Soil Use and Management (2001) 17, 179±187 DOI: 10.1079/SUM200173 Removal of rock fragments and its effect on soil loss and crop yield, Tigray, Ethiopia J. Nyssen 1 *, Mitiku Haile 2 , J. Poesen 1 , J. Deckers 3 & J. Moeyersons 4 Abstract. When the farmers of the Highlands of Tigray (northern Ethiopia) consider rock fragment cover in their ®elds to be excessive, they remove some of them. In addition, large amounts of rock fragments of all sizes are removed from ®elds for building stone bunds. Semi-structured interviews indicate that the farmers are often reluctant to take away the smaller rock fragments (i.e. < 5 cm across) from their ®elds, since they believe these bene®t soil moisture conservation and protect topsoil from erosion. A ®eld experi- ment was carried out on a Vertic Cambisol (average slope: 0.125 m m ±1 ), 2 km east of Hagere Selam (subhu- mid climate). Rock fragments were totally, partially or not removed from the 12 runoff plots (5 m 3 6 m) before the beginning of the 1999 cropping season, during which a local mixture of wheat varieties (Triticum spp.) was sown. After harvest, erosion rates were assessed by measuring deposited sediment volume in trenches at the lower side of each subplot, and grain and straw yields were assessed. We found a signi®cant negative relationship between rock fragment cover and soil loss by water erosion. However, the resulting positive relationship between rock fragment cover and grain and straw yield was weak. This might be explained by the fact that the plot did not suffer from drought due to soil and climatic conditions. Detailed analysis showed that cover by medium and large rock fragments (> 2 cm diameter) showed an optimum per- centage cover above which crop yields decrease. A recommendation resulting from this study is to rely on the farmers' experience: smaller rock fragments should never be removed from the surface of ®elds during soil and water conservation works; instead rock fragment rich soil can be used to top the stone bunds. Keywords: Rock fragments, size, erosion, water conservation, crop yield, farmers' attitudes, Ethiopia INTRODUCTION T he importance of rock fragment cover in the protection of arable land from soil erosion by water is well known (Wischmeier & Smith 1978; Ro Èmkens 1985). In this paper, `rock fragments' includes all mineral particles > 5 mm in diameter. Poesen et al. (1994) review the main sub-processes inducing the bene®cial effect of rock fragment cover in reducing rill and interrill erosion, which are: (1) reduction of soil erodibility by protection of the soil surface against raindrop impact and ¯ow detachment; (2) reduction of soil surface sealing; and (3) retardation of overland ¯ow velocity thus reducing its detachment and transport capacity. Although high rock fragment cover is an important feature in the ®elds of the Ethiopian Highlands, it has not received due attention in research. In his adaptation of the Universal Soil Loss Equation for Ethiopian conditions, Hurni (1985) takes rock fragment cover into account, without however mentioning the evidence on which his estimates of reduced soil loss due to rock fragment cover are based. Poesen & Lavee (1994) indicated that generally, rock fragments increase the rate of in®ltration into cultivated land by protecting the soil from raindrop impact and thus from surface sealing and crusting. Increase of in®ltration rates depends on factors such as position and size of rock fragments, the % rock fragment cover and also on the structure of the ®ne earth. Effects of rock fragment cover on productivity can be positive, negative or ambivalent (see Figure 10 of Poesen & Lavee 1994). In the semiarid areas of Ethiopia, moisture conservation techniques were shown to induce a substantial increase of crop yield (Mitiku & Fassil 1996). Since the abolition of feudality in the 1970s, after many years of war and several famines, there exists a popular will in Tigray (Northern Ethiopia) to manage the natural legacy; participation of peasants in soil and water conservation (SWC) is important (Hurni & Perich 1992). A recently introduced technique in northern Ethiopia, the building of stone bunds (Nyssen 1998) involves the use of many rock fragments of different sizes that are generally taken from the neighbouring ®elds. For this purpose, the farmers build strong walls with large rock fragments, using medium sized 1 Laboratory for Experimental Geomorphology, K.U. Leuven, Redingen- straat 16, B - 3000 Leuven, Belgium. 2 Makalle Á University, P.O.Box 231, Makalle Á, Ethiopia. 3 Institute for Land and Water Management, K.U. Leuven, Vital Decosterstraat 102, B - 3000 Leuven, Belgium. 4 Royal Museum for Central Africa, B - 3080 Tervuren, Belgium. * Corresponding author. E-mail: jan.nyssen@geo.kuleuven.ac.be J. Nyssen et al. 179