Water Infiltration, Conductivity and Runoff under Fallow Agroforestry on Sloping Terraces

Appropriate management of available water supplies is essential to prolong the growing season and optimize the effectiveness with which rainfall is used for agricultural production. The present study examined the impact of planting tree fallows ( Alnus acuminata , Calliandra calothyrsus and Sesbania sesban) on the degraded upper sections of sloping terraces on water infiltration and subsequent runoff on a Haplic ferralsol in southwestern Uganda. Infiltration measurements, done by a tension infiltrometer, were conducted under 3-year old tree canopies, under maize stands grown adjacent to trees, and under sole maize ( Zea mays ). Runoff from confined plots of agroforestry (trees on upper and crops on lower terrace sections) and sole crop systems were measured by the tipping bucket method mounted with counters. Measurements of runoff was done during the long rains of 2003 and short rains of 2004. Infiltration was invariably higher under agroforestry systems (P <0.001) than sole cropping, particularly under Alnus and Calliandra systems. A similar pattern was observed for saturated hydraulic conductivity (Ksat), which was greater in all tree-based systems except Sesbania than in the sole crop (P <0.01). The Ksat values were 1.3, 2.2, 1.0 and 0.8 cm h-1 respectively under Alnus, Calliandra, Sesbania and sole crop systems. Of even greater significance is the ability of trees to reverse the typical gradient in soil hydraulic properties observed on sloping terraces. Saturated hydraulic conductivity was consistently higher on the upper terrace than the lower terrace in the tree-based systems (P <0.01), whereas the reverse was true for the sole cropping system. During rainfall events of <10 mm, runoff accounted for only <4% of total rainfall in all treatments. The reduction in runoff relative to the sole crop control was 64, 84 and 96 in the Alnus, Calliandra and Sesbania systems. During high rainfall events (>10 mm) the effect of agroforestry was more dramatic, reducing runoff relative to the sole crop by 92, 76 and 91, respectively under Alnus, Calliandra, and Sesbania systems. Results demonstrate the ability of trees to break loose the hard compacted soils on the upper terrace so as to increase infiltration. The resultant increase in infiltration coupled with physical barriers of ground litter combine to reduce runoff under agroforestry systems