Gwendolyn Gyssels

Impact of plant roots on the resistance of soils to erosion by water: a review

Vegetation controls soil erosion rates significantly. The decrease of water erosion rates with increasing vegetation cover is exponential. This review reveals that the decrease in water erosion rates with increasing root mass is also exponential, according to the equation SEP e b RP where SEP is a soil erosion parameter (e.g., interrill or rill erosion rates relative to erosion rates of bare topsoils without roots), RP is a root parameter (e.g., root density or root length density) and b is a constant that indicates the effectiveness of the plant roots in reducing soil erosion rates. Whatever rooting parameter is used, for splash erosion b equals zero. For interrill erosion the average b-value is 0.1195 when root density (kg m 3) is used as root parameter, and 0.0022 when root length density (km m 3) is used. For rill erosion these average b-values are 0.5930 and 0.0460, respectively. The similarity of this equation for root effects with the equation for vegetation cover effects is striking, but it is yet impossible to determine which plant element has the highest impact in reducing soil losses, due to incomparable units. Moreover, all the studies on vegetation cover effects attribute soil loss reduction to the above-ground biomass only, whereas in reality this reduction results from the combined effects of roots and canopy cover. Based on an analysis of available data it can be concluded that for splash and interrill erosion vegetation cover is the most important vegetation parameter, whereas for rill and ephemeral gully erosion plant roots are at least as important as vegetation cover.

Characteristics and controlling factors of bank gullies in two semi-arid mediterranean environments

Abstract Bank gullies are gullies that are formed due to a height drop caused by a terrace or a river bank, which develop by headward retreat in erodible hillslopes. This study aims (i) to investigate the morphology of actively eroding bank gullies, i.e., geometrical characteristics resulting from past erosion and active erosion processes shaping the gully, and, (ii), to find relationships with environmental site characteristics, such as topographical parameters, material properties and climate. The ultimate goal is to identify the most important controlling factors of past and present bank gully erosion. Fifty-five active bank gullies formed in different lithologies by various erosion processes have been selected in the Guadalentin basin and the surroundings of Guadix (Southeast Spain). For each bank gully site, geometrical and topographical parameters of both the channel and the drainage basin were measured. Erosion features indicating activity at the gully head, such as tension cracks, plunge pools, undercutting, fluting, piping and rill or sheet erosion on sloping side walls were mapped, and samples were taken from distinct lithological layers that were considered to influence the type and intensity of erosion processes. A relationship could be shown between the presence of piping and fluting and a number of material characteristics, including particle size distribution, dispersion behaviour and electrical conductivity. On the other hand, lithology appeared not to be a differentiating factor on gully development in the long run, as expressed by the total eroded volume ( V ). This parameter was most strongly related to the drainage basin area in which the entire bank gully had been formed ( A o ), explaining 66% of the variance. The relationship is V =1.75* A o 0.59 . No significant difference was found between regression lines through sub-datasets of different soil textural classes. Finally, multiple regression was used to include both topographical parameters and material characteristics in an explanatory and/or predictive equation for the total eroded bank gully volume. The results of the analyses using the entire dataset, including the sites in the Guadalentin as well as in the Guadix area, have been compared with the results for the separate study areas. Differences are not only related to topographical and lithological characteristics, but may also be the consequence of a different climate in the two areas.

Short-term bank gully retreat rates in Mediterranean environments

Abstract In this study, short-term headcut retreat was monitored from 46 active bank gullies, selected in the Guadalentin and the Guadix basin in Southeast Spain. The measurements were carried out manually using an orthogonal reference system fixed by erosion pins around the gully heads, between April 1997 and April 1999 with a 1-year interval. The average volumetric retreat rate for all gullies was 4.0 m 3 year −1 , corresponding with an average linear retreat rate of 0.1 m year −1 , but more erosion took place during the first monitored year (1997–1998) compared to the second (1998–1999). An interplay of spatial variations in rainfall distribution and tension crack activity is assumed to be responsible for the important difference in annual headcut retreat, compared to the small difference in annual rainfall amounts. Statistical analysis showed that the present drainage-basin area ( A p ) was the most important topographical factor explaining average gully headcut retreat rate, both in terms of annual eroded volume ( V e ) and annual linear retreat ( R l ), and expressed by the power relationships V e =0.04 A p 0.38 ( R 2 =0.39) and R l =0.01 A p 0.23 ( R 2 =0.39). The V e – A p relationship was compared with the relationship between original drainage-basin area ( A o ) and total eroded bank gully volume (Vol), i.e. Vol=1.71 A o 0.60 ( R 2 =0.65). The importance of runoff generation from a drainage basin is shown by the positive correlation of linear headcut retreat and the runoff curve number (CN), representative for the conditions in the drainage basin. High CN values tend to coincide with higher annual eroded volumes in the relationship between present drainage-basin area ( A p ) and annual eroded volume ( V e ), but this effect was not observed in the relationship between original drainage-basin area ( A o ) and total eroded bank gully volume (Vol). Stepwise multiple regression selected the relevant environmental parameters explaining annual eroded volume and linear retreat. In both equations, the present drainage-basin area explained the largest part of the variation. The CN was selected as another common parameter. Height of the headcut was the second most important variable explaining annual eroded volume, indicating the role of energy transfers and undercutting at the headcut. Linear retreat was further explained by the average slope of the present drainage-basin area, representing the effect of decreasing transmission losses and increasing flow velocity with steeper catchment slopes, and by the sand content, decreasing the cohesion of the soil material, promoting soil fall and headcut retreat. Spatial extrapolation of the measured volumetric retreat rate of 4.0 m 3 year −1 revealed that active bank gully heads contribute up to 6% of the sediment yearly filling up the Puentes reservoir. Estimated gully ages (i) based on the ergodic principle, and (ii) by linear extrapolation of actual gully retreat rates in the past, range between 63 years and 1539 and between 64 and 1720 years, respectively. The high correlation between the gully ages estimated by the two methods is attributed to the fact that most gullies have not reached the evolutionary stage of significantly declining retreat rates. Since medium-term gully retreat rates are more dependent on drainage-basin area compared to the short-term retreat rates obtained in this study, the estimated gully ages represent maximum values, assuming that present land-use and climate conditions prevailed over the last two millennia.

The impact of sowing density of small grains on rill and ephemeral gully erosion in concentrated flow zones

Despite the fact that soil erosion by water causes considerable on-site and off-site problems, farmers in Europe are reluctant to adopt prevention and control measures when such measures require additional labour and material inputs. This paper documents the impact of multiple sowing of small grains on concentrated flow erosion rates and grain production for a winter triticale field (X Triticosecale Wittmack ex. A. Camus) in the Belgian loess belt. Multiple sowing refers to drilling more than once in zones of concentrated flow erosion in order to increase the total root mass in this zone. Multiple sowing strongly altered the morphology of erosion channels and reduced soil loss significantly. Statistical analysis confirmed that differences in channel dimensions could be explained by the seedling density. Doubling the root mass in the topsoil by multiple sowing, resulted on average in a reduction of soil loss by 42% for the whole growing season. For the winter period, soil loss reduction, mainly attributed to the triticale roots, amounted even to 53%, showing the tremendous impact of seedling roots on soil erosion by concentrated overland flow in the early stages of vegetation growth. Furthermore, total grain yield in the multiple drilled zones was not significantly smaller compared to the conventionally drilled parts of the field. Grain size of cereals was slightly smaller. The results of this case study indicate that double sowing in concentrated overland flow zones may be a viable soil erosion control technique. # 2002 Elsevier Science B.V. All rights reserved.