Development of an ArcMap Toolbar for Regional Evapotranspiration Modeling 1

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1 Development of an ArcMap Toolbar for Regonal Evapotranspraton Modelng 1 Shujun L, Davd Tarboton, Mac McKee Abstract Evapotranspraton (ET) s mportant to hydrologsts as well as regonal planners. Methods for the calculaton of ET requre land use as well as weather and solar energy nputs. In ths paper we present an ArcMap toolbar for the calculaton of spatally dstrbuted ET from ground based weather data. The toolbar provdes functonalty to nterpolate weather data over the regon of nterest and calculate reference ET from a selecton of standard reference methods. Ths s then combned wth the crop coeffcent based on land cover to estmate potental ET for use n hydrologc modelng and water use plannng. 1 Introducton Evapotranspraton (ET) s an elementary component of the regonal water and energy balance. Its spatal and temporal dstrbuton s of mportance to water resources managers, planners, and researchers. ET s dffcult to measure, and systematc measurements at regonal scale are rare. In many cases, regonal ET data needs to be estmated. In the past years, some researchers explored the possblty of dervng regonal ET from remotely sensed data (Prce 1990, Sucksdorff and Ottle 1990, Bastaanssen 2000). Whle remote sensng approaches can provde hgh spatal resoluton and coverage, they provde lower temporal resoluton lmted by the frequency of satellte overpasses and sky condtons. Furthermore almost all these methods are dagnostc rather than predctve. In other words, ths category of approach can not furnsh us wth ET nformaton under the possble future change of ether ndvdual clmate factor or the land surface characterstcs. However, n many cases lke long term water resources plannng, the ET needs to be estmated under gven scenaros wth smulated or estmated clmate and planned land use patterns. Takng nto consderaton the above facts, ET s commonly estmated from ground meteorologcal data wth avalable land cover nformaton through the conventonal reference ET - crop coeffcents approach. Ths approach also allows to use smulated meteorologcal data from meteorologcal models or statstcal approaches as well as observed data to derve ET seres. The meteorologcal varables drvng the physcal process of ET are readly avalable through routne montorng networks such as MesoWest and Natonal Clmatc Data Center(NCDC). Establshed methods (Jensen et al. 1990)exst to calculate pont-specfc reference ET from pont meteorologcal data. Wth the support of GIS technology, t s possble to extend these methods to get spatally dstrbuted ET over a regon. ArcObjects (Zeler 2001), part of the ESRI ArcGIS package, provdes a way to ntegrate GIS wth external models. It provdes an object system through whch the functonalty n the end- 1 Presented at 23 rd ESRI Internatonal User Conference, July 7-11, 2003, San Dego, Calforna - 1 -

2 user-applcaton of ArcGIS can be accessed by programmng. Wth ArcObjects t s techncally feasble to develop an ntegrated envronment for the specfc task of regonal ET modelng. Ths paper presents an approach for modelng regonal and long-term ET from wdely avalable ground meteorologcal data and crop patterns mplemented as an ArcMap toolbar (ArcET) usng the ESRI ArcGIS ArcObjects modelng envronment. 2 Development of ArcET 2.1 Conceptual Framework of the GIS-based approach for regonal ET modelng The conceptual Framework of the GIS-based approach for regonal ET modelng s gven n fgure 1. The goal s to calculate crop evapotranspraton for every grd cell wthn the doman of a gven study area. Inputs consst of tme seres of pont meteorologcal data, land cover/land use data and crop coeffcent tables. The meteorologcal data s nterpolated to grds usng nterpolaton procedures approprate for the quantty beng nterpolated. These are then used to compute a grd of reference ET. The crop coeffcent tables are used to provde a crop coeffcent at each grd cell based upon land cover/land use data. Combnng the crop coeffcent wth reference ET provdes the estmated ET at each grd cell. Clmate Statons & Observatons ( Ponts ) Tabulated Crop Coeffcents DEM Land Use (Polygons) Crop coeffcent ( Grd ) Clmate varables ( Grds ) Reference ET ( Grd ) Crop ET ( Grd ) Fgure 1. Illustraton of the GIS-based approach for regonal ET modelng (The boxes depct the ArcET functon modules) - 2 -

3 2.2 Development of the toolbar to mplement the above approach Functonalty desgn ArcET conssts of the followng four modules shown n fgure 2 wth specfc functonalty as follows: (1) Interpolatng grds of spatally dstrbuted meteorologcal parameters from pont measurements (2) Reference ET calculaton at each grd cell (3) Generatng crop coeffcent grds from land cover/use nformaton (4) Grd calculatons to get crop ET Pont-based Clmate Parameters (Database) Jon spatally Spatal Interpolatons (ArcObjects) Statons Map (shapefle) 1 2 Met -Parameter Rasters DEM (Grd) Raster Readng Out (ArcObjects) (VB ADO) Access DB Tmax(req.) Tmn(req.) Tdew(opt.) Ea(opt.) RH(opt.) SR(opt.) WS(opt.) Lat(cal.) Ele(req.) RefET(cal.) Other Felds Reference ET Module (RefET COM) Raster Wrtng Back (ArcObjects) (VB ADO) Jon spatally Land Use/Cover Map (shapefle) 3 4 Reference ET Maps Crop-specfc Kc Curves (Database) Geodataset Converson (ArcObjects) Crop Coeffcents Maps Raster Calculaton (ArcObjects) Crop ET Maps Fgure 2. Overall desgn of ArcET The followng sectons dscuss the functonalty and desgn of each module

4 Module 1: Interpolatng grds of spatally dstrbuted meteorologcal parameters from pont measurements ArcGIS provdes a selecton of two-dmensonal spatal nterpolaton methods through ArcObjects. The specfc nterpolaton scheme chosen needs to recognze the physcal attrbutes of the quantty beng nterpolated and the relatonshp to topography. ArcET mplements nterpolaton schemes for temperature, humdty, solar radaton and wnd. 1) Interpolaton of Temperature Temperature s fundamental to the calculaton of ET. The dependence of temperature upon elevaton represented n terms of a lapse rate lead us to mplement Elevatonally Detrended Krgng for the nterpolaton of temperature. In ths scheme temperature at any locaton can be expressed as a combnaton of two components: a trend vertcally varyng wth elevaton and a horzontal random resdual T ( u) = TH ( u) + TV ( u) (1) where T s the vertcal trend V T s the horzontal resdual H The vertcal trend at any locaton u wthn the study s assumed to be a functon of elevaton T V ( u) = f ( Z( u)) (2) here Z (u) s the elevaton at locaton u Takng a lnear regresson relatonshp, f ( Z( u)) = az( u) + b (3) then, T V ( u) = az( u) + b (4) In above formula, a and b are regresson coeffcents obtaned by developng a lnear regresson relatonshp between the measured temperature at each staton and the staton elevaton over the whole set of statons at each calculaton step. The horzontal component at any on-gage locaton s nterpolated from the resdual dfference between the measured temperature and the estmated trend value from the above lnear regresson at each staton usng ordnary Krgng T H n ( u) = w T ( g ) (5) = 1 H where T ( g ) = T( g ) [ az( g ) b] (6) H + Then, the temperature at any locaton u wthn the study s estmated as n T ( u) = az( u) + b + w T ( g ) (7) where = 1 H - 4 -

5 u represents any locaton wthn the study area g represents the th gage locaton among consdered gages surroundng u n s the total number of gages whose measurements are consdered to be spatally correlated to the value at locaton u w s the weght of the th gage Z (u) s the elevaton at locaton u The operatonal procedure for temperature nterpolaton conssts of the followng steps: (1) Establsh a regonal regresson relatonshp between temperature and elevaton from temperature observatons and staton elevatons for each calculaton tme step T = a Z + b t t (2) Remove the elevatonal trend ( T ) from the measured values based on the above regresson, and get the resduals at gage locatons, T = T T (3) Interpolate the resduals ( T ) wth ArcGIS bult-n ordnary Krgng to get the horzontal ' resdual ( T ) of temperature at all other locatons (4) Use DEM and the regonal regresson relatonshp between temperature and elevaton to get the vertcal trend (T ) at all other locatons (5) Obtan the estmaton of temperature ( T ) at any locaton by addng the above two terms, T = T + T 2) Interpolaton of Humdty Data Humdty nformaton may be expressed n terms of dew pont temperature, relatve humdty or vapor pressure. We convert relatve humdty and vapor pressure to dew pont temperature, then apply the temperature nterpolaton scheme. 3) Interpolaton of measured solar radaton When solar radaton data s avalable, nverse dstance weghtng (IDW) s used to nterpolate solar radaton data. When there are no solar radaton measurements the emprcal approach recommended n FAO56 (Allen et al. 1998) based upon the temperature range s adopted. ( T T ) Ra R K S = S max mn (8) Where T max s maxmum ar temperature[ o C] T mn s mnmum ar temperature[ o C] R s extraterrestral radaton []MJm -2 d -1 ] a - 5 -

6 K S s an emprcal adjustment coeffcent varyng from 0.16 for nteror locatons to 0.19 for coastal regons. 4) Interpolaton of measured wnd speed When wnd data s avalable, nverse dstance weghtng (IDW) s used to nterpolate between the pont measurements. When there s no wnd data then a general value of wnd speed s used over the entre area (Allen et al. 1998) Module 2: Conduct reference ET calculaton at cell level Once the grds of ndvdual meteorologcal varable are avalable, they are read nto a personal database on a cell-by-cell bass. Then reference ET calculaton wth a partcular method can be conducted over the database. The calculated values stored n the reference ET feld of the personal database are used to generate reference ET grds. To ensure that users from any regon can apply ths extenson for ther work, a selecton of reference ET calculaton methods are ncluded n the extenson as a method lbrary. Users may select a partcular method based on the data avalablty and regonal clmate & terran condtons. The followng methods are avalable: 1. FAO 56 Penman-Monteth (grass ) 2. Standardzed ASCE Penman-Monteth equaton(s) (short / tall grass ) 3. Hargreaves SCS modfed Blaney-Crddle equaton 5. Prestley-Taylor Module3: Generatng crop coeffcent maps from land cover/use nformaton In order to obtan the correspondng dstrbuted crop coeffcent nformaton, the followng operatons must be performed: 1. Look up exstng crop types n the provded land use shapefle 2. Provde crop coeffcents nformaton correspondng to exstng crop types 3. Jon tme seres crop coeffcents to specfed land use polygons 4. Convert the joned crop coeffcent attrbute n the feature layer to a raster Ths module mplements procedures to perform these GIS operatons automatcally, and provde the modeler an nterface to nput or edt crop coeffcent for avalable crop types. Module 4: Grd calculatons to get crop ET The crop ET grds are obtaned by multplyng crop coeffcent grds wth reference ET grds, usng the IMapAlgebraOp raster calculator nterface. The total ET wthn user provded zones s then summarzed n an ET table

7 2.2.2 Interface desgn To free the modeler from complcated nteractons wth ArcGIS, all the functons for ET modelng are ncorporated nto a custom ArcGIS toolbar wth categorzed menu tems (fgure 3). Fgure 3. ArcET toolbar nterface The nput data preparaton and control settngs are accessed through the general Settngs menu tem under the regnal ET Estmate menu (fgures 4-6). These provde the nterface to control the basc data preparaton. Fgure 4. Indvdual nterface for data nput - 7 -

8 Fgure 5. Indvdual nterface for reference ET method settngs Fgure 6. Indvdual nterface for runnng control - 8 -

9 Once the preparaton work s done and settngs s saved, the "Get Reference ET" menu nvokes the modelng process. Durng the generaton of crop coeffcent maps, some user s nteractons are requred to check provded data or append new data. The nterface shown n fgure 7 s used for ths purpose. Fgure 7. Indvdual nterface for generatng Crop Coeffcent maps Once the reference ET has been calculated the "Get Crop ET" menu nvokes the crop ET calculaton process. The nterfaces and GIS functonalty of the toolbar have been programmed n Mcrosoft Vsual Basc wth GIS-related functonalty coded through reference to the follows: ESRI Object Lbrary ESRI ArcMap Object Lbrary ESRI Spatal Analyst Shared Object Lbrary ESRI Spatal Analyst Extenson Object Lbrary The program s packaged as a dynamc lnk lbrary (DLL) that can be easly customzed nto the man nterface of ArcMap accordng to the standard procedure provded by ESRI

10 3 Use of the toolbar The toolbar descrbed above provdes the capablty to perform ET calculatons from wthn ArcGIS. For the calculaton of reference ET only, users are requred to nput the followng data: (1) An ESRI shapefle to show the locaton of clmate statons wthn the regon of nterest (2) pont-measurements of meteorologcal varables n the format of tabulated tme-seres (3) DEM n ESRI GRID format for the area of nterest To obtan crop ET, the followng data are also requred: (4) An ESRI polygon shapefle for Land Use/Cover nformaton (5) A DBF table wth crop coeffcents data The modelng results are provded n the formats of (1)monthly reference ET maps as fgure 8 (2) monthly crop ET maps as fgure 9 (3) tabulated monthly ET nformaton zone by zone as fgure 10 Fgure 8. Monthly reference ET maps Fgure 9. Monthly crop ET maps

11 Countes Hydrologc Unt Codes (HUCs) Watersheds / Basns Fgure 10. Tabulated monthly ET nformaton The toolbar, together wth nstructons for ts use are avalable from Summary An ArcMap toolbar (ArcET) was developed to provde an ntegrated GIS envronment for the dervaton of the spatal dstrbuton of ET. ArcET facltates the mplementaton of varous pont-specfc reference ET methods at regonal scale through seamless ntegraton of GIS wth ET models and thus provdes an effectve and effcent operatonal tool to get the spatal and temporal dstrbuton of evapotranspraton. Acknowledgement Ths work was supported by the USGS through a Natonal Insttutes for Water Resources Research grant. We would lke to thank Mr. Crag Mller from Utah Dvson of Water Resources for hs valuable suggestons n functonalty desgn of ths tool and provdng data for testng the tool

12 Reference Allen, R. G., L. S. Perera, D. Raes, and M. Smth Crop evapotranspraton: Gudelnes for computng crop water requrements. Food and Agrculture Organzaton of the Unted Natons, Rome. Bastaanssen, W. G. M SEBAL-based sensble and latent heat fluxes n the rrgated Gedz Basn, Turkey. Journal of Hydrology 229: Jensen, M. E., R. D. Burman, and R. G. Allen, edtors Evapotranspraton and Irrgaton Water Requrements. Amercan Socety of Cvl Engneers, New York. Prce, J. C Usng Spatal Context n Satellte Data to Infer Regon Scale Evapotranspraton. IEEE Transactons on Geoscence and Remote Sensng 25: Sucksdorff, Y., and C. Ottle Applcaton of satellte remote sensng to ertmate areal evapotranspraton over a watershed. Journal of Hydrology 121: Zeler, M., edtor Explorng ArcObjects. Envronmental Sysytem Research Insttute, Redlands. Author Informaton Shujun L PhD Graduate/Research Assstant Cvl and Envronmental Engneerng Utah State Unversty Logan, UT Phone: (435) Emal: shujunl@cc.usu.edu Davd Tarboton Professor, Cvl and Envronmental Engneerng Water Program Coordnator, Utah Water Research Laboratory Utah State Unversty, Logan UT Ph: (435) Fax: (435) Emal: dtarb@cc.usu.edu Mac McKee Professor, Cvl and Envronmental Engneerng Utah Water Research Laboratory Utah State Unversty, UMC Canyon Road Logan, Utah Phone: (435) Fax: (435) E-mal: mmckee@cc.usu.edu