| dc.description.abstract | Diminished soil fertility is among the leading causes of reduced crop productivity in many parts of south-western Uganda. Farmers perceive soil erosion as the main cause of declining soil fertility. However, these farmers’ perceptions sharply contradict earlier studies which had indicated that soil erosion was a minor problem in the area. The premises of this study, therefore were to i) determine the land uses/cover within L. Bunyonyi sub-catchment; ii) determine the magnitude of soil and nutrient losses under different land uses and landscape position in L. Bunyonyi sub-catchment. iii) characterize soil erosion hotspots within L. Bunyonyi sub-catchment, Kabale District. iv) assess the efficiency of various soil and water conservation practices in reducing runoff, soil and nutrient losses. Both field assessment and Landsat images were used to characterize the major land uses within the catchment. Soil erosion was measured by constructing erosion traps in four different land uses specifically annuals, perennials, grazing and woodlots located at three landscape positions (summit shoulders, mid-slope and foot slope). Soil and Water Assessment Tool (SWAT) model was used to identify the erosion hotspots as well as determining the efficiency of some of the soil and water conservation interventions. The soil and water conservation practices studied were mulching, trenches, ridges and control. The major land uses/cover by 2014 were: small scale farmlands (64.6%), Tropical High Forest (2.2%), grassland (0.09%), Open water (12.2%), wetlands (2.6%) and woodlots (18.5%). Soil erosion varied significantly (p=0.05) across land uses and landscape positions, the highest occurring in the woodlots and land position. Runoff ranged between 45 and 300 m3 ha-1yr-1 with the lowest 47 m3 ha-1yr-1 under perennials at summit shoulders and highest 298 m3 ha-1yr-1 under perennials, at mid-slope. Soil loss was generally very low ranging from 28 kg ha-1yr-1 at midslope of grazing land to 807 kg ha-1yr-1 at midslope of woodlots. All the land uses significantly influenced loss of all the nutrients under study. Nutrient losses ranged as follows: nitrogen, 389-1879 g ha-1yr-1; phosphorus, from 139-684 g ha-1yr-1; potassium, 828-6833 g ha-1yr-1; magnesium, 277-1679; and calcium, 565-2528 g ha-1yr-1. Consistently, woodlots experienced the highest losses for all the nutrients measured. The erosion hotspots represent 18.6% area of the entire catchment for both surface runoff and sediment yield were located in Rubaya and Kyamuhanguzi sub counties. The erosion hotspots mainly occur under ferraltic soils located on steep to very steep slopes. The brighter spots are generally under regosols and forested areas. Mulching was the most effective practice; reducing runoff from 207.8 (control) to 82.1 m3ha-1yr-1, a 60% decrease. Trenches were the second efficient measure in controlling erosion reducing it to 148.4 m3ha-1 yr-1 equivalents to 29% of the control. Smallscale farmland and woolots were the dominant land use/cover in L. Bunyonyi sub-cathment; the woodlots lost more soil and nutrients than other landuses; SWAT model was effective in simulating the hydrograph, runoff and soil and nutrient losses; the erosion hotspots were mainly on woodlots of eucalyptus under ferralsols with steep sloes. Mulches and trenches were efficient in controlling soil runoff and nutrient losses (p ≤ 0,05). Future studies should focus on leachate and L. Bunyonyi sedment and nutrient dynamics. 9 | en_US |
