspiratio. This is cotrary to the kowledge that the residetial areas should have lower ETa. This ca be explaied by the pixel based remote sesig techiques. The pixels of urba area icludes ot oly the costructio but also the water body, street trees ad grass parcels, which all have very high evapotraspiratio. Therefore, the estimated ETa i urba area represeted the mixed effectio of all these thigs.

7 4.4. Temporal distributio of ETa Fig. 5 shows the time series daily evapotraspiratio of differet lad covers over Hebei Plai for the cloud free days. Due to little differece betwee the values obtaied o March 4, 25 ad March 5, 25, mea values for these two days were take i the figure show. Sice the images obtaied are limited, so the temporal aalysis of daily evapotraspiratio is partly represetative. 7 Daily Evapotraspiratio (mmday -1 ) Time (day) barre or sparsely vegetated forest shrublad water croplad grasslad urba ad built-up Figure 5.Time series average daily evapotraspiratio of differet lad covers over Hebei Plai As the figure show, maximum values of all kids of lad cover types are i the summer seaso (17 August, 24), the mea values is betwee 4. ad 5.8 mm day -1. The, autum comes with relatively a little bit small daily evapotraspiratio as compared to summer seaso, the mea of which is about 3.4 to 5.4 mm day -1 (21 September, 24). The miimum values for all kid of lad cover types are appearig i the witer seaso, with a rage of mea values that betwee.5 ad.8 mm day -1. I sprig, with the thawig of the lad ad the aabiosis of vegetatio, the daily evapotraspiratio icrease slowly to a rage about mm day -1 o early March (4 March, 25 or 5 March, 25). I later March (25 March, 25), due to the dry climatic coditio ad limited soil moisture over Hebei Plai, little chage or decrease for the daily evapotraspiratio were observed from the figure as compared to the early March. 5. SUMMARY AND CONCLUSIONS Evapotraspiratio is amog the most importat compoets of basi-scale water cycle. It is also the most difficult items to be determied, especially i a big area. Remote sesig techiques provide a possibility to deal with this challege from space. Several algorithms were developed to estimate the regioal evapotraspiratio based o the satellite obaservatio. Surface Eergy Balace System (SEBS) was used to calculate the daily evapotraspiratio over Hebei Plai i this paper. The accuracy aalysis shows that there is a good agreemet with the SEBS modeled ETa ad groud truth ETa measured by lysimeter for most of the selected cloudy free days, which verified that the SEBS is a useful ad accurate tool to estimate the regioal evapotraspiratio. Without doubt, due to the pixel by pixel based remote sesig techiques, some error must occur, especially, whe the coarse resolutio images were used. Spatial-temporal distributio of ETa over Hebei Plai was discussed based o the daily actual evapotraspiratio o cloud free days durig August, 24 to March, 25 by combiig the up-to-date lad cover. Maximum ETa values of all kids of lad cover types over Hebei Plai are i the summer seaso, the mea is betwee 4. ad 5.8 mm day -1. I autum, a

8 relative small ETa values is domiat over study area as compared to summer seaso, which is about 3.4 to 5.4 mm day -1 (21 September, 24). The miimum values are appearig i the witer seaso, with a rage of mea values that betwee.5 ad.8 mm day -1. I sprig, with the thaw of the lad ad the aabiosis of vegetatio, the daily evapotraspiratio icrease slowly to a rage about mm day -1 o early March (4 March, 25 or 5 March, 25). I later March (25 March, 25), due to the dry climatic coditio ad limited soil moisture over Hebei Plai, little chage or decrease for the daily evapotraspiratio were observed. Hebei Plai is oe of the most importat agriculture farm lads i Chia. The domiat crop types over this area are the witer wheat followig by summer maize. Due to the various irrigatio performaces i differece place ad high spatially distributio of raifall, the soil moisture i the root zoe i differet site is highly differet. Therefore, a spatial distributio patter of ETa over Hebei Plai is difficult to be obtaied. As far as differet lad cover types were cocered, most of the high ETa values are foud i barre or sparely vegetated area ad water body. I additio, a certai amout of evapotraspiratio ca be foud i urba area, which ca be explaied by the water body, street trees ad grass parcels that located i urba area. ACKNOWLEDGEMENTS This work was performed durig M.Sc research of the first author at the Departmet of Water Resources, Iteratioal Istitute for Geo- Iformatio Sciece ad Earth Observatio (ITC), the Netherlads. The stay was partially fuded by the Miistry of Lad ad Resources (MLR) of Chia ad Istitute of Hydrogeology ad Evirometal Geology (IHEG), Chia. The first author is pleased to ackowledge these fiacial supports, as well as the assistace ad support provided by ITC. REFERENCE 1. Pema, H. L., Natural evaporatio from ope water, bare soil ad grass, Proceedigs Royal Society of Lodo, Series A. Mathamatical ad Physical Scieces, Lodo, Moteith, J. L., Evirometal cotrol of plat growth, Academic Press, New York, Jese, M. E., R. D. Burma, et al., Evapotraspiratio ad Irrigatio Water Requiremets, ASCE Mauals ad Reports o Egieerig Practices. America Society of Civil Egieers, New York, Norma, J. M., W. P. Kustas, et al., A two-source approach for estimatig soil ad vegetatio eergy fluxes from observatios of directioal radiometric surface temperature, Agricultural ad Forest Meteorology, Vol. 77, , Bastiaasse, W. G. M., M. Meeti, et al., A remote sesig surface eergy balace algorithm for lad (SEBAL) 1. Formulatio, Joural of Hydrology, Vol , , Su, Z., The Surface Eergy Balace System (SEBS) for estimatio of turbulet heat fluxes, Hydrology ad Earth System Scieces, Vol. 6, 85-99, Su, Z. ad C. Jacobs, ENVISAT: actual evaporatio. BCRS Report 21: USP-2 Report , Beleidscommissie Remote Sesig (BCRS), Delft, Su, H., M. F. McCabe, et al., Modelig evapotraspiratio durig SMACEX: comparig two approaches for local- ad regioal-scale predictio, Joural of hydrometeorology, Vol.6, , Timmermas, W. J., J. va der Kwast, et al., Itercompariso of eergy flux models usig ASTER imagery at the SPARC 24 site (Barrax, Spai), ESA proceedigs WPP-25: SPARC fial workshop, Eschede, Gutma, G. ad A. Igatov, The derivatio of the gree vegetatio fractio from NOAA/AVHRR data for use i umerical weather predictio models, Iteratioal Joural of Remote Sesig, Vol.19, , Va de Gried, A. A. ad M. Owe, O the relatioship betwee thermal emissivity ad the Normalized Differece Vegetatio Idex for atural surfaces, Iteratioal Joural of remote sesig, Vol.14, , Brutsaert, W., Evaporatio ito the atmosphere. Reidel, Dordrecht, Zhag, Z. H., J. S. Shi, et al., Water-savig agriculture ad sustaiable groudwater use i Hebei Plai, Sciece Report, Istitute of Hydrogeology ad Evirometal Geology, CAGS: p.32 (i Chiese), Zhag, Z. H., D. Shi, et al., Evolutio of Quaterary groudwater system i North Chia Plai, Sciece i Chia Series D (Earth Scieces), Vol.4, (i Chiese with Eglish abstract), Zhag, Z. H., D. Shi, et al., Evolutio ad developmet of groudwater eviromet i North Chia Plai uder huma activities, Acta

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