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...esCampbell model (with and without stability correction) and the Monteith model.sCalculation procedures were nearly identical to those used by Kjelgaard et al. (1996). ThesCampbell model is given as (=-=Campbell, 1977-=-):sIn z — d + Zpj ^ In z -d + Zm + M^MsI J V jsk-uswhere z is tlie anemometer height (m), d is the zero plane displacement (m), ZH and ZMsare the roughness lengths for sensible heat and momentum (m), ...

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...amce in the red and near-infrared bandssthrough a spectral vegetation index, such as the Normalized Difference Vegetation Indexs(NDVI; Rouse et al., 1974) or the Soil Adjusted Vegetation Index (SAVT; =-=Huete, 1988-=-).sIn the 1999 study, fc was estimated by destructive plant sampling and fit to the NDVIswith a three-piece linear regression:sf j , = 0sN D V Is< - 1 0sfc =2.36(NDVl)-0.24 0.10 < NDVI < 0.23sfc = 0.9...

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...(W m'^), G is the soil heat flux density (W m'^), pa is the density of dry air, assumedsconstant at 1.19 (kg m'"') (AZMET), Cp is the specific heat of dry air, assumed constant ats1013 (J kg"' °C*') (=-=Jensen et al., 1990-=-), y is the psychrometric "constant" (kPa °C"'), A issthe slope of the saturated vapor pressure—temperature relation (kPa °C''), rc is the canopysresistance, assumed constant at 10 and 250 (s m'') for...

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...izes the SDD for meteorological variability' (Idso et al., 1981;sJackson et al., 1981); and the water-deficit-index (WDI), which is essentially the CWSIsthat accounts for background soil temperature (=-=Moran et al., 1994-=-; Clarke; 1997).sIdso et al. (1977) showed an inverse relationship between seasonal cumulativesSDD and final yield of wheat using six irrigation treatments. Walker and Hatfield (1979)sfound similar re...

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...e has been linked to crop physiological processes such assphotosynthesis (Jackson, 1984); thermal emittance is generally a function of canopystemperature, which has been linked to evapotranspiration (=-=Jackson, 1982-=-). Remotessensing is advantageous over in-situ sensing techniques in that large cropped areas can besscanned quickly, non-invasively, and non-destructively. Spatial resolution that otherwiseswould not...

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...s (i.e., hectares containing one-meter grids)sis possible with remote sensing: hence, remote sensing would lend itself to precisionsagricultui-e, particularly by providing images of field conditions (=-=Moran et al., 1997-=-).s1.3.3 Canopy Temperature-Based IndicessThe use of canopy temperature to detect crop water stress has been ofsconsiderable interest since the 1960s, when the development of infrared thermometrysallo...

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...sair temperature (K), and fiiction velocity (m s"') respectively. Like if/n and vj/M, C > 0 forsstable and C < 0 for unstable conditions. Sensible heat H issPaCpfe-Ta) H = (A.4)sand u* is defined as (=-=Campbell, 1985-=-)s* u = kusIn Z — d + ZJY^ + ¥M L JsEquation (A.l) can be simplified by combining it with equation (A.5):sIns% =sJ A z - ds+szp isI 2 * k usFor unstable conditions, \j/H is given as (Kjelgaard et al.,...

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...face is warm and dry relative to a cool, transpiring crop, causing sensible heat losssfrom the soil to the crop and atmosphere. In this case, more complex multicomponentsmodels may be required (e.g., =-=Shuttleworth and Wallace, 1985-=-). Moran et al. (1994)saddressed the consequences of this assumption through simulation models and foundsthe greatest error to occur for mid values of fc; however, they stated that a number ofsassumpt...

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... soil moisture deficit causes A.ETcsto fall below A.ETp. Assuming the instantaneous latent heat flux ratio during thesmaximum diumal atmospheric demand is similar to the daily latent heat flux ratios(=-=Jackson et al., 1983-=-), and converting latent heat flux to depth, the ratio (ETc / ETp) isssubstituted for (A.ETc / A.ETp) in equation (A.l 1). The resulting expression is tenned thesSoil Water Stress Index (SWSI) because...

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...lutionsrequirements for irrigation management (Gage, 2000). Airborne platforms could providesmeaningful data if near-continuous coverage was possible; however, this would clearlysbe cost prohibitive (=-=Clarke, 1997-=-). Most research in canopy temperature-based indicesswas ground-based as hand held IRTs were used; Keener and Gardner (1987) describessuch a device with an internal processor that calculates the CWSI....

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...c "constant" (kPa °C"'), A issthe slope of the saturated vapor pressure—temperature relation (kPa °C''), rc is the canopysresistance, assumed constant at 10 and 250 (s m'') for / and u, respectively (=-=Ehrler, 1973-=-;sKeener and Gardner, 1987). Appendix A gives procedures for calculating ra- The Rn termsis assumed a firaction of the total incoming short-wave solar radiation Rs, and G issassumed a fraction of Rn,"...

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...iency.s97sINTRODUCTIONsRemote sensing is an efficient method for detecting crop water stress on a sitespecific basis if canopy temperature measurements are available at sufficient spatialsresolution (=-=Jackson, 1984-=-; Moran et al, 1997). Satellite and aircraft remote sensingsplatforms generally lack the timeliness, repeat frequency, or spatial resolution requiredsfor irrigation management, and data acquired by th...

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...fied period (along wdth irrigation, rain, runoff, leaching factors,sand deep percolation), the crop water need is estimated. This is commonly known as thes"checkbook method" of irrigation management (=-=Martin et al., 1990-=-). These estimates ofsETc, however, do not account for local microclimates or within-field variability of soilsand plant characteristics (Moran and Jackson, 1991). This can be alleviated when ETcsesti...

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...aused by some adverse condition, such as a soil moisture deficit, and issgenerally deemed detrimental to growth and development. Final crop yields have beensrelated to total consumptive use of water (=-=Doorenbos and Kassam, 1979-=-). When cropswater stress occurs, atmospheric demand is not being met because of stomatal closure,sand the total consumptive use of water is reduced. Thus, the detection and avoidance ofscrop water st...

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...e timeliness, repeat frequency, or spatial resolution requiredsfor irrigation management, and data acquired by these platforms carry a greatersprocessing requirement than if acquired from the ground (=-=Moran, 1994-=-). Self-propelledscenter pivot and linear move irrigation systems can provide a platform for ground-basedsremote sensing (Phene et al., 1985) and variable-rate application needed for site-specificsirr...

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...ry 15 minutes throughout the 24 hour period duringsmost of the season; instrument height was 2.0 m from furrows. A daily CWSI value wassobtained by averaging measurements between 1400 and 1600 hours (=-=Idso et al., 1982-=-).sVolumetric soil moisture was measured using neutron scattering, a capacitancesprobe, and time domain reflectometry (TDR). Tnvo neutron access tubes were placed inseach plot; measurements were taken...

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... the treatment irrigated when soil matric potential reached 50 kPa. Thesesresults were also observed for potatoes (a crop extremely sensitive to water stress) in assimilar study at the same location (=-=Shae et al., 1999-=-). Although both studies reportedsbetter WUE when irrigations were timed based on soil matric potential than on thesCWSI, the former is an in-sitii method and so does not have the advantages of remote...

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...is used; thesCWSI may be valid for partial canopy cover if off-nadir IRT measurements are possible.sIn either case, soil background may still appear during times of water stress because ofsleaf wilt (=-=Jackson et al., 1986-=-). Moran et al. (1994) extended the CWSI concept bysaccounting for soil temperature using the same energy balance principles used in thesCWSI, and defined the Water Deficit Index (WDI). A nadir-lookin...

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...season (Slack et al.,s1996). Cumulative GDD were calculated following the procedures of Snyder (1985),swhere the lower and upper threshold temperatures for cotton were 12.8 and 30.0 °C,srespectively (=-=Brown, 1991-=-). The mid season basal crop coefficient as given was correctedsfor local climatic conditions (Allen et al., 1998).s77sThe CWSI was compared to the SWSI; comparisons are given in terms of slope,sinter...

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...ed on 26 April 1998 (DOY 116) on east-west raisedsbeds spaced 1.0 m apart on a laser leveled 1.3 ha field. The soil is classified as a CasasGrande series, with sandy loam or sandy clay loam textures (=-=Post et al., 1988-=-). The studyswas part of a larger remote sensing experiment consisting of two nitrogen levels in asrandom block design with 16 plots; however, measurements pertinent to this article weresmade only in ...

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...reasing ra (eq. A.l). Thes68sopposite is the case for unstable (enhanced) conditions. The Campbell model withoutsstability correction is simply VI/H = H/M = 0.sAn expression for stable conditions is (=-=Businger, 1975-=-)s¥H = VM = (A.2)swhere is an atmospheric stability correction parameter, defined ass^ ^ - kzgH CA.3)sPaCpTgU ^swhere g, H, Ta, and u* are the gravitational constant (9.81 m s'^), sensible heat (W m"^...

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...asured leaf temperatures of unstressed cottonsusing wire thermocouples. Thus, the baseline can be computed using only airstemperature, vapor pressure deficit, and a crop-specific slope and intercept (=-=Idso, 1982-=-;sGardner et al., 1992a). Reliable estimates of meteorological parameters required for thestheoretical CWSI were not widely available outside of research farms until the late 1980sswhen networks of au...

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...ariability (CTV), which is the range of canopystemperatures measured in a field (Clawson and Blad, 1982); the crop-water-stress-indexs(CWSI), which normalizes the SDD for meteorological variability' (=-=Idso et al., 1981-=-;sJackson et al., 1981); and the water-deficit-index (WDI), which is essentially the CWSIsthat accounts for background soil temperature (Moran et al., 1994; Clarke; 1997).sIdso et al. (1977) showed an...

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...idedsmeteorological data on an hourly basis. Rainfall was recorded by an in-field rain gage.sThe fraction of vegetation cover (fc) was correlated to the normalized differencesvegetation index (NDVI) (=-=Rouse et al., 1974-=-) and verified by weekly destructive plants103ssampling in three locations of each plot, which also gave estimates of plant height.sVolumetric soil moisture was measured and estimated between measurem...

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...nter pivot and linear move irrigation systems can provide a platform for ground-basedsremote sensing (Phene et al., 1985) and variable-rate application needed for site-specificsirrigation management (=-=Sadler et al., 2000-=-). Since these systems pass over a field atsregular intervals, an on-board remote sensing system conceivably could providesinformation on crop conditions that meets repeat frequency and spatial resolu...

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...s at 0.05, 0.10, 0.15, ands0.20 m depths. The TDR probes were multiplesced to a Tektronix model 1502C cablestester; volumetric soil moisture was retrieved -every hour using the TACQ softwaresprogram (=-=Evett, 1998-=-). The upper and lower lim-its of volumetric soil moisture availablesto the crop (assumed field capacity 0fc and wilting point 0wp, respectively) were estimatedsat each location by taking the maximum ...

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...strategies tosyield (Hussman et al., 1998). Nonetheless, crop water use can be simulated because it isslinked to measured canopy temperature, allowing estimates of f DEP through a soil watersbalance (=-=Hatfield et al., 1984-=-; Kjelgaard et al., 1996; Barnes et al., 2000).sThe present research investigates a more direct approach of relating f DEP to thesCWSI. Jackson et al. (1981) relates the CWSI to crop evapotranspiratio...

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...ssman et al., 1998). Nonetheless, crop water use can be simulated because it isslinked to measured canopy temperature, allowing estimates of f DEP through a soil watersbalance (Hatfield et al., 1984; =-=Kjelgaard et al., 1996-=-; Barnes et al., 2000).sThe present research investigates a more direct approach of relating f DEP to thesCWSI. Jackson et al. (1981) relates the CWSI to crop evapotranspiration (ETc), wheresCWSI valu...

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...a greatersprocessing requirement than if acquired from the ground (Moran, 1994). Self-propelledscenter pivot and linear move irrigation systems can provide a platform for ground-basedsremote sensing (=-=Phene et al., 1985-=-) and variable-rate application needed for site-specificsirrigation management (Sadler et al., 2000). Since these systems pass over a field atsregular intervals, an on-board remote sensing system conc...

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...kPa °C"'), A issthe slope of the saturated vapor pressure—temperature relation (kPa °C''), rc is the canopysresistance, assumed constant at 10 and 250 (s m'') for / and u, respectively (Ehrler, 1973;s=-=Keener and Gardner, 1987-=-). Appendix A gives procedures for calculating ra- The Rn termsis assumed a firaction of the total incoming short-wave solar radiation Rs, and G issassumed a fraction of Rn," i.e., Rn = 0.7Rs, and G =...

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...247 for plot 12 (followingsdrizzle on DOY 245). Cloud passages reduced Rs by 50% or more, and several minutessmay be required for the canopy temperature to reach equilibrium following a change insRs (=-=Pennington and Heatherly, 1989-=-). With frequent changes in Rj, the canopy energysbalance was disrupted, but the net effect was usually reductions in measured canopystemperature relative to calculated lower and upper temperature lim...

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... the crop. Crop water use is quantified assevapotranspiration (ETc), which is presently most conveniently estimated as die productsof a crop coefficient (Kc) and a reference evapotranspiration (ETo) (=-=Allen et al., 1998-=-). Insthe past two decades, networks of meteorological stations have been installed in manysagricultural regions in the United States to provide estimates of ETo on a weekly, daily,sor even hourly bas...

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...f DEP), which issmore practical and universally understood in timing and determining application depthss(Martin et al., 1990) and for relating the effect of irrigation management strategies tosyield (=-=Hussman et al., 1998-=-). Nonetheless, crop water use can be simulated because it isslinked to measured canopy temperature, allowing estimates of f DEP through a soil watersbalance (Hatfield et al., 1984; Kjelgaard et al., ...