Prediction of Rainfall Variability Impact on Water Erosion Intensity under Four Soil Management Practices at Nsukka, Nigeria

Abstract

Anthropogenic activities and natural factors are majorly responsible for soil degradation. These degradations are bound to increase with change in climate, thereby making some of the current soil management practices unsustainable in the future. Therefore, this study was aimed at predicting the impact of rainfall variability on water erosion intensity and to identify the most sustainable soil management practices in Nsukka Local Government Area of Enugu State, Nigeria. Four soil management practices were analysed using Water Erosion Prediction Project (WEPP) model. Soil samples were collected from profile pits dug on fallow land, range land, manually cultivated land and tractorized cultivation, at 0-20, 20-40 and 40-60cm depths with two replications each. The samples were analysed for their physical and chemical properties. Climate data collected from the Nigeria Meteorological Agency, Abuja from 1981 to 2010 were used as a baseline climate scenario while projected climatic data for Representative Concentration Pathways (RCP) 4.5 and 8.5 outputs scenarios from World Climate Research Programme (WRF) model were used as future (2041 to 2070) climate scenarios. The future and historical rainfall, minimum and maximum temperatures were tested for trends and also used to simulate mean monthly and annual soil losses and runoffs for Nsukka. The trend test at 95 % confidence level for the historical data set showed a significant trend for the mean monthly rainfall and a non-significant seasonal trend with a Kendall S statistics of 119.00 and 6.00 respectively. The test also showed significant trend for future temperatures but was not significant for both historical temperatures and future rainfalls for RCP 4.5 and 8.5 although there were high variations. The soil properties and climate data for historical and projected scenarios were incorporated into the WEPP model as inputs and ran for the different soil management practices. The model predicted highest runoff and soil loss of 1097.47 mm and 32.974 Mg ha-1 yr-1 under tractorized and fallow land managements respectively while the lowest amount of runoff and soil loss of 594.92 mm and 0.005 Mg ha-1 yr-1 were recorded by rangeland. Highest runoff and soil loss were also predicted under the historical climate at 1190.92 mm and 38.294 Mg ha-1 yr-1 respectively while the projected RCP 8.5 produced the lowest amount of runoff and soil loss. The test of sustainability using Least Significant Difference (LSD) revealed that rangeland would be the most sustainable in the future. It also showed a statistically significant difference in the amount of soil losses and runoffs from the different land management practices and rainfall regimes (historical and projected climate conditions) with the historical climate posing the greatest threat to both runoffs and soil losses. The study concluded that range land would be the most sustainable land management in the future as its soil loss was less than the soil loss tolerance level (1.3 Mg/ha/yr) for Nsukka, runoffs and soil losses from the future rainfall would generally be lower than that of the baseline and as such, the various management practices with little modifications are considered sustainable.