Effects of Rice Straw Size on Flow Velocity and Rill Erosion: A Laboratory-Scale Experiment

Abstract

The residues of rice production could be used as a mulch to reduce the effects of rill erosion on long and steep hillslopes. However, there is a need to identify the most effective size of this residue to apply as a countermeasure of rill erosion, exploring its effect on hydraulic variables. Several investigations have focused on the anti-erosive effects of other crop residues, while experiences on rice straw applications to reduce rill erosion are still lacking. To fill this gap, this study has measured the variability in flow velocity, stream power and the resulting soil loss in a rill covered by rice straw. Flume experiments simulating rill erosion have been carried out comparing soil loss among treatments with rice straw (dose of 3 tonnes ha⁻¹ and lengths between 20 and 70, 80 and 130, or 140 and 190 mm) and a non-mulched control. Moreover, a multiple regression model that predicts soil loss for a rill cover with rice straw of a given length has been proposed. The application of rice straw reduced the soil loss by at least 20% compared to bare soils. The most suitable size of the applied straw was 90 to 130 mm, which reduces soil loss by 45%. Finer straw (20 to 70 mm) did not significantly improve the soil’s resistance to rill erosion. The beneficial effects of straw must be ascribed to the reduction in flow velocity due to the presence of straw, as shown by accurate power equations regressing the soil loss to this variable. In spite of some limitations (small experimental scale, local environmental conditions, and low incorporation level of the substrate), the results are useful for land managers and hydrologists for soil conservation in hillslopes subjected to intense rill erosion and with similar climatic and hydrological and geomorphological conditions as the case study.

1. Introduction

Production of rice, an essential food for half the world’s population, annually produces around 700 million tonnes of straw [1]. The sustainable management of this large amount of residues can be a challenge. Rice straw can be used as a mulching material for soil conservation in areas that are prone to surface runoff and erosion [2,3,4]. Long and steep hillslopes may be viable sites for this use to mitigate the impact of concentrated flow and rill erosion.

Recent investigations have highlighted that crop residues can modify overland flow hydraulics by increasing surface roughness and reducing flow energy. The effects of straw mulch on soil erosion have been explored in many studies worldwide, also for rice straw [5,6,7]. Research has demonstrated that the factors influencing the hydrological effectiveness of straw mulch are numerous. These drivers include methods and seasons of mulch application as well as doses and the length of this residue left on the soil [8,9]. Regarding the latter variables, specifically the use of rice straw, numerous researchers have evaluated its effectiveness in reducing runoff generation and soil loss [3,10,11]. To cite the most recent studies based on experimental and small-scale flumes, the lab-scale study by [5] found that rice straw was more effective at reducing runoff than soil loss and suggested a large cover of soil with the mulch. The experiment reported by [12] demonstrated that soil loss decreases with the length of rice straw, which has a lower influence on surface runoff. The experimental study by [3] measured the lowest soil loss with a straw size of 10 mm and developed a simple linear function to predict soil loss in a deforested area. However, these studies did not focus on rill erosion and did not explore how two key parameters of the concentrated flow in rills (namely, the water velocity and stream power, commonly used in studies on rill erosion [13,14] are influenced by the size of rice straw applied on the ground. For instance, the study of [8] showed that straw incorporation reduced runoff generation mainly by increasing infiltration on sloping soils, but nothing was said about the process hydraulics. Despite these advances, the specific influence of rice straw length on rill flow velocity and stream power as drivers of soil erosion due to overland flow remains poorly documented. This knowledge is not only important to better understand the hydraulics of rill erosion but is also useful for land managers to select the most effective characteristics of rice straw for soil mulching. To the authors’ best knowledge, no flume experiments exploring the effectiveness of various lengths of rice straw at reducing soil erosion with a specific attention on flow velocity and stream power have been published thus far. The importance and novelty of this study lie in the analysis of the effects of mulch size on rill erosion, rarely captured in field conditions, under the controlled flume experiments. This goes beyond the previous investigations that only focused on the most effective mulch doses.

To fill this gap, this study evaluates how rice straw of different sizes influences the flow velocity and stream power, as well as the mulch effects on soil loss in a rill with a variable soil slope. Therefore, the investigation is carried out by combining controlled experiments with regression-based modelling to assess how different straw sizes affect the rill hydraulics and soil detachment process. The study specifically aims at exploring: (i) the variability in flow velocity, stream power and the resulting soil loss in a rill covered by rice straw (dose of 3 tonnes ha⁻¹) with length between 20 and 70, 80 and 130, or 140 and 190 mm in comparison to non-mulched soils; and (ii) the relations between the soil loss and key hydraulic parameters—flow velocity and stream power—for a rill cover with rice straw with a given length. The experiments have been carried out in a flume simulating the erosion process in a rill with soil sampled from deforested sites of Northern Iran. These sites, where vegetation was almost completely removed to build high-voltage towers, are representative of landscapes subjected to the impacts of severe rill erosion due to intense human action [15]. The identification of a cheap and environmentally sustainable soil conservation technique limiting these impacts is useful for landscape managers and hydrologists facing land degradation.

2. Materials and Methods

2.1. Study Area

The soil for the flume experiments on rill erosion was sampled from deforested hillslopes in Saravan Forest Park (close to Rasht city, Guilan Province, Northern Iran, 37°09′22′′ N; 49°41′46″ E) at a mean altitude between 50 and 250 m (Figure 1). According to the Köppen–Geiger classification, the climate is typically Mediterranean, Csa type [16] with an annual average temperature of 16.3 °C and rainfall of 1360 mm. The main species in the study area include Danae racemosa, Juncus glaucus, Parrotia persica, Artemisia annua, Pteris cretica, and Mespilus germanica. Soils under these species, such as Mespilus germanica, were affected by rill erosion after deforestation.

Soil is uniform with a silty clay loamy texture (sand 12.9%, silt 47.8% and clay 39.3%). Mean bulk density, aggregate stability, and organic matter content are 1412 ± 31 kg m⁻³, 0.23 ± 0.05 mm, and 1.18% ± 0.04%, respectively.

2.2. Soil Sampling and Flume Experiments

For each flume experiment for rill erosion simulation, 2.5 kg of soil was collected in sampling points randomly chosen in the studied hillslopes. Topsoil (0–200 mm) was excavated to collect the soil after removing litter, rocks, roots and weeds, according to the procedure by [17]. The samples were transported in bins to a laboratory and placed as an erodible bed of a slope-adjustable hydraulic flume (length of 3.5 m, width of 0.2 m and height of 0.2 m) made of steel (Figure 2). In the flume, the bed filled with soil was scoured by the simulated concentrated flow under different slopes and soil conditions (non-mulched and mulched with rice straw of different sizes).

Before each experiment, the soil of the flume bed was saturated with water until ponding and then left to dry for 24 h to have a water content equal to the field capacity. After the experiment, a 20-mm surface layer of soil was removed from the flume and replaced by a new layer of dry soil and again subjected to a wetting–drying cycle (24 h).

2.3. Flume Experiments

Before all experiments, the water discharge was set to 0.11 ± 0.03 L s⁻¹ using a graduated cylinder and stopwatch in three replications. This value is the mean rill flow after a rainfall intensity between 90 and 110 mm h⁻¹ in the area [3], as recorded by field measurements.

Before each run, flow discharge and soil surface levelling were standardized to ensure consistent initial conditions. This design ensures comparability across treatments and statistical validity for detecting differences among straw lengths and slope gradients.

During each experiment, the flow velocity (V, m s⁻¹) and water depth (h, mm) were measured. The mean V was measured in three replications using the fluorescent dye technique. The flow regime was checked by calculating the Reynolds’ number, based on the measured values of V and h. Based on this number, the measured V was corrected by a coefficient of 0.6, 0.7 or 0.8 for the laminar, transition and turbulent flow regimes, respectively, according to [18]. The mean h was measured in six replications (at two cross sections, 3 and 0.5 m upstream of the flume outlet, and three points per section, 0.05 m from the sides and in the centre of the flume) using a digital depth with accuracy of 1 mm [15,19]. The scouring experiment ended five minutes after its start. The water flow was collected and measured in tanks placed at the flume outlet. After each experiment, the collected water was mixed, and five samples were collected. The samples were dried in an oven for 24 h at 105 °C and then weighed. The soil loss was calculated as the product of the sediment concentration (sediment weight by sample volume) and the collected runoff.

The stream power (Ω, kg s⁻³), defined as the rate of energy expenditure against the bed and banks of a stream per unit area [13,20], is a critical factor in soil detachment and transport [14,21]. This variable was calculated by the following equation:

$$\mathrm{\Omega }=\rho gRSV=\tau V$$

(1)

where ρ is the water density (kg m⁻³), g is the gravity acceleration (m s^–2), R is the hydraulic radius (m), S is the slope gradient (m m⁻¹), and τ is the shear stress.

2.4. Experimental Design

One flume experiment was conducted for each of four ‘soil conditions’ and four ‘soil slopes’. The four soil conditions were: three classes of rice straw length (20–70, 80–130 and 140–190 mm) and a soil without cover (considered to be a control). A straw of the desired size was applied to the flume bed as a mulch cover and was not mixed with the surface soil to form a protective layer on the ground. For each test, the straw was evenly distributed by hand to ensure a homogeneous surface cover, avoiding clustering or overlap between straw pieces. The position of treatments and slope combinations in the sequence of experiments was randomized to minimize systematic bias and temporal effects (e.g., temperature and humidity changes). Deep mixing would disrupt this surface barrier, defeating the specific purpose of using mulch to prevent erosion by reducing raindrop impact and slowing overland flow. The applied straw dose was 3 tonnes ha⁻¹, which was the most effective dose in reducing soil loss in the study area according to a previous study by [3] in the same environmental conditions. We deliberately chose to explore only one straw dose to limit the number of flume experiments. Testing a single, representative dose allowed us to focus on isolating the specific influence of straw length on rill hydraulics and erosion rates. However, considering that ample literature has explored the effects of straw dose on rill hydraulics and erosion, this is a minor limitation of the study.

The three soil conditions simulated in the experiments will be referred to as BS for bare soil—that is, non-mulched soil—and RS/20–70 mm, RS/80–130 mm, and RS/140–190 mm—that is, soil mulched with rice straw with length class of 20–70, 80–130 and 140–190 mm, respectively. The four soil slopes (4.4%, 7.5%, 16.9%, and 22.7%) were selected based on the typical values in the hillslopes of the study area. Each experiment was conducted in five replicates. Therefore, the experimental design consisted of 80 flume experiments: four soil conditions (three classes of straw length and control) × four soil slopes × five replications.

2.5. Statistical Analysis

A two-way ANOVA followed by Tukey’s test (p < 0.05) was applied to identify significant differences in the flow velocity, stream power and soil loss (response variables) among the four conditions, with four levels—BS, RS/20–70 mm, RS/80–130 mm, and RS/140–190 mm—and four soil slopes—4.4%, 7.5%, 16.9%, and 22.7% (independent factors). Before ANOVA, the equality of variance and normal distribution of samples were checked using the Shapiro–Wilk and Levene tests, respectively. Both tests demonstrated that the ANOVA assumptions were met. ANOVA and other tests were carried out using OriginPro software, release 2025b, OriginLab Corporation, Northampton, MA, USA.

The coefficient of determination (R²) was used as a statistical measure to quantify how well the non-linear regressions explain the variability of the soil loss (dependent variable) based on the flow velocity and stream power (independent variables). R² indicates the proportion of the total variance that is captured by the model, providing a measure of its explanatory power.

3. Results

3.1. Variability of Flow Velocity, Stream Power and Soil Loss with Soil Condition and Slope

According to ANOVA, the differences in flow velocity, stream power and soil loss were always significant among the soil conditions studied (F > 10.1; p < 0.001) and slopes (F > 6.50; p < 0.001). The interaction between these two factors was significant for soil loss (F = 93.2; p < 0.001), but not for flow velocity (F = 0.42; p = 0.92) and stream power (F = 0.67; p = 0.73) (

Table 1. Results of two-way ANOVA applied to water flow velocity, stream power and soil loss measured in flume experiments for bare soils and sites with application of rice straw with different sizes (study area: Saravan Forest Park, Guilan Province, Iran).

Factor	Degrees of Freedom	Sum of Squares	Mean Square	F-Value	p-Value
Flow velocity
Soil condition	3	1.52	0.51	45.2	<0.0001
Soil slope	3	0.22	0.07	6.50	0.001
Interaction	9	0.04	0.00	0.42	0.92
Stream power
Soil condition	3	68.3	22.8	10.1	<0.0001
Soil slope	3	65.6	21.9	9.67	<0.0001
Interaction	9	13.6	1.51	0.67	0.73
Soil loss
Soil condition	3	736,335	245,445	2193	<0.0001
Soil slope	3	745,744	248,581	2221	<0.0001
Interaction	9	93,867	10,430	93.2	<0.0001

Note: values in bold are significant (p < 0.05).

).

In more detail, the maximum flow velocity was measured in bare soils, and this velocity, as expected, increased with soil slope (from 0.33 ± 0.12 m s⁻¹, 4.4%, to 0.56 ± 0.16 m s⁻¹, 22.7%). Flow velocity decreased in the soils mulched with rice straw, and a size of straw between 20 and 70 mm resulted in the lowest values (in the range 0.08 ± 0.04 m s⁻¹, 4.4%, to 0.16 ± 0.06 m s⁻¹, 22.7%) (Figure 3).

The same trend as for the flow velocity was noticed for the stream power. More specifically, the highest stream power was measured for the bare soil (from 1.14 ± 0.53 kg s⁻³, 4.4%, to 4.97 ± 3.23 kg s⁻³, 22.7%), while the soils mulched using rice straw with size between 20 and 70 mm showed the minimum values (from 0.28 ± 0.17 kg s⁻³, 4.4%, to 1.47 ± 1.08 kg s⁻³, 22.7%). Also, this variable increased with soil slope (Figure 3).

The soil loss in the absence of mulching was again higher compared to the treatments. To be more precise, in bare soils, a loss between 335 ± 0.12 g (4.4%) and 723 ± 15 g (22.7%) was measured in the flume experiments. After the application of rice straw, the soil loss decreased to a minimum of 186 ± 7.45 g, 4.4%, to 378 ± 9.76 g, 22.7%. As for the hydraulic variables, soil loss monotonically increased with slope under all conditions (Figure 4).

3.2. Relationships Among Soil Loss, Flow Velocity, and Stream Power Under Variable Soil Conditions and Slope

For all soil conditions, soil loss was strongly correlated with both flow velocity (R² > 0.97) and stream power (R² = 0.99) (Figure 5). The regression analysis gave the following equations associating the response variable (soil loss) with the hydraulic parameters (flow velocity and stream power):

SL = 1659 V^1.51    (for BS, 2a)

SL = 2366 V^1.01  (for RS/20–70 mm, 2b)

SL = 2025 V^1.11  (for RS/80–130 mm, 2c)

SL = 919 V^0.84  (for RS/140–190 mm, 2d)

Additionally, the following are established:

SL = 290 Ω^0.52   (for BS, 3a)

SL = 270 Ω^0.43  (for RS/20–70 mm, 3b)

SL = 306 Ω^0.43  (for RS/80–130 mm, 3c)

SL = 296 Ω^0.32  (for RS/140–190 mm, 3d)

where SL (soil loss) is expressed in g, V (flow velocity) in m s⁻¹ and Ω (stream power) in kg s⁻³.

Figure 5. Power regressions between soil loss (dependent variable), water flow velocity, and stream power (independent variables) measured in flume experiments for bare soils and sites with application of rice straw with different sizes (study area: Saravan Forest Park, Guilan Province, Iran). Legend: BS = bare soil, RS/20–70 mm = rice straw (20–70 mm); RS/80–130 mm = rice straw (80–130 mm); RS/140–190 mm = rice straw (140–190 mm).

Figure 5. Power regressions between soil loss (dependent variable), water flow velocity, and stream power (independent variables) measured in flume experiments for bare soils and sites with application of rice straw with different sizes (study area: Saravan Forest Park, Guilan Province, Iran). Legend: BS = bare soil, RS/20–70 mm = rice straw (20–70 mm); RS/80–130 mm = rice straw (80–130 mm); RS/140–190 mm = rice straw (140–190 mm).

4. Discussion

4.1. Variability of Flow Velocity, Stream Power and Soil Loss with Soil Condition and Slope

Erosion peak in rills formed in bare soils resulted in 10.3 tonnes ha⁻¹ (the latter value measured in the steepest soils), if simply scaled to a unit area of land, uniformly subjected to this erosion form. This value can be considered as the effect of an erosive event following a return period of many years. Therefore, the rill erosion rates measured in the experimental area are close to or even slightly higher compared to the tolerance limit for agricultural areas (about 10–12 tonnes ha⁻¹ yr⁻¹, [22,23]). This indicates the need for effective soil conservation techniques that contrast land degradation and, specifically, the impact of rill erosion in those deforested sites.

The simulations at the experimental flume have shown the effectiveness of rice straw application against soil loss due to rill erosion. Mulching reduced the rill erosion by over 20% (values averaged among the four slopes) compared to the soil loss measured in bare soils. This effectiveness was recorded at the coarsest rice straw size (130 to 190 mm). The reduction in straw length resulted in a further decrease in soil loss, −45 to −46%, again in comparison to the rills without treatment. Since the difference in rill erosion between the two finer straw sizes was non-significant, the study suggests that the length of straw between 90 and 130 mm is sufficient to achieve appreciable mulch effectiveness against rill erosion. In other words, the use of fine residues is not beneficial. As expected, the effectiveness of rice straw application slightly increases with slope steepness, and this increase is more pronounced at the coarsest straw size (−12% for the mildest slopes and −33% for the steepest soils).

Among the scarce studies exploring the effects of straw size on soil loss in rills, research has shown that mulch influences the stability of rills under extreme rainfall. However, excessive straw doses could potentially concentrate surface flow with a subsequent increase in rill erosion [24]. The study of [25] showed that shorter straw lengths (30–50 mm) were more effective in reducing soil loss after mulching, while a medium size (80–100 mm) performed better when straw was incorporated in the soil surface. The investigation of [26] also showed the role of straw incorporation that may significantly reduce soil erodibility in rills due to the increase in its critical shear stress. However, incorporation of straw below an application dose of 9 tonnes ha⁻¹ (much higher than the experimental dose in this study) does not yield additional benefits and, in some cases, slightly reduces the soil’s resistance to erosion.

In our study, the very short time elapsed from the straw application to the flume experiments (three days) was insufficient for an effective incorporation of the organic residues into the soil. Therefore, the consequent improvement in the physico-chemical properties of the treated soils was not evident. This consideration allows us to ascribe the reduction in rill erosion observed after soil mulching to the decrease in flow velocity due to the presence of rice straw on the soil surface. The flume measurements are in line with our expectations: in comparison to the values measured in the bare soil, the straw application reduced the flow velocity by 22% for the coarsest residues, data averaged among the four slopes. The mean flow velocity in the soils treated with straw at the finest sizes was lower by 63% (90–130 mm) to 75% (20–70 mm) compared to the values measured in the bare soils. It is worth noting that the reductions in the stream power after rice application are very close to the corresponding decreases measured for the flow velocity (−22% for the coarsest residues, and −63% to −74% for finer sizes). Since the stream power is given by the product of the flow velocity and the shear stress, this result again confirms that the effectiveness of the mulching treatment is quite exclusively due to the mechanical action of straw on the concentrated flow. In other words, the straw segments slow down the surface runoff with a consequent reduction in the entrapment of soil particles and, thus, of soil loss.

4.2. Relationships Among Soil Loss, Flow Velocity and Stream Power Under Variable Soil Conditions and Slope

The regression analysis showed that soil loss in each rill can be associated with excellent accuracy by simple equations using the flow velocity or stream power as input variables. Despite the very low differences between the stream power and flow velocity for the experimented straw sizes and soil slopes, the equations regressing the soil loss and flow velocity are slightly more accurate compared to the expressions using the stream power.

The equations with V as the independent variable show that soil loss exponentially increases with flow velocity, but this increasing rate (equal to the exponent of V in the equations) is not proportional to the straw size. For instance, the exponent of Equation (2a) is higher than the exponent of Equation (2b). In contrast, in the expressions based on Ω as a predictor, the exponent follows the gradient RS/140–190 mm < RS/80–130 mm (the latter equal to RS/20–70 mm) < BS, which corresponds to the variability in soil loss among the four soil conditions. Because stream power is computed from flow velocity and hydraulic radius, minor measurement errors in these variables may propagate into stream power estimation. However, the repeated measurements and calibration of discharge prior to each run minimized such uncertainty, ensuring consistency across all experimental replicates.

This result confirms the tight associations between erosion and hydraulic variables governing the rill formation processes and is in line with existing literature shedding light on how straw mulching influences these hydraulic variables. In more detail, a higher flow velocity in bare soil is directly associated with increased soil loss; in this condition, the absence of protective cover enhances the erosive effects of raindrop impact, soil detachment for runoff and easier transport of soil particles [27]. According to [28], the straw application significantly reduces flow velocity (from 30% to 55% compared to bare soil). This reduction in turn lowers the erosive force of the overland flow and thereby of soil loss.

Also, the relationships between stream power and soil detachment have been widely explored by research, although direct studies focusing solely on straw size are very limited. The literature clearly indicates that straw mulching alters the hydraulic characteristics of surface runoff, which also modifies the relationship between stream power and soil detachment capacity [13,14]. Specifically, straw increases the threshold stream power that is required to initiate soil detachment, thereby enhancing erosion resistance [29]. The combined effects of straw mulching on flow velocity and stream power also reduce overall runoff and sediment yield.

4.3. Practical Implications and Main Limitations

The study has shown the viability of rice straw as a mulching material for soil conservation in deforested lands prone to rill erosion. The small scale of the laboratory flume experiment allows control of hydraulic conditions and precise measurements of the related variables, such as the flow discharge and velocity, and the slope gradient. This also enables easy testing of erosion models through repeatable observations of process evolution.

However, the results should be validated by proper upscaling in the field, especially regarding the effects of the most effective straw size identified in this investigation (somewhat contrasting other literature studies, although this research was conducted in different conditions and with other substrates). This upscale would allow capturing the variability of climatic, hydrological and geomorphological characteristics in real conditions. In the field, higher erosion rates can be observed on longer hillslopes, where the concentrated flow may have a higher sediment transport capacity [30]. According to [31], small working scales—such as the experimental flume of this study—can show biased runoff and erosion rates compared to hillslopes.

As outlined above, only one application dose was deliberately tested in this study for the sake of simplicity. Flume experiments with other doses of rice straw are, however, welcome to give land managers and agronomists insight into the most effective combination of doses and sizes. Although the application doses of straw for mulching have been widely investigated, optimal straw lengths and application rates vary depending on specific conditions [5,12]. Therefore, there is a further need to explore whether it is convenient to increase the dose and reduce the size, or the reverse option leads to lower erosion.

The experimental tests were carried out immediately after straw application, which prevented the beneficial effects of substrate incorporation in the soil. Straw incorporation into the soil may offer better protection against erosion under certain conditions compared to surface mulching [32,33]. In this regard, the application of organic matter decomposing from the buried residues could increase the soil stock. As a consequence, important physical properties of soil, such as the aggregate stability, porosity and bulk density, may noticeably improve [34]. Further experiments after some weeks or a few months should evaluate whether the organic matter supply to soil after straw application is able to improve those soil properties and, thus, reduce surface runoff and soil loss.

A combination of straw mulching with other soil conservation techniques (e.g., minimum tillage or cross-slope cultivation) may significantly reduce the hydrological and erosive response of soil compared to traditional practices [35]. Field experiments on this effectiveness are suggested to explore the synergistic effects of combined soil management.

5. Conclusions

The flume experiments simulating rill erosion in deforested lands of Northern Iran have shown that the application of rice straw (3 tonnes ha⁻¹) reduces the soil loss by at least 20% compared to bare soils. The most suitable size of the applied straw is 90 to 130 mm, which reduces soil loss by 45%. Finer straw (20 to 70 mm) does not significantly improve the soil’s resistance to rill erosion. The beneficial effects of straw must be ascribed to the reduction in flow velocity due to the presence of straw, as shown by accurate power equations regressing the soil loss to this variable.

The study shows some limitations associated with the small experimental scale, only one tested dose of straw and one simulated flow discharge, as well as low incorporation of the substrate. It must also be emphasized that these results are derived from small-scale flume experiments under controlled conditions. The influence of field-scale variability in rainfall, soil structure, and slope geometry may significantly alter erosion dynamics. Therefore, the transferability of these findings to natural hillslopes requires validation through field experiments. Overall, in spite of these limitations, the results are useful for land managers to identify the most effective straw size for soil conservation in deforested lands.