Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices
Introduction
The concentrated flow erosion component of many physically-based soil erosion models (e.g. CREAMS, Knisel, 1980; KYERMO, Hirschi and Barfield, 1988; WEPP, Nearing et al., 1989; PRORIL, Lewis et al., 1994; EGEM, Woodward, 1999) relies on the same basic soil detachment relationship (Lane et al., 1987). In this relationship, two parameters express the soil's ability to resist erosion forces applied by concentrated runoff: soil erodibility during concentrated flow, Kc and critical flow shear stress, τcr. In what follows, when referring to the soil's ability to resist erosion by concentrated runoff in general (not specifically in terms of Kc or τcr), the term soil erosion resistance during concentrated flow, or shortly soil erosion resistance is used. Model predictions of concentrated flow erosion (i.e. rill and gully erosion) are generally very sensitive to the input values of Kc and τcr in the basic soil detachment relationship (e.g. Alberts et al., 1995). Therefore, a good estimation of these parameters for different soil types under different conditions is crucial for accurate prediction of soil loss by rill or gully erosion. In those models, Kc and τcr values can be defined by the user but typically they are derived from reference tables. Values reported in these reference tables are often only guide values, measured in different environments or acquired through empirical relationships, hereby often neglecting the spatial and temporal variability of soil erosion resistance. Stroosnijder (2005) recently pointed to the fact that there is a crisis in soil erosion measurement and modelling because there are still insufficient empirical data of adequate quality to represent the large inter-annual, intra-seasonal and spatial variation in soil erosion rates. This lack of knowledge certainly applies to the erosion resistance of soils during concentrated flow, which has attained much less attention than the well-studied rill and interrill erodibility (Poesen et al., 2003).
As almost any soil property seems to influence the soil erosion resistance during concentrated flow (see Knapen et al., 2006, for a summary of soil and environmental factors influencing Kc and τcr), the temporal (as well as spatial) variability of soil erosion resistance is not fully understood yet. Nevertheless, research revealed that a limited number of factors mainly controls the temporal variations in soil erosion resistance during concentrated flow. Nachtergaele and Poesen (2002) for instance concluded from the results of their laboratory flume experiments on undisturbed samples from different soil horizons (Luvisol) that soil detachment during concentrated flow throughout the year can be estimated reasonably well from soil moisture content alone. Other studies (e.g. Brown et al., 1989, Brown et al., 1990, West et al., 1992, King et al., 1995, Morrison et al., 1994, Shainberg et al., 1996) indicated that soil detachment during concentrated flow decreases considerably with time after tillage as a result of consolidation, i.e. any natural process (e.g. wetting and drying) that helps increase the soil stability through building cohesional strength. Experimental research revealed that other processes responsible for the intra-seasonal variations in soil erosion resistance during concentrated flow include root growth (Mamo and Bubenzer, 2001a, Mamo and Bubenzer, 2001b; Gyssels et al., 2006; De Baets et al., 2006), soil surface sealing and crusting, freeze–thaw effects (Van Klaveren and McCool, 1998) and decomposition of incorporated crop residue (Van Liew and Saxton, 1983, Brown et al., 1990). Although all these controlling factors were shown to have an effect on soil erosion resistance during concentrated flow under controlled laboratory conditions, the overall variability in the field resulting from the complex interaction of all these factors has not been quantified yet.
As tillage disturbance places the topsoil in a loose, erodible condition, applying any kind of conservation tillage (i.e. tillage treatments with crop residue management through reduced disruption of the soil compared to conventional ploughing) can be assumed to increase topsoil resistance to concentrated flow erosion. The increasing recognition of the need for conservation management systems requires knowledge on soil erosion resistance for different tillage practices. Conservation tillage practices differ in their degree of soil disturbance by tillage with different soil erosion resistance as a consequence. Nonetheless, research revealed that different conservation tillage systems all reduce soil loss during concentrated flow erosion (e.g. Laflen et al., 1985) by (1) slowing runoff and absorption of some of the forces that are usually applied to the soil surface (e.g. Cogo et al., 1984, Giménez, 2003) and (2) by decreasing the soil erodibility (Hussein and Laflen, 1982, Franti et al., 1985, Franti et al., 1999, West et al., 1992, King et al., 1995) and increasing the critical flow shear stress (Laflen et al., 1985, King et al., 1995). It is recognized that the effect of no tillage is small for a consolidated soil as crop residue decomposes over time (Brown et al., 1989, Brown et al., 1990, Norton and Brown, 1992, West et al., 1992, King et al., 1995). Yet, little information is available on the intra-seasonal variations of the beneficial effect of no-tillage on soil erosion resistance during concentrated flow.
This study aims to improve our understanding of the intra-seasonal variations of the erosion resistance of soils in loess in temperate climates during concentrated flow for different tillage practices. More specifically, the objectives of this study are:
- (1)
to investigate the intra-seasonal variation of soil erosion resistance during concentrated flow in terms of Kc and τcr for a typical winter wheat field on a loess-derived soil in central Belgium;
- (2)
to quantify the effect of applying conservation tillage on Kc and τcr over time for loess-derived soils;
- (3)
to determine the main soil and environmental parameters controlling the intra-seasonal variability of soil erosion resistance during concentrated flow for loess-derived soils.
Section snippets
Conceptual framework
In current process-based soil erosion models, the rate at which soil particles are detached by concentrated runoff is usually modelled as a function of the amount by which the erosive power of the water flow (expressed with a hydraulic variable, e.g. flow shear stress, stream power, flow discharge) exceeds the resistance the soil offers to this flow erosivity. Most often flow shear stress is used as hydraulic predictor variable to estimate the soil detachment capacity. The resulting soil
Study site
In order to measure the soil's erosion resistance during concentrated flow under naturally occurring conditions typical for the European Loess Belt, undisturbed topsoil samples were collected at regular time intervals in a winter wheat field at Huldenberg in the Belgian Loess Belt, near the city of Leuven (50°49′57″N, 4°36′2″E). The soil at the field site is a typical silt loam soil (Typic Haploxeralf or Haplic Luvisol) with 12% clay, 53% silt and 35% sand. A winter wheat field, following a
CP, DT and ST compared
Fig. 4 shows differences in soil detachment for CP, DT and ST on 16 March 2005. Soil erodibility during concentrated flow (Kc) is largest for the conventionally ploughed topsoil. The erodibility values, i.e. the slopes of the regression lines in Fig. 4, significantly differ for the three tillage practices (p < 0.01) and decrease with a decreasing degree of topsoil disturbance by tillage: Kc(CP) > Kc(DT) > Kc(ST). Residue seems to increase the critical flow shear stress whereas depth of tillage (DT
Conclusions
Our laboratory experiments on soil detachment by concentrated runoff for loess-derived soils showed an important variability in soil erodibility throughout the year (Fig. 5), with variations in both Kc and τcr of about an order of magnitude. When comparing the temporal variability in Kc with the spatial differences in Kc values between soils of different textures measured in the WEPP erosion experiments (Alberts et al., 1995), having a maximum difference of a factor 3, the importance of
Acknowledgment
This research was funded by the Fund for Scientific Research Flanders (F.W.O-Vlaanderen). The Belgian team of the SOWAP (Soil and Surface Water Protection using Conservation Agriculture in Northern and Central Europe) Life project and the Van Acker family are kindly thanked for enabling sampling at the three field plots of the Huldenberg field site and for providing weather data and useful background information on agricultural management.
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