Rainfall-runoff-sediment relationship in vegetation fallows in the rainforest zones of southern Nigeria

Studies that examined rainfall-runoff-sediment association in fallow vegetation of varying ages are not adequately documented in the literature. The present study evaluated rainfall-runoff and rainfall-sediment association on vegetation fallows; 3-year old, 5-year old and 10-year old in the rainforest zone of southern Nigeria. Measurements of rainfall producing runoff and sediment were carried out from the months of March to November in 2012 rainy season. Sixty-one significant rainstorms were registered, accounting for a total rainfall amount of 1,359 mm. Sixty-one (61) storms of 5.2 mm and above generated measurable runoff on the 5-year old and 3-year old plots, while sixty (60) on the 10-year fallow plot. Out of the sixty-one (61) storms recorded in the entire experiment, fifty-four (54) produced significant sediment on the 5-year fallow plot, forty-four (44) generated sediment on the 3-year fallow plot, while, sediment loss was observed and measured in thirty-six (36) of the rainstorms on the 10-year old fallow plot. Pearson’s correlation showed that rainfall had strong, positive and significant relationship with runoff on all the plots. The correlation coefficients in the fallow plots ranged from 0.887 – 0.949and were significant at 0.01 confidence level. The result further revealed that 5.0 to 40.0 mm of rainfall generated the most runoff events experienced on all the plots. Rainfall also had strong, positive and significant relationship with sediment loss. The correlation coefficients in the fallow plots ranged from 0.863 – 0.965 and were significant at 0.01 confidence level. It also showed that 9.0 to 40.0 mm of rainfall yielded the most sediment on all the treatments. Rainfall was observed to have strong relationship with runoff and sediment on all the treatments accounting for a greater variation in erosional losses.