A Soil-Canopy-Atmosphere Model for Use in Satellite Microwave Remote Sensing

Transcription

1 University f Suth Carlina Schlar Cmmns Faculty Publicatins Earth and Ocean Sciences, Department f A Sil-Canpy-Atmsphere Mdel fr Use in Satellite Micrwave Remte Sensing Venkataraman Lakshmi University f Suth Carlina - Clumbia, vlakshmi@gel.sc.edu Eric F. Wd Bhaskar J. Chudhury Fllw this and additinal wrks at: Part f the Earth Sciences Cmmns Publicatin Inf Published in Jurnal f Gephysical Research, Vlume 102, Issue D6, 1997, pages Lakshmi, V., Wd, E. F., & Chudhury, B. J. (1997). A sil-canpy-atmsphere mdel fr use in satellite micrwave remte sensing. Jurnal f Gephysical Research, 102 (D6), Jurnal f Gephysical Research 1997, American Gephysical Unin This Article is brught t yu fr free and pen access by the Earth and Ocean Sciences, Department f at Schlar Cmmns. It has been accepted fr inclusin in Faculty Publicatins by an authrized administratr f Schlar Cmmns. Fr mre infrmatin, please cntact SCHOLARC@mailbx.sc.edu.

2 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D6, PAGES , MARCH 27, 1997 A sil-canpy-atmsphere mdel fr use in satellite micrwave remte sensing Venkataraman Lakshmi, 1 Eric F. Wd, and Bhaskar J. Chudhury 2 Water Resurces Prgram, Department f Civil Engineering and Operatins Research, Princetn University, Princetn, New Jersey Abstract. Reginal and glbal scale studies f land-surface-atmsphere interactins require the use f bservatins fr calibratin and validatin. In situ field bservatins are nt representative f the distributed nature f land surface characteristics, and large-scale field experiments are expensive undertakings. In light f these requirements and shrtcmings, satellite bservatinserve ur purpses adequately. The use f satellite data in land surface mdeling requires develping a hydrlgical mdel with a thin upper layer t be cmpatible with the nature f the satellite bservatins and that wuld evaluate the sil misture and sil temperature f a thin layer clse t the surface. This paper utlines the frmulatin f a thin layer hydrlgical mdel fr use in simulating the sil mistures and sil temperatures. This thin layer hydrlgical mdel is the first step in ur attempt t use micrwave brightness temperature data fr reginal sil misture estimatin. The hydrlgical mdel presented here has been calibrated using five years (1980-!984) f daily streamflw data fr the Fdngs Creek catchment. The calibrated parameters are used t validate the daily streamflws fr the next 5 year perid ( ). The cmparisn f surface sil mistures and surface temperatures fr the perid f the Intensive Field Campaigns (IFCs) during the First ISLSCP (Internatinal Satellite Land Surface Climatlgy Prject) Field Experiment (FIFE) in 1987 is carried ut and yields gd results. The thin layer hydrlgical mdel is cupled with a canpy radiative transfer mdel and an atmspheric attenuatin mdel t create a cupled sil-canpyatmsphere mdel in rder t study the effect f the vegetatin and the sil characteristics n the Special Sensr Micrwave Imager (SSM/I) brightness temperatures. The sensitivities f the brightness temperatures t the sil and vegetatin is examined in detail. The studies shw that increasing leaf area index masks the plarizatin difference signal riginating at the sil surface. 1. Intrductin Surface sil misture is perhaps the mst imprtant indicatr f land surface respnse t atmspheric frcing and prvides feedback t the atmsphere. The prper estimatin f sil misture wuld greatly enhance ur understanding f land-surface-atmsphere interactins. Surface sil misture plays an imprtant rle in partitining rainfall int infiltratin and runff. The land surface evapratin and transpiratin depend n the amunt f sil misture available. Tgether, surface temperature and sil misture determine the land surface heat and water balance. In large-scale mdeling, the sil misture and temperature are imprtant in deciding the depth f the planetary bundary layer, circulatin/wind patterns [Mahfuf et al., 1987; Lanicci et al., 1987; Zhang et al., 1982] and reginal water and energy budgets. It is therefre imprtant fr imprved mdeling f these quantities and the use f bservatinal data n scales cmparable t the mdeling scales. Satellite data are useful in this regard. Research in and Nw at General Sciences Crpratin, Labratry fr Atmspheres, NASA Gddard Space Flight Center, Greenbelt, Maryland. 2Nw at Hydrlgical Sciences Branch, NASA Gddard Space Flight Center, Greenbelt, Maryland. Cpyright 1997 by the American Gephysical Unin. Paper number 96JD /97/96JD $ utilizatin f remtely sensed data is imprtant fr the purpses f understanding spatial variability and reginal scales and in verifying land surface parameterizatins [Wd, 1991]. Prjects like the Glbal Energy and Water Experiment (GCiP) Cntinental Scale Internatinal Prject (GCIP) invlve the develpment and testing f hydrlgical mdels n a cntinental scale ver the suthern plains f the United States. The availability f data sets fr the validatin f cntinental scale sil misture simulatins wuld be very useful. There are many advantages t remte sensing as a methd f sil misture determinatin as cmpared t field sampling. Field sampling is pint based and des nt give a clear picture f the variatin f the sil misture ver an area. Accuracy (difference between the actual sil misture pattern and the interplated sil misture pattern) f interplatin schemes depends n the clseness f the sampled sil misture data pints and the hetergeneity f the sil misture distributin. In the case where the crrelatin lengths f sil misture are smaller than the measurement spatial interval, grund-based data cllectin may result in the biased sampling f the sil misture. Satellite remte sensing ffers spatial cverage and a certain tempral frequency in mnitring sil misture frm space. The micrwave frequencies f the electrmagnetic spectrum are the mst sensitive t the variatins f sil misture [Schmugge, 1985] due t the plar nature f water. This change in dielectric cnstant f the sil (caused by changes in sil misture) is recrded as changes in the radiatin emitted by

3 6912 LAKSHMI ET AL.' SOIL-CANOPY-ATMOSPHERE MODEL the sil in the micrwave regin. Our chice here t use passive micrwave remte sensing was dictated by the presence f the Special Sensr Micrwave Imager (SSM/I), a micrwave sensr with glbal cverage and daily tempral cverage. In additin, the SSM/I is a very stable, sensitive, and wellcalibrated sensr [Hllinger et al., 1990], which makes it a very appealing chice. In additin, the SSM/I is the nly remte sensr at this time which can be used fr sil misture sensing. The mtivatin fr the develpment and testing f a thin layer mdel fr land hydrlgy stems frm the desire t use satellite remte sensing fr purpses f sil misture estimatin. The SSM/I is a fur-frequency (19, 22, 37, and 85 GHz), dual plarizatin (hrizntal and vertical except fr 22 GHz, which has nly hrizntal plarizatin) sensr. The reslutin f the SSM/I is abut 56 km at 19 GHz and 33 km at 37 GHz, which are the frequencies being used fr sil misture sensing. The surface temperature and sil misture frm the bundary cnditins fr micrwave radiatin emanating frm the sil. The penetratin depth f the SSM/I sensr wuld be in the rder f ne tenth f the wavelength t a wavelength [Ulaby et al., 1986], which means the cntributin thickness t the micrwave radiatin wuld be 0.1 t 1.5 cm in the case f 19 GHz and 0.08 t 0.8 cm in the case f 37 GHz. Therefre the hydrlgical mdel shuld predict the surface temperature and sil misture fr a thin upper layer (1 cm). Mst f the hydrlgical mdels have an upper layer which crrespnds t the rt zne depth [Famiglietti et al., 1994a, b; Liang et al., 1994]. This may be anywhere between 5-10 cm and 50 cm. We are specifically interested in the tp 1 cm layer. Als, these mdels d nt validate with regard t bth sil misture and sil temperature. An missin in bth f these mdels is the adequate parameterizatin f misture gradient driven flux frm the lwer layer t the upper layer t replenish the sil misture f the upper layer. This aspect f sil misture dynamics is very imprtant when mdeling the sil clumn, especially during the mrning (0600) verpass f the SSM/I sensr, prir t which the upper layer has been depleted by evapratin during the previus day and is replenished during the nighttime hurs when the sil evapratin is lw. A thin layer mdel f sil hydrlgy with a 1 cm upper layer based n assumptins validated by cmparisn with a mre detailed finite element apprach [Mahrt et al., 1984] t minimi?e truncatin errrs is used. The purpse f this paper is t develp a thin layer hydrlgical mdel fr water and energy balance that can be used t predict the tp 1 cm layer sil misture and the surface temperature and t understand the prcesses and the sensitivities f the SSM/I brightness temperatures t vegetatin and sil misture. This is fllwed by investigatins f the effect f hetergeneities in land surface characteristics and rainfall in- put n simulated brightness temperatures [Lakshmi et al., 1996a] and simulatins f reginal scale surface sil mistures using the thin layer hydrlgical mdel fr a perid f a year (August 1987 t July 1988) and estimatin using SSM/I 19 and 37 GHz brightness temperature data and their cmparisn [Lakshmi et al., 1996b]. 2. Thin layer sil hydrlgical mdel This sectin describes the thin layer hydrlgical mdel. The mdel is divided int tw layers: the tp layer is 1 cm thick and the bttm layer is 99 cm thick. The mdel includes infiltratin f rainfall, runff, bare sil evapratin frm the tp layer, the Z I O Z 2 q2 Qb Figure 1. Representatin f the thin layer mdel f sil hydrlgy. exchange fluxes between the tp layer and the bttm layer, subsurface drainage frm the bttm layer, and transpiratin by vegetatin frm the bttm layer. The water balance fr the tw sil layers (1.0 cm tp layer thickness) and the tp canpy interceptin strage is given by Figure 1 dc dt =P-Pn do Zl- - -= Pn - E - R - ql,2 (1) do2 z2- - -= qi,2- q2- T- Qb C(0 -< C _< S) is the amunt f intercepted water n the canpy, S being the canpy strage capacity; the units fr S and C are in millimeters; 0 (0r --< O --< Os) and 02(0r --< 02 --<--< Os) are the vlumetric sil mistures f layer 1 (with thickness z ) and layer 2 (with thickness z2), q,2 and q2 are the exchange fluxes frm layer 1 t layer 2 and drainage frm layer 2; rts are present in the bttm layer and extract misture fr transpiratin frm layer 2 nly, T is the transpiratin assumed t cme ut f layer 2 nly, R is the runff, and Pn is the net precipitatin reaching the sil surface, which is given by Pn = P - S,, ifp >_ S,, where P is the precipitatin, and P = 0 if P < S,, where S, is the available strage in the canpy given by S, = S - C. The initial cnditins fr the abve set f differential equatins is given by C(t = 0): C ø, O (t = 0) = 0, and 02(t = 0) = 0. The water table is assumed t lie belw the bttm layer, and the dynamics f the water table are nt mdeled. Als, the capillary rise frm the water table is nt cnsidered. Since the bject f this study is t simulate the tp 1 cm layer sil misture, it is assumed that the changes in the depth f the water table d nt affect the tp layer sil misture. Hwever, when the water table is clse t the surface (such as an area adjacent a stream channel), this assumptin will break dwn. The mdel is nt being used at a

4 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL 6913 fine spatial reslutin t simulate the sil mistures clse t 3. Canpy Radiative Transfer Mdel stream channels; it is used fr a catchment in an average sense. The radiative transfer mdel fr the vegetatin canpy is a The parameters in the fllwing paragraph deal with the part f the cupled sil-canpy-atmsphere mdel used in the sil-vegetatin system which is built n the physics t capture brightness temperature simulatins as well as in the sensitivity the diurnal cycle f the land surface respnse t atmspheric studies examining the rle f hetergeneities n brightness frcings. The sil hydraulic parameters deal with the mve- temperatures. The canpy radiative transfer mdel described ment f misture in the sil; the sil thermal prperties char- in this sectin fllws the descriptin f Chudhury et al. acterize the sil surface interactin with lng and shrtwave [1990]. The land surface hydrlgical mdel (described abve) radiatin. The vegetatin parameters determine the amunt f transpiratin and the base flw parameters help in estimating the runff at the catchment utlet. The incming radiatin (shrtwave and lngwave) is cmputed using Beer's law attencmputes the sil misture and surface temperature f a 1 cm layer, which serves as the bttm bundary cnditins fr the canpy radiative transfer mdel. The micrwave radiatin riginating frm the sil surface is mdulated by the verlying uatin t the bservatins the tp f the canpy [Chudhury vegetatin canpy, resulting in the canpy-tp brightness tembatlllbtlull bu fll l llt U, perature values. This canpy-tp brightness temperature (mi- [Eaglesn, 1982; Latchet, 1975]. The aerdynamic resistances crwave radiance) is attenuated by the atmspheric water vafr the transfer f misture and heat fllws that f Bmtsaert pr befre it reaches the satellite sensr. [1982]. The ptential evapratin frm bare sil, ptential evapratin frm the canpy strage, and the ptential transpiratin f the canpy is cmputed using ener balance [Famiglietti et al., 1994a; Lakshmi, 1995]. The actual evapra- The sil-canpy temperature T and the tp layer sil misture 0 are used in the canpy scattering mdel [Chudhury et al., 1990] t cmpute the canpy-tp hrizntally and vertically plarized brightness temperatures. tin frm bare sil (E) is cntrlled by the sil hydraulic The radiative transfer mdel treats the interactin f micrprperties, cnductivity and diffusivity [Mahrt and Pan, 1984]. wave radiatin frm the sil with the vegetatin branches, The sil cnductivity and diffusivi are cmputed using the stems, and leaves. The mdel is based n a high-frequency Brks-Crey relatins [Brks and Crey, 1964] and the paapprximatin: the extinctin crss sectin area f the scatrameters crrespnding t the apprpriate sil types [Rails et terer equals their gemetrical shadw area. The mdel als assumes that there is n transmissin f radiatin by the stems al., 1982]. The actual transpiratin (T) f the vegetatin is a and the branches. All radiatin n the stems and branches is functin f the ptential, sil misture f the lwer layer, absrbed. The mdel is analytic and prvides an expressin fr wilting pint, and a transitin pint which se es as a switch the canpy-tp brightness temperature using the tw-pint be een ptential and sil cntrlled [Neghassi, 1974]. The Gaussian quadrature, which results in a system f tw cupled actual evapratin frm the canpy strage depends n the rdinary differential equatins with the bttm bundary cnrati f the amunt f misture in the strage and its capacity ditin dictated by the sil misture and sil-canpy temperaand the ptential evapratin [Rutter et al., 1975]. The strage ture and the tp bundary cnditin dependent n the radiacapaci (S in millimeters) is related t leaf area index (L) tin frm the sky incident n the canpy. This micrwave [Dickinsn, 1984; Sellers et al., 1986] by S = 0.2L [Dickinsn, radiatin is then attenuated by the atmspheric water vapr 1984]. Leaves cvered with a film f water (having intercepted befre it reaches the satellite. water n them) are assumed nt t transpire [Rutter et al., The canpy-tp brightness temperature TB( /, p) is related 1975]. The exchange f sil misture between the tp layer and t the at-satellite brightness temperature TB(A, /, p) fr the lwer layer (q 1,2) is gverned by gravity (sil cnductivity) zenith angle / f the sensr, plarizatin p (hrizntal r and the sil misture gradient (sil diffusivity). The sil cnductivity and diffusivity is cmputed using the greater f 0 and 02, i.e., where the sil misture mvement riginates [Mahrt vertical), and A (the altitude f the radimeter) by T (A, 7, P)= q'a( A, 7)T (7, P)+ Tatm(A, T) (2) and Pan, 1984]. The drainage frm the lwer layer (q2) is the where %(A, 7) is the transmissivity f the atmsphere, and cnductivity f the lwer layer. The verland runff (R) is the Tatre(A, T) is the radiatin entering the radimeter frm the sum f the infiltratin excess and the saturatin excess. The atmsphere. infiltratin excess is decided by the infiltratin capacity depen- The canpy-tp brightness temperature Tu(7, p) will be dent n the sil hydraulic prperties: cnductivity and diffu- derived fllwed by the derivatin and discussin f r (A, 7) sivity times the sil misture gradient be een the surface and and T tm(a, T). The radiative transfer equatin is given by the tp layer [Mahrt and Pan, 1984]. The saturatin excess is [Chudhury et al., 1990; Stephens, 1988] as the difference be een the sil misture and the saturatin sil misture fr the case when the sil misture f the tp layer exceeds the saturatin value. There is n surface runff r infiltratin when the air temperature is less than 273 K, as the precipitatin is cnsidered t be in the frm f snw. The subsurface flw frm the lwer layer (Q) cnstitutes the base flw and is expressed using the ARNO nnlinear flw equatins [Francini and Pacciani, 1991; Liang et al., 1994]. ter the water balance is cmputed, the ener balance is reslved t yield the temperatures f the bare sil surface, vegetatin and the cmpsite f the sil, and vegetatin canpy. Fr cmplete details regarding the hydrlgical mdel, the reader is referred t Lakshmi [1995]. di(x, /x)= k(/x) -I(x Ix) +- tx dx ' 2 ß P(tx, tx')i(x, pc') dtx' + (1 - t(/x))t0 (3) -1 where I(x, tx) is the radiance at depthx within the canpy (the tp f the canpy is taken as x - 0, and the bttm f the canpy is taken t be x = 1) at an angle whse csine is /x (/x = cs (3/), /x > 0 fr radiatin directin tward sil and /x > 0 fr radiatin directin away frm sil), k(/x) is the

5 6914 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL extinctin cefficient, r(/ ) is the single-scattering albed, P(/, / ') is the phase functin (the prbability that the element will scatter incident radiatin at/ ' t directin/ ), and T is the sil-canpy temperature. The bundary cnditins are given by I(0, /x)= Tsky I(1, -/.t): R(/ )I(1, / ) + (1 -R(i ))T where Tsky is the intensity f atmspheric radiatin incident n the tp f the canpy and R (/ ) is the reflectivity f the sil. The first cnditin states that the dwnwelling radiatin at the tp f the canpy is the sky radiatin, and the secnd cnditin states that the upwelling radiatin at the bttm f the canpy equals the emissivity f the sil plus the incident radiatin reflected at the sil surface. The canpy-tp brightness temperature is given by (4) T, /, p) = I(0, -/ ) (5) The abve differential equatin is slved by using the twpint Gaussian quadrature [Chandrashekar, 1960] t yield the brightness temperatures. The brightness temperature is given by a linear cmbinatin f Tsky and T as TB( /, p) -- ATsky q- (1 - A)T 0 (6) where A is defined as the effective reflectivity f the silcanpy system [Chudhury et al., 1990]. The plarizatin difference index is defined as the difference between the hrizn- tally plarized and the vertically plarized reflectivity f the sil-canpy system [Lakshmi, 1995]. Fr details regarding the derivatin f the brightness temperatures, the reader is refrred t Chudhury el al. [1990] r Lakshmi [1995]. The p- larizatin difference index is a better estimate f sil misture than the plarizatin difference f brightness temperatures since it des nt depend n surface temperature, air temperature, r precipitable water used t cmpute satellite brightness temperature. The values f the plarizatin difference index are multiplied by 100 in all figures, tables, and discussins. This is dne fr the sake f cnvenience in the interpretatin f the numerical results and des nt change the results in any way. T (A, 3,, p)= r,(a, ),)T (),, p) + Tsk (7) The average (T ) and plarizatin difference (A T) brightness temperature are defined as - 1 T, = [T,(A, 3,, V)+ T,(A. 3,, H)] AT: T,(A, 3,, l/)- T,(A, 3,, H) 5. Hydrlgical Mdel Testing and Validatin This sectin describes the applicatin f the thin layer hydrlgical mdel n a catchment fr simulatin f water and energy fluxes. The descriptin f the catchment, the surces f the data, the calibratin f the parameters, and the validatin f the results using bserved data are described belw Site Descriptin The purpse f the mdeling effrt was t carry ut a 10 year simulatin ver the Kings Creek catchment alng with validatin. The First Internatinal Satellite Land Surface Climatl- gy Prject (ISLSCP) Experiment (FIFE) was a land-surfaceatmsphere experiment carried ut n a 15 x 15 km site near Manhattan, Kansas [Sellers et al., 1992]. This area is cvered by tallgrass prairie, and it cnsists f rlling hills. The gals f the experiment (as utlined by Sellers et al., [1992]) were t carry ut upscale integratin f mdels frm a plant scale t a scale amenable t the use f remtely sensed satellite data and t test applicatins f satellite data and validate hydrlgical mdels f land surface prcesses. The hydrlgical mdel described abve is applied t the 11.7 km 2 Kings Creek catch- ment lcated in the nrthwestern crner f the FIFE site. The field experiment was carried ut in fur distinct duratins (termed as IFCs (Intensive Field Campaigns)) during IFC-1 (June 1-6, 1987), IFC-2 (June 25 t July 11, 1987), IFC-3 (August 6-21, 1987), and IFC-4 (Octber 5-16, 1987). The simulated surface sil mistures and temperatures are cm- pared with the bservatins during these perids n an hurly basis Data and Parameters The hurly meterlgical data crrespnding t Tpeka, Kansas, are taken frm Earth lnf's NCDC (Natinal Climate Data Center) Surface Airways data prduct. The variables used here frm that database are air temperature, dew pint 4. Atmspheric Attenuatin Mdel temperature, air pressure, wind speed, clud height (defined as The canpy-tp brightness temperatures underg atmspheric attenuatin due t atmspheric xygen and water vathe height f the lwest sky cver layer mre than ne-half paque), ttal sky cver, and wind speed. The 10 year (1980- pr befre resulting in the at-satellite brightness temperatures 1989) hurly rainfall data were btained frm the Tuttle Creek (T (A, /, p)). The ptical thickness is cmputed n the basis rain gauge. The rainfall data fr the duratin f the FIFE IFCs f the ttal precipitable water vapr in the atmspheric clumn were btained frm the FIFE Infrmatin System (FIS) and I/ (in millimeters) [Chudhury, 1993] and is related t the used in the simulatins. The same was the case fr the meteatmspheric transmissivity (r,). The effective radiating tem- rlgical data fr the perids during the Intensive Field Camperature f the atmsphere is related t the air temperature T, paigns where bserved data were available fr Kings Creek; and the ttal precipitable water. The ttal precipitable water thse data were used in place f the Tpeka Surface Airways and the effective radiating temperature are used t cmpute data. The incming shrtwave radiatin is mdeled using the the sky temperature Tsky, which serv es as the upper bundary tw-stream apprach utlined by Dubayah el al. [1990] and cnditin n the canpy. The at-satellite brightness tempera- Dubayah [1992]. It is crrected fr clud cver effects using an ture (T (A, ', p)) is cmputed using the canpy-tp bright- empirical crrectin factr 1 - (1 - K)N, where K = ness temperatures (T ( /, p)) fr plarizatinp (hrizntal r z, z is the clud base altitude in kilmeters and vertical), altitude A, atmspheric attenuatin r,(a, 3,), and N is the fractin f sky cvered with cluds [Eaglesn, 1970]. atmspheric radiatin entering the radimeter Tatm( l, The incming lngwave radiatin is given by,-t, 4, where, (apprximated t be equal t Tsky ) [Slaby et al., 1981] as is the clear sky atmspheric emissivity dependent n atm- spheric water cntent [Ids, 1981] given by e, = e [Bras, 1990] and e is the vapr pressure in millibars, (s)

6 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL 6915 Table 1. Parameters fr the Thin Layer Hydrlgical vlumetric sil misture cntents are 0.12 and The min- Mdel imum stmatal resistance equal t 100 s/m. The base flw Parameter Value parameters fr the ARNO mdel Q ax, 0,, and Q* [Francini and Pacciani, 1991] and the expnent A in the transpiratin Albed (sil), as 0.15 relatinship [Neghassi, 1974] are calibrated using the bserved Albed (vegetatin), a v 0.20 daily flws at Kings Creek U.S. Gelgical Survey stream Emissivity (sil), es 1.00 Emissivity (vegetatin), e v 1.00 gauge data. Rughness length (sil), Z, s m 5.3. Results and Discussins Rughness length (vegetatin), Z,s 0.07 m Zer plane displacement (sil), ds 0.0 The hydrlgical mdel described in the previusectins is Zer plane displacement (vegetatin), dv 0.25 m Tp layer thickness, z 0.01 m used t simulate the water and energy fluxes fr the Kings Bttm layer thickness, z 0.99 m Creek catchment fr a perid f 10 years ( ) n a Leaf area index, L biweekly LAIs hurly time step. The parameters are calibrated using the first Minimum stmatal resistance,?mt 100 s/m 5 years ( ), and the validatin is dne fr the!985- Prsity, 0, streamflws. The calibrated parameter values are used t Residual Sil Misture, Or 0.02 Brks Crey Parameter, m 0.2 generate the streamflws, sil mistures, and surface temper- Air entry suctin head, c 0.2 m atures at an hurly time step fr the entire 10 year perid. The Saturated hydraulicnductivity, K, 1.89 x 10-6 ms - sil mistures fr the time perid crrespnding t the fur Transitin sil misture, 0* 0.12 Intensive Field Campaigns (IFCs) in FIFE 1987 are cmpared. Wilting sil misture (vlumetric), There has been n adjustment r initializatin f the sil Meterlgical data hurly data, Tpeka, Kansas Rainfall data mistures at the start f the IFCs. hurly data, Tuttle Creek Streamflw data daily, Kings Creek Calibratin and validatin f streamflws. The hurly streamflws are simulated ver the first 5 year perid ( ) and aggregated t daily values. The parameters are ptimized using the rt-mean-square f the difference between T, is the air temperature, and rr is the Stefan-Bltzmann cn- the bserved flws (btained using the daily discharge values at stant. The incming lngwave radiatin is crrected fr clud Kings Creek) and the simulated values ver the 5 year perid. effects using the fractin f clud cver N as N 2 This results in the values f t, Dmax, Or,, * Qt, * (ARNO mdel), [Tennessee Valley Authrity (TVA), 1972]. The data fr the air and A (transpiratin parameter) as 3.38 mm/h, 0.15, 0.06 temperature, vapr pressure, clud cver, and clud base alti- mm/d, and 0.50, respectively. The rt-mean-square f the tude are btained frm the Earth Inf Surface Airways data set errr f the streamflw ver the perid is 1.7 and fr Tpeka, Kansas. 1.6 mm ver the perid (see Figures 2 and 3). The The vegetatin data have been btained frm the University variatins f the daily averaged simulated vlumetric sil misf Maryland reprcessed NOAA glbal vegetatin index ture fr the tp and bttm layers are shwn in Figure 4. The (GVI) data prduct [Gward et al., 1994]. This NOAA GVI results f the streamflw calibratin and validatin are pltted has been put tgether frm measurements made by the ad- as time series fr and in Figures 2 and 3, vanced very high reslutin radimeter (AVHRR) nbard respectively. The agreements between the simulated (slid NOAA plar rbiting satellites. The data cmprises three lines) and the bserved streamflws (dtted lines) at the Kings years (1983, 1987, and 1989) f biweekly cmpsite bserva- Creek gauging statin are reasnable. The disagreements tins. The leaf area index is cmputed using the nrmalized culd be due t the use f rainfall data frm the Tuttle Creek difference vegetatin index (NDVI) using a Beer's law kind f rain gauge which is abut 20 km frm the FIFE site (caused by variatin [Baret and Guyt, 1991]. The values f the leaf area the nnavailability f the hurly rainfall data at the Manhattan index fr the years ther than 1983, 1987, and 1989 are taken rain gauge fr the perid under study). In general, the simuas the average f the values frm 1983, 1987, and 1989 data. lated streamflw verestimates the bserved streamflw, but it Missing perids embedded in the 1983, 1987, and 1989 data are shws gd qualitative agreement. Since the bjective f the estimated by simple interplatin. The reslutin f the NDVI study is nt t match the simulated and the bserved runffs GVI is abut 16 km at the equatr. The ther data used in this but t simulate a realistic variatin f the tp sil layer misstudy have been tabulated in Table 1. ture, the streamflw results are acceptable. In the case f the The sil type at Kings Creek is silt lam. The values f the calibratin time perid (Figure 2) it can be seen in the year residual and saturated vlumetric sil misture cntents are 1982 (June 24 and July 1, Julian days 175 and 182, respectively) taken t be 0.02 and 0.50, respectively; the Brks-Crey pa- that the bserved streamflws are 8.6 and 50.3 mm, respecrameter is equal t 0.2; the air-entry suctin head is 0.20 m; tively. On examinatin f the rainfall recrds f the Tuttle and the saturated hydraulic cnductivity fr silt lam is 6.8 Creek gauge there is n rain between June 16 (Julian day 167) mm/h [Rawls et al., 1982]. The albed f bare sil and vegeta- and June 25, 1982 (Julian day 175), and the ttal rain n July tin is taken as 0.15 and 0.20; the emissivity f bare sil and 1, 1982, is 7.8 mm (there is n rain n June 28, 29, and 30, vegetatin are taken t be unity [Famiglietti al., 1994a]. The Julian days ). This shws that the rain gauge at Tuttle zer plane displacement fr bare sil and vegetatin are zer Creek des nt recrd the strm that results in these large and 25 cm, respectively; the rughness lengths fr bare sil and streamflws. The ther plts in Figure 2 shw better agreevegetatin are 1 mm and 7 cm, respectively. The average daily ment. The same is the case in Figure 3. Figure 4 is the cunair temperature cmputed using the Earth Inf data set is terpart f Figure 3 giving sil mistures. The bserved streamtaken t be the sil temperature 5 cm depth (fr use in flw n May 17, 1986 (Julian day 137), is 18.1 mm, and there grund heat flux calculatin). The initial interceptin strage is is n rainfall bserved at the rain gauge between April 28 taken t be zer. The values f the transitin and wilting (Julian day 118) and May 31, 1986 (Julian day 151). On the

7 ._._ 6916 LAKSHMI ET Air.' SOIL-CANOPY-ATMOSPHERE MODEL _. ½- Day (1980) Day (1981 ) w- ½- Day (1982) 0 ' Day (1983) Day (1984) Figure 2. Observed (dtted line) and simulated (slid line) daily discharges (in millimeters) fr the cali- ther hand, between August 20 (Julian day 232) and Octber Sil misture cmparisns. The average f the sil mis- 1, 1989 (Julian day 274), there is a huge verestimatin f the tures bserved ver the Kings Creek catchment (btained usstreamflw. This is because f a very large base flw estimatin ing the FIFE Infrmatin System, FIS database) is pltted that results frm the increase in sil misture f the bttm againsthe simulated sil misture fr IFC-I thrugh 4 fr layer due t 297 mm f rain that is recrded by the rain gauge bth the tp (Figure 6) and the bttm (Figure 7) layers. The (see Figure 4). The increase the bttm layer sil misture bservatins are made at a depth f 25 and 75 mm at the causes increased simulated streamflw (cmpared t bserved Bwen rati statins (2, 8, 10, 12, and 14 crrespnding t grid streamflw) in the secnd half f 1986 (frm Julian day 200 t numbers 1916, 3129, 3414, 2915, and 2516). At each statin, 300). The tp layer sil misture shws much greater daily there are five measurements f sil misture crrespnding t variability than the bttm layer sil misture. This is cnsis- the center, nrth, suth, east, and west (distance apprximately tent with the expecte dynamics. In additin there is a greater 30 m in each case). These are averaged t btain the catchment number f increases in the tp layer sil misture in 1987 averaged sil misture fr cmparisn with the simulated sil where the simulated streamflw is greater than zer fr mst misture. The simulated sil mistures crrespnd t a tp f the year. This is als the case fr the increased number f layer f 10 mm (1 cm) thickness and a bttm layer f 990 mm streamflw events frm Julian day 200 t 300 in In (99 cm) thickness. The 25 mm bserved sil mistures are summary, the hydrlgical mdel estimates the streamflw pltted against the tp layer simulated sil misture, and the with reasnable accuracy as cnsistent with the rainfall data. 75 mm { bscrved sil m )isture,, arc pltted againsthc bttm

8 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL 6917 Day (1985) O 100 nn,-,,, Day (1986) Day (1987) Day (1988) Day (1989) Figure 3. Observed (dtted line) and simulated (slid line) daily discharges (in millimeters) fr the validatin perid ( ). layer simulated sil misture. The bserved sil mistures are pltted individually at the beginning f the day (i.e., if the bservatins are n Julian day 152, June 1, 1987, they are pltted crrespnding t hur 1, day 152) since the time f day at which the bservatins are made is nt available. Als, the bservatins are made nce daily and d nt capture the tempral dynamics f the sil misture variatin. The tempral variatin is reflected in the simulated sil misture. Hwever, nte that the abscissa (in days) is ver a range f 6 days fr IFC-1, 17 days fr IFC-2, 16 days fr IFC-3 and 12 days fr IFC-4. Therefre tempral variatins will appear emphasized in IFC-2 and 3 and appear much mre gradual in IFC-4 and IFC-1. The fur panels in Figure 6 are fr IFC-1 thrugh IFC-4. It is interesting t nte that each f them displays different nuances assciated with sil misture dynamics. In the case f IFC-1, there is virtually n rainfall as seen frm the rainfall data fr the IFCs in Figure 5 (nly ne hur with 0.2 mm f rain n June 2 at 0900). The bserved sil misture exhibits a very slw decrease (drydwn) ver time. The simulated sil misture remains relatively cnstant fr the duratin f the IFC. This is because there is replenishment f the lss in sil misture (due t bare sil evapratin) f the upper layer by diffusive flux frm the bttm layer. The bttm layer hlds a larger amunt f misture cmpared t the tp layer (capacity f the tp layer is equal t 10 mm x 0.50 (prsity) = 5 mm; capacity f the bttm layer is equal t 990 mm x 0.50 (prsity) = 495 mm), and therefre the bttm layer sil mis- ture shws little decrease when it supplies the tp layer with misture (even if there is a 5 mm flux f misture frm the bttm layer t the tp layer, the bttm layer sil misture cntent decreases by nly 0.005). In the case f IFC-2, there is n rainfall n June 25 and 26 (Julian days 176 and 177), and the tp layer sil misture shws a decrease. There is rainfall n June 27, and the sil mistures increase. On June 27 at 2200 there is a 9.65 mm rainfall, and the tp layer sil misture

9 6918 LAKSHMI ET AL.' SOIL-CANOPY-ATMOSPHERE MODEL Day (1985) Day (1986) Day (1987) Day (1 988) Day (1989) Figure 4. Simulated tp layer (slid line) and bttm layer (dtted line) daily averaged sil misture fr increases frm t This is expected since the rainfall panel f Figure 7. There is rainfall between June 27 (I78) t wets the tp layer f the sil almst instantly. The ttal rainfall June 30 (181), after which there is n rain fr a perid f 3 days n June 27 is mm. This rainfall is reflected in the b- frm July I t 3 ( ). We can bserve the drydwn in the served 25 mm sil misture the next day (June 28), which tp layer sil misture frm n July 1 t n July 4. increases frm 0.22 n June 27 t 0.31 n June 28. There are There are again perids f rain n July 4 (abut 0.2 mm) and tw imprtant bservatins here. The immediate increase July 5 (8.9 mm), after which there is n rain till the end f the the 25 mm sil misture is less than the increase seen in the 1 IFC, and the sil misture f the tp layer exhibits a drydwn cm sil misture. The 25 mm sil misture shws a mre (there is an hur f 1.5 mm f rain n July 7, hence the spike gradual change as ppsed t the 1 cm sil misture. On the arund day 187). The 25 mm bserved sil misturexhibits ther hand, the 1 cm sil misture respnds n much shrter behavir that agrees with the rainfall input and the simulated 1 cm sil misture. (quicker) timescales. This cannt be cmpletely verified since nly daily bservatins f the 25 mm sil misture are avail- IFC-3 behaves similar t IFC-2 in that there are perids f able. After the tp layer gets wet in respnse t the rainfall rain and perids f drydwn when the sil misture decreases. input, gravity and the sil misture gradient between the tp The case fr IFC-4 is similar t the nes discussed abve. layer and the bttm laye results in mvement f the misture There is n rain between Octber 5 and 13, but there is rainfall t the bttm layer. Hwever, since the bttm layer is 99 cm in 2 hurs f Octber 13 and 14. Thugh this rainfall is very thick, the increase in sil misture f the bttm layer due t slight (1.27 and 0.51 mm), it des increase the sil misture f this incming sil misture is very slight, as seen in the secnd the tp layer. The rainfall n Octber 15 increases the upper

10 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL 6919 IFC-1,,,, 1 s Julian Day June 1-6, IFC-2..., Julian Day June 25-July 11, 1987 IFC Julian Day August 6-21, 1987 IFC-4,,,, Julian Day Octber 5-16, 1987 Figure 5. Precipitatin (in millimeters) fr the Kings Creek gauge fr IFC-1 thrugh IFC-4. layer sil misture, after which it drps ff due t drainage int the bttm layer. The simulated sil misture shws cnsistent agreement with the bserved rainfall and the bserved 25 and 75 mm sil mistures. Since the tp 1 cm layer sil misture is affected by rainfall almst instantaneusly, it is imprtant that the sil misture accunting scheme perates n hurly time steps (as in the case here) s that the tempral dynamics is captured. Surface temperature cmparisns. The bserved hurly surface temperatures frm the statins (three super autmated mesnet Statins (AMS) 31, 3, and 7) in grid numbers 2123, 2139, 2428, and 3221, respectively, are half hurly bservatins averaged t hurly values. The bserved (dts) and simulated (slid line) are pltted in Figure 8. Hwever, during the midday hurs the simulated values f surface temperatures are larger than the bserved values. The IFC-1 cmparisn shw

11 6920 LAKSHMI ET AL.' SOIL-CANOPY-ATMOSPHERE MODEL IFC Julian Day June 1-6, 1987 IFC Julian Day June 25-July 11, 1987 IFC Julian Day August 6-21, 1987 IFC O Julian Day Octber 5-16, 1987 Figure 6. Mean (nce a day) bserved (25 mm depth, dts) and (hurly) simulated (tp layer, lines) vlumetric sil misture fr IFC-i thrugh IFC-4. reasnable agreements fr mst f the hurs except fr a few hurs f June 4 (Figure 8) (the furth peak in panel i f Figure 8). There are disagreements between the bserved and the simulated surface temperatures f 5 K r mre between 1000 and 0600 n that day. The wind speed cntrls the resistance f the bare sil evapratin and the vegetatin transpiratin. The aerdynamic resistances are inversely prprtinal t wind speed. The lwer the wind speed, the higher the value f the aerdynamic resistance fr bare sil and vegetatin. During the day f June 4, 1987, the wind speeds between 1000 and 0600 ranged frm 1.4 t 2.1 m/s. This range, when cmpared t the wind speed variatin during the same time n June 5 (4.7 t 11.2 m/s) results in a much larger aerdynamic resistance, thereby reducing the evaptranspiratin. It is because f this that the simulated surface temperatures n June 4 shw a marke disagreement with the bservatins. The same expla-

12 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL 6921 IFC Julian Day June 1-6, 1987 IFC Julian Day June 25-July 11, 1987 IFC-3. m E c Julian Day August 6-21, 1987 IFC-4 E Julian Day Octber 5-16, 1987 Figure 7. Mean (nce a day) bserved (average f 25 and 75 mm depth, symbls) and (hurly) sin (bttm layer, lines) vlumetric sil misture fr IFC-1 thrugh IFC-4. natin hlds fr July 1, 2, and 3 (panel 2, Figure 8); August 10, 11, and 13 (panel 3, Figure 8); and Octber 11 and 12, 1987 (panel 4, Figure 8). The hurs f verestimatin cincide with lw values f wind speed (which reduce the simulated evaptranspiratin) cupled with high values f incming slar radiatin (at midday hurs). The high slar radiatin (and hence net radiatin) results in large values f sensible and grund heat fluxes (since the latent heat is small), and this increases the surface temperature t preserve the energy budget. Hwever, fr mst part, the simulated surface temperatures d agree well with the bserved values. Figure 9 shws the scatter f the simulated surface temperatures with the bserved val- ues. The rt mean square errr ver IFC-1 is 5.7 K; IFC-2 is 5.5 K; IFC-3 is 5.9 K; and IFC-4 is 3.6 K. On examining Figure 9, we can bserve that there is mre verestimatin f the bserved temperatures than underestimatin. The thin tp layer culd be a factr in the reduced simulated evaptranspiratin which results in high midday temperatures. Since we are especially interested in the 0600 bservatins, which cincides with the SSM/I ascending rbit verpass, cmparisns between the bserved and the simulated values at 0600 are meaningful. The rt-mean-squar errr fr the 0600 surface temperatures are 1.8, 0.8, 1.5, and 1.2 K fr IFC-1, IFC-2, IFC-3, and IFC-4, respectively. The rt-mean-square

13 6922 LAKSHMI ET AL.' SOIL-CANOPY-ATMOSPHERE MODEL IFC Julian DayJune 1-6,1987 IFC Julian Day June 25-July 11,1987 IFC-3 c Julian Day August 6-21, 1987 IFC hur Octber 5-16, 1987 Figure 8. Time series plt f bserved hurly surface temperatures (dts) and simulated hurly tp layer temperatures (lines) fr IFC-1 thrugh IFC-4. errr fr all the 0600 surface temperatures lumped tgether frm all the IFCs is 1.3 K. Figure 10 shws the scatterplt between the bserved and the simulated 0600 surface temperatures. The agreement is very gd. sensitivity study as they have the mst significant effect n the plarizatin difference index. These sensitivities will help us in understanding the factrs that cntribute t the plarizatin difference index bserved by the SSM/[. 6. Sensitivity f Radiative Transtar t Vegetatin and Sil Misture The cupled land surface hydrlgy-canpy radiative transfer mdel is used t study the sensitivity f the plarizatin difference index t changes in leaf area index, sil misture, and vegetatin parameters. The vegetatin parameters-stem area index and canpy misture cntent are chsen fr the 6.1. Effect f ¾egetatin Parameters The effect f the stem area index (7) and the canpy misture cntent (rnc) n the range f the plarizatin difference index zxy (X100 as explained earlier; dented by DY in all figures), the leaf area index (LAI), and the sil misture range (between residual and saturatin) is shwn in Figure 11 fr the 19 GHz case (a similar variatin is seen fr the 37 GHz case and is nt shwn herc) The stem area index varies acrss the

14 LAKSHMI ET AL.' SOIL-CANOPY-ATMOSPHERE MODEL 6923 figure panels frm 0 t 0.6 frm left t right, and the canpy misture cntent varies frm 0.65 at the tp (crrespnding t a turgid leaf) t 0.05 at the bttm (crrespnding t a dry leaf). The branch t stem area rati (X) is set t 2.7, crre- spnding t the vegetatin type shrubs [Whittaker and Wd- well, 1967; Whittaker et al., 1974]. The lines drawn in the figures (btained by simulatin) crrespnd t sil misture at saturatin (the line t the right, i.e., greatest value f DY fr a given LAI) and at residual (least value f DY fr a given LAI). These lines frm the bunding curves in between which values (f A Y) fr ther sil mistures lie. The stem area increases and the range between the simulated saturatin sil misture cntent A Y and the residual sil misture cntent A Y curves decreases fr a given leaf area index. There is a greater decrease in the plarizatin difference index fr the saturatin sil misture than fr the residual sil misture. The plarizatin difference signal riginates the sil surface and prpagates thrugh the vegetatin. This plarizatin difference signal caused by the biplar nature f the water mlecule is greatest fr a saturated sil. The increase leaf area index and/r stem area index attenuates this signal. This attenuatin f the plarizatin difference index is greater fr the saturatin sil misture A Y curve as ppsed t the residual sil misture A Y curve fr bth the increase in leaf area index and the stem area index. At sufficiently high leaf area index (in this case 7.0) the plarizatin difference index f the residual and the saturatin sil misture curves (fr all stem area indices and leaf misture cntents f 0.65 and 0.35) cincide. Increasing canpy misture cntent als decreases the plarizatin difference signal, as seen frm the figures mving IFC Observed Surface Temperature (K) IFC Observed Surface Temperature (K) v u) IFC Observed Surface Temperature (K) IFC-4 E _ Observed Surface Temperature (K) Figure 10. Scatterplt f 0600 bserved surface temperatures and simulated tp layer temperatures fr IFC-1 thrugh IFC-4. v IFC-1.-- E Observed Surface Temperature (K) IFC Observed Surface Temerature (KI v E m IFC-2.. '..! ß,.: E v Observed Surface Temperature (K) IFC-4 E._ Observed Surface Temerature Figure 9. Scatterplt f bserved hurly surface temperatures and simulated hurly tp layer temperatures fr IFC-1 thrugh IFC-4. bttm t tp. When the leaf is dry (m c = 0.05), the plarizatin difference index stays high, even at high leaf area indices. Turgid leaves absrb the micrwave radiatin and plarizatin difference emitted frm the surface f the sil [Chudhury et al., 1990]. Therefre an increase in the canpy misture cntent attenuates the plarizatin difference index. The abve bservatins are cmmn t bth 19 and 37 GHz plarizatin difference indices. Table 2 shws the variatin f the simulated maximum range f the plarizatin index (A Y), i.e., the difference between the plarizatin index fr the sil saturated case and the sil dry case fr varying the stem area index and the canpy misture cntent. As the bservatins frm Figure 11 shwed, the range decreases with increasing stem area index and increasing canpy misture cntent. Furthermre, the range is greater fr the 19 GHz case as ppsed t the 37 GHz case, but fr a dry leaf (m c = 0.05), they are almst identical. This shws that the vegetatin exerts greater influence in mdulating the plarizatin difference signal fr 37 GHz as ppsed t 19 GHz. In additin, fr the case f a dry leaf and stem area index equal t zer, the plarizatin difference index is identical (13.0) fr bth the 19 and the 37 GHz frequency. In this case, there is almst n influence exerted by the vegetatin n the plarizatin difference signal riginating frm the sil. Hence fr the case f zer stem area index and zer canpy misture cntent, the plarizatin difference index fr the 19 and 37 GHz differs nly due t the difference in the dielectric prperties f water, which are a functin f frequency (the Fresnel reflectivity is fr 19 GHz and fr 37 GHz fr vlumetric sil misture f 0.50) and is small.

15 6924 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL i/ SAI--O mc-. 5 SAI rnc=0.65 SAI=0.6 mc= SAI=O 0 mc- 5 SAI=0.3 mc= 35 SAi=O 6 inc= SAI--O 0 SAI=0.3 SAI=O 6 : rnc- 5 rnc=o.05 : mc=o DY (19 GHz) DY (19 GHz) DY (19 GHz) Figure 11. Sensitivity f 19 GHz plarizatin difference index (DY) t the leaf area index (LAI) fr different values f stem area index (SAI) and canpy misture cntent (me) Sensitivity t Leaf Area Index and Sil Misture a range f leaf area indices and (2) sensitivity with respect Our aim in this sectin is t shw the affect f varius leaf area index fr a range f sil misture values. The sensibiphysical variables n simulated plarizatin difference intivities f the plarizatin difference-index A Y t leaf area index L and sil misture 0 can be expressed as a A Y/OL, and dices. The plarizatin difference indices derived frm the SSM/I will be used t ascertain sil misture values [Lakshmi 0 A Y/O 0 will bth be a functin f the vegetatin parameters, et a/ 1996bl. We are trvin t utline the fact that there are the stem area index and the canpy misture cntent that were sensitivity issues invlved in the sensitivity f the plarizatin examined in the previus sectin. This sectin wiii examine these sensitivities fr a stem area index f 0.3 and a canpy difference index: (1) sensitivity with respect sil misture fr misture cntent f The sensitivity f the plarizatin difference index t the leaf Table 2. Maximum Simulated Range Residual t Saturated Sil Misture Cntent (0, - 0r) fr Plarizatin Difference Index (AY X 100) Leaf Area Index (L) = 0.75 Stem Area Canpy Index Misture AY (x 100) AY (x100) (X) (m ) (19 GHz) (37 GHz) Branch t stcm area rati (,/) fr 19 and 37 GHz. area index a A Y/OL has t be evaluated at a fixed value f sil misture. It will be a functin f the sil misture and the leaf area index (in the neighbrhd f which it is being evaluated). This can be seen in the tp half f Figure 12, in which the variatin f plarizatin difference index fr 19 GHz has been pltted againsthe leaf area index fr different values f the sil misture cntent ranging frm 0.02 (residual) t 0.50 (saturatin) fr 10 increments, and it can be seen frm the figure that as the sil misture decreases, O A Y/O L decreases; that is, O1 < 02 '----> TJ L,O=01 < T/ L,O=02 (9) In the case f 19 GHz (Figure 12), fr L = 1.0, OAY/OL is 1.1 fr 0 = 0.02 (residual sil misture) and 3.5 fr 0 = 0.5 (fr saturatin sil misture). The value f O AY/OL is cmputed

16 LAKSHMI ET AL.: SOIL-CANOPY-ATMOSPHERE MODEL 6925 _ c ! S^1=0.3 mc SAI=0.3 mc=0.35!!!! i!! DY (19 GHz) Figure 12. Sensitivity f 19 GHz plarizatin difference index (DY) t leaf area index fr different sil misture cntents between 0.02 (residual) and 0.50 (saturatin) and fr vlumetric sil misture cntent fr different leaf area indices between 0.0 and 7.0 fr stem area index and canpy misture cntent at 0.3 and 0.35, respectively. using the A y values fr the leaf area index L f 0.5 and 1.5. In the case f 0 taking up intermediate values between 0.02 and 0.5, the value f OAY/OL lies between 1.1 and 3.5; 1.8 fr 0 = 0.12; 2.7 fr 0 = 0.21; and 3.3 fr 0 = In the case f 37 GHz the values are similar but slightly higher fr 0 = 0.5, OAY/OL = 3.6. This shws that the 37 GHz frequency has an increased sensitivity t leaf area index. The sensitivity t leaf area index f the plarizatin difference is maximum fr the case f saturatin sil misture and minimum fr the case f residual sil misture. Hence we can write < < O O Os In additin, as the leaf area index increases, the sensitivi W t leaf area index decreases (at a ed value f sil misture) that is, L < L 2 / l,0 > /,0 Using the abve values, fr a value f 0 = 0.21, the value f O Y/OL is 2.7 frl = 1.0; 1.5 frl = 2.0; 0.9 frl = 3.0; and 0.4 fr L = 5.0. Hwever, fr higher sil misture cntents, the sensitivi f the plarizatin difference index t leaf area index still remains significant even when the leaf area index increases. This can be seen in Figure 12, fr example, fr , fr L = 1.0, OAY/OL = 1.8; L = 2.0, OAY/OL = 1.0; and frl = 3.0, OAY/OL = 0.6 fr the 19 GHz case. In the case fr the crrespnding values are 3.3, 1.9, and 1.1 fr L -- 1, 2, and 3, respectively. S, fr the leaf area index f 3, 0 A Y/OL is 1.0 fr but nly 0.6 fr 0 = The crrespnding values fr the 37 GHz case are 1.7, 0.8, and 0.5 fr L = 1, 2, and 3, respectively (fr 0 = 0.12) and 3.3, 1.7, and 1.0 fr L - 1, 2 and 3, respectively (fr 0 = 0.31). At higher values f leaf area index, the plarizatin difference signal gets attenuated by the vegetatin canpy. The strength f the plarizatin difference qign l it fimcticm c f the sil misture cntent. At higher sil misture cntents the plarizatin difference signal des nt get cmpletely attenuated at larger values f leaf area index, and therefre there is still sensitivity t the leaf area index. On the ther hand, fr lw sil misture cntents the plarizatin difference signal is lw t begin with and gets attenuated with increasing leaf area index t a degree that further changes in leaf area index d nt affect the signal, hence reducing the sensitivity. The sensitivity f the plarizatin difference index t the sil misture O AY/00 is evaluated at a fixed value f leaf area index. The sensitivity f 0 A Y/O 0 is a functin f the leaf area index and the sil misture cntent in the neighbrhd in which it is evaluated. The variatin f the plarizatin difference index with sil misture fr different leaf area index values ranging between 0 and 7 at increments f 0.5 is presented in bttm half f Figure 12. It can be seen that as the leaf area index increases, the sensitivity f the plarizatin difference t the sil misture 0 A Y/O 0 decreases (fr a given 0); that is, ( OAY' ( OAY' Li < L 2 ---> O0,] œ:œ, The value f the 19 GHz OAY/00 fr 0 = 0.12 is fr L = 0; 17.7 frl = 1.0; 11.5 frl = 2.0; and 4.2 frl = 4.0. It can be seen that as the leaf area index increases, the sensitivity rapidly decreases. This is expected since increasing leaf area index masks the plarizatin difference signal f the sil misture. The 37 GHz O AY/00 als behaves similarly. The values f OAY/00 are 22.9, 10.4, 6.25, and 1.0 crrespnding t L f 0, 1.0, 2.0, and 4.0, respectively, fr 0 = It can be seen that the sensitivity values f 0 A y/ 0 are lwer fr the 37 GHz than fr the 19 GHz. This is expected, as the 19 GHz has greater sensitivity t the sil misture due t its lnger wavelength. The decrease f 0 A y/ 0 with leaf area index results in almst zer sensitivity when high values f leaf area index (like L : 7.0) are apprached. This can be seen in Figure 12; the A Y versus 0 curve fr L is almst a straight vertical line. As the sil misture increases frm residual sil misture cn- tent, O AY/00 first increases and then decreases. This can be seen by bserving the slpe 0 A y/ 0 f the A Y versus 0 curve (fr a fixed L). As the sil misture increases (fr a given leaf area index), the sensitivity f the plarizatin difference t the sil misture increases. After a certain sil misture cntent, the plarizatin difference signal gets saturated, and further increases in sil misture d nt result in crrespnding increases in plarizatin difference index. The results fr the 37 GHz case shw similar variatins. The figure fr the 37 GHz

17 6926 LAKSHMI ET AL.: SOI1.-CANOPY-ATMOSPHERE MODEL case is nt shwn here because the results fr 37 GHz qualitatively resemble that fr 19 GHz and has n new features. 7. Cnclusins A thin layer mdel f hydrlgy with cmplete water and energy budgets has been presented here. The mdel is built n the framewrk f Mahrt el al. [1984] fr inclusin f a thin upper layer. The parameterizatins include the misture gradient driven flux fr diffusin f water frm the lwer layer t the upper layer t replenish the misture lst during evapratin. It shuld be pinted ut that in applicatin f the Mahn and Pan [1984] scheme, the cnclusins f Mahrt and Pan [1984] hld gd fr clay type f sils and will nt wrk fr sand. The sil pe in ur applicatin is silt lam whse prperties are clser t clay than t sand, hence the apprpriateness f this apprach. The hydrlgical mdel is applied ver a 10 year perid. The bserved daily streamflws frm 1980 t 1084 are used t calibrate the mdel parameters. The simulated streamfiw, are validated ver the perid. The cmparisns between bserved and simulated streamflws have been gd given the fact that the rain gage used fr rainfall input was abut 16 km away frm the catchment. The aim f this paper was nt t develp a mdel fr accurate streamflw predictin. The aim f this paper is t develp a scheme t predict the surface temperature and sil misture with a sufficient degree f accuracy fr the 1 cm surface layer. The streamfiv, is cmpared t determine that the hydrlgical mdel behaves prperly with the rainfall prcess. The mdel-simulated sil mistures and surface temperatures arc cmpared fr the time perids during the fur IFCs in FIFE. This mdel prduces values f the 1 cm layer sil mistures which agree with ur understanding f hydrlgs'. This hydrlgical mdel shws a definite prmise in estimating sil misture and can be used alng with micrwave satellite data. The 19 and 37 GHz plarizatin difference indices have a greater range between the residual and the saturated sil misture levels, shwing greater sensitivity t sil misture variatins. The sensitivi ' f the plarizatin difference index t the sil misture is affected by the leaf area index; an increase in leaf area index decreases this sensitivity'; an increase in sil misture results in increased sensitivity fllwed by a decrease in sensitivity at high sil mistures. Amng the vegetatin parameters, the stem area index and the canpy misture cntent affect the plarizatin difference the greatest. An increase in the stem area index and/r the canpy misture cntent results in a masking f the plarizatin difference signal riginating at the sil surface. We wish t cautin the reader(s) that a straight frward regressin type f analysis (between sil misture and plarizatin difference index) may nt wrk withut due attentin t the sensitivities and uncertainties that are studied in this paper. Ignring these may result in incrrect results. It is prpsed that this mdel can be used in cnjunctin with passive micrwave satelille data fr sil miqturc estima tin. The 19 and 37 GHz Special Sensr Micrwave hnager (SSM/I) brightness temperature data are prpsed t be used fr the study. This des nt imply that the micrwave brightness temperature data are the nly way t estimate sil mistures. We wuld emphasize at this pint that the use f satellite data is useful given its spatial and tempral cverage, and it can be used in cnjunctin with hydrlgical mdeling t achieve better estimates f sil misture. MicrwaYe satellite data at lwer frequency (6.6 GHz) has been used in the past t infer sil misture [Owe et al., 1992] and sil wetness [Chudhu' and Mntelib, 1988]. These, hwever, use simple regressin-based relatins between sil misture (r antecedent precipitatin index API) and brightness temperature. In the case f higher frequencies (such as the 19 and 37 GHz frequencies that we prpse t use). a simple inversin may nt be very effective. It is desired that a cmplete mdel f sil hydrlgy prviding the surface temperature and sil misture alng with a radiative transfer mdel fr the plant canpy and an attenuatin mdel fr the atmsphere wuld be used t simulate the SSM/I brightness temperature and subsequently help in retrieving sil mistures frm bserved brightness temperatures. The mdel f thin layer sil hydrlgy will help in this regard. Acknwledgments. The page charges were paid by the Sunder Research Team (Jel Sus- kind). This is gratefull 3 ackm wledgcd. Retirerices Baret, F., and G. Guyt, Ptentials and limits f vegetatin indices fr LAI and APAR assessment, Remte Sens. Envirn., 35, , Bras, R. L., Hydrlgv: An Intrductin t Hydrlgic Science, Addisn- Wesley, Reading, Mass., B ks, R. 1t.. and A -I. (Mrcy. t tydraulic p pcrties [ prus media, Hvdri. Pap. 3, Cl. State [ niv., Frt Cllins, Cl., i964. 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