Comparison of soil and infiltration properties of range and afforested sites in northern Morocco

Forest Ecology and Management, 3 (1980/1981) 295--306 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

295

COMPARISON OF SOIL AND INFILTRATION PROPERTIES OF RANGE AND AFFORESTED SITES IN NORTHERN MOROCCO

ERWIN R. BERGLUND', ABDELAZIZ AHYOUD* and M'HAMMED TAYAA** *Direction des Eaux et For~ts et Conservation des Sols, Tetouan (Morocco) **Institute o f Agronomy and Veterinary Medicine Hassan H, Rabat (Morocco)

'Present address: College of Forestry, University of Minnesota, St. Paul, MN 55108 (U.S.A.) (Accepted 13 October 1980)

ABSTRACT Berglund, E.R., Ahyoud, A. and Tayaa, M., 1981. Comparison of soil and infiltration properties of range and afforested sites in northern Morocco. Forest Ecol. Manage., 3 : 295--306. The double-ring infiltrometer proved to be a simple, rugged, and effective tool for comparing extreme hydrologic site conditions in northern Morocco. Soil characteristics and infiltration rates were determined for heavily (H) and moderately (M) grazed and afforested (F) sites. Soil physical properties were similar except site F had significantly more clay and organic matter and a lower bulk density than the other sites. Mean infiltration rates were well-fitted by Horton's model. Superior site F infiltration was attributed to the site's higher organic matter content and lower bulk density. Regression analyses revealed positive relationships between infiltration and clay and organic matter contents that were attributable to vegetative influences. A negative relationship existed between infiltration and soil bulk density. The regression of final to initial infiltration rates was positive with the coefficient of determination decreasing with improved site conditions. Afforestation can be an effective watershed rehabilitation technique in Morocco. The results of this study indicate that forest vegetation and absence of grazing create favorable surface conditions which enhance infiltration.

INTRODUCTION Morocco's natural resources are an important component of the country's economy. Forests (8.6%), alfa grass (Stipa sp.) zones (5.3%), rangeland and unproductive areas (73.2%), and much of the remaining agricultural areas are used for producing 26 million animals of which over 75% are sheep and goats (Challot, 1960). Booming populations, urban areas, and demands for increased agricultural production have stimulated the construction of irrigation and potable water reservoirs throughout the country. However, the effectiveness of existing and future reservoirs may be severely reduced because of sedimentat i o n p r o b l e m s . M. L a h l o u ( 1 9 7 9 ) r e v e a l e d a s u s p e n d e d s e d i m e n t d i s c h a r g e in

0378-1127/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

296 the Sidi Salah sub-basin of the Tleta watershed {between Tangiers and Tetouan) to be 2000 m3/km 2 per year. The Oued Nekor River near E1 Hoceima discharges an estimated 4958 m3/km 2 per year and threatens a reservoir now under construction to a life of 10 years or less {Anonymous, undated). In general, A. Lahlou (1977) estimated sediment deposits in 16 reservoirs to range from 200--3300 m3/km 2 per year. These sedimentation rates result, in part, from Morocco's intensive range practices. Intensive grazing coupled with collective land ownership often results in marginal or degraded range conditions. The soil may reflect these changes by altered infiltration/surface r u n o f f rates and basic physical soil properties {Hanson et al., 1970; Branson et al., 1972; Gifford, 1978). Increased surface r u n o f f on degraded sites provides increased opportunities for surface and streambank erosion, channel scouring, and elevated sediment yields. Afforested sites, on the other hand, have been shown to protect the soil surface, ameliorate soil properties, and enhance infiltration (Harcharik and Kunkle, 1978). In 1978 a dam was completed on the Tleta watershed located on the western slopes of the precipitous Rif mountains. The 833 mm annual rainfall in this watershed causes severe surface erosion and the Government of Morocco is planning to implement mechanical and vegetative erosion control measures to reduce sediment yields to the reservoir. To date, numerous rehabilitation measures {contour trenching, check dams, afforestation, etc.) have been implemented and recommended in Morocco but verification of effectiveness is limited. The objective of this study (Ahyoud, 1978) was to compare the hydrologic effectiveness of an afforested site with two rangeland areas in the Tleta watershed by evaluating the infiltration rates and basic soil properties. STUDY AREA The Tleta watershed is on the western portion of the Rif Mountains in northern Morocco. The 18 200 ha basin is located equidistant between the cities of Tangiers and Tetouan at 5 ° 39'W longitude and 35 ° 35'N latitude (Fig. 1). The west-northwest exposure of the basin provides a Mediterraneantype climate highly influenced by the Atlantic Ocean. A mean annual rainfall of 833 m m occurs principally from November to March with a maximun~ m o n t h l y rainfall occurring in December. Mean m o n t h l y temperatures range from l l ° C in January to 25°C in August. Elevations in the basin range from 24 m to 680 m at the top of Jbel Haya with a mean elevation of 171 m (Tayaa, 1977). Geological formations are representative of the western Rif Mountains with over 95% of the basin underlain by a marl formation from Cretaceous and Tertiary periods. The ridges forming the eastern and southern boundaries of the watershed are sandy marls, non-calcareous sandstones, and quartzite deposits from the Oligocene epoch (Anonymous, 1977). Agriculture, rangelands, and afforestation occupy 48, 42, and 2% of the basin, respectively.

297 5°39'W Gibraltar Atlantic Ocean b CeoaUt Scale 30krn

\ J~La nache

\\\

••vv ~

\..

TLETA Watershed

Fig. 1. Location of the Tleta watershed in northern Morocco and the three study sites H, M, and F. Three study units were selected within the non-agricultural areas based upon preliminary soils information and field observations (Fig. 1). Site H, at approximately 110 m elevation, is a heavily used area under continual collective grazing and occupies 3.5% of the basin. Woody vegetative crown cover averages 12.5% of the site and is dominated by suppressed (5--25 cm) d o u m palm (Chamaerops humilis L.), Lavandula stoechas L., Cistus crispus L., Calycotome villosa (Poiret) Link., and Phyllyrea augustifolia ssp. media (L.) Rouy. Watteuw (1978) reported t h a t the soils are hydromorphic with iron accumulations and t h a t they have a low porosity, massive structure and minimal quantity of roots. Textural analyses indicated the soils are clay loams (Table I). Site M, at approximately 125 m elevation, is a moderately' degraded brushland area subjected to continual grazing and is characteristic of approximately 25% of the basin. Woody vegetative crowns cover 41.3% of the site and range up to 2 m in height. Dominant w o o d y vegetation includes Olea europaea L., Calycotome villosa (Poiret) Link., Pistacia lentiscus L., and some scattered d o u m palm. Herbaceous vegetation is dominated by Anthozanthum odoraturn L., Atractylis sp., and Asphodelus microcarpus Salsmand Viv.

298 TABLE I Results of soil textural analyses, organic matter content, and bulk density for the 1--7 cm soil depth, vegetative crown cover and slope for the three sites in the Tleta basin where n = sample size of infiltration points, ~ = mean value and CV = coefficient of variation (%). All three soils fall within the clay loam class H, doum palm (n = 50) Site characteristic Soil textural class (%) Clay (< 2 urn) Silt, fine (2--20 ~m) Silt, coarse (20--50 um) a Sand, fine (50--200 um) Sand, coarse (200--2000 ~m) Total sand (20--2000 urn) Soil organic matter content (%) Soil bulk density (g/cm3) b Vegetative crown cover (%) Slope (%)

20.9 24.8 12.2 26.3 15.8 54.3 1.47 1.44 12.5 0--10

M, brushland (n = 61)

F, afforestation (n = 43)

CV

~

CV

~

CV

24.4 33.5 32.6 31.6 36.1

28.7 30.4 12.3 12.4 16.2 40.3 1.77 1.42 41.3 5--25

23.7 17.4 35.0 45.2 37.0

39.0 23.2 11.0 14.4 12.4 37.8 2.70 1.22 99 5--40

20.8 22.4 45.5 39.6 43.5

33.3 6.9 42.4

37.9 8.5

26.3 13.1

aThe French soil classification system identifies coarse silt (20--50 ~m) which is otherwise included as fine sand (20--200 ~m) in the international system. bSample size for bulk density measurements was 2n. Site F, at a p p r o x i m a t e l y 2 5 0 m e l e v a t i o n , is an a f f o r e s t e d site w h e r e grazing is e x c l u d e d . A l e p p o p i n e (Pinus halepensis L.) was p l a n t e d in 1 9 6 2 a n d occupies 1% o f t h e basin. R e m n a n t s o f d o u m p a l m t o p p e d soil p e d e s t a l s indicate severe e r o s i o n p r o b l e m s p r i o r t o 1 9 6 2 t h a t m a y have s t i m u l a t e d t h e a f f o r e s t a t i o n e r o s i o n c o n t r o l e f f o r t . W o o d y v e g e t a t i v e c r o w n c o v e r is d o m i n a t e d b y t h e 5 - - 8 m A l e p p o p i n e t h a t t o t a l l y o c c u p i e s t h e site. O t h e r p r e v a l e n t species o f t e n s u p p r e s s e d ~ y t h e p i n e include Calycotome villosa (Poiret) Link., Olea europaea L., Pistacia lentiscus L., Phyllyrea augustifolia ssp. media (L.) R o u y , a n d Cytisus arboreus (Desf.) DC. N o n - w o o d y v e g e t a t i o n include Festuca caerulescens Desf., Hyparrhenia hirta (L.) S t a p f . , Anagallis arvensis L., Echium pycnanthum P o m e l , Pallenis spinosa (L.) Cass., Scabiosa sp., Pulicaria sp., and Cynara sp. T h e soil, c o v e r e d b y 2 - - 3 c m o f litter, is well s t r u c t u r e d w i t h a b u n d a n t m a c r o p o r e s f r o m r o o t c h a n n e l s a n d f a u n a l a c t i v i t y . T h e soil t e x t u r e at site F is finer t h a n at sites H a n d M w i t h m e a n clay c o n t e n t s o f 39.0, 20.9 a n d 28.7%, r e s p e c t i v e l y . T h e organic m a t t e r c o n t e n t averages 2.70% a n d b u l k d e n s i t y averages 1.22 g / c m 3. METHODS D o u b l e - r i n g i n f i l t r o m e t e r s in c o n j u n c t i o n w i t h soil a n d v e g e t a t i o n characteristics w e r e used t o o b t a i n m e a s u r e m e n t s at e a c h site. T h e s e i n s t r u m e n t s w e r e c h o s e n b e c a u s e t h e y are e f f e c t i v e f o r p r o v i d i n g relative ( c o m p a r a t i v e ) in-

299 filtration rates (Branson et al., 1972; Orr, 1975; Brechtel, 1976), easy to construct, inexpensive, quite insensitive to mechanical damage while being transported, and uncomplicated for difficult field conditions. On the other hand, the double-ring infiltrometer overestimates actual infiltration rates because of ponded water and the absence of raindrop impact (Hewlett and Nutter, 1969}. Sampling points were r a n d o m l y located for sites H and M in areas accessible to Land Rovers. The afforested site, F, was systematically sampled at approximately 20 m intervals along established trails where 5--10 m long perpendicular accesses were cut into the vegetation to locate each sample point. In total, infiltration was measured at 50, 61, and 43 sample points for sites H, M, and F, respectively. Relative infiltration measurements were obtained with 25 cm high doublering infiltrometers having inside and outside ring diameters of 18 and 36 cm, respectively {Brechtel, 1976). Four perforated steel supports 20 cm long stabilized the concentric rings and assured equal water levels in the outer ring compartments. The supports extended from the top of the rings to 5 cm from the b o t t o m to facilitate both horizontal placement and an average 5 cm depth for each sample. An approximately 17 cm head of water was maintained in the inner ring to accomodate measurements at high infiltration rates. A stopwatch determined the time necessary for the incremental additions of water to infiltrate while the outer ring water level was kept equal to t h a t of the inner ring by repeated additions of water. Readings continued untill the infiltration rate became constant, whereas other researchers have used fixed sampling durations of 60 min (Rauzi and Hanson, 1966), 70 min (Linnartz et al., 1966), 120 min (Rhoades et al., 1964), and 140 min (Woodward, 1943). The mean sampling durations and standard deviations (in parentheses) for sites H, M, and F were 98 (22), 94 (30), and 109 (21) min, respectively. Two soil cores were taken at each side of the infiltrometer in the mineral soft. The sample cylinder was 6.0 cm high with a 5.4 cm inside diameter. A smaller 1.0 cm high ring was placed on top of the sample cylinder so that the surface 0.1--1.0 cm of soil could be sliced away to reduce surface roughness and organic matter irregularities. During the dry fall and wet mid-winter periods as m a n y as 20 sample attempts were necessary to obtain two satisfactory cores as cautioned by Blake (1965). These cores were placed in sealed containers and provided the basis for the laboratory analyses. Percent canopy cover for a 10 m radius zone around the sample point was visually estimated. Bulk density was determined for each soil sample. The two soil samples per site were then mixed and passed through a 2 mm sieve.' One soil subsample was used for a particle size analysis employing the Robinson pipette technique (Day, 1965) while another subsample was analyzed for percent organic carbon by the Walkley and Black (1934) method. Varied infiltration rates at each sampling point made it impossible to obtain field measurements at precise time intervals. To facilitate s.ummarization of the data, infiltration curves were drawn for each sample point and values

300

were interpolated and extrapolated at fixed periods of 0, 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 25, 30, 45, 60, 75, 90, 105, and 120 min. A mean curve of infiltration vs. time was thus derived for sites H, M, and F. Infiltration means, standard deviations, and skew coefficients were calculated for each time period per site. The skew coefficients for sites H, M, and F ranged from 1.16 to 2.59, 1.65 to 2.98 and 0.90 to 1.67, respectively, and indicated positively skewed distributions. Natural logarithm transformations tended to normalize the data and resulted in respective skew coefficient ranges of--0.13 to 0.19, --0.13 to 0.00, and 0.99 to 0.25. This procedure facilitated statistical comparison between the sites at 5 and 1% probability levels. Infiltration rates approaching a minimum value at 2 h reflect the characteristic infiltration capacity or hydraulic conductivity at field saturation (Slack and Larson, 1979). The characteristics of the surface soil are critical for influencing infiltration rates (Satterlund, 1972). Therefore, initial and final infiltration rates for each sampling point were correlated with respective site characteristics of soil texture, soil organic matter, and soil bulk density. RESULTS AND DISCUSSION

Soil comparison Basic soil comparisons between the heavily grazed doum palm (H), moderately grazed brushland (M), and afforested (F) sites are shown in Table II. The three sites fell within the general clay loam soil class but all three sites were different in clay content, with site F having the highest percentage (39.0%) and site H the least (20.9%). Site M had a greater percentage of fine silt TABLE II Statistical comparison of site characteristics for the heavily grazed (H), moderately grazed (M) and afforested (F) sites. Sites connected by lines within a given site characteristic are not significantly different at the 1% level of probability Site characteristic

Statistical comparison of site characteristics Greater than

Soil textural class (%) Clay (< 2 urn) Silt, fine (2--20 urn) Silt, coarse (20--50 ~m) a Sand, fine (50--200 ~m) Sand, coarse (200--2000 ~m) Total sand (20--2000 urn) Soil organic matter content (%) Soil bulk density (g/cm 3)

F M M H H H F H

Less than M H H F M M M M

H F F M F F H F

aThe coarse silt content between the three sites was not significantly different at the 5% level of probability.

301

(30.4%), while site H had a significantly greater percentage {54.4%) of sand size particles ( 2 0 - - 2 0 0 0 pm). Thus, site H, having coarser soils, may be expected to have a greater infiltration rate. Further analyses revealed site F soils as having a significantly greater quantity of organic matter (2.70%) and a significantly lower soil bulk density (1.22 g/cm3). This difference in organic matter content results from the undisturbed forest cover in contrast to the grazed collective lands where forage is constantly being removed (Satterlund, 1972; Harcharik and Kunkle, 1978). Lower soil bulk densities are enhanced by organic matter, root channels, and soil fauna activities. In general, site H had coarser textured soil with low organic matter content (1.47%) and high soil bulk density {1.44 g/cm3). Site M had medium textured soils with statistically equal organic matter content (1.77%) and soil bulk density (1.42 g/cm 3) as site H. Site F had fine textured soil with a high organic matter content {2.70%) and low soil bulk density (1.22 g/cm3).

Infiltration comparisons Curves of mean infiltration rates for sites H, M, and F are illustrated in Fig. 2. Application of the Horton (1940) empirical model + (fo -- fc)e - K t where ft = infiltration rate at t h {mm/h), fc = infiltration rate at t = 2 h, }Co= infiltration rate at t = 0 h, and K = constant derived at t = 0.25 h, resulted in ft = fc

1500Site

120C

Horton

model

= 226 +1213e-6.24t

F

ft

M

ft =

65 + 129e -5'28 t

H

ft"

43 +136e -5"49t

900 E E

c o ,T, 600 o

°°~

•

~T_ 300 site F" M

0

0.5

1.0 1"5 20 Time (h) Fig. 2. M e a n infiltrationcurves from double-ring infiltrometers for the heavily grazed (H), moderately grazed (M), and afforested (F) sites.

302

r values of 0.987, 0.989, and 0.971 for sites H, M, and F, respectively. This model has been shown to give excellent fit to infiltration data by numerous researchers (Gifford, 1976; Collis-George, 1977). The initial infiltration rate (fo) for the afforested site (F) was significantly greater (1% level) than that of sites H and M (Table III) by 8.0 and 7.4 times, respectively. Similarly, the final infiltration rate (fc) at 2 h -- or, the hydraulic conductivity at field saturation -- was significantly greater (1% level) for the afforested site (225.9 mm/h) in comparison to the heavily (43.1 mm/h) and moderately (65.0 mm/h) used sites. Sites H and M were statistically equivalent at all times. This major difference in infiltration rates illustrates that forest vegetation and absence of grazing create favorable surface soil conditions which enhance infiltration (Anderson et al., 1976). The significantly greater quantity of organic matter (2.70%) in the forest soil promoted a distinct soil structure throughout the upper 20 cm of soil in contrast to the heavily (1.47%) and moderately (1.77%) used collective grazing land. Furthermore, soil aeration and resistance to degradation are intensified by the development of clay--humus complexes (Duchaufour, 1970). The significantly lower soil bulk density (1.22 g/cm 3) in the forestland reflects not only the higher organic matter content but also the abundant root and animal channels in comparison to the heavily (1.44 g/cm 3) and moderately 1.42 g/cm 3) used sites. Equivalent infiltration rates for sites H and M indicate similar hydrological conditions despite apparent vegetation differences. TABLE III Initial (f0) and final (fc) infiltration rates resulting from double-ring infiltrometer readings on heavily grazed (H), moderately grazed (M), and afforested (F) sites where ~ = mean value, CV = coefficient of variation (%) for the untransformed data, and n = sample size H(n=50)

fo (mm/h) fc (mm/h)

179.3 43.1

CV 115 114

M(n=61)

F(n=43)

~ 194.0 65.0

.~ 438.9 225.9

CV 125 127

CV 87 73

Infiltration--soil comparisons Initial (f0) and final (fc) infiltration rates were compared by regression analyses and they were compared to the measured soil parameters (Table IV). The low correlation coefficients (r) reflect complex and highly variable interrelationships in comparing simple infiltrometer measurements to basic soil properties. However, four important relationships are evident. First, the combination of the three sites revealed positive relationships between f0 and clay content (%) and fc and clay content (%) where each explained approximately 21% of the variation. This is contrary to results of Baver (1933) and Harrold et al. (1976), who indicate that infiltration should decrease with increases in

303

TABLE IV C o r r e l a t i o n c o e f f i c i e n t ( r ) f o r l i n e a r r e g r e s s i o n a n a l y s e s c o m p a r i n g i n i t i a l (f0) a n d f i n a l (fc) infiltration rates per site and for the three sites combined to soil properties and the c o m p a r i s o n o f fc t o f0 a

f0 Clay Fine silt Coarse silt Fine s a n d Organic m a t t e r Bulk density Initial soil moisture fo

a,

&

H

M

F

* * * * * * *

* * * *

* * * * * * *

0.412 --0.392 *

H+M+F 0.464 * * * 0.425 --0.534 *

H

M

F

* --0.373 * * * * * 0.858

* * * *

* * * * * --0.367 * 0.687

0.417 --0.436 * 0.817

H+M+F 0.457 * * * 0.468 --0.592 * 0.789

indicates values of r 2 < 0.100.

clay content. They key to this reversal is probably the present ecological condition of the three sites. According to Rauzi and Zing (1956), the texture of surface soils can only be effectively correlated with infiltration if the vegetable cover is in the same condition. The positive correlations with clay content u n d o u b t e d l y reflect the ameliorated site conditions under the afforested site (F) which also had a significantly higher clay content. Second, site M and the combined sites indicated positive correlations with organic matter that explained 17--22% of the variation in f0 and fc- This reflects the important role of organic matter in developing humus--clay complexes for enhancing soil structure and macroporosity (Duchaufor, 1970). Heavy grazing can reduce organic matter and, thus, infiltration (Osborn, 1952; Sant, 1966). Third, f0 and fc had negative correlations with soil bulk density on sites M and the combined sites with fc having the same relationship on site F. The correlations explained 14--35% of the variation. The marked increase in the r value for the combined sites (--0.592) reflects both the broader range of data for sites H, M, and F and also the greater uniformity in the bulk densities as shown b y the markedly lower coefficients of variation (Table I) -- especially in sites H and M. Many studies, such as Rauzi and Hanson (1966), Thompson (1968), and Tromble et al. {1974), have shown h o w intensive land use (grazing) increases soil bulk density, reduces soil pore space, and reduces infiltration. The relatively high soil bulk densities and associated low infiltration rates on sites H and M support field observations of rapid surface puddling and overland flow during storms. Fourth, a distinct positive correlation exists between f0 and fc. It is greatest for site H (0.858) and least for site F (0.687). Two aspects are.evident. First, fc values will be greater for those sites having higher initial infiltration rates.

304

This is a simple b u t important fact in conducting watershed reconnaissance surveys. Second, correlations decreased from site H (heavily grazed) to site F (afforested). This corresponds to improved site conditions from site H to F. In sparsely vegetated areas, site H, infiltration is more closely related to physical soil characteristics (Gifford, 1978) such that a high f0 would imply a high fc. Site F infiltration rates are c o n f o u n d e d between physical soil properties and vegetation so that the f0 and fc comparison is not as well defined. CONCLUSIONS

The double-ring infiltrometer appears to be a rugged yet effective tool for comparing hydrologic conditions of different sites. Infiltration rates for the afforested (F) site were significantly greater (1% level) than the heavily (H) and moderately (M) grazed sites. The ability of undisturbed forests to p r o m o t e favorable hydrologic conditions cannot be disputed. The softs of site F had significantly more clay b u t also significantly higher percent organic matter and low soil bulk density. This afforested site of 15-year-old Aleppo pine on a formerly degraded area provided an excellent multi-storied cover (100%), a 2--5 cm litter layer, and a porous, well-structured soil. In comparison to sites H and M, the afforestation rehabilitation effort appears to have been successful in improving infiltration conditions of the site. The relative state of vegetative cover on the grazed sites can be a misleading indicator of infiltration rates. Site M had more w o o d y cover than site H (41.3 and 12.5%, respectively), b u t the soil bulk densities and organic matter contents were statistically equivalent. Gifford (1978) similarly observed that when cover is sparse the physical soil properties are critical in modelling infiltration. For sites H and M, the continuous collective grazing apparently reduced infiltration conditions to the same level due to surface disturbances irrespective of the degree of vegetative cover. In general, watershed recovery, in terms of improved soil physical characteristics and infiltration characteristics, appears to be feasible in the Tleta basin in less than 15 years after establishment of dense vegetative cover and management of grazing. ACKNOWLEDGEMENTS

This study was supported by the University of Minnesota Project funded b y the U.S. Agency for International Development (contract No. AID/NE C-1279) and l'Institut Agronomique et V~t~rinaire Hassan II, Rabat, Morocco.

REFERENCES

Ahyoud, A., 1978. E t u d e de l'infiltration relative dans le bassin versant du Tleta. L ' I n s t i t u t A g r o n o r n i q u e et V~t~rinaire Hassan II, Rabat, Morocco. M~rnoire de troisi~rne cycle, 168 pp.

305 Anderson, H.W., Hoover, M.D. and Reinhart, K.G., 1976. Forests and water: effects of forest management on floods, sedimentation, and water supply. U.S.D.A. Forest Service, Pacific Southwest Forest and Range Experiment Station, Berkeley, CA. Gen. Tech. Rep. PSW-18,115 pp. Anonymous, 1977. Schema d'am~nagement du bassin versant du Tleta. Minist~re de l'Agriculture et de la R6forme Agraire du Maroc et FAO. Rapport Terminal du Project MOR 71/536. Anonymous, Undated. Am~nagement de la revi~re du Nekor (seuils de sedimentation). Ministate de l'Equipement et de la Promotion Nationale, Division d'Exploitation (A. Lahlou, Chef), 107 pp. Bayer, L.C., 1933. Soil erosion in Missouri. Mo., Agric. Exp. Stn., Bull. 349, 66 pp. Blake, G.R., 1965. Bulk density. In: C.A. Black (Editor), Methods of Soil Analysis, Part I. Agronomy Series No. 9. American Society of Agronomy, Madison, WI, Ch. 30, pp. 374--390. Branson, A., Gifford, G.F. and Owen, J.R., 1972. Rangeland Hydrology. Society for Range Management, Denver, CO, Range Sci. Ser. No. 1, 84 pp. Brechtel, H.M., 1976. Application of an inexpensive double-ring infiltrometer. In: S.H. Kunkle and J.L. Thame~ (Editors), Hydrological Techniques for Upstream Conversation. F A O (Rome). Conservation Guide No. 2, pp. 99--107. Challot, J.-P., 1960. Consequencies du patfirage sur la stabilit6 du sol et sur la production foresti~re au Maroc. Fifth World For. Congr. Proc., pp. 1714--1718. Collis-George, N., 1977. Infiltration equations for simple soil systems. Water Resour. Res., 13(2): 395--403. Day, P.R., 1965. Particle fractionation and particle-size analysis, In: C.A. Black (Editor), Methods of Soil Analysis, Part I. American Society of Agronomy, Madison, WI, pp. 545--567. Duchaufour, P., 1970. Pr6cis de P~dology. Masson et Cie, Paris, 481 pp. Gifford, G.F., 1976. Applicability of some infiltration formulae to rangeland infiltration data. J. Hydrol., 28(1) 1--11. Gifford, G.F., 1978. Use of infiltration equation coefficients as an aid in defining hydrologic impacts of range management schemes. J. Range Manage., 31(2): 115--117. Hanson, C.L., Kuhlman, A.R., Erickson, C.J. and Lewis, J.K., 1970. Grazing effects on runoff and vegetation on western South Dakota rangeland. J. Range Manage., 23(6): 415-420. Harcharik, D.A. and Kunkle, S.H., 1978. Forest plantation for rehabilitating eroded lands. In: FAO (Rome), Special Readings in Conservation. FAO Conservation Guide No. 4, pp. 83--101. Harrold, L.L., Schwab, G.O. and Bonduront, B.L., 1976. Agricultural and forest hydrology. Ohio State University Agricultural Engineering, Columbus, pp. 51--52. Hewlett, J.D. and Nutter, W.L., 1969. An Outline of Forest Hydrology. University of Georgie Press, Athens, 137 pp. Horton, R.E., 1940. An approach toward a physical interpretation of infiltration capacity. Soil Sci. Soc. Am., Proc., 5: 399--417. Lahlou, A., 1977. Envasement des retenues des barrages du Maroc. Minist~re des Travaux Publics et des Communications, Direction de l'Hydraulique, Division Exploitation, Service Gestion des Eaux. Notes de 460 ~ 480, 161 pp. Lahlou, M., 1979. Etude du r~gime des sediments en suspension dans les eaux de ruisellement au niveau du bassin versant de Sidi-Salh-region du Tangerois. L'Institut Agronomique et V6t~rinaire Hassan II, Rabat, Morocco. M~moire de troisi~me cycle, 208 pp. Linnartz, N.E., Hse, C. and Duvall, V.L., 1966. Grazing impairs physical properties of a forest soil in central Louisiana. J. For., 64: 239--243. Orr, K., 1975. Recovery from soil compaction on bluegrass range in the Black Hills. Trans. Am. Soc. Agric. Eng., 18(6): 1076--1081.

306 Osborn, B., 1952. Range soil conditions influence water intake. J. Soil Water Conserv., 5: 128--132. Rauzi, F. and Hanson, C.L., 1966. Water intake and runoff as affected b y intensity of grazing. J. Range Manage., 19(6): 351--356. Rauzi, F. and Zing, A.W., 1956. Rain maker helps prove a theory. Soil Conserv., 21 : 228-229,240. Rhoades, E.D., Locke, L.L., Taylor, H.M. and McIlvain, E.H., 1964. Water intake on a sandy range as affected b y 20 years of differential cattle stocking rates. J. Range Manage., 17(4): 185--190. Sant, H.R., 1966. Grazing effects on grassland soils o f Varanasi, India. J. Range Manage., 19(6): 362--366. Satterlund, D.R., 1972. Wildland Watershed Management. John Wiley and Sons, New York, NY, 370 pp. Slack, D.C. and Larson, C.L., 1979. Modeling infiltration: the key process in water management, runoff, and erosion. International Conference on Agricultural Hydrology and Watershed Management in the Tropics, 20--24 November, Ibadan, Nigeria, 22 pp. Tayaa, M., 1977. Watershed management practices for rehabilitation of critical areas. A memoire for the diploma Ingenieur d'Etat, submitted to the University of Minnesota (U.S.A.) and l'Institut Agronomique et V~t~rinaire Hassan II, Rabat, Morocco, 168 pp. Thompson, J.R., 1968. Effect of grazing on infiltration in a western watershed. J. Soil Water Conserv., 23(2) 63--64. Tromble, J.M., Renard, K.G. and Thatcher, A.P., 1974. Infiltration for three rangeland soil-vegetation complexes. J. Range Manage., 27(4): 318--321. Walkley, A. and Black, I.A., 1934. An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci., 37 : 29--38. Watteuw, R., 1978. Carte de reconnaissance p£~dologique du Tangerois. Ministate de l'Agriculture, Direction de la Recherche Agronomique Section P£~dologie, Tanger, Maroc (unpublished map). Woodward, L., 1943. Infiltration capacities of some plant--soil complexes on Utah rangelands. Am. Geophys. Union Trans. (Part II): 468--473.