Deriving Surface Water Storage and Curve Numbers from Rainfall–Runoff Relationships in Conventional and Minimum Tillage Systems in Gwanda, Zimbabwe

Abstract

Soil water availability is one of the major constraints limiting crop productivity under semi-arid conditions in sub-Saharan Africa. Crop models are tools that can be used to explain and predict the effect of improved technologies on runoff and soil water availability, and their impact on crop productivity. The study hypothesized that minimum tillage treatments (planting basins and ripper) retain more rainwater and reduce runoff generation compared to conventional tillage treatments in maize-based cropping systems. Runoff plots were established on-farm and surface runoff was collected after each daily rainfall event. Surface water storage and curve number for each conventional and minimum tillage treatment were derived from the runoff and rainfall amounts measured over two growing seasons. Daily rainfall events of 9–76 mm generated runoff in both conventional and minimum tillage treatments. Planting basins retained more rainwater (12–19%) and reduced runoff generation (40–51%) than the conventional and ripper tillage treatments. Runoff generation in the tillage treatments varied with soil texture. Conventional and double ploughing treatments recorded more runoff (11–12%) in loamy sands than in sandy soil. Surface water storage and curve numbers from tillage treatments were consistent with runoff results and with conventional treatments, having higher curve numbers than minimum tillage practices. Conventional and ripper tillage practices have similar runoff potential as demonstrated by their curve numbers generated in this study. Curve numbers of 75–76 for conventional and 72–74 for minimum tillage systems are practical under light-textured soil and a land slope of <2% when conventional and minimum tillage practices are implemented.