The effect of narrow grass strips in controlling soil erosion and runoff on sloping land
The study was carried out on twelve runoff plots installed at Kabete Campus Field Station, University of Nairobi, on a 10% natural slope of eutric Nitisol to assess the effect of grass strips in controlling soil loss and runoff on sloping land. Four treatments, a control plot without grass strip and three different widths (0.5, 1.0 and 1.5 m.) of Nandi setaria (Setaria anceps), were tested under natural rainfall during 1982 and 1983, simulated runoff, and simulated rainfall of 80 mm/hr. Results showed that the treatment effect was highly significant in reducing both soil loss and runoff under natural rainfall. Annual soil loss for the control plot was 97.7 t/ha. while for the 0.5, 1.0 and 1.5 m. wide grass strips annual soil losses were 35.4, 35.6 and 17.8 t/ha. (36, 36 and 18%) respectively. Annual runoff from the control plot was 100~. (20%) and percentage runoff for the 0.5, 1.0 and 1.5 m. wide grass strips were 56, 44 and 24% of the control piot, respectively. The difference between the three different grass strip widths was not stat~stically significant although annual soil loss from the 0.5 m. wide grass strip was double that from the 1.5 m. wide grass strip. The simulated runoff test showed that soil losses from the plot with 0.5 m. wide grass strip were only 4.5, 7.8 and 41.1% of the soil losses from the control plot under the three application . -4 -4 -4 rates of 1.5 x 10 ,2.5 x 10 and 3.6 x 10 3 -1 m s respectively •• Runoff from the 0.5 m. wide grass strip were 1.9, 3.5 and 50.2% of the runoff from the control plot for the same application rates. Under heavy simulated rainfall, soil losses from the plot with 1.5 m. wide grass strip were 17.1, 25.1 and 36.7% of the soil losses from the control plot for dry, wet and very wet antecedent soil moisture conditions, respectively. Runoff from the plot with 1.5 m. wide grass strip were 32.7, 41.0 and 46.0% of the runoff from the control plot for the same three applications. Efficiency of grass strips in controlling soil loss and runoff increased from the 0.5m. wide grass strip to 1.5 m. wide grass strip and decreased from the dry to very wet antecedent soil moisture conditions. Strip width was exponentially related to soil loss and runoff with coefficient of determination (r2) of 0.96 for soil loss and 0.88 for runoff under dry antecedent soil moisture condition. Sediment was deposited on the plots with grass strips, with most of the deposition taking place on the uphill edge of the grass strip, extending two metres in one and half years. There was little deposition in the grass strips. The maximum mean depth of deposition was 7.6 cm. at the 0.5 m. wide grass strip. Much of scour took place on the upper section of the plots. In almost all the plots the ground slope was reduced by about 2% Two thirds of sediment transported was in sand size aggregates although the soil contained more than 50% clay. Grass strips had no influence in the trapping of different sized aggregates. Of the six erosivity indices tested, the amount of storm (A), in mm., was highly correlated (r = 0.88) with soil loss followed by the El1S(r=0.86) and the EI30 (r=O.73) indices. The KE> 25 index gave the least correlation coefficient (r=O.41). The ~nnu~l r~inf~ll ~rosivi~y f~e~or (R) for R~b~~~ w~s est.imated to be 246 m~t-cm/ha-hr and the soi l, erodibility factor (K) was 0.21 t-ha-hr/ha-m-t-cm. which wa s higher than that from previous findings.