FILTRATION STUDIES IN THE I'KCOS RIVER WATERSHED, NEW MEXICO AND TEXAS

IIILLARD I,. SMITH AND LUKA B. LEOPOLDl

Sot1 Conservation Service, U . 8. Dppartment o j Agricul ture

Received for publication October 11, 1941

THE WATERSHED PIZODLEM

From its source in central New Mexico to its confluence with the Rio Grande, 525 miles to the south, the Pecos River watershed is usecl prcdoniinantly for grazing, and those lands adjoining the river valleys are intensively cultivated under irrigation. The grazing lands have been subjected to varying degrees of Overuse that has caused depletion of the vegetal cover.

The only long-time solution to widespread overuse of natural resources is con- servation through management. Watershed management, in broad terms, is a system of regulated use to ensure the yield of good-quality water and a t thc same time allow the maximum use of range and other agricultural resources. The condition of the land, the frequent damage by flood, and the rapid diminution of the capacity of storage reservoirs are indications of the fact that past use of soil and water resources has not been on a basis of sustained yield. Since grazing is the major use of lands in the Pecos basin, the main task in this area is to effect a system of range control that will allow rehabilitation of the depleted plant cover and thereafter maintain range lands in a condition of sustained productivity.

PURPOSE OF INFILTRATION STUDIES

As in all the major watersheds in southwestern United States, vegetation in the Pecos basin is more or less delicately adjusted to t]opography and climate. Graz- ing pressure has reduced forage production and in some places encouraged re- placement of forage species by unpalatable hcrhs and shrubs. Tlic change in vegetal cover has been partly the cause of, and certainly has been accompanied by, important changes in the rate of watcr intake of the soils. Organic matter, scarce enough in soils of semiarid areas under ordinary conditions, has been de- creased by the replacement of grasses by weeds and shrubs. Plant litter on the soil surface, structure of the A horizon, and dispersion of the clay are all affected by the changes in vegetation.

As part of a flood control study, infiltration experiments were conducted in an attempt to evaluate these changes. The data obtained were used to project the effect of a change of vegetal cover in terms of a change in the rate of infiltr a t' ion.

SOILS AND VEGETATION

Bounding the watershed on thc north and n ~ a r l y all along the western edge are In thc mountain

1 Associate soil scientist and assistant agricultural engineer, Soil Const,rv:ition Service, Albuquerque, New Mcxico. Reagm, who was chief of the party in charge of the infiltromcter studies, and of thc lab- oratory technicians, who analyzed the soil samples.

195

ranges of mountains, rising in places to 12,000 feet elevation.

The authors gratefully acknomlcdge tllc ass~stancc of C. A .

196 HILLARD I,. SMITH AND LUNA 33. LEOPOLD

areas thc soils arc stoiiy, shallow, dark in color, high in organic material, and show some profilc dcvclopmc~nt. These arms rceeive 20 or more inches of precipita- tion and arc covcrcd by conifcroiis f o r d s .

In the central portion of‘ the watershed ephemeral streams fiowing cast from the mountains have tlissectcd a formerly extensive alluvial apron of coarse clastics, leaving n rolling plain uiidei+~in by limestone and red beds covered in places with pcdimeilt gi~avcls, and with terrace clcposits near the strcam valleys. The soils of this cacntral portion arc characteristically shallow and calcareous, have textures varying from clay loani to clay, and show little profile develop- mcnt. The soils may be classified as brown or redtlish-brown soils. The Vegetation is preclomi- nantly short grass of turf-forming variety, iiiclucling blue grama (Boute lom gracilis), galleta (Ililaria jamesii) , arid bliicstcm (Agropyron smithii) . Improper use of t hese grasscd plains has cmwd suppression of the dcsiraLle palatable grasses arid has cricour:tgcd annual weeds, snakeweed, arid rabbit brush.

In the southern portion of tlic watershed where the aiinual precipitation is 12 to 16 inches tlic soils arc shallow :ind calcareous. Thc vegetation is desert grass- land and dcsmt shrub associatioils of which creosote bush (Couillea tridentata) and s:iltl)ush (Atr iptcx spp.) nrcl c~haracteristir. The soils of tlic area may be broadly classified as tlcscrt soils.

The average annual precipitation varies from 14 to 18 inches.

SELECTION O F 1’LOTS FOIL INFILTRATION STUDIES

Since tlic Pccos River \vat (>rshCd compriscs 37,000 square miles, large varia- tions in soil cliarnctc~risti(~s and vegetal ( ~ v w may bc cxpccted. The funds and the timc nvnilahlc for infiltration studiw allowcd only a liniitcd nuinbcr of tests; thereforc, I 2 diffcimt c.ombin:itions of soils and vcgetat,ion were selccted for study. Ihc l i mrnhinaticm of soil type and vegetal cover was dcsignatcd as a

tudy to nieasurc the cllecst of vcxgctal density pling cstrcmcs of density. Plots nvre sclcctcd on each o f d ( ~ i i ( x vcgc+ttioii and areas of spar,;c or no vogetation.

~ o i k i n t , cffwt ol :i program of rang(’ nianagcmcnt, is an increase in ity, this fnctor furnishcs an index to thc prospeclive value Gf a

It rmist not IJC :tt;sumcd that an incr(wx1 in vegetal dciisity ricccssarily is synonymous with nn i!ic.re:isc in foragci production. For rxamplc, in a blue grama ~ ~ a r i q ~ , ovri.grnziiig in:~y i~icrcasc t he grourid density because continual

nd cro1q)ing of‘ tlicl own m w r lorces n change from an original tylxl t o mort o f a :,oci-forining t y 1 ) ~ . J3i i t in this type of cover such rd t i r c tlic plniit vigor ( G ) , decreasc thc volumc growth of usable

for:ig(x, :tnd diminish th(. surfncc littcr, which alonc is a potent factor in keeping surfncc~ pores of thc1 soil opcn and tlhits incrcnsing infiltration. Again, ovcrusc of a tobosa grass range’ n1:Ly aicouragc iq11:~c~mcrit of tobosn by burro grass which, tliougti ~iaving a 1iiglic.r density tliari I lic totiosa, produces less iisahle forage and nct u:Llly ticcrcxics t h r . in[iItrntion rntc. On(. scxt of (y~crimc~nts in this study s l i o ~ (,ti th:it Four plots o f burro g r a ~ with (is p c ~ c m t avc~agc’ dcnsity had an

atiun .” It wa:, tlic. plan of tlii 011 ( W I I htr:.,tllln hy stratuni to includr a

INFILTRATION I N PECOS RIVER WATERSIIED 197

erage infiltration rate of 0.50 inch an hour, while nearby tobosa plots on the e soil had an infiltration rate of 2.20 inches an hour, in spite of having a

In the plots tested, bare or nearly bare areas were compared with areas in ich the vegetation was in relatively good condition. Except in certain vegetal es, in which the initial effect is an artificial incrcasc in density accompanicdiby er plant vigor and volume growth, or where grazing has caused replacement of

nch grasses by less desirable turf grasses, density is oiie indication of range

lightly lower clensity.

condition.

allowed. were chosen immediately outside the fence. plot was chosen more or less a t random.

In the selection of plots, fenced areas were chosen within which no grazing \vas For example, plots were selected in a fenced graveyard, and other plots

The actual location of an individual

LL a x

10 20 30 40 50 60

TIME IN MINUTES AFTER BEGINNING OF RAINFALL

PIC. I . TYPICAI, INFILTRATION RUN, P ~ c o s WATERSHED STUDY

Plot 4, rim 011 \wt soil April 10,1940; soil, slinllow, gravel sandy loam; vegetation, dewr 1 grassland, density 17 per cent; slope 9 per cent.

CONDUCT O F T H E FIELD INVESTIGATION

The data summarized in this paper were obtained by the use of an artificial rain applicator developed hy the Department of Agriculture. It employs the use of a 12- by 30-inch plot surrounded by thin galvanized metal plot borders, 6 inches high. The plot was covercd by a canvas tcnt, to keep the wind from blow- ing the raindrops. The machine, called thc “North Fork” type, produces rain- drops smaller than those of natural rain and gives a rate of infiltration somewhat greater than that under conditions of natural rainfall.

The intensity of water application was held constant during thc rim, and the average rate was determined by calibration runs before and after application of rainfall to the plot. For the calibration, the plot was covered by :L metal pan that collected all water applicd to the plot.

Runoff was collected in a can equipped with a hook gage for mc,asuring the volume of water, and the time of each reading was recorded with a stop watch.

198 HILLARD L. SMITH AND LUNA B. LEOPOLD

Readings were taken a t half-minute intervals while the rate of runoff was chang- ing, and a t intervals of 1 to 5 minutes after the rate had become more or less constant.

The wgctation on the plot was clipped to approximately 1 inch in height, but no natural litter was rcmoved from the plot surface.

After each test was coniplctcd the data were plotted in the form of a graph, a typical example of which is presented in figure 1. A smooth curve was drawn through the points showing the instantaneous rate of runoff a t a given time after the beginning of rainfall application. The apparent infiltration rate a t any time was computed by subtracting the instantaneous rate of runoff from the cor- responding rate of rainfall.

An initial “dry” ian was made on the soil under conditions of field moisture. Twenty-four hours later, water was again applied to determine the infiltration rate for the soil at field moisture capacity.

Soil moisture samples were taken from within the wetted area outside the plot borders before both the dry and the wet run. A second wet sample was taken inside the plot after completion of the wet run. Soil profile samples for analysis wcre taken alongside each plot.

No test was run less than 50 minutes.

LABORATORY PROCEDURE

Mechanical analyses were made by the pipette method as outlined by Olm- stead and others (9), except that the pretreatment was omitted. Instead of using sodium oxalatc alone as a dispersing reagent, a mixture of sodium silicate and sodium oxalate was used. The dispcrsion ratio (percentage of dispersed clay) was determined by the procedure outlined by Volk (ll),which is based on the 5 - p clay instead of the silt plus clay as determined by Middleton (8). The pH de- termination was made on a l : 2 soil-watcr suspension, by means of a glass elec- trode. Moisture equivalent was obtained by the method of Rriggs and McLane (2). Organic matter content was determined by a modification of the Schollen- bcrgcr rapid-titration rnc%hotl (lo), as outlined by Allison (1). Moisture samplcs wcrc collccted in 16-ounce tin cans and dried in the oven at 110 C . for 40-48 ho11rs.

I

BASIC DATA

Bot11 a dry run arid a wet run w e r ~ made on each of 264 plots. Soil analyses wcre made of samples taltcri from 126 of the plots, and the st,atisticaI analysis em- ployed thr data obtained. For ’he invchgation of the effect of vegetal density, coniposit,c curves wcre drawn using a11 the 264 plots.

In order t o have comparable data, so far as initial soil-moisture conditions were concerned, only the wct runs wcrc used for the comparisons that follow. The effect of initial soil moistiire is discussed sc.parately.

The filial ratc of infiltration of tlic 126 soil profiles analyzccl varied from 0.05 to 5.64 inches an hour. Thc mean was 1.03 inches, and the median vias 0.85 inch an hour.

TABJ,E: I

Relation betwecn final inji1trutio)L rate jor wet r u n unci ucge lu l d c n s i i g

comparnble soil profiles I_-__ ~

Descrl,-sIirrrb . . . . . . . . . .

G rasslnn d

Desert - s I i i ~ u ~ ~ . . . . . . .

Descrt-shrub (tobosn grass:

MEAN U E N b l l ' i

1s 1 \ 3 7 2

( 6 1 20 4 7 7 0

1 5 1 2 \ 20 5

Bnrc

Desert-slirub (burro grass)

Grassland (blue gr:Lmn) . . . . . .

Desert-shrub . . . . . . . . .

Desert-sliluL . . . . . . . . . . . . .

Desert-shrub , . . , , . . . . . . . . . . . .

Grasslnnd . . . . . . . . . . . . . . . . . .

Woodland . . . . . . . . . . . . . . . , . . . .

Woodland. . . . . . . . . . . . . . . . .

' 2.2 { I 19.2

, { I 13.7 4G.2

>IE.\N FLNhL 1NPIL-

L'LC4TION RATI:

i8z . iI ir .

0.81 1.06

0.75 0.74

0.27 1.96

0.27 0 I 95 1 .S7

0 . 7 l 0.87 4.54

4.07 1.2.1

0 .35 0.53 2.05

0.37 2.24 2 .4s

0.19 0.50

0.29 0.08

0.3s 0.75

0.53 0.96

0.24 0.64

1.11 2.39

0.85 1.10

0.44 1.23

.1111111 I: ,I X l l N b

11 12

3 7

3 G

3 11 4

5 5 2

L 4

'3 5 3

3 3 3

2 1

3 3

3 9

(i G

4 7

3 4

2 3

2 3

- ~~

AIJ,I. lllllill

c r , ?)i t

-~

8 1 0

1 25

f3:LI c'

5s

I h r e 20 58

10 25 SO

SO 30

9 2s 60

Ba1 e 70 72

l la1c 70

B:UC 75

I3 arc 10

1331 P

12

3 3!)

27 GO

12 25

19 40

I u c( i&

na1 c 13

I3:lI.P

15

U:ii e S

I3m c 8

25

2 13 65

33 1

1 12 37

Bale 53 40

I3:rle 62

1 3 n l e 45

Ba1 c 1

13m e 2

1 8

5 40

2 21

7 29

~- ~~

J Izh i - mum

iir /hi . -~

1.48 2.06

1.16 1 . 4 0

0.45 3.33

0.45 3.33 2.79

I . 00 1 . I 4 5.64

7.17 2.56

0.50 1.03 2.66

0.59 4.20 3.38

0 . 3 2 I . -12

0.40 1 .29

0 . 4 0 1.23

0 . 00 1.65

0.5G I .27

1.44 2.88

1.33 1 .83

0.51 1.G9 __

i m , ' h r .

0.12 0 . 3 6

0 .44 0.25

0.17 0.28

0.17 0.20 1.60

0.37 0.65 3 . 1 4

I .70 0 . G G

0.15 0.45 1.50

0.24 0.61 0.80

0.06 0 .14

0.09 0.29

0.05 0 . '$3

0.30 0.40

0.20 0.15

0.48 1.50

0.37 0.61

0.36 0.77 __

199

200 HILIARD 1,. smm AND LUNA ~ 3 . LEOPOLD

ItESULTS OF THE INVESTIGATION

EffLct of vegetal density

Tablo 1 shows that thc vcgctal density is one of the most important factors governing the infiltration rate. As the density of cover increascs, so also does the rate of intake. For esamplc, in dcsert-shrub type on n given soil, a series of plots having average densities of 0 (that is, barc plots), 18 and 37 pcr cent, had final infiltration values of 0.27, 0.95, and 1.87 inches an hour, rcqcctively. Withill each stratum thc same relation was truc in general.

An exarnplc of a composite curve of infiltration rate showing effect of vegetal density is prcscntrd in figurc 2.

The effcct of vcgrtal density in increasing infiltration rate may be attributed to a nuInbcr of factors, the relative importance of which has not been evaluated. Vegetal cover tends to break the impact of raindrops on the soil surface. The

TIME IN MINUTES A F T E R BEGINNING OF R A I N FALL

FIG. 2 . TYPICAL ConwosIm CURVES OF INFIL~I~RATJON IIATE SHOWING EWECT OF

VEGIWATIVIC L)I,:NSITY

18 runs on wet soil; mil, deep clny loam-light cln);; vegetation, desert shrub; vicinity of Itoswcll. New Rlcxico.

iniportance of sealing of surface pores by inwash of colloidal particles has been notcd by Lomderniilk (7), and by Dulcy and Kclly (4). Tllc vcgctal density probably affects tlie organic matter in thc soil, which has bcc~i shown by Free, Browning, and Musgravc (5) to bc correlat!cd with intiltration ratc.

That this inclmse of infiltration rate with vegetal density is more than inere chance is shown by tlie results of a corrclation analysis in which the data from 126 plots wmc ustd. Thc correlation coefficient was computcd to be 0.46, whereas the value of T would be 0.23 at thc 1 per cent level of significancc?

__ _ _ ~~

SI -~ -

~~

z 7 = - -

d [ N 2x2 - (ZX)21[N ZY2 - (Z1’)21 N = iiuiubei of plots, 1% in this b t u t l y X = iato of infiltinliori 1’ = otllrl vn l lnr l t , I c , 0 l ~ : L l l l C 1natlr1

T y p e o j vcp4ataon The mean rates of infiltration for wet runs on the 24 grassl:md, 39 woodland,

and 63 desert-shrub plots wcre 1.55, 1.24, :mcl 0.71 inchcs an hour. The dif- ference in thc average final ratc of infiltration of tlic g m s ~ l ~ i i c l and woodlard plots, 0.31 inch, is not significant. The diffcrcncc of 0.84 inch betwcen the grass- land and shrub is highly significant, as is that of‘ 0.52 iiwh I.xt\\eeii the woodland alld the shrub.

Organic maltcr

In tlic profiles studiccl, thc avciagc orgarlic, iixitter content 713s about, 2.5 pcr cent. Though thc infiltration rat(. tcncfctl t o inrreaie v 7 i t h increase of organic matter, coml)nt:ition of t l ~ simple correlation cocffiritmt showed no statistical significance. As notcd earlier, otlicr workers liar e s l i o ~ r i significant correlation between thcse factors. There arc logical rcasoiis M liy t l rcw should be relatcd. For example, i l i i n gcncrally scccptecl fact t h t organic mat aggregation of soil particles, which in tuin is closi‘ly allied with water inovcment phenomena In this study, organic iimttw of the surface :-inch was found to be correlated uitl-i vcgctal dcniity (T = 0.26), arid this was corrclatcd with the dispersion ratio of tlic first :-inch. Thcrc was, in turn, corrclalion hetwccn organic InsttPr and dispersion ratio atJ both tlic first] (0- - :-inch) depth ( r =

Thc ctispersion ratio was significantly cori.c,latcd with the infiltration ratc.

These intcrrc~liztionships hctwccii soil strwtuic, orgstnic matter, vegetation, dispersion, and water movcmcnt arc complex plicnomena not fully expressed by the computation of siinplc liiicar corrrlntion.

The grassland plot\ had a mcmi organic matter coiilcnt of 3.69 per cent; the woodland, 2.98; and the desert-shrub, 1.77. The difference of approximately 0.7 per cent betneeri th r grassland the the woodland plots is significant. The difference of npproximatcly 1.9 pcr ccnt brtmccn tlw dcscrt-shrub and the grass- land plots is highly significant, as is that of‘ 1.2 per ccnt bctwvccn thc woodland and the desert-shrub plots.

oiid (i- -15-inch) dcpth (r = -0.27).

SoaE tcxctu~c

The inean final infiltration ratcs of tlic soil profilcs studied vary nith soil tes- There is a statistical significance only in the clif?’crences

The texlure ture, as shown in tahlc 2 . between clay and clay loam, and between clay and 5andy loam. inff uences infilti ation probably through t he pheiiornc~~on of colloid swelling.

Dispcmon The infiltration rate is significantly corrclated xvith the amount of dispersed

clay in the soil in both the first and sccoiid depths (7 = -0.30 and -0.35 re- spectively). It is also correlated with the ratio of dispersed clay to total clay content in both dcpths. In this study it was found that the dispersion ratio varicd inversely as the organic matter.

CORRELATION O F VARIOUS FACTORS AT TWO DEPTIIS I

~- ~ ____ - ~

11 XTUKL

- -~

Sandy loam . .

Clay loam Clay

Sandy clay loam

~ -~ _- _ _ _

_ _ _ _ ~ _ - ___.

N O I I I I ~ K or runs l N r ~ ~ ~ $ ~ ~ & f T E

- -~ ___ i n / h r .

13 1 42 20 1.22 36 1.07 57 0 86

______ Inliltration

Depth zone ~ FACTOR

Vcgetal density

Depth zone ______

* Significant at thc 5 per ccnt point. '*Significant at thc I per cent poilit.

Vegctal density. . . . . . . . . Organic matter . . . . . . . .

Dispersed clay . . . . . . . . . Dispersion ratio. . . . . . . . Clay ( 5 - p ) . . . . . . . . . . . . . . Clay (2-p). . . . . . . . . . . Silt+clay. . . . . . . . . . . . . Moisturc equivalcnt . . . .

Slope . . . . . . . . . . . . . . . . . .

._____..__

Dispersion rat],

0-1 inch __-

0.46" 0.07 0.14

-0.30" -0.31" -0.06 -0 01 -0.15 -0.15

Depth zone

0-1 inch

-0.10 -0.53

___ t-1s

inche! __

-0.2

Moisture equivalent

0.08

-0.35"'K -0.31"" -0.19" -0.15 - 0.30"" -0.12

Depth zone

0.26 0.04

-0.10

0-P inch

*-15 inches

0.65

0.22

INFILTR BTION I N PECOS IiIVElZ W 4TERSHED 203

EVALUATION O F FLOOD CONTROL hlEASURES I N TERMS OF INFILTRATION

has been shown (3) to have an important effect on vegeta- vegetation to respond to grazing rcduction depends on soil

acteristics, precipitation, erosion conditions, and other interrelated factors. the recent flood control survey of the Department of Agri-

ture, estimates of potential forage production were made during the field milistioil. From the estimates of incrcased vcgetal density expectcd as a ult of controlled grazing, estimates of consequent increased infiltration rates

he effect of a change in infiltration rates as a result of in- creased density, a synthetic hydrograph was constructed to approximate as closely as possible the flood hydrograph from a tributary to Pecos River, thc Macho Draw, from which came a significant portion of the damaging flood of June, 1937. It was estimated that the infiltration rate at the time of the flood averaged 0.30 inch an hour. Under proper management, the increased vegetal density probably would increase the infiltration rate to 0.50 inch. This increase would, for the conditions assumed, decrease the flood peak 20 to 30 per cent. Similar coniputation of flood peak reduction for other tributaries showed com-

that estimates of possible increase of vegetal dcnsity for a error, but under the assumed conditions, this study has

crease of infiltration rate, with other conditions remaining The

computed from the infiltration data presented in this paper.

ant, may decrease the peak of flash floods by as much as 25 pcr cent. nificancc of such a decrease in flood peak is self-evident.

SUMMARY

Artificial rainfall was applied to 264 plots, 12- by 30-inches, representing vari- 1 types in the Pecos watershed. In general the soils were heavy in tcxture, slightly alkaline in reaction (pH 7.6-8.5),

d developed under an annual rainfall of 12-18 inches. The predominant

Soil samples from 126 plots were analyzed in the laboratory, and the data The investigation showed a highly significant

itivc linear correlation between the final infiltration rate and vegetal density. tion showed a highly significant negative correlation with

The 5-p clay nificant negative correlation with the final rate of infiltration.

eta1 types were desert shrub, grassland, and piiion-juniper woodland.

analyzed by statistical methods.

n t of dispersed clay, and silt plus clay.

REFEREKCES

(1) ALLISON, I,. E. 1935 Organic soil c:lrbon by reductiorl of chromic acid. Soil S c i .

(2) BRIGGS, 1;. ,J., AND MCLANC, J. W. 1911 nloisturc cquivalcnt determinations and their application.

(3) COOPIALRIDER, C. IC., AND HENDRICICS, B. A. 1937 Soil erosion and strcnm flow on range and forest lands of the Upper Rio Grande watershed in rclation to land re- souices and human welfnrc. U. s. Dcpt. h g r . Tcch. Bul. 567.

40: 311-320.

i l m e r . SOC. Bgron. Proc. 2: 135-147.

204 HILLARD L. SRlITIl AND LUNA B. LEOPOLD

(4) DULCY, F. L., AND KELLY, L. L. 1939 Thc cffcct of soil type, slope, and surface conditions on iiit:tkc of water.

(5) FREE,, G. R., BROWNING, G. M , AND MUSGRAVE, G . W. 1940 Relative infiltration and related physical characteristics of certain soils. 1J. S. Dept. Agr. Tech. Bul. 729.

(6) L k N T O W , J. L., AND FLORY, E. R. 1940 Fluctuating forage production. Soil Conserv. 6: 137-144.

( 7 ) Lo\vDERnlILIi , W. C. 1930 Influence of forest litter on runoff, percolation, and erosion. Jour. Forestry 25: 474-491.

(8) ILIIDDLETON, 11. E. 1930 Properties of soils nhich influence soil eiosioii. U. S. Dcpt. Agr. Tech. 13111. 178.

(9) OLMSTTXD, L. B., ALEXANDER, L. T., AND MIDDLETON, 11. E. 1930 A pipette method of mcchanical mnlysis of soils bascd on improvctl dispersion piocediirc. U. S. Dcpt. h g r . Tech. 1 3 ~ 1 . 170.

(10) SHOLLENRXRGCB, C. J. 1931 l>c+mnination of soil organic matin ' Soil 8 c i

(11) VOLT<, G. M. The method of determination of degree of dispersion of the clay fraction of soils as used in investigation of abnormal characteristics of soils in Rcgion 8 of the Soil Conservation Scrvice. Soil Sci. SOC. ATW. Proc . 2: 561-565.

Ncbr. Agr. ESP. Sta. Res. Bul. 112.

31 : 483--486. 1037