Modeling the effects of a partial residue mulch on runoff using a physically based approach

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Abstract

A partial covering mulch of residue on the soil strongly affects runoff dynamics, which consequently substantially reduces runoff amount. Experiments were conducted in la Tinaja (Mexico) on runoff plots (RPs) (20 m2) of four different treatments (bare, unplanted with 1.5 t ha−1 of residue, planted with 1.5 and 4.5 t ha−1 of residue), to characterize mulch effects. During one crop cycle, rainfall and runoff flow were recorded at a 20 s time step. Soil moisture, crop leaf area index, saturated hydraulic conductivity and sorptivity were also measured. Mulch increased the infiltration rate of the topsoil layer, concentrated overland flow and slowed it down by increasing roughness and pathway tortuosity. The physically based model developed accounts for these mulch effects on runoff. The model consists of a production and a transfer module. Each RP is considered as a micro-catchment drained by a single channel. The production module accounts for rain interception by the plant and the mulch, soil retention and infiltration. The excess rainfall that cannot infiltrate defines runoff and is concentrated in the channel. The transfer module governs runoff flow out of the RP according to Darcy–Weisbach's law. The model was calibrated on 12 events (five parameters). Fitted parameters provided high Nash efficiencies ranging from 0.721 to 0.828. Both runoff hydrographs and volumes were well simulated. A sensitivity analysis was carried out on eight parameters and a partial validation was done on 14 independent events. The model can be used as a predictive tool to assess the effect of various types of mulch on runoff. All its parameters are physical and can be measured or derived from literature. The model can also simulate inner variables of interest (water depth in the channel, infiltration in the channel and the hillslopes, etc.) at any time during rainfall.

Introduction

Experimental knowledge about the effects of a mulch of vegetative residue on runoff is well established. Savabi and Stott (1994) proved that this porous media can store significant amounts of liquid water. Rutter et al. (1971) and later Scopel et al. (1998) showed that a mulch partly intercepts the rain and cuts down the quantity of water reaching the soil. Gilley and Kottwitz, (1994) observed an increase in soil retention capacity due to the modification of soil microtopography by mulch elements. Also, mulch elements act as a succession of barriers that block runoff and increase roughness (Gilley et al., 1991, Gilley and Kottwitz, 1992, Weltz et al., 1992). Consequently, runoff pathways are generally more sinuous, and runoff flow velocity lower on mulched soils (Abrahams et al., 1994, Poesen and Lavee, 1991). Finally, mulch tends to develop and strengthen topsoil structure through soil protection, macro-fauna activity and the incorporation of organic matter, which usually provides a high infiltration rate (Rao et al., 1998, Scopel et al., 1998, Valentin and Bresson, 1992, Zachmann and Linden, 1989).

However, very few authors have attempted to formalize or model these effects. Moreover, their models did not address all the previously listed effects but focused only on certain specific points. Gilley et al. (1991) modeled the uniform water flow on an impervious surface (∼7 m2) covered with glued residue, with the help of Darcy Weisbach's law. Yu et al. (2000) used Manning's equation to simulate overland flow on a mulched impervious soil (108 m2), at a 1 min time step. In both cases, modeling was in good agreement with experimental data and contributed to the determination of a friction factor that varies according to the type of residue used. However, the important role of the mulch in infiltration processes was not taken into consideration. Bristow et al. (1986), and later Bussière and Cellier, 1994, Gonzalez-Sosa et al., 1999, developed two similar mechanistic vertical 1D models to simulate the heat and water regimes of mulched soil. They simulated rain interception by mulch and percolation to the soil, and assumed that the rain that could not infiltrate into the soil was directly evacuated from the system, without considering runoff flow dynamics on the soil. The lack of accurate experimental data made calibration and validation of runoff processes impossible in these models.

Based on the early experimental results of Scopel et al. (1998), the present study consists in quantifying and modeling all the important effects (rain interception, soil infiltration and retention, velocity and pathway geometry of runoff flow) of a partial-covering mulch of corn residue on runoff. The methodology adopted consists of two parts: (1) obtaining a set of data that describe runoff dynamics at a short time step (20 s) on middle-scale runoff plots (RPs) (20 m2), (2) developing a physical model that accounts for the main processes that drive runoff on mulched soils, and calibrating and validating the latter on the experimental data.

Section snippets

Theory

Because we worked on a soil that tended to crust, we assumed that the topsoil layer strongly controlled infiltration (Vandervaere et al., 1998). Based on this concept, the model developed is composed of a production module for estimating runoff volume and a transfer module for assessing its flow dynamics to the plot outlet. The system on which the model was applied consisted of a RP that was simplified to a micro-catchment of two hillslopes drained by a single central channel (Fig. 1). The

Material and methods

Experimental data (rain and runoff dynamics), soil, mulch and plant properties, and parameters describing runoff flow were required to calibrate and validate the model. This information was obtained from measurements on different plots and is described in the subsections below. The methodology adopted for modeling consisted of three steps: (1) calibration of runoff production and transfer modules by means of an iterative procedure; (2) sensitivity analysis; (3) validation of the model.

Experiments

The results of the artificial runoff experiment are presented in Table 4. Tortuosity increased almost linearly with mulch biomass, its average from 1.09 on RP0 to 1.46 on RP4.5P (Table 4). Corresponding effective slope decreased from 0.064 to 0.048. Friction factor, derived from velocity and flow depth (Eq. (11)), was strongly affected by mulch biomass and increased on average from 0.20 on bare soil to 2.45 on RP4.5P. Width occupation varied from about 0.15 on mulched plot to approximately 0.40

Discussion

In this work, specific experiments were designed to assess quantitatively the main effects of a partial covering mulch of corn residue on runoff. Measurements showed that runoff was dramatically cut down by mulch, even for a small amount of residue. Runoff coefficient was reduced by 50% on average by applying only 1.5 t ha−1 of residue. This behavior was due to both short-term and long-term effects. At the cycle scale, the mulch stored up to 1.6 mm by rain interception, which reduced runoff

Conclusion

The objective of this work was to quantify and model the main effects of residue mulch on runoff processes. The experimental layout showed that runoff was dramatically cut down by mulch, even for a small amount of residue. In the short run (0–1 year), mulch intercepted rain, enhanced water flow concentration by dam effect, and slowed down runoff flow by increasing roughness and pathway tortuosity. In the long run (4 years), mulch ensured high topsoil water conductivity and sorptivity.

Acknowledgements

Financial and technical support from Centro Internacional de Mejoramiento del Maı́z y Trigo (CIMMYT) as well as Institut National de la Recherche Agronomique (INRA) is gratefully acknowledged. Special thanks to Jean-Claude Gaudu for his efficient technical assistance.

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