The effect of alfalfa cover on production runoff at the scale of erosive plots using a rain simulator

Abstract

Introduction

Vegetation is an effective and universal way for water and soil conservation. In general, runoff and can be mitigated by vegetation cover to increase water infiltration, enhance the sediment trapping, reduce the kinetic energy of raindrop impact, increase the surface roughness, decrease the flow velocity, and control the runoff hydraulic properties. The impact of vegetation cover on runoff generation has been studied at different spatial scales, from experimental scale to watershed. Most of the studies have showed the effectiveness of vegetation cover in the surface runoff reduction. Considering the difficulty and economic infeasibility of covering the entire surface of an area in a soil conservation project at arid regions, one may question that how much of a soil surface should be covered to have an optimal impact on runoff production? The objective of this study is to evaluate the effect of various ratios of alfalfa coverage on runoff generation under simulated rainfall condition.

Materials and methods

This research was carried out using a completely randomized design with a treatments based on the ratio of alfalfa coverage (0%, 35%, 65% and 90%) under artificial rainfall conditions. The studied soil was collected from the surface horizon of farmlands in Kashan and its texture was sandy loam. All experimental plots were 2 meters long, 0.55 meters wide, and 0.35 meters deep with a metal runoff collector. The alfalfa seeds were planted at a spacing of 15×15 cm in the 35% plot, 10×10 cm in the 65% plot, and 5×10 cm in the 90% plot. Finally, all plots were subjected to a simulated rainfall with an intensity of 90 mm/hr for 70 min duration. During the experiment, the runoff gathered via the collector at the end of the plot was collected at 5-minute intervals. The Kolmogorov–Smirnov, one-way ANOVA and Duncan’s tests were eventually applied for data analysis.

Results and Discussion

Results showed that the plant cover treatments were most effective in runoff reduction at the first 15 minutes, and performed progressively poorer in this respect towards the end of the experiments. This is because at the late time steps, infiltration rate has reached its final rate and the soil cannot passes water at a higher rate. There was a significant difference (P <0.01) between the coverage ratio treatments (0%, 35%, 65% and 90%). Also, the bare plots had the highest average runoff rate and were categorized into a separate group (a). The plots with 35%, 65%, and 90% coverage ratios had average runoff rate of respectively 0.329 mm min-1, 0.222 mm min-1 and 0.112 mm min-1, and were placed in separate groups as well. In the bare soil treatments, a high amount of rainfall drops on the soil surface without the vegetation interception, exceeds the infiltration rate and subsequently the runoff is produced. Also, soil sealing and crusting processes due to raindrop impacts could reduce the infiltration and generate higher runoff.

Conclusion

The current study evaluated the influence of alfalfa coverage ratios on the runoff production in the experimental condition subjected to artificial rainfall. Higher coverage of alfalfa were associated with a delay and reduction in runoff generation.

According to our foundings, we have obtained a practical threshold of coverage ratio for runoff reduction: a minimum of 65% of alfalfa coverage, which enables resonable decresing of flow surface runoff. This coverage threshold could be considered a beneficial criterion for the soil and water conservation practices in the arid area where the dense vegetation establishment is difficult and more expensive than lower coverage.

Results and Discussion

Results showed that the plant cover treatments were most effective in runoff reduction at the first 15 minutes, and performed progressively poorer in this respect towards the end of the experiments. This is because at the late time steps, infiltration rate has reached its final rate and the soil cannot passes water at a higher rate. This finding is consistent with the results of Wu et al. (2011), who reported the ability of alfalfa cover to improve the soil resistance against erosion and sediment transport. There was a significant difference (P <0.01) between the coverage ratio treatments (0%, 35%, 65% and 90%). Also, the bare plots had the highest average runoff rate and were categorized into a separate group (a). The plots with 35%, 65%, and 90% coverage ratios had average runoff rate of respectively 0.329 mm min-1, 0.222 mm min-1 and 0.112 mm min-1, and were placed in separate groups as well. In the bare soil treatments, a high amount of rainfall drops on the soil surface without the vegetation interception, exceeds the infiltration rate and subsequently the runoff is produced. Also, soil sealing and crusting processes due to raindrop impacts could reduce the infiltration and generate higher runoff.

Conclusion

The current study evaluated the influence of alfalfa coverage ratios on the runoff production in the experimental condition subjected to artificial rainfall. Higher coverage of alfalfa were associated with a delay and reduction in runoff generation.

According to our foundings, we have obtained a practical threshold of coverage ratio for runoff reduction: a minimum of 65% of alfalfa coverage, which enables resonable decresing of flow surface runoff. This coverage threshold could be considered a beneficial criterion for the soil and water conservation practices in the arid area where the dense vegetation establishment is difficult and more expensive than lower coverage.