In-field runoff as affected by runoff strip length and mulch cover

• More rainfall events of low amount, short duration and low intensity.
• Rainfall positively relate to runoff; increase in length and mulch reduce runoff.
• Collectable runoff from bare narrow has more advantage for infiltration in basins.
• Runoff from wide full mulched infiltrates more of the rainwater on the runoff area.
• Fine sandy loam hold water following rain; access by the roots during dry spell.

The purpose of the study was to quantify and predict the rainfall–runoff relationships in the technique of in-field rain water harvesting (IRWH) during maize growing season on Bainsvlei Kenilworth ecotope. The plot set-up was performed according to different runoff strip lengths (RSL) and mulching level (ML) treatments. The growing season received more rainfall events of low amount and high percentage of rain events of short duration (<60 min) and low intensity (≤10 mm h−1) compared to long-term. This reveals that the considerations of only relatively large and intense rain events for in-field runoff (Ro) estimation would failed to provide reliable runoff estimation under IRWH system. The mean runoff comparison from runoff strip length shows that the narrow (RSL-1) plots generate significant more runoff than the wide (RSL-3) plots with mean Ro from the RSL-1 being nearly 3 times more than that from RSL-3 plots that is due to lower water storage capacity and surface friction in the narrow runoff strips. The bare management harvested more in-field runoff water and showed a significant difference from 39% and 96% mulch level cover. From a linear regression analysis, runoff was significantly affected by amount of rainfall, mulch level, runoff strip lengths and peak rainfall intensities with R2 of 0.69. The model shows that the main characteristics of the rainfall event (amount and peak intensity) are positively related to runoff, whereas an increase in runoff length and addition of mulch cover both reduce in-field runoff water. The performance of the model yielded good results (D = 0.97) provides relatively satisfactory results with MAE value of 0.71 mm over the growing period. By analyzing, probability curves for in-field runoff processes during the growing season, simulated long-term Ro showed the amount of in-field runoff collectable water from a bare narrow plot (RSL-1) has more advantage for infiltration in the basins, while wide strips (RSL-3) with full mulch cover (96%) infiltrate more the rainwater on the runoff strips. The overall results obtained explain that the physical property of the fine sandy loam soil with gentle slope (<1%) of this experimental site enhances infiltration due to soft crust formation in the top soil surface. This implies that the soil has an ability to hold considerable amount of water following a wet period or after a rain event which was then accessed by the roots during short dry spells.