Effects of Grazing and Trampling on Soil Deterioration Around Recently Drilled Water Holes in the Sahelian Zone

· )' r 6 Effects of grazing and trampling on soil deterioration around recently drilled water holes in the Sahelian Zone C. Valentin ! In 1937 Stebbing pointed out that drier climatic conditions were lead ing to the southward encroachment of the Sahara (20). His observations were almost immediately rejected by an Anglo-French commission, which argued that in most cases soil degradation was caused by the action of man (18). Because of the protracted drought of the 1970s in the Sahelian belt, Stebbing's caution recently was brought into sharp focus. According to many scientist~ overgrazing and trampling, especially near watering points, are the major causes of soil degradation and, consequently, deser tification of this area (6, 9, 13, 26). However, published material relating accelerated erosion to animal husbandry remains scarce. Additional in formation is essential. Scientific attention has focused on a large region of northern Senegal, which as recently as 30 years ago was called the "FerIo Desert." At that time, despite the continuous presence offodder, livestock were herded out of the area as ponds dried up after the rainy season. The Ferlo was thus deserted by nomadic herdsmen 9 months out of each year. Once numer , ,. ous deep wells were drilled in the 1950s, the seasonal migrations of no madic communities declined rapidly, and this region was subjected to permanent pastoralism. Such a radical change in pastoral practices affected many aspects of the human and natural environments. To document such changes, studies recently were carried out in that area. The area provided a large-scale ex perimental site for an interdisciplinary team composed of investigators of 51 EFFECTS OF GRAZING AND TRAMPLING ON SOIL DETERIORATION 53 52 C. VALENTIN several international, Senegalese, and French institutions, including soci General average: 291.1 32.1 average till 1969: 313.0 35.3 ologists, economists, physicians, nutritionists, veterinarians, animal average from 1970 to 1979: 194.4 44.8 breeders, botanists, and soil scientists. E 800 Here, I present major findings relevant to grazing and trampling g, 700 POCOR 16°38' N ~ around the waterholes on soil deterioration, namely sealing, soil compac 600 14°S6'W c 500 tion, and wind and water erosion. .§ 400 ~ 300 Study area ~ .......-r~~r:""'T'"'Op~~~-r;: 200 h.-,.............,.....................-y-..............-r.................,r-r-¥......--T:..........'""'T'":"'"'T"T".................. 2 The study area (Figure 1), which covers 10,000 km , sustains a human population estimated at 30,000 and about 100,000 bovine livestock(16). The arid rangelands are scattered, with 13 drilled waterholes, 25 km apart. Water is raised from the largest water table in West Africa, the Maestrichtian, which is 200 m deep. General average: 476.7 :t 39.3 average till 1969: 517.4 :t 40.0 The area has a very hot, dry season that lasts 9 months a year. Rainfall average from 1970 to 1979 : 330.0 :t 42.8 E 900 is concentrated between July and September. In this zone data can be E aOo misleading because of the wide variability of climatic events. For exam ;;- 700 L1NGUERE II) 15° 23' N ple, from 1920 to 1969 average annual precipitation ranged from 300 mm C 600 15°0rw '§ 500 ~-ts¥\.t:'M.M-d~~ · ro 400 V\P~fPv ~ 300 c V· II) 200 L...--.-~~"""'T-:-""'"...........,.-:...............'T":""""""........-::.........--~""""T::-.....-,-~~~~U;; years z « General average: 481.6 .i 44.3 w l100 average till 1969 : 519.5 ~ 44.9 eE1000 MATAM average from 1970 to 1979 : 304.4 !. 63.5 u =- 900 15° 3a'N ro 13°1~'W o C aoO '§ 700 » ~ ::: ¥.c',;rL~=-=::..:-=.::~:t::l~:::jJ,=-=~~~::v~.,;;~-\---1H---- ~ 4oo -h=~------t=---------_+--+_----_T_____r.-140 400 r 300 200 u .... Z c::( -J General average .... ___ Average from the first data to 1969 c::( _0- Average from 1970 to 1979 Figure 2. Rainfall and its variability, in three towns bordering the study area (24). 100 200 K"" I I Figure 1. General location of the study area. , , 54 rI' C. VALENTIN i EFFECTS OF GRAZING AND TRAMPLING ON SOIL DETERIORATION f in the northern part of the area (Podor) to 500 mm in the southern part was measured with three replications to investigate possible soil compac (Linguere and Matam). But a general and prolonged rainfall deficit of tion and to relate it to trampling. Soil samples were collected for mechan about 40 percent was recorded throughout the 1970s (Figure 2) (24). In ical and sieve analysis to determine deflation. Likewise, the amounts of 1981 rainfall returned roughly to normal (358 mm in Tatki, 348 mm in l, organic matter were determined. Tessekre, and 433 mm in Revane). ,L The study area encompasses two regions: the sandy Ferlo and the .t Soil surface features in the distant zones cuirassed Ferlo. The sandy Ferlo in the northern part of the Ferlo Desert I: includes smooth dead ergs of the Quarternary era. Deep ferruginous soils Ii Surprisingly, it was outside the zone affected by grazing and trampling developed on these fossil sand dunes. Aristida mutabilis, Schoenfeldia 1; that sealing most often occurred. However, the magnitude of the sealing gracilis, and Cenchrus biflorus are characteristic species of the steppic t varied widely from one study area to another (Table 1). Most bare and in grass layer. Fodder production ranges from 400 to 900 kg of dry matter/ha durated spots in Tessekre were attributed to ancient and ruined termite (8). Thorn shrubs and small trees (Commiphora africana, Balanites mounds. Long trenches that were dug elsewhere revealed other possible aegyptiaca) are scattered over the landscape. patterns. Often in Tatki a broken calcareous horizon formed a shallow' The cuirassed Ferlo, in the southeastern part of the study area, has a dome just below the crusted areas. Likewise, bare spots in Revane oc more marked relief. An iron cuirass, which has been partly destroyed by curred mostly where the irregular boundary of the gravelly layer was erosion, is underlain by tertiary sandstones. Shallow and gravelly ferrugi closest to the surface. Thus, soil surface heterogeneity often was induced nous soils occur on that iron pan. They include extensive areas with sealed by a lateral variation of the underlaying soil. surfaces, such that the herbaceous layer (Zornia glochidiata) is very scan Detailed studies of morphology, as well as sieve analysis, clearly dem ty and unproductive. Fodder production usually does not exceed 100 kg onstrated that sandy micromounds that bound the bare spots were aeolian dry matter/ha (8). Pterocarpus lucens forms monospecific communities in deposits. Provided that rainfall was normal, as in 1981, runoff washed that arid woodland. down sediments from these mounds, and crusted areas were covered again with sand. Consequently, an important areal reduction of bare sur Methodology face was observed. Three representative drilled waterholes were selected for this study Soil surface alteration in the affected zones along a transect from northwest to southeast in accordance with the cli matic gradient. Two waterholes are located on the sandy Ferlo, the third Ii (i Sandy Feria. Closer to the drilled holes, cattle tracks became more pro on the cuirassed Ferlo. Topsoil textures, determined on natural sites, vary nounced and numerous. Because the·hooves of the herds pound micro from sand in Tessekre to loamy sand in Tatki and sandy loam in Revane ~ mounds and spread the sand over the crusted spots, the whole surface be (Figure 3). E came completely covered with a sandy layer. Within a ring located be Grazing and trampling were assumed to be related to the density of tween 0.5 km and 2.0 km from the watering points, wind-sorted surface~ livestock near the waterholes. Two major concentric areas were identi ~ were marked out by sand ripples. Evidence of an accelerated deflatiof fied. The affected zone, from the vicinity of the waterholes to a distance from 5 to 7 km, is where cattle impact the most. The undisturbed zone, I ~ beyond the previous circle, are areas where cattle rarely wander. Because 2 of this very low use, these rangelands were considered as natural refer ! zone~ \ Table 1. Areal extent and average diameter of bare and crusted spots in the ences for environmental features. .j undisturbed by trampling. The study encompassed two areas: a morphological outline of soil sur Areal Extent Average Diameter face processes (sealing, sand drifting, rills, and so on), studied in relation Distance to oj Bare and oj Bare and to the distance to the watering place and season (before and after the Waterhole Crusted Surfaces Crusted SurJaces rainy season), and a quantitative assessment of several soil characters. On I Study Site (km) (%) (m) several points of a radial line drawn from the waterhole (denoted by Km Tessekre 7 8 2 0), bulk density of the topsoil, albeit often covered with sand deposits, Tatki 5 49 11 I Revane 8 52 12 .}I -=r- 56 C. VALENTIN EFFECTS OF GRAZING AND TRAMPLING ON SOIL DETERIORATION -57- 100 o ICLAY 1 1,80 I~ . --:= :.--:::-- .- 90 I--:--::=t:::::.=---- 10 ,,+'+: ......... )( TATKI 80 +/ I l'" ~;ite: Sorting index: + TESSEKRE + TESSEKRE 1.57 Q) 70 + b/ Cl / i 0" TATKI 1.63 1,70 III , . I /x E 60 + I I 0 REVANE 2.12 Q) I \ ~ i )( ~ 50 ,- l il II Q) " i M .~ 40 / / iii I /- 0 E 1,60 + i/ (,) '5 30 , , ........ \\)( E // \ ~ C) U 20 I ,,' 0 +, ~ :~/ , I I > , 10 ~~,o 4J +-- ~~-;i~ ·en 1,50 o - ~ , c o o 0 It) o It) N N Q) 0. ~. (millimeter) o ~. ~. ~. g. g. o N o' "C Very Coarse Medium Fine Very ICoarse Fine ~ )( coarse fine I SANDI ISILTI II :;:, m 1,40 Figure 3. Grain size distribution curves and sorting indexes of samples (0-5 cm), H from the zones undisturbed by trampling.

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