Browsing by Subject "USLE"

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    Soil erosion in Andean Cropping Systems: The impact of Rainfall Erosivity
    (2002) Sonder, Kai; Leihner, Dietrich
    Rationale and methods The Andean region of Colombia, 30% of the country, has about 15% of the tot. population and 50% of the rural population. A great part of the countries food-crops are grown on the hillsides of the region, which are severely degraded or threatened by degradation. The objectives of the present study were: a) to determine the applicability of the USLE rainfall erosivity factor to the Andean Region, particularly the energy-intensity term b) to calculate long-term erosivity data c) to establish the long-term erosivity of two soil types in the research area d) to evaluate the yield and soil conservation performance of several cassava-based cropping systems. Research was carried out at two sites in the Cauca department in the southwest of Colombia. Santander de Quilichao is located at 3o 6' N, 76 o 31' W, at an altitude of 990 m a.m.s.l., with an annual precipitation of 1,789 mm and an average temperature of 23.7 o C. Mondomo, which lies 2o 53' N, 76 o 35' W at an altitude of 1,450 m a.m.s.l., has an annual precipitation of 2,133 mm and an average temperature of 18.2 o C. The soils at both sites belong to the inceptisols, which form about 77% of the soils of the Cauca departments. In 1987 erosion plots were established at both sites on slopes of between 7% and 20% consisting of 8 treatments and 3 repetitions at Quilichao with 2 at Mondomo. During the research in this study, the 8 treatments comprised: 1) Continuous bare fallow, 2) Traditional cassava-based rotation, 3) Continuous sole cassava, 4) Cassava-based rotation with minimum tillage and mulch, 5) Cassava-based rotation with two previous years of bush fallow, 6) Cassava-based rotation with vetiver grass barriers, 7) Cassava-based rotation with legume strips, and 8) Cassava-based rotation with improved fallow element. Rainfall erosivity Drop size distribution measurements with a Joss-Waldvogel Distrometer showed that the USLE R factor is applicable for the research region as no significant differences were found between the measured kinetic energy of rainfall events and calculated values according to the USLE. The average annual r-factor values during the 12 year research period for Quilichao and Mondomo were 10,037 and 9,016 MJ ha-1 mm h-1 a-1 respectively. A highly significant relationship was found between a modified Fournier index based upon average monthly rainfall amounts and the equivalent monthly r-factor values. Soil losses K-factor values measured of 0.017 t h MJ-1 mm-1 at Quilichao and 0.011 t h MJ﷓1 mm-1 at Mondomo may be regarded as being between medium and low, although soil losses on the bare fallow plots were very high due to the extreme erosivity of the climate. Total soil losses in Quilichao on the bare fallow from 1986 to 1998 were 1,840 t ha-1. In Mondomo it was 2,380 t ha-1. From June 1994 to July 1997, the 7 cropped treatments in Quilichao showed susceptibility to erosion in the following order (from high to low): Sole continuous cassava, bush fallow >> farmer rotation, legume strips > improved fallow > minimum tillage, grass barriers. In Mondomo the susceptibility to erosion for the same period was: Sole continuous cassava >> legume strips >> bush fallow > improved fallow, farmer rotation > grass barriers, minimum tillage. When calculating soil loss for a representative plot only the minimum tillage and grass barrier treatments reached levels below the tolerable average annual soil losses under both the Quilichao and Mondomo conditions.The erosivity and erodibility values for the whole 12-year duration of this project showed that there is a highly significant relationship between annual soil loss amounts and the R-factor of the USLE. The continuous bare fallow plots showed strong declines of organic matter at both sites. Yield performance The two conservation treatments, minimum tillage and grass barriers, showed no significant differences regarding yield compared to the widely used continuous cassava system. The farmer rotation treatment showed the highest yield levels, but there were no significant differences between this treatment and the minimum tillage treatment at Mondomo and Quilichao. Conclusions The results confirm the high erosivity of the climate and support the necessity of maintaining permanent soil cover. The proven applicability of the R-Factor of the USLE for the region and the long-term soil erodibility values determined should enable the potential erosion risk to be estimated and appropriate soil conservation measures offered. Of the cropping systems evaluated, both the minimum tillage and the vetiver grass barrier treatments proved to be interesting alternatives to the local cropping systems as they reduced soil erosion to a sustainable level and at the same time reached or even surpassed the yields of the traditional cassava monocropping. All the other systems would lead to degradation if long-term permanent cropping was practised.