Spatial Patterns of Infiltration on a Bench Terraced Hillslope in Kigezi Highlands, South-Western Uganda

Abstract

The spatial patterns of infiltration on bench terraced hillslopes are not fully understood. Moreover, these patterns are pivotal for assessing hydrological processes affecting water distribution, surface flow and soil erosion, and groundwater recharge. Our objective was to determine the spatial patterns of steady-state infiltration rates on a bench terraced hillslope; and assess the influence of soil properties on the infiltration process. We employed Geo-Information Science (GIS) to stratify the terraced bench hillslope into three slope position sequences, namely, Upslope, Middle slope, and Downslope. Besides, in each slope position segment, a bench terrace was randomly selected; and later stratified into three plot segments, namely, Upper (A), Middle (B), and Bottom (C), for purposes of conducting infiltration experiments. At each plot segment, three infiltration experiments were replicated to ascertain the intra-bench and inter-bench terrace steady-state infiltration rates. The experimental field design consisted of three slope position sequences, three plot segments, and three replicates, hence, leading to a total of twenty-seven (27) experiments. The infiltration measurements were conducted using a standard double-ring infiltrometer. We sampled soil for two different depths, namely; 0-15 cm and 15-30 cm, at each infiltration experiment to establish its relationship to the infiltration process. Using a Spatial Autocorrelation tool embedded in ArcGIS 10.2, we analysed the spatial patterns of infiltration along the hillslope. Furthermore, a Two-Way analysis of variance (ANOVA) was used to determine differences between the means of the infiltration rates across the hillslope sequences. Descriptive Statistics were used to investigate the Degree of Variability in infiltration rates. Results showed that beyond 15 m, infiltration rates vary significantly (P<0.018) from 5 cm hr-1 to 40 cm hr-1 between the bench terraced hillslope sequences, with a mean rate of 21 cm hr-1. Intra-bench steady-state infiltration rates varied higher in the Bottom (C) compared to Upper (A) and Middle (B) segments, especially in the Upslope sequence, where rates varied from 6 cm hr-1 to 20 cm hr-1 with a coefficient of variation (CV) of 45%. The clay content in the soil had a significant influence (P<0.003) on infiltration in the study sites of the hillslope with a correlation coefficient (R, r2 = -0.5). We concluded that the spatial patterns of infiltration on a bench-terraced hillslope are influenced by slope position. Infiltration is higher at the Downslope and Middleslope bench terraces as compared to the Upslope bench terrace. Soil texture and the structural condition of the bench terraces themselves play a cardinal role in the variations in infiltration. Within the bench terraced land parcels, infiltration is higher on the downslope and lower in the upslope positions. Since percent clay content increased with slope, it was noted as a fundamental soil characteristic affecting infiltration within the bench terraces. Our recommendation was that in bench terraces where steady-state infiltration was high such as in downslope and middle slope positions, the terrace bunds should be strengthened with grass and stone embarkments to consolidate them coherently. This strategy may reduce terrace bund breakages and consequently counter surface flow on the bench terraced hillslope. Besides, in areas where low steady-state infiltration rates were recognised, such as the upslope position, agronomic practices should be enhanced.