EVAPOTRANSPIRATION IN FOREST STANDS OF THE SOUTHERN APPALACHIAN MOUNTAINS J. L. KOVNEE Southeastern Forest Experiment Station, Forest Service, USDA

Transcription

1 EVAPOTRANSPIRATION IN FOREST STANDS OF THE SOUTHERN APPALACHIAN MOUNTAINS J. L. KOVNEE Sutheastern Frest Experiment Statin, Frest Service, USDA Experiments at the Cweeta Hydrlgic Labratry in the high rainfall belt f the Suthern Appalachian Muntains have demnstrated that manipulating the frest vegetatin can affect streamflw frm the treated watersheds. In general, it has been fund that remval f the frest vegetatin in part r in ttal prduces increased streamflw. A summary f these studies was given at the 1954 meeting f the Gergia Academy f Sciences and published in the Gergia Mineral News Letter Vl. 7, N. 4, Winter Streamflw, hwever, must be regarded as a residual frm precipitatin after the return f misture frm the grund t the atmsphere. Fr example, when averaged ver 1 a number f years, the strage change is negligible and the annual water balance equatin may be written as ' P Ev = R!... ', (1) Where P = precipitatin, Ev = evaptranspiratin and R = streamflw. It is much mre apprpriate t cnsider the relatinship between evaptranspiratin and vegetatin and t examine sme f the characteristics f evaptranspiratin revealed by the Cweeta studies. This is particularly imprtant since such data are extremely difficult t btain. It is practically impssible t directly measure evaptranspiratin frm frest stands. In additin, the mathematical thery f evapratin had nly been develped fr a free liquid surface r a permanently saturated slid surface. The extensin t drying f slids as in the case f the remval f water frm the sil has nt been successful. Evaptranspiratin actually includes three separate prcesses interceptin, transpiratin, and evapratin. Climatlgists are inclined t grup all three tgether as being respnsible fr the return f misture t the atmsphere with the accmpanying latent heat energy f vaprizatin. In scientific investigatins f natural land areas, each prcess will have t be studied separately in rder t arrive at basic principles fr manipulating vegetatin in rder t alter the water balance. The Cweeta experiments culd nt be expected t prvide infrmatin f this kind with techniques f measurement presently available. In the data that fllw, all evaptranspiratin phenmena are cmbined. The cmplete water balance equatin fr any time perid, neglecting deep seepage, may be written as P = Ev + R + S 2 Si (2) Where Si and Sg are initial and final sil reservir strage values, respectively. Since the strage cycle nrmally has a 12-mnth perid, the difference fr such a perid is usually small in cmparisn with the ther quantities. This quantity can be regularly minimized by Reprinted frm the Bulletin f the Gergia Academy f Science Vl. XV; Pages 80-85

2 selecting an arbitary hydrlgic year. At Cweeta the perid frm May 1-April 30is used since maximum sil reservir strage regularly ccurs arund May 1. The greatest variability in the strage factr is due t grund-water strage. Actually this can be estimated frm the derived depletin curve and used in (2). The nly unknwn is then the s-called retentin strage, which must be very small since sils are in the range f field capacity as f May 1. The difference between annual precipitatin and streamflw adjusted fr grundwater strage fr a hydrlgic year at Cweeta is thus a gd estimate f annual evaptranspiratin mmm Pr ICij ita.in m**~1 nr d >-H< mm*** U U Hi Years FIGURE 1. Annual precipitatin and P-R fr Watershed 2 In fig. 1 annual precipitatin and P K are given fr watershed 2, which has an area f 31 acres and an apprximate mean elevatin f 2800 feet. Estimated average annual evaptranspiratin is 40 inches against 29 inches f streamflw. Althugh precipitatin during the 18-year perid varied frm 47.5 inches t 84.2 inches, evaptranspiratin ranged frm 35.6 t 43.9 inches. This cnservative aspect f the evapratin prcess has been recgnized in ther humid parts f the wrld and has led t the bservatin that streamflw is a residual f the water cycle precipitatin that has escaped the attractin f the sun's energy. This fact has anther implicatin. If evaptranspiratin can be reduced, fr example, by altering the vegetatin t a new stabilized cnditin, then cnsistent gains in streamflw can be expected each year. Since the evaptranspiratin is dependent n climatic factrs such as wind mvement and humidity, it is mre than likely that it will change with elevatin. Fig. 2 shws the relatinships between elevatin and estimated average annual evaptranspiratin n sme Cweeta watersheds. Evidently evaptranspiratin decreases with

3 ho n l """ >, "^ \, ^ ^^ 0 ^* cm -S 20 2 K "^^>v "^^.w^ ^^> ^.»» h 36 3S 1*0 U2 lib 56 IjB U Maximum elevatin in hundred feet FIGURE 2. Annual P-R vs. elevatin f watershed. elevatin, drpping frm 33 inches at 3000 feet t 20 inches at 5200 feet. This is a rather large change and may be influenced by ther factrs assciated with elevatin such as vegetatin and sil depth. The relatinship indicates the advantages f high elevatin areas as a surce f water supply. Nt nly are lsses t the atmsphere less, but precipitatin generally increases with elevatin. The breakdwn f evaptranspiratin int its cmpnents fr a lw elevatin frested watershed can nly be dne n an apprximate basis. Interceptin studies n frest stands similar t thse n Cweeta indicate that annual interceptin n 70 inches f precipitatin amunts t abut 9 inches. On tw watersheds which were clearcut the first year, increase in streamflw amunted t 17 inches. Since heavy accumulatin f cut material was left n the grund, it may be assumed that interceptin and evapratin remained abut the same and that the increase in streamflw came frm cessatin f transpiratin. Thus transpiratin may be estimated at 17 inches. The ttal evaptranspiratin is estimated at 32 inches, leaving a balance f 6 inches fr evapratin. Admittedly, this is a very rugh breakdwn but it des give sme idea f the rder f magnitude f the different cmpnents f water lss. It is practically impssible t btain reliable estimates f mnthly values f evaptranspiratin fr a natural frest stand. Nne f the Cweeta data prvides this kind f infrmatin. The annual value des prvide a reliable cntrl, hwever, and it is nt t difficult t make a reasnable distributin f the mnthly values. Thrnwaite 's frmula fr cmputing ptential evaptranspiratin was used, since sil misture is seldm limiting at Cweeta, n accunt f high yearrund rainfall. The frmula results in a yearly ttal f 27.7 inches, which is smewhat lw. The mnthly values were adjusted accrdingly

4 t a 31-inch annual ttal, and sme minr crrectins made n the basis f lcal evapratin pan data. The distributin need nly be apprximate fr the fllwing discussin. The tp curve in fig. 3 represents the mnthly march f evaptranspiratin fr an iindisturbed frested watershed. The experiments at Cweeta have demnstrated that cutting the frest vegetatin has decreased evaptranspiratin. Fr example, we will cnsider the treatment which cnsisted in cnverting a watershed frm a frested t a field cnditin. The change in vegetative cver has resulted in a decrease in annual evaptranspiratin amunting t 10 inches. Incidentally, the previus bservatin regarding the cnservative nature f evaptranspiratin has been brne ut in this experiment. Once the cver was fairly well stabilized, annual increases have regularly apprximated 10 inches, ranging frm 8-12 inches. Mnthly values f evaptranspiratin have been estimated n a basis f an annual ttal f 22 inches. It is assumed that the mnthly evapratin values fr the drmant seasn wuld remain relatively unchanged. The lw^er curve in fig. 3 represents the new march f evaptranspiratin, arid the area enclsed by the tw curves represents the annual decrease f 10 inches. Fig. 3 shuld be a gd gen- W I FIGURE 3. J F M A M J J A S O N D Mnths Estimated mnthly evaptranspiratin befre and after treatment Watershed 17.

5 eral descriptin f the relative behaviur f evaptranspiratin under the tw kinds f vegetative cver n the watershed, since the same climatic factrs cntrlling water transprt frm the grimd t the atmsphere prevail. The immediate evaptranspiratin draft (except interceptin) is n the ttal sil reservir strage which will reflect the mnthly changes indicated in fig 3. Mnthly increases in streamflw, hwen J8 FIGURE 4. Mnths Average mnthly increase in streamflw vs. average mnthly decrease in evapratin fr Watershed 17. ever, cannt be predicted frm fig 3. Frtunately, the experimental methd using a cntrl watershed makes it pssible t estimate by regressin analyses the average mnthly increases in streamflw based n 15 years f recrd. Fig. 4 is a cmparisn f estimated mnthly increases in streamflw and decreases in evaptranspiratin. It is quite bvius that the latter bear little direct relatinship t the frmer. In fact, ver half f the streamflw increase ccurs during the drmant seasn, when the treatment actually had little direct effect n evaptranspiratin. The smallest increase, 0.36 inch, ccurs in April, with May abut the same at 0.37 inch. The largest increase f 1.65 inches takes place in January, with December a clse secnd at 1.50 inches. The largest

6 change in evaptranspiratin belngs t July and amunts t 2.3 inches. Sme advantage has been gained in smthing ut the evaptranspiratin curve in that sizeable increases in streamflw ccur during Octber, Nvember, and December, when flws are nrmally very lw. In rder t explain the mnthly phase difference between evaptranspiratin and streamflw, it is necessary t understand the water cycle in this regin. Let us begin arund the middle f April, when the whle sil prfile is at "field capacity" and the water table is at its highest level. The trees begin t leaf ut, intercepting sizeable quantities f precipitatin, and active transpiratin begins t remve large amunts f readily available water frm the upper sil mass by virtue f extensive rt systems, laterally and in depth. When the trees are remved, as in this experimental treatment and replaced by lesser vegetatin, smaller amunts f water are remved frm the sil. Meanwhile, the water table begins t drp in the summer perid as grundwater is depleted by streamflw and transpiratin draft f vegetatin in cntact with the water table. Current net precipitatin cntributes very irregularly t permanent grundwater as lng as it is retained and expended in the upper layers f the sil. As a result, mnthly streamflw ttals cntinue t decline withut interruptin at a decreasing rate right thrugh the grwing seasn. Each year, hwever, sme large strms r a successin f smaller strms suceeded in vercming the misture deficit and cntributing t grundwater. Generally, the prcess des nt take place ver the whle watershed but is restricted t lcal areas. With a lesser type f vegetatin, the direct transpiratin draft is reduced and the water table des nt drp s fast. This, f curse, results in increased streamflw. Als, with a smaller sil misture deficit nw, a few mre summer strms will cntribute t grundwater and hence raise the water table abve nrmal cnditins and increase streamflw. Beginning in the fall, arund the first f Octber, the trend in sil misture depletin is reversed, althugh streamflw may cntinue t drp. The frest vegetatin reacts t the climatic change, ntably lwer temperatures, t reduce its transpiratin draft. Evapratin als declines, and net precipitatin, which nw exceeds the cmbined ttal, is effective in reducing retentin strage. Later in Nvember frsts remve the fliage, making transpiratin negligible and decreasing interceptin, s that the sil misture build-up prceeds mre rapidly. By the end f December, the sil mass is at "field capacity." Nte that sme accretin t grundwater may take place during this perid and cntinues during the winter. The cntributin accunts fr increases in streamflw which shw up during this perid. Streamflw in the winter mnths will nw respnd directly t the amunt f precipitatin, and the increases in streamflw are maintained in January and February. After the middle f March and thrugh April the water table is at maximum rise. This accunts fr the reduced increases in streamflw in March and April.