## 3 Results and Discussion
### Runoff Generated in the Four Tillage Treatments  
Tillage effects on runoff generation were significant (\(p<0.001\)), and the planting basin treatment had the lowest runoff amount recorded (Figure 3). The grid of 0.9 m \(\times\) 0.6 m  
Figure 2: Daily rainfall events that generated runoff from conventional and double ploughing, ripper, and basin tillage treatments. (SD = standard deviation). -spaced planting basins created a higher surface roughness than the other tillage treatments, and this facilitated more rainwater harvesting and infiltration, hence the reduced runoff generation recorded. The surface roughness created by the basin grid increased surface water storage potential, a result consistent with rainfall simulation experiments by [40]. Using the Tukey test to unravel the tillage effects on runoff amount generated, results showed that runoff amounts in CP and DP, and DP and ripper treatments were similar (Table 2, Figures 3 and 4). Runoff from CP was significantly (\(p<0.05\)) higher than that from the ripper treatment (Figure 3; Table 2). The confidence intervals for DP-CP and Ripper-DP cross the zero line, and this indicates that the runoff amounts from these two tillage treatments are not significantly different (Figure 4). Previous studies showed that conventional ploughing temporarily increased the porosity and infiltration capacity of the surface soil layer [41; 42; 43]. However, these soil properties are quickly lost after a few rainfall events as the rainy season progresses. As the season progresses, soil porosity decreases, resulting in more runoff generation from conventionally ploughed treatments [12; 44]. In the ripper treatment, furrows created by ripping were filled with soil as the season progressed, which resulted in reduced rainwater harvesting and hence increased runoff generation. In addition, soils in south-western Zimbabwe form surface crusts [45], which could have promoted high runoff generation, particularly in the conventionally ploughed treatments. The heavy rainfall events recorded during experimentation [12] could have promoted soil surface sealing and crusting, which in turn increased runoff generation from conventionally ploughed and ripper treatments. [PERSON] et al. [46] found that soil crust formation reduced infiltration rates and cumulative infiltration in a Loess Plateau Soil. In this study, runoff generation increased with an increase in the amount of rainfall recorded in each event in all till \begin{table}
\begin{tabular}{l c c c c} \hline
**Tillage Pair-Wise Comparison** & **Estimate** & **S.E.** & **T-Value** & \(p\)**-Value** \\ \hline DP-CP & \(-0.06431\) & \(0.11470\) & \(-0.561\) & \(0.9435\) \\ Ripper-CP & \(-0.33331\) & \(0.11470\) & \(-2.906\) & \(0.0209\) * \\ Basins-CP & \(-0.91634\) & \(0.11470\) & \(-7.989\) & \(<\)0.001 *** \\ Ripper-DP & \(-0.26900\) & \(0.11470\) & \(-2.345\) & \(0.0911\) \\ Basins-DP & \(-0.85203\) & \(0.11470\) & \(-7.428\) & \(<\)0.001 *** \\ Basins-Ripper & \(-0.58303\) & \(0.11470\) & \(-5.083\) & \(<\)0.001 *** \\ \hline \end{tabular} Statistical significance codes: *** = 0.001; * = 0.05.  
\end{table} 
Table 2: Pair-wise comparison of runoff generated from different tillage treatments across sites and seasons. S.E. = standard error, \(T\)-values = test statistics, \(p\) value = probability value. Figure 3: Runoff generated from the four tillage treatments across experimental sites and across growing seasons (2006/07 and 2007/08) in Gwanda and Insiza districts, Zimbabwe. Means with the same letter are not significantly different (\(p<0.05\)). A crossed blue square in each box plot represents the treatment mean. As for the proportion of average runoff-generating rainfall amounts recorded during experimentation, the runoff was 6% in CP and DP, 5% in ripper, and 3% for the planting basin treatments. It was reported that storms in excess of 50 mm of precipitation volume produced about 40% of the runoff in a 20-year period in Texas, United States, even though the storms were about 10% of the precipitation events [47, 48].