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WATER YIELDS OF FOREST, MAQ UIS AND GRASS C OVERS I N SEMI -ARID REGION S :
A LITERATURE REVIEW
A. Y. SBACHOR11 and A . MtCHAELI
INTRODUCTION
The constant development of the economy and popu·
lation in Israel has brought about an ever-increasing demand on the water supply. This water C0116umption is mostly supplied from ground-water sources through .natural recharge occurring during the winter.
The increased consumption rate has brought about increased pumping rates in the mountain regions.
Additional afforeotation programmes are in the plan·
ning stage. As a result of decreaoed grazing of the mountainous areao by sheep and goats, the chaparral .:over has increaoed considerably in density.
The depletion of ground water accompanied by the increase of evergreen vegetation, both forested and chaparral, concentrated the attention to the probable .e.B'ect ofincreued water consumption by this vegetation cover type and thereby decreasing ground-water recharge. It has been claimed that the evergreen vege·
tation through its deep root system utilizes moisture throughout the summer and during dry winter periods from water in soil waohed into the cracks in the rocks -or from water temporarily stored above relatively impervious lenses. This water is deducted from the recharge available to the ground water, coming from precipitation occurring after the periods of consump·
"lion. On the other hand, it hu been suggested that with annual and graosy vegetation this phenomenon io confined to the shallow soil horizons abo'Ve the rock because of the ohallow root system of this type of vegetation. Because of the practical importance of this problem, plans for a comprehensive research programme in Israel were drawn up.
In view of the world-wide interest in this problem and the large amount of material available, it wao decided that, in the .first place, a study should he made of the pertinent and quantitative evidence available from abroad. This Wll8 necessary to determine whether -the probable order of magnitude of the differences in water yields from different vegetation cover types are
of ouch a magnitude 118 to justify the efforts involved in a local study.
This article contains the abbreviated results of ouch a study.
L ITERATURE REVIEW
In order to obtain a quantitative evaluation of the differences of water yields from different types of vegetation coven, it was intended to make the evalua·
tion for the following cover types: forested, woodland and chaparral, grassy and herbaceous, bare arell8.
A comprehensive review of available data up to 1960 was made, which included
157
references, from which .iL wM iutended to select tbe relevant studiee that could be used for a quantitative evaluation.The criteria for a relevant study were 118 follows.
1. The study should contain quantitative evaluation of either: (a) all the components of the water balance;
(b) precipitation and water losses (evapotranspiration and interception) ; (c) precipitation and total runoff (ourface runoff
+
deep seepage) or water yields.It is sufficient to have only partial water balance valueo [(b) or (c)]; if the precipitation and water losses are known, water yield may he deduced and vice versa.
In the ease that ouly a fraction of water yield or water loues was measured. the relative composition of the water balance may be affected quantitatively.
Therefore, for the purpose of this review, such studies were not taken into consideration for the quantitative evaluation, but only considered qualitatively.
2. The study must have had a duration of at least a full year, or a closed hydrological season. A study of a shorter duration may bias the relations of water losses to water yields and present an incomplete picture t. A. Y. Sba.cborl S. at Lba Soil Ero1ieo Experinnntal St.tioll,
R
.. pia lattlutteo£ Aplou.lttll'(l. Emek-Bdtr (luael).
467
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Physiology of the plam cover/Phy1 iologi6 de la cou1.•erture veghale
representing the short period only. Even though annual values were taken into consideration, average annual values based on longer periods were preferred.
After the review began it became evident that the most important studies for this purpose were the studies conducted in regions having an annual precipitation of less than 1,400 mm.fyr. This amount is the highest annual precipitation ever recorded in the wettest Galil ee region in Israel. Therefore the review was concentrated on studies from rainfall regions below 1,400 mm.fyr. A summary description is given in Table 1 of the important studies and results obtained also in the higher rainfall regions (although these
were not taken into consideration for the final quanti ..
tative analysis). Resnlts of studies conducted in regions having similar hydrometeorological conditions to Israel were preferred, but it became evident that the number of such studies was too small and therefore all studies conducted in rainfall range lower than 1,400 mm.fyr.
having met the previous requirements-were taken into the final evaluation. In two of the studies from California which contained a great range of precipi·
tation values, the average annual values were broken into groups of years having similar annual rainfall values. From the few studies reviewed it was observed that the water losses from chaparral type vegetation TADloE 1. Studies and results obt
a
ined in high rainfall regions.Pt.oe aad
name olet.,rdy
1. North Fork, Calif.
2. Bao• Lake, Calli.
3. San Dimas, Calif.
(plots)
4. San Dimas, Calli.
5. San Dimas, Calif.
(lysimeters)
46H
29, 28
29
31, 29, 25
25 25, 6, 34
1938/39 1938/39 1939/40 1939/40
1941/42 1942/43 1943/44 1944/45
1953/56
Avnage of 1949/56
1957/58
Chaparral Bare Chaparral Bare Chaparral Burned Pine Bare Pine Bare Pine Bare Pine Bare
Ceanothus, Chamice Ceanotbua, Chamice Cbamice Bare Chaparral oak Defoliated oak Coulter pine Cham
i
ee Scrub oak Ceanothus Grass Bare Coulter pine Cham.ice Scrub oak Ceanothus GrassBare
Annual pre-Aoaual d pi�tioo
{::.)
yield(mm.)
625 214
625 250
1 035 635 1 035 721
830 420
1 280 970
1 285 704
1 285 1 000 1 292 663 1 292 1 049
978 366
978 615
1 260 719 1 260 881
1 080 560
!
1 080 565 Average 558
1 080 550
1 080 650
525 25
500 280
523 124
I
523
�!
Average 92 523523 64
523 132
523 322 1 230 591
!
1 230 570
1 230 590 Average 588 1 230 600'
1 230 810
1 230 996
Dif'fflreQoe, iD.
watuyi.eld.
(mm.)
36 86
246
386 249 162
92 255
40 230
222 40S
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Place aud aame of ttudy
6. Hopland Watershed, Calif.
Monroe, Volfe Can·
yon San Dimas, Calif.
8. Black Mesa, Colo.
9. Fruser, Colo.
10. Fool Creek, Colo.
U. Parish Creek, Utah
12. Wagon Wheel Cap, Colo.
13. White River, Colo.
14. Fish Creek. Calif.
1S: Coweeta Hydrologi·
cal Laboratory, N.C.
(a) Watershed 17 Clear cut kept cut
(b) Watershed 13 Clear cut. natura) regrowth
(c) Watershed 19 Rernoval of understory
A. Y. Shachori and A. Michaeli Water yielch of forur, maquis and gron covers
RtJertoce'
3, 4
25
27
10, 12, 23, 36. 37
11, 13, 10, 23
7,1
2, 26
26,33
15
9, 20, 14, 16
9, 14
9, 14
9, 14,16
You
1957/58 1957)58 1958}59
1956 1960 19S7 19S7 19S8
1947/49
1910/18 1922 1920/27
1924
5 years ht year 3-14 years 1938 1st 2nd 4th 6th 10th 14th
htyear 2nd year 3rd year
Ve1etatlve cover
Oak cbaparral Bllrtled
Removed riparian vegetation 10%ofnrea iuMonroe
Aspen Spruce Grass
Lodgepole pine Complete cut
Lodgepole pine fir. 11pruce SO% cut Lodgepole, fir,
spruce, 50% cut Lodgepolo, fir,
�pruee, SO % cut
Aspen and grass Grass Bore
Pine. 6r, aspen, spruce Cut, natural regrowth Cut, natural regrowth
Spruce
Damage by bark beetles
S. Cal. chaparral Burned
Calih. oak, hickory Clear cut kept cut Clenr cut ltept cut Oak, hickory Clear cut Natu.ral regrowth Natural reg·rowth Natural regrowth Natural regrowtb Natural regrowth Oak, hickory SO % cut strips 50 o/o eut otripo SO o/o cut atrips
Aanu11.l pre- Aunual Ditfcrenee io
eipitatioa water yield water yield•
{mm,) (mm.) (�.)
1 320 812
1 320 940 128
Increase of water yield by90%
Notgiveu (500)' (330)3 (255)'
620 262
620 334 14
7SO(av.) 289
7SO 396 107
7SO 330
750 416 86
7SO
7SO 54
1 34U 773
1 340 874 101
1 340 970 197
S40(av.) 1S7
222 65
183 26
20 % increase (+ 29%)
Av. 1 830
Av. 1 830
1 830 430
1 830 280
480 305 230 190 150 120
127 205 102
469
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Phy•iology of me planl cover/Physiologie d4 la couverrure vlgerale
Plac:ll\ ._lkl Rt'fereoor' Ylfar Vt"Jetative Coi)Y('I'
cipitatioo
Aonu..lpre-
Annualwater yield Dilf'ereDce io water yield•
oame
of study (mm.) (�) (111m.)16. Calhoun, S.C. 24 1953 S. pines and broad leaf 1 237 444
Sedge (grato) 1 237 500 56
Bare 1 237 546 102
1952/54 S. pineo and broad leaf 1 040 462
Sedge (grass) 1 040 626 164-
11. FemowExp.Fore&t,. 38 6th year Calib. 2nd growth 1 522 848
W.Va. Broad leaf forest on- 1 522 940 92
derstory grass
18. Sand dunes, N.J. 21 Pine seedlings, 7 years (590)' old
Young pine. 15 years (570)'
Mature oak (560)'
Mature pine, 100 years (530)3
Bare (438)'
19. Fylde Water Works, 17, 18, 19 Sitka spruce 965 313
Preoton (England) Gran 1 040 647 (334) 270'
20. Queens College, near 30 Pine (556)'
London (England) Gr8S8 (464)' (92)
21. Emmental Valley Forett 1 641 785
(Switzerland) 2/3 graos 1 719 1 021 236
22. Castricum 1ysimc:· 1946/57 Pine 825.6 107.8
ten (Holland) Oak 825.6 100.5
nnnf\ V"'Setation R25.1\ 14R.5
Bare 825.6 247.4 139.6
23. Winterthal, Bramke 35 Forest, (Winterthal) 1 253 674
(Germany) Cut (Bramke) 1 221 700 (26)
24-. Sierra Anehe, Ariz. 27 13 years Chaparral, gr ... 550 50 (a) Natural drainage (av.)
(b) Open eans Chaparral 770 119
Grass 770 153 34
Bare 770 250 97
Chaparral 656 25
Crass 656 52 27
Bare 656 13 21
Chaparral 562 99
Bare 562 94 5
25. Dilldown Forest, 32 1951/55 Quereus llicifoJia and H10 820
Del. other
26. Munroc Canyo1l 25, 29 Southern California 686 305
chaparral
�: :.�J\:::{1:;t)���o�r:!!!'o:�::�::rtid�.
3.
Ev•potra•bpir•tion value•.
4.
Corn:ctiqCor n.inf•ll dilfeuoe..
470
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A. Y. Shachori and A. Michadi Water yields of forul, m a quia and gran cot: er&
are equal to, or not much greater than, that of a forest (pine) (in the range of 400-1,400 mm.fyr. annual precipitation), and therefore it was possible to combine the two cover types into one population.
On the other hand, it was observed that ouly a small number included true bare areas. In many of the studies where it was meant to have a bare area, it actually contained some grassy or herbaceous cover. This is especially true in areas where forest or chaparral had been removed by cutting or burning and the annual herbaceous or grassy cover had been allowed to grow (in a few studies, even the woody vegetation was allowed to regrow). Therefore, in order to obtain meaningful results, these two types of cover, the grassy herbaceous and barren areas., were grouped into a second population.
The results of the different studies were analysed in order to find whether there existed a significant difference in the water losses or yields of these two groups of data, obtained from different parts of the world. The data was gathered in such a manner as to
enable the quantitative analysis of water yields as a function of annual precipitation in each population (Table 2). It is important to note that this review does not deal with riparian or phreatophytic vegetation. All riparian vegetation studies indicate that the evapotrans
piration of such vegetation is very high and it is a subject by itself.
From 14 different studies, there have been found 20 direct comparisons between the water yields of forest, woodland or chaparral covered areas to those of grassed and barren areas.
The report also reviewed additional studies that were not used in the final analysis, either because the annual precipitation was greater than 1,400 mm./yr., or because of their riparian nature,. or as a result of having an incomplete balance. However, these studies were important enough to he mentioned as they aided in showing the relation between water yield differences and precipitation.
The summary of these 26 studies is presented in Table l.
TABLE 2. Data for quantitative analysis (for graphic representation of these values see Fig. 1).
Loutloo Detail
I Rei,No.
San Dimao, Calif. Oak, defoliated 2S San Dimas, Calif. Lysimetero (5-yr. 25,6
av.)
San Dimao, Calif. Lyoimeters (5-yr. 25, 6 av.)
San Dimas, Calif. Lyaimeters 25,6
(1957 /58 av,)
San Dimas, Calit. Lyiimeters 25,6 (1957/58 av.)
San Dimas, Calif. Ploto 25, 29, 31
North Fork, Calif. 1938/39 29,28
North Fork, Calif. 1939/40; 1936/37 29,28
North Fork, Calif. Unbumed 29, 28
North Fork, Calif. Burned 29, 28
Bass Lake, Calif. 1941/42 29
Bass Lake, Calif. 1942/43 29
Bau Lake, Calif. 1943/44 29
Bass Lake, Calif. 1944/45 29
Hopland, Calif. 1957/58 3, 4
Monroe Canyon, Calif. 1938/53 25,29 Siena Anche� Ariz. 13 yr. av. 27,26
Wagon Wheel Gap, 1910/27 2, 26
Colo.
Frazer, Colo. 36, 37, 10, 12, 23
Parioh Creek, Utah 1947/49 7,1
Parish Creek, Utah
Winterthal (Germany) 35
Fylde (England) 17,18,19
Calhoun, S.C. 1953 24
Dilldown For .. t, Del. 1951/55 32(a),32 (h),32(c) Fool Creek, Colo. 1956 11, 13, 23, 10
Precipi�
t«tioa
<�-)
525 523 523 1 230 1 230 1 080 625 1 035 830 1 285 1 292 978 1 260 1 320 686 550 540 620 1 340 1 253 965 1 040 1 410 750
25 92 92 588 588 558 214 635 420 754 663 366 719 812 305 50 157 262 773 674 313 462 820 330
No. oFpoi.at ondla,ram
10 11 12 13 14 15 16 17 18 19 20 21 22 23
Preclpi·
t.tioD.
(�.)
500 523 523 1 230 1 230 1 080 625 1 035 1 280 1 285 1 292 978 1 260 1 320
530 620 1 340 1 340 1 040 1 040 750
Grn.ed aod
bare DifFerence ill water
Waw yield ( ....
.
)280 132 322 810 996 650 250 721 970 1 000 1 049 615 881 940
222 334 874 970 647 626 416
No. olpoi-ot
00
di.gnl!Q ��)
1 255
2 40
230 222 408
4 92
5 36
6 86
8
9 246
10 386
11 249
12 162
13 128
16 17 18 18 20 21 23
65 74 101 197 334 164 86
471
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Phpwh>gy of lhe plant cover/Phy•iologie de la couv..W,.. vEglrok
Q UANTITATIVE EVALUATION OF T H E DATA
COMPARISON BETWEEN WATER YIELDS
(
TOTAL RUNOFF
)The values used for the quantitative comparison of water yields as related to precipitation
are
given inTable 2.
The quantitative evaluation of the relationship of the water yields from the two different cover groups, with varying precipitation, showed that the values can be fitted by two linear regressions, with a high degree of correlation (see Fi«. 1.)
For forest, woodland and chaparral:
I 100
R, = 0.805 (P-398) T = 0.972.
Forest, woodhmd, moqvi 0 Grass or bar•
1,'1,3 location of s.tudies (see Toble l)
For grassed and bare areas:
R1 = 0.920 (P-281) T = 0.972.
where � = annual water yield in mm., P = annual precipitation in mm., r = coefficient of corrclation.
The linear regression of the woody vegetation indicates lower water yields than the regression of the grassy vegetation. Also, there is an indication that with increa
sing precipitation, the difference between water yields increases.
DIFFERENCES IN WATER YIELDS ( TOTAL RUNOFF )
In Figure 1, the average difference between the two regression lines-in the 800-1,000 mm. annual rainfall range-is about 120 rum.
I 000
Mixture of .,�ui and J,orboc;cO\IS v�getation
} �ot
oiedof catchment '" northern isroel ;:clu Ah1.1. Gosh catchment in Jsroel pu:domimmtfy r•gre�1ion
bore end ga�s ��oq..,ahonl
10
/
6°
18A• E
<
� 0
0
!
"' ,_
472 900
800 700 600 500 400 300 200
lOO 00
A.G
@
P {ptt>eipitntion in n)ll\,)
...
1 1....
20
Fie. 1. Comparison of water yield$ between forest, woodland, maquis-covered areas to grassed Dr hare areas under various precipitation
co
nditions. (1, 2, 3 etc. = location of studies in Table 1).Page 494 sur 549 https://unesdoc.unesco.org/in/documentViewer.xhtml?v=2.1.19…&multi=true&ark=/ark:/48223/pf0000019549/PDF/019549mulo.pdf
A.
Y. Shachori GndA.
Michaeli Water yields of forest, Dlaquis and grass coversHowever, in a graphic representation of the diffe
rences in terms of total runoff (or water yields) shown in Figure 2, in which 20 direct comparisons are plotted against rainfall, the following indications may he found:
1. In all studies, the total runoff from hare or grassed areas si greater than that from forested woodland or maquis·covered areas.
2. The magnitude of the difference indicates a tendency to increase with increasing rainfall.
3, In the rainfall range of 500-800 mm.fyr., there are seven pairs of direct comparisons, two of which, both frGm Southern California, show an average dif
ference of 242 mm.fyr., the other five pairs show an average difference of 60 mm.fyr.
4. In the rainfall range of 800-970 mm.fyr. there are no data.
5. In the rainfall range 970-1,400 mm. /yr. the differences show a rather wide scatter from 86 mm. to 408 mm.
The average difference of the 13 pairs
is
213 mm.400
350
300
250
200
0 150
]
lOO. 0 �
� 50
§.
...
0:
4 0.0
- 50
0 0
P (PJecipitation in mm.)
© I
© 'lA
© 16
© 2
© 17
5
@
6. In conclusion, the data indicate that there are average differences of at least 60 mm. in the low rainfall range (500-800) and differences of at least 100 mm. in the higher rainfall range 970-1,400 mm.fyr.
DISCUSSION
The analysis of the different studies reviewed indicates a smaller water yield from forest-woodland maquis
covered areas than from grass or barren areas. In general, there
is
no certainty that quantitative results obtained in this review will he applicable to the local conditions. However, in Figure 1, the regression line of the woody vegetation indicates that there was no water yield at a precipitation level of 400 mm., a quantity which may be the minimum requirement for woody vegetation development.The cessation of water yield under grassy vegetation
© 23
11
@
© 20
21
@
6 4
@@ 3A ©
10
@
9 3 @
@ 18A
@ 12
@ 13
@ 18
@
F1c. 2. Di£rerences between water yields of forest, woodland m<�qui1 and grassed or bare aceas,undervarious precipitation conditions.
473
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Physiology of <he pl<>m <()!)fr/ Phy•iolo&i• de lo couvorturo vls&alo
was at a precipitation level of 280 mm. Tbis range may also correspond to the lower level of precipitation which will support an annual vegetation cover.
The local ecological significance of these rainfall values (400 and 280 mm.) are discussed by Professor M. Zohary in Geobutany (1955, p. 202·203): "In Israel, it is possible to observe that the boundaries ef specific plants and plant communities fit the general pattern of specific isohyets • . • • The boundaries of the 3 phyto·
geographic regions are none other hut the limits of many specific plant communities in the country, for instance-
the cessation of the natural typical woodland-maquis of Quercus·Pictacia type is between 350-400 mm. The cessation of the typical dwarf shrub and grass steppe of Poterium Spinosum is between 300-350 mm., and the community of Artemision between 150·200 mm . . . . The isohyet of 350 mm. is a critical line of many living phenomena in the country, Le., the disappearance of the woodland-maquU and other Mediterranean societies, the disappearance of extensive agriculture, etc. The isohyet of 200 mm. is also a critical line of some other phenomena of life."
Yet, it should be noted that these ranges are general, and deviations which might be greater than these absolute values may arise as a result of different distri·
bution of precipitation within the season. and other local conditions.
The application of the results of this review to local conditions is also supported by catchment studies in Northern Israel (Goldscbmidt, 1961) in which water balance studiea for three catchments are given:
Y arkon river catchment: R, = 0.06 (P-360);
Jordan and Litani catchments R, = 0.90 (P-360);
Ein Ziv catchment: R, = 0.88 (1'-490);
where R, = total runoff in mm. fyr., P = average annual rainfall in mm.fyr.
These catchments are covered by a mixture of forest-woodland-maquis, grasses and cultivated areas, The regressions developed for these catchments all fall within the envelope of the two regressions presented in this review.
In addition, the work of the Small Watershed Research Team (Michaeli, Shachori and Rosenzweig, 1962;
Shachori, Michaeli and Segal, 1960a, 1960b) indicates water balance data for a few catchments in northern Israel to be as shown in Table 3.
The relations of total runoH· to annual rainfall, for the three catchments-although ouly two years of records are available for each catchment-indicate that the two catchments with the denser maquis vege·
tation (Bustani with 76 per cent and Fallah with 56 per cent) fall within the envelope of the two regressions presented. These values are marked 8 in Figure 1.
Abu-Gosh catchment, which is covered mostly by grasses and low shrubs and a large portion of bare area on 86 per cent of its area, while ouly 14 per cent of its
474
area is in planted young forest, indicates even a higher total runoff under dry conditions than indicated by the regression line for bare and grass areas. Ahu-Gosh values are marked S in Figure 1. These slightly higher values are probably due to concentration of rainfall in a short period and due to shallowness of soil mantle.
TABLE 3. Water balance data
... Rainfall Total nmofi' Ev�po-.
( ... /YT.) (DUJL/yr.)
trt::J.J;•:.)on
Upper Fallab catchment 705 294 411 (56 per cent forest maq•is) 579 241 338 Bustani catchment (76 per 515 182 333 cent forest muquis) 630 252 378 Abu-Go•h catchment (86 440 227 213 per cent grass and eulti- 241 59 182 vation. 14 per cent
planted forest)
Supporting evidence for the local use of the regression calculated from experiments conducted elsewhere is provided by three independent local sources:
1. The coincidence of the ecological boundaries of woody and herbaceous communities in Israel, with the isohyets corresponding to the annual transpira
tion of two vegetation types derived from the rcgres·
sion lines (Zohary, 1955).
2. The fact that relationship between runoff and rain
fall found in three local catchments of mixed woody acd herbaceous vegetation (Gold•chmidt, 1961) falls between the two regression lines obtained for the 20 experiments conducted abroad. In addition, two-year records of rainfall-runoff data for two catch·
ments of mixed woody and herbaceous vegetation (Shachori, Michaeli and Segal, 1960o, 1960b) also fall between the two regressions.
3. The values obtained from a predominantly herha·
ceous catchment in Israel (Michaeli, Shachori and Rosen7.weig, 1959) indicate similar-or even slightly higher-water yields to those predicted by the regres·
sion line obtained from similar foreign catchments.
Obviously, final conclusions cannot be drawn only from results obtained in this review. Although in this review, due mainly to insufficiency of pertinent data, four cover types were grouped into two populations, in a local study it is important to compare water yields from all four types of vegetation, namely: pine forest;
woodland, maquis; annual herbaceous and grassy vegetation: bare areas.
In conclusion, it must be emphasized that the results of such a review, in a detailed study which may follow it, will have to he tested against the economic conside
ration involved in afforestation, range re·seeding and
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A. Y. Shachori and A. Michaeli
W oter yi.u. of foro#, maquis
and grau cover•grazing. A comprehensive consideration of such prob
·
!ems will have to include economic investigations and other intangible economic benefits (like tourism or aesthetic values). It will have to consider not only the potential differences in water yields but the feaai·
bility of actually recovering such differences in the
various parts of the country. Resnlts obtained from a detailed local study will provide an important means hitherto lacking in the general planning and land use classification. It will also assist in arriving at an efficient mnltiple use programme for the mountain areas of the country.
R E S U M E
Rendemenls hydriques de surfaces couvertes
deforets, de maquis er de formations herbacees dans les regions semi·
arides d'apru les �tudes publi�es sur
cellsquestion
(A. Y. Schachori et A. Michaeli)En se fondant sur les donnees deja publiees, les auteurs evaluent les differences de rendement hydrique que I' on peut observer entre les surfaces couvcrte.e de forets, de bois ou de chaparral, d'une part, et les regions couvertes de graminees ou denu�es de toute ,.egetation, d'autre part. Leur but est de determiner si ces differences de rendement hydrique soot suffisantes pour justifier une etude similaire dans les regions semi-arides d'lsracl.
Ils decrivent bri�vemcnt 26 etudes importantes et en resument les resultats, en insistant plus particu·
litrement sur les travaux eonsacres a des regions semi·
arides, comme celles de l'oucst des ll:tats-Unis.
Les resultats de ces etudes semblent indiquer que Ies surfaces couvertes de forets, de bois ou de chaparral peuvent etre considerees comme un groupe, et les regions denudees ou couvertes de graminees comme un autre.
En evaluant quantitativement Ies rendements hydri
ques de ces deux types de vegetation differents en fonction des precipitation&
(j
usqu'a 1 400 mm), ilest possible de mettre en evidence deux •egressions lineaires :
a)
Pour les forets, les bois et le chaparral : R1 = 0,805 (P-398 mm) r
= 0,972b) Pour les formations herbacees et les surfaces nucs : R, = 0,920 (P-281 mm) r = 0,972 oil R1 = Rendement hydrique annuel en mm
P = Precipitations annuelles en mm
r
= Coefficient de correlation.La comparaison des deux regressions montre que, dans les conditions de pluviosite caracteristiques des regions scmi-arides, il existe une difference moyenne de 120 mm par an entre les rendements hydriques correspondant .. ces deux types de vegetation. Les di.tferenccs des rendements hydriques par rapport a la hauteur des precipitations soot cependant tr�s inegales;
mais on peut dire en resume que, pour une hauteur de precipitations de 500 a 800 mm, Ies differences moyennes des rendements hydriques pourraient etre de 60 mm au moins et que, pour une hauteur de precipitations superieure (970-1 400 mm), clles pourraient etre de 100 mm au moins par an. .
Les donnees publiees sur les rendements hydriques des bassins de reception du nord d'lsra�l soot presentees a l'appui de !'idee que les resnltats ci-dessus soot appli
cables a cette region et justifient une etude locale complete.
D I S C U S S I O N
H. J_ DE BoER. If two variables are supposed to be statisti·
cally related, then the closeness of this relationship is judged by computing the value of the conelation coefficient. If this value proves to be lat'ger than 0.95 then there are two possi
bilities. The first one is that the relationship is not a statis·
tic.al one hut a functional one. This means that one of the variables ooly depends on the other one. The second poasi ..
bllity is that bias ia introduced.
A. SIIACBORI. It seems that the relationship is functional A
regression relationship shows that the total runoff does depend on amount of rainfall. The correlation coefficient here is used as an index of that dependency. It may not be confined to the mo.st rigorous statistical requirements but nevertheleu it is used as auch in many other similar studies by other workers, and is accepted as the degree or scatter of the individual points from the regression lines..
H. J. DE BOER� It appears that the experiments mentioned are not quite independent of each
othet.
475
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Physiology of 'M plaru coverJPhysiologi.e de la coun11ure veget ale
A_ SHACBORI. This may be. true, and therefore a statistical ooiilparison between two regcession lines cannot be made, but
as shown graphically in Figure 2, a difference between pairs of measurements is mnde in order to obtain some quantitative meo.slll"e of the dillerence of the variom studies.
A.
VERNET.
Au voisinage des zones desertiques, le tapis vegCtal s'ouvre de plus en plus et l'Cvaporation augmente par rapport3
la transpiration. Or l'Cvapotranspiration est large·ment fonction de la nature du sol; eJle est en p&rticulier moin
dre sur sable que rur limon. ll est cu"tieux que ceci ne p-rovoqu.c pas une grande variabilite dans rctude statistique de mCmc que les autres ca.ractt?:ristiques du sot
A. SB.ACRORI. There are very few studies of much lower rain
fall th�tn 5-400 IJllll. /yr. The :study is a very general one, where an effort was made to put most of the pertinent available data on a "common denominatoru. IT there hod been more data available it might have shown that effect. In fact the Ahu
Go�h values in Figure 1 do show this effect to have some extent.
It seems, howeva, tbat the type and density of vegetal cover wou1d he rellected by the quantity of water avail able for e,rapotranspiration more than soil type.
G. F. MAluuNK:. Your figures concern whole years. Are you s,ure that �M is approxi..mately 0?
I suppose that the different level of your lines is for the greater part due to root growth, the higher line belonging to the vegetation with the shallow roots. What is your explanation?
A. SB.ACDOlll. (1) We ha-ve tried, in the various !fitudies, to differentiate what portion of A M goes into ETI and what portion to Rt. In semi-arid regions, most of A M in root zone after winter would c::ventu�ly be used up in summer in ET before the next winter. In eases where differentiation was not possible, the data was not used in quantitative analysh.
(2) Most probably, I think, this would be the main factor.
Also, we must keep n i mind the fact that herbaceous vegeta
tion in semi-arid regiow dries completely during a long dry su:m.rner.
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Discussion section
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