Estimating actual evapotranspiration using water budget and soil water reduction methods

Transcription

1 Estimting ctul evpotrnspirtion using wter budget nd soil wter reduction methods Dvid S. Chnsyk 1, Emmnuel Mpfumo 2, nd Crystl L. A. Chikowsky 1,3 1 Deprtment of Renewble Resources, University of Albert, Edmonton, Albert, Cnd T5K 1V5; nd 2 Soil Science nd Plnt Nutrition, School of Erth nd Geogrphicl Sciences, University of Western Austrli, Crwley 6009, Austrli. Received 16 September 2005, ccepted 3 April Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. Chnsyk, D. S., Mpfumo, E. nd Chikowsky, C. L. A Estimting ctul evpotrnspirtion using wter budget nd soil wter reduction methods. Cn. J. Soil. Sci. 86: Studies on estimtion of ctul evpotrnspirtion on disturbed lnds re scrce nd yet such dt re essentil in hydrologic modeling. Our study compred the vribility of estimtes of ctul evpotrnspirtion (AET) from reclimed site in northern Albert using the simplified wter budget (WB) nd soil wter reduction (SWR) methods. The AET estimtes from the simplified wter budget eqution (AET 1 ) required field soil wter content, precipittion nd runoff. The AET estimtes from the soil wter reduction method (AET 2 ) required dily potentil evpotrnspirtion (PET), field cpcity (FC) wter content, minimum field-mesured wter content, nd field wter content. Soil wter ws mesured using neutron moisture guge every 2 wk during the growing seson for 2-yr period. The verge AET 1 nd AET 2 estimtes for 2001 were 1.9 nd 1.4 mm per dy, respectively, wheres those for 2002 were 2.1 nd 1.2 mm per dy, respectively. The pired t-tests to compre AET 1 ginst AET 2 indicted significnce differences (P 0.05) in 5 out of 11 mesurement dtes, especilly during high rinfll periods. Overll AET 2 estimtes were more vrible thn AET 1 estimtes. Therefore, the lrger vribility of AET 2 estimtes imply less relibility of sptilly verged AET 2 estimtes for use in regionl nd globl circultion models. Key words: Extrctble wter, reclimed lnd, wter budget, soil wter reduction, potentil evpotrnspirtion Chnsyk, D. S., Mpfumo, E. et Chikowsky, L. A Estimtion de l évpotrnspirtion réelle d près le biln hydrique et les méthodes de réduction de l eu du sol. Cn. J. Soil Sci. 86: Peu d études se sont ttrdées à estimer l évpotrnspirtion réelle des sols perturbés. Il s git pourtnt d une donnée essentielle pour l modélistion de l hydrologie. Les uteurs ont compré l vribilité des estimtions de l évpotrnspirtion réelle sur un site resturé du nord de l Albert en recournt à l méthode simplifiée du biln hydrique et ux méthodes de réduction de l eu du sol. Pour estimer l évpotrnspirtion réelle vec l éqution simplifiée du biln hydrique (ETR 1 ), il fut connître l teneur en eu du sol sur le terrin, l importnce des précipittions et le tux de ruissellement. Pour estimer l évpotrnspirtion réelle vec l méthode de réduction de l eu du sol (ETR 2 ), on doit svoir l cpcité de rétention d eu, l teneur en eu minimle et l teneur en eu du terrin. Les uteurs ont mesuré l teneur en eu du sol vec un humidimètre à neutrons une semine sur deux pendnt l période végéttive pour une période de deux ns. En 2001, l vleur estimtive moyenne de ETR 1 et de ETR 2 s étblissit respectivement à 1,9 et à 1,4 mm pr jour, contre 2,1 et 1,2 mm pr jour en Les pires de test t servnt à comprer les deux prmètres révèlent un écrt significtif (P 0,05) à 5 dtes sur onze, principlement lors des pluies bondntes. Dns l ensemble les estimtions de ETR 2 vrient plus que celles de ETR 1. L plus grnde vribilité de ETR 2 signifie que les vleurs estimtives de ce prmètre donnent une moyenne sptile moins fible pour les modèles d hydrologie régionux et globux. Mots clés: Eu extrctible, terres resturées, biln hydrique, réduction de l eu du sol, évpotrnspirtion potentielle Globl rinfll verges 750 mm ech yer, of which t lest 60% is returned to the tmosphere s evpotrnspirtion (ET), nd thus mking ET the lrgest component of the terrestril hydrologicl cycle (Bumgrtner nd Reichel 1975; Droogers 2000). Evpotrnspirtion is the only term tht ppers in both the surfce energy blnce eqution nd the wter blnce eqution. Relible ET estimtes re often required for solution to problems in hydrology, gronomy, forestry nd lnd mngement nd for the improvement of globl circultion models (Ytes 1997). The mesurement of ET is lwys difficult (Oliver 1985) nd techniques include wter blnce methods using weighing lysimeters, nd meteorologicl methods such s the eddy correltion 3 Current ddress: Albert Environment, 10th Floor Oxbridge Plce, Street, Edmonton, Albert, Cnd T5K 2J nd Bowen-rtio methods, which re very time consuming, lborious nd/or costly (Burnsh 1995; Droogers 2000). As result, vrious methods of estimting ET hve been developed, nd re generlly clssified into direct nd indirect methods (Shrm 1985; Htfield 1990). Direct ET estimtion methods include those tht utilize weighing lysimeters, wheres indirect methods include those bsed on the concept of ctul evpotrnspirtion (AET) versus potentil Abbrevitions: AET, ctul evpotrnspirtion; AET 1, ctul evpotrnspirtion from wter blnce method; AET 2, ctul evpotrnspirtion from soil wter reduction method; ASW, vilble soil wter; AWHC, vilble wter holding cpcity; ESW, extrctble soil wter; FC, field cpcity; PET, potentil evpotrnspirtion; SWR, soil wter reduction method; TDR, time domin reflectometer; TSW, totl soil wter; WB, wter blnce method; WP, wilting point

2 Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. 758 CANADIAN JOURNAL OF SOIL SCIENCE evpotrnspirtion (PET) nd utilize meteorologicl dt (Shrm 1985; Htfield 1990). A common method of AET estimtion is tht of solving terms in the wter blnce eqution below: n SWi = SWo + Pi Qi ETi wi i= 1 ( ) where SW i = finl soil wter on dy i; SW o = initil soil wter on dy i 1; P i = precipittion; Q i = surfce runoff; ET i = ctul evpotrnspirtion; w i = deep percoltion nd by-pss flow; n = totl number of dys in the period of concern. This method is pplicble to smll plots (~ 10 m 2 ) or to lrge ctchments (~ 10 km 2 ), nd it my cover periods rnging from week to yer (Trmbouze et l. 1998; Rn nd Kterji 2000). In rid nd semi-rid res with smll slopes nd/or thick ground cover surfce runoff could be neglected, especilly if precipittion intensities re generlly low (Holmes 1984; Rn nd Kterji 2000). Trmbouze et l. (1998) suggested tht soil wter blnce must be estimted t lrge number of sites in order to chieve good field evpotrnspirtion estimtes. Another common method of AET estimtion involves the use of coefficient to scle down the PET under wter-limited environments. The first step involve clculting PET from meteorologicl dt using equtions bsed on the erodynmic theory nd energy blnce, often referred to s combintion models (Penmn 1948; Monteith 1965). The most widely used form of the combintion equtions is the Penmn-Monteith eqution (Monteith 1980). Quite often the PET is then used to estimte AET fter employing soil wter reduction fctor, which is bsed on vilble or extrctble soil wter (Slbbers 1980). Despite widespred use, reserch on the comprison of vribility of estimtes from different AET methods t different times of the yer hs mostly been conducted in griculturl nd forest environments (Brr et l. 1997; Krongo nd Shrm 1997; Trmbouze et l. 1998; Droogers 2000; Rn nd Kterji 2000), nd none conducted on reclimed lndscpes. However, these lndscpes present different dynmics of soil wter movement nd retention from griculturl nd forest environments s result of unnturl reconstruction of soil profiles. Disturbed lndscpes from mining of oil snds hve to be reclimed nd re-vegetted to the pre-mining or nturl stte. For exmple, in northern Albert the stndrd prctice during reclmtion involves cpping the overburden mteril such s tiling snds with minerl nd/or orgnic mteril (pet) tht cn sustin growth (Leskiw nd Moskl 1997). The cpping mteril is sometimes obtined from distnt loctions nd is often of different ge nd composition, nd my result in substntil vrition of the point AET estimtes cross the reclimed lndscpe. Further, oil residues re sometimes present in the tilings snd overburden mteril of the reconstructed profile nd my potentilly crete pockets of wter repellency cross reclimed lndscpes, nd thus cuse substntil vrition in wter retention cross the lndscpe. The mgnitude of vrition (1) determines the relibility of sptilly verged AET estimtes for use in regionl nd globl circultion models. Therefore, to provide representtive estimtes for reclimed res would require understnding the vribility of point estimtes cross such lndscpes nd how this differs with method of AET estimtion. The objective of this study ws to compre ctul evpotrnspirtion estimtes from reclimed lndscpe using two techniques: the wter budget eqution (WB) nd the soil wter reduction (SWR) technique bsed on PET clcultion from Penmn-Monteith combintion eqution. MATERIAL AND METHODS Site Description The study ws conducted in 2001 nd 2002 on the reclimed slopes of the Southwest Snd Storge Fcility of Syncrude Cnd Ltd., pproximtely 50 km north of Fort McMurry, Albert, Cnd (ltitude N, longitude W). The tilings fcility mesures 25 km 2 nd is considered to be one of the lrgest tilings ponds in the world. The fcility ws constructed in series of lifts or benches, ech composed of long moderte forwrd slope nd short subtle bck slope, ech bench seprted by smll wterwy. The dominnt vegettion found on site included white sweet clover (Melilotus lb Desr.), yellow sweet clover (Melilotus officinlis L.), white clover (Trifolium repens L.), sow thistle (Sonchus rvensis L.), fireweed (Epilobium ngustifolium L.) nd horsetil (Equisetum rvense L.). Other vegettion species included timothy grss (Phleum prtense L.), lflf (Medicgo stiv L.), slender whetgrss (Agropyron trchyculum L.), rspberry (Rubus ideus L.), strwberry blite (Chenopodium cpittum L.), white spruce [Pice gluc (Moench) Voss], jck pine (Pinus bnksin Lmb.), spen (Populus tremuloides Michx), Siberin lrch (Lrix siberic Ledeb.) nd dogwood (Cornus rcemos Lm.). The topsoil ws sndy cly lom to cly lom textured nd cm deep overlying tilings snd. The experimentl re mesured bout 650 by 400 m with n verge slope of 4.5%, divided into four trnsects running from the first wterwy upwrds to the tilings pond (i.e., est-west direction), spced 50 m prt (Fig. 1). Thirteen luminum ccess tubes were instlled in ech of the four stright-line trnsects running upslope (Fig. 1). Soil wter mesurements were conducted every 2 wk using the CPN 503 neutron moisture guge strting t 15-cm depth nd extending down to 195 cm in 10-cm depth increments. Not ll tubes could be instlled to this depth due to instlltion difficulties. The limittions of using neutron probes to mesure ner-surfce soil moisture re recognized. However, Chnsyk nd Neth (1988) showed tht mesurements with neutron probe t depth of 20 cm clerly extend to the ground surfce. Therefore we re confident tht the soil moisture of the cm root zone ws totlly mesured. Tube loctions 6 nd 13 were excluded from the nlyses becuse they routinely hd free wter in them, indicting shllow wter tble (n unwelcome complicting fctor). Bulk density ws mesured using CPN 501 moisture/density guge lowered down ech ccess tube. Pressure

3 CHANASYK ET AL. ESTIMATING ACTUAL EVAPOTRANSPIRATION 759 ) E D MET sttion North C B A Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. b) ~ 650 m wterwys 100 m m 25 m 25 m Slope E D C B A Toe Fig. 1. Lyout of neutron ccess tubes () 3-D schemtic, (b) pln view.

4 Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. 760 CANADIAN JOURNAL OF SOIL SCIENCE plte nlysis ws conducted on soil smples collected 3 m wy from ccess tubes in 2001 for determintion of the field cpcity (0.033 MP) nd wilting point (1.5 MP) wter content (Topp et l. 1993). Avilble wter holding cpcity (AWHC) ws determined s the difference between field cpcity (FC) nd wilting point (WP). An utomted wether sttion ws instlled on site in mid-july 2001 for mesuring mximum, minimum, nd verge tempertures, solr rdition, reltive humidity, wind speed, wind direction nd soil tempertures. Potentil evpotrnspirtion (PET) ws estimted using the Penmn- Monteith eqution (Monteith 1980) from the dily climte dt obtined from this wether sttion. The PET estimtes were used in the estimtion of ctul evpotrnspirtion (AET 2 ) using the soil wter reduction (SWR) method. In 2001, AET 2 vlues were clculted for four mesurement dtes strting 2001 Jul. 23 fter the wether sttion ws instlled. In 2002, AET 2 vlues were clculted for ll seven mesurement dtes strting 2002 Jun 04. Avilble Soil Wter, Extrctble Soil Wter nd Actul Evpotrnspirtion A depth intervl of cm ws chosen s representtive of the zone where most wter ws extrcted by plnts. The vilble soil wter (ASW, in mm) nd extrctble soil wter (ESW, in mm) on ech mesurement dte nd t ech ccess tube loction were computed. Ech ASW vlue ws determined s the difference between the field cpcity (0 120 cm intervl) nd wilting point (WP) soil wter for tht depth intervl. Ech ESW vlue ws computed s the difference between the field totl soil wter nd the minimum single field soil wter cross ll tube loctions tht were mesured throughout the 2-yr study period. The conversion of WP to mm for the cm depth intervl ws determined s the sum of products of bulk density, grvimetric wter t WP nd depth increment. ASW vlues for ech of the 44 ccess tube loctions were computed. Also ASW vlues for ech slope position were determined s the verges of four vlues representing ech trnsect to give totl of eleven vlues for ech dte. Actul evpotrnspirtion (AET, in mm d 1 ) estimtes were determined using the following two methods, the wter blnce method (WB) nd the soil wter reduction method (SWR). The WB method estimted dily ctul evpotrnspirtion (AET 1, mm d 1 ) from reduced eqution of the form: N 1 AET1 = TSWi 1 TSWi Pi 1 N ( + )+ + i= 0 ( ) where TSW i = initil totl soil wter (mm); TSW i+1 = totl soil wter on the next mesurement dy (mm), P i = ccumulted precipittion between ith dte of initil soil wter mesurement nd the ith + 1 dte (mm), N = number of dys in the period. This ssumes tht the surfce runoff nd deep percoltion re insignificnt components of the wter blnce, nd tht ll precipittion infiltrtes. This ssumption is (2) justified since frmes instlled on-site to monitor surfce runoff from vrious points on the study re indicted very little runoff during the growing seson. Furthermore, previous reserch in res tht receive less thn 450 mm precipittion hve shown tht the mount of rinfll-induced surfce runoff is often negligible (Neth nd Chnsyk 1996; Hrms nd Chnsyk 1998; `Chnsyk et l. 2003). Therefore it cn be ssumed tht surfce runoff mounts were insignificnt during summer. Also, we ssumed tht deep percoltion ws insignificnt bsed on dt from selected points on the study site collected using rel-time time domin reflectometer (TDR) probes. The dt indicted tht there ws little movement of wter beyond the interfce lyer between the topsoil nd the tilings snd (depth of pproximtely cm). For exmple in 2001 the volumetric moisture content (%) from TDR probes plced 5 cm below the interfce did not chnge by much (i.e., difference between mximum nd minimum mesurement ws less thn 1.5 % on vol/vol bsis), nd in 2002 volumetric moisture content lso chnged little (i.e., difference between mximum nd minimum mesurements ws generlly less thn 5% on vol/vol bsis) (Chikowsky 2003, unpublished thesis). Therefore, regrdless of whether the conditions were wet or dry, the movement of wter from the topsoil into the subsurfce tilings snd ws limited. The soil wter reduction method (SWR) pplied the extrctble soil wter concept to compute dily ctul evpotrnspirtion (AET 2, mm d 1 ) s described below, N 1 TSWi TSW m AET2 = PETi N. TSW fc TSW i= 1 m where TSW m = minimum field mesured totl soil wter (mm); TSW fc = totl soil wter t field cpcity (mm); TSW i = totl soil wter on the ith dy (mm); N = number of dys between soil moisture redings; PET = potentil evpotrnspirtion (mm d 1 ). The extrctble soil wter ws used insted of the conventionl wy of using vilble soil wter becuse the ltter lcks recognition of the dynmic spect of wter vilbility (Ritchie 1981; Shrm 1985). Sttisticl Anlysis All sttisticl nlyses were conducted using the `SAS sttisticl pckge (SAS Institute, Inc. 2000). For ech mesurement dte pired t-test ws used to compre AET 1 ginst AET 2. The test first involved evluting the homogeneity of vrince using the folded F-test. If the vrince ws homogeneous mong the two groups the t-test ssuming equl vrinces ws conducted, wheres if the two groups differed in their vrinces then t-test for unequl vrinces ws conducted nd this involved use of the Stterthwite correction procedure. Multiple liner regression ws lso performed using SAS REG procedure (SAS Institute, Inc. 2000) to investigte the dependence of AET 2 on totl soil wter nd PET. The regression coefficients of terms tht were significnt t the 0.05 significnce level were included in the liner model. (3)

5 CHANASYK ET AL. ESTIMATING ACTUAL EVAPOTRANSPIRATION 761 Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. Tble 1. Monthly precipittion verge monthly ir temperture nd long-term norml Precipittion (mm) Averge temperture ( C) Preceding Rinfll Month LTN z LTN z rinfll event mount (mm) April N/A N/A Jul My N/A N/A Aug June N/A N/A Aug July Sep August My 28 Jun September Jun Jul Totl / verge N/A N/A Jul Aug Aug Sep z Long-term norml dt ws obtined from Environment Cnd Fort McMurry Airport Wether Sttion ( ). Tble 2. Totl soil wter (TSW, mm) for the 0- to 120-cm depth intervl throughout 2001 nd 2002 growing sesons TSW for Tubes Dte Precip. z Avg. SD CV 2001 dt Jun. 09 N/A Jun. 20 N/A Jul Jul Aug Aug Sep dt Jun Jun Jul Jul Aug Aug Sep Note: The vlues underlined re the minimum mesured TSW vlues throughout the 2-yr study period. N/A, vlues were not vilble; meteorologicl sttion not set up yet. Tble 3. Wter retention prmeters (field cpcity, wilting point, vilble wter holding cpcity, in mm) for the 0- to 120-cm depth intervl Tubes Prmeter Avg. SD CV FC WP AWHC Lowest TSW Lowest/WP Lowest TSW = lowest totl soil wter (mm) mesured over the 2-yr period. Lowest/WP = rtio clculted s lowest totl soil wter (mm)/wilting point (mm). RESULTS AND DISCUSSION Totl Soil Wter nd Avilble Soil Wter Monthly precipittion mounts in both 2001 nd 2002 were lower thn long-term norml except for July 2002, which received more thn long-term norml (LTN) for tht month (Tble 1). Also, in both yers the wettest month ws July, which lso hd highest verge temperture. The totl soil wter in 2001 for the 0- to 120-cm depth intervl generlly decresed until Jul. 04 (dy of yer 185) nd then slightly incresed nd reched stedy level in mid-august (Tble 2). Totl soil wter in 2002 ws generlly greter thn in 2001, minly due to the higher mount of precipittion in the former yer. For ll but four tubes, the lowest TSW mesured during the 2-yr study occurred in September 2001; the remining four loctions fell to the lowest recorded TSW on 202 Jul. 16. The verge TSW t ech mesurement dte vried cross the study re with coefficients of vrition rnging between 29 nd 42%.

6 762 CANADIAN JOURNAL OF SOIL SCIENCE Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. Tble 4. Dily ctul evpotrnspirtion (AET 1, mm/dy) during the 2001 nd 2002 growing sesons using wter budget pproch Avg. PET Tubes Dte Precip z (mm d 1 ) Avg. SD CV 2001 dt Jul Aug Aug Sep dt Jun Jun Jul Jul Aug Aug Sep z Precipittion since lst mesurement dte. Initil dtes were July 04 for 2001 nd My 24 for Actul evpotrnspirtion (AET, mm d 1 ) = (difference in ccumulted soil wter between dte in question nd preceding dte + precipittion)/number of dys. Note: AET clcultion ssumes tht surfce runoff nd percoltion = 0; SD = stndrd devition; CV = coefficient of vrition (%). Tble 5. Dily ctul evpotrnspirtion (AET 2, mm/dy) during the 2001 nd 2002 growing sesons using neutron probe dt, soil wter method nd extrctble wter concept PET Tubes Dte Precip z (mm d 1 ) Avg. SD CV 2001 dt Jul Aug Aug Sep dt Jun Jun Jul Jul Aug Aug Sep Note: The bsence of AET 2 dt for 2001 Jul. 04 ws becuse the wether sttion hd not been instlled t the time nd therefore PET estimtes were not vilble for the AET 2 clcultion; SD = stndrd devition; CV = coefficient of vrition (%). AWHC computed for n re round ech ccess tube rnged between 208 nd 222 mm for the 0- to 120-cm depth intervl (Tble 3). This vrition ws primrily due to vrition in the bulk density mong tube loctions. The verge rtio of the lowest TSW nd WP is ner 1.0 (0.9). These results re n encourging indiction tht the lbortorydetermined WP my indeed be good indictor of the minimum TSW tht might be expected in the field from this type of soil. Vribility of Evpotrnspirtion Estimtes As expected the ctul evpotrnspirtion estimtes from the WB (AET 1 ) nd the SWR methods (AET 2 ) were generlly lower thn PET vlues obtined using the Penmn-Monteith eqution. In both yers the mximum AET 2 vlues were less thn corresponding AET 1 vlues. The verge AET 1 vlues for the entire study re during the mesurement period were 1.9 nd 2.1 mm per dy for 2001 nd 2002, respectively Tble 6. Results of pired t-tests for compring AET 1 ginst AET 2 for ech mesurement dte in 2001 nd 2002 Dte in Significnce Dte Significnce 2001 t-vlue level (P) in 2002 t-vlue level (P) Jul NS Jun ns Aug ** Jun * Aug NS Jul <0.0001*** Sep NS Jul * Aug ns Aug * Sep ns *, **, *** Significnt t 0.05 level, 0.01 level, nd level, respectively; NS = non-significnt t 0.05 level. (Tble 4). However, the verge AET 2 estimtes were less thn AET 1 estimtes nd these were 1.4 nd 1.2 mm per dy for 2001 nd 2002, respectively (Tble 5). For ll mesurement dtes, the verge AET 1 estimtes rnged between 1.1 nd 2.8 mm per dy, with CV rnging

7 CHANASYK ET AL. ESTIMATING ACTUAL EVAPOTRANSPIRATION 763 Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. Estimted Actul Evpotrnspirtion (mm d -1 ) Estimted Actul Evpotrnspirtion (mm d -1 ) Yer 2001 Yer 2002 June 4 July 23 b June 18 b b Aug 14 Jul 16 b Jul 29 Aug 27 Mesurement Dte Mesurement Dte Aug 13 AET 1 method AET 2 method AET 1 method AET 2 method b Aug 27 Sep 21 Sep 13 Fig. 2. Estimted ctul evpotrnspirtion rtes for 2001 nd 2002.using the soil wter budget (AET 1 ) nd the soil wter reduction (AET 2 ) methods bsed on moisture mesurements in top 120 cm. between 14 nd 57% (Tbles 4 nd 5). The vribility of AET 1 estimtes is comprble with the results reported in other studies, lbeit in griculturl environments. For exmple, within griculturl fields severl reserchers reported CV vlues between 15 nd 46% (Villgr et l. 1995; Rghuwnshi nd Wllender 1997; Hupert nd Vnclooster 2004) found CV vlues up to 42%. The AET 2 estimtes verged between 0.6 nd 2.0 mm per dy with CV rnging between 74 nd 94%. This vribility in AET 2 ws lrger thn tht of AET 1 estimtes from the soil wter blnce pproch. This is possibly due severl resons: first, the different vegettion types (grsses, trees nd shrubs) on site likely gve different lef re indices (LAI) which directly ffects the frction of irrdince intercepted by vegettion round ech ccess tube. This in turn ffects the ltent energy flux nd therefore the trnspirtion rte. Further, the vegettion types hve different cnopy resistnces due to differences in cnopy wter potentils, such tht PET vlues from Penmn-Monteith combintion eqution likely vried

8 764 CANADIAN JOURNAL OF SOIL SCIENCE Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. Actul evpotrnspirtion (AET 1, mm d -1 ) No correltion r = 0.08; P = 0.41; N = cross the study site s evidenced by other reserchers (Rn et l. 1997; Rn nd Kterji 2000). The high vrition in both AET estimtes suggests implicitly tht the smpling frequency hs to be quite intensive to obtin good rel men AET estimtes. The pired t-test results in Tble 6 nd Fig. 2 indicte tht the AET 1 method gve significntly greter (P 0.05) AET vlues thn the AET 2 method in 5 out of 11 mesurement dtes, with n verge difference of 0.5 mm per dy in 2001 nd n verge difference of 0.9 mm per dy in The lrgest differences between AET 1 nd AET 2 were obtined when low rinfll period preceded the mesurement dte. For exmple, rinfll mount week before 2001 Aug. 14, 2001 ws 5.2 mm, nd tht rinfll before 2002 Jul. 16 ws 3.0 mm, s shown in Tble 1. Our results re the opposite of wht ws reported in humid ctchments, where AET from wter blnce nlysis ws smller thn AET from wter reduction method by n verge 0.5 mm per dy (Krongo nd Shrm 1997). The difference my be due to drier climtic conditions in our study. The Penmn- Monteith method hs been reported to hve tendency to underestimte the potentil evpotrnspirtion under dry conditions due to wind dvection (Smith et l. 1996), nd 1:1 line Actul evpotrnspirtion (AET 2, mm d -1 ) Fig. 3. A sctter plot of ctul evpotrnspirtion estimtes obtined using the wter budget eqution (AET 1 ) versus AET vlues obtined using the extrctble soil wter reduction method (AET 2 ). therefore my hve resulted in ctul evpotrnspirtion estimtes from the soil wter reduction method being lower thn tht from the wter blnce method. When correltion nlysis ws conducted between AET 1 nd AET 2, the results indicted wek correltion (P = 0.08) between the AET vlues computed by the two methods (Fig. 3). This result concurs with the results of Krongo nd Shrm (1997), who reported poor reltionship between AET bsed on the wter blnce versus AET bsed on soil moisture reduction method. Their reson for the difference ws tht the wter reduction method ws incpble of incorporting runoff in the AET estimtion. Although this my hold true under humid conditions under their study, in our study the conditions were not s humid nd very little runoff occurred throughout the 2-yr study period. The difference between AET 1 nd AET 2 estimtes must be ttributed to differences in the components of the equtions used in their clcultions. For AET 1, the component terms include totl soil wter nd precipittion, wheres for AET 2 they re soil wter, potentil evpotrnspirtion nd soil wter retention. Furthermore, the AET 2 method does not ccount for the evportion occurring immeditely fol-

9 CHANASYK ET AL. ESTIMATING ACTUAL EVAPOTRANSPIRATION 765 Cn. J. Soil. Sci. Downloded from by on 11/23/17 For personl use only. lowing precipittion, whereby wter my be lost directly from wet soil nd lef surfces. Usully the technique used to mesure soil wter (e.g., the neutron moisture meter used in this study) is such tht mesurements re not conducted when it is rining, rther, mesurements re tken t lest dy fter rin event. This mens tht ny intercepted precipittion tht is lost vi evportion would not be included in the AET 2 clcultion, but would be n integrl component of the AET 1 clcultion. According to Hnks (1992) there re severl possible reltionships between AET/PET rtio nd soil wter frction. One extreme condition ssumes tht wter is eqully vilble to plnts for the entire soil wter rnge between field cpcity nd residul wter content or wilting point, wheres the other extreme condition ssumes liner reltionship between AET/PET nd soil wter frction such tht vilble soil wter decreses linerly s the soil dries out. However, the correct sitution is likely in between the two extremes. Further, the rel sitution devites from the two extremes due to the fct tht the reltive soil wter concept ssumes uniform or verge soil wter content, which is seldom found. Also, fter rinfll event the root zone my be prtilly wet such tht evpotrnspirtion my not be limited for few dys despite the verge soil wter of the root zone indicting limittions. A study conducted by Denmed nd Shw (1962) indicted tht if climtic drying demnds were high, such s on cler dys, then it is possible tht n extreme condition ssuming liner reltionship to soil wter between field cpcity nd wilting point would occur. However, when demnds re low, such s on cloudy dys, they reported tht soil wter between field cpcity nd wilting point ws eqully vilble for plnt uptke s indicted by n AET/PET rtio tht remined close to 1.0. We believe tht our results closely resemble the ltter condition s indicted by the lck of liner reltionship between AET/PET nd ESW frction (dt not shown), nd lso the fct tht the northern Albert climtic conditions re cool nd subhumid, nd thus hve low climtic drying demnds. In fct the results of this study re pplicble elsewhere, soil wter models tht use the soil wter reduction method my be grossly overestimting soil wter due to underestimtion of AET. However, others hve reported contrsting results. For exmple, Brr et l. (1997) showed tht using n ET method tht included soil wter limittion to ET produced worthwhile improvement in hydrologicl performnce of the Semi-distributed Lnd Use-bsed Runoff Processes (SLURP) model. They concluded tht the choice of the method of AET estimtion ffects the bility to simulte strem flow nd even uncertin estimtes of soil wter re reported to improve the estimtion of AET. In our study, reducing PET to determine AET by soil wter function gve highly vrible evpotrnspirtion estimtes. Therefore if this method is to be used in reclimed res with substntil vribility, much lrge number of loctions for soil wter determintion would be required to obtin good estimte of the eril evpotrnspirtion. CONCLUSIONS Estimtion of ctul evpotrnspirtion depends on the ssumptions of the methods used. The ctul evpotrnspirtion rtes estimted using the soil wter reduction technique (AET 2 ) were generlly smller thn corresponding vlues obtined using the wter budget eqution. The verge AET vlues estimted using the wter budget eqution (AET 1 ) were 1.9 nd 2.1 mm per dy, for 2001 nd 2002, respectively, wheres those estimted using the wter reduction technique (AET 2 ) were 1.4 nd 1.2 mm per dy for 2001 nd 2002, respectively. The difference in the results from the two AET methods is most likely due to differences in how the wter budget equtions re prmeterized. For exmple, the lck of inclusion of precipittion in the AET 2 estimtion mens tht evportion of intercepted precipittion is not ccounted for in this method, but is inherent in the AET 1 method. These differences in estimtion of AET cn substntilly influence the output from hydrology models tht the user must be wre of these differences. The AET 1 /PET rtio ws not relted to the ESW frction, suggesting reltively low climtic drying demnds on the study site. The lrger vribility of AET 2 estimtes thn AET 1 estimtes imply tht the former my be less relible in obtining sptilly verged AET estimtes for use in regionl nd globl circultion models. ACKNOWLEDGMENTS We grtefully cknowledge funding from Syncrude Cnd Ltd. nd the Nturl Sciences nd Engineering Reserch Council (NSERC). Thnks to Clr Qulizz (Senior Environmentl Scientist with Syncrude) for ll her efforts on the project, nd to Dine Brdshw, Neil Reid, Elise Prker, Sbrin Rock nd Noble Donkor for ssistnce with field dt collection. Brr, A. G., Kite, G. W., Grnger, R. nd Smith, C Evluting three evpotrnspirtion methods in the SLURP mcroscle hydrologicl model. Hydrol. Proc. 11: Bumgrtner, A. nd Reichel, E World wter blnce: men nnul globl, continentl nd mritime precipittion, evportion, nd run-off. Elsevier, Amsterdm, the Netherlnds. 179 pp. Burnsh, R. J. C The NWS river forecst system-ctchment modeling. Pges in V. P. Singh, ed. Computer models of wtershed hydrology. Wter Resources Publictions, Highlnds Rnch, CO. Chikowsky, C. L. A Soil moisture regime nd slinity on tilings snd storge fcility. M.Sc. thesis in Lnd nd Wter Resources, Deprtment of Renewble Resources, University of Albert, Edmonton, AB. 135 pp. Chnsyk, D. S., Mpfumo, E. nd Willms, W. D Quntifiction nd simultion of surfce runoff from fescue grsslnd wtersheds. Agric. Wter Mnge. 59: Chnsyk, D. S. nd Neth, M. A Mesurement of nersurfce soil moisture with hydrogenously shielded neutron probe. Cn. J. Soil Sci. 68: Denmed, O. T. nd Shw, R. H Avilbility of soil wter to plnts s ffected by soil moisture content nd meteorologicl conditions. Agron. J. 54: Droogers, P Estimting ctul evpotrnspirtion using detiled gro-hydrologicl model. J. Hydrol. 229: Hnks, R. J Applied soil physics: Soil wter nd temperture pplictions. 2nd ed. Springer-Verlg, New York, NY. 176 pp. Hrms, T. E. nd Chnsyk, D. S Runoff response from two reclimed wtersheds. J. Amer. Wter Resour. Assoc. 34:

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