Effectiveness of land use and soil conservation techniques in reducing runoff and soil loss in Europe and the Mediterranean

Runoff and soil loss caused by water erosion are major drivers of soil and land degradation throughout Europe and the Mediterranean. This study aims to better understand and quantify (1) plot-scale annual runoff (Ra) and annual soil loss (SLa), (2) the factors that control Ra and SLa over the wide range of environmental conditions in Europe and the Mediterranean, and (3) the effectiveness of different land use types and soil and water conservation techniques (SWCTs) in reducing Ra and SLa. A more detailed discussion of the knowledge gaps with respect to plot-measured Ra and SLa in Europe and the Mediterranean, and the specific objectives for this research is given in chapter 1.A database of Ra and SLa data measured throughout Europe and the Mediterranean on bounded runoff and soil loss plots under natural rainfall, with a measuring period (representative for) at least one year was compiled from the literature. The resulting database contains plot data from 1 409 plots, corresponding to 9 297 plot-years from 239 plot-measuring stations throughout Europe and the Mediterranean. The database contains Ra data for 804 plots (corresponding to 5 327 plot-years) and SLa data for 1 056 plots (corresponding to 5 327 plot-years) under conventional land management practice (see chapter 2). Furthermore, also Ra data for 287 plots (corresponding to 1 713 plot-years) and SLa data for 356 plots (corresponding to 2 035 plot-years) where SWCTs were tested were collected (see chapter 7). This study is both the largest compilation of plot SLa data in Europe and the Mediterranean to date, and the first to systematically include Ra data and data on plots where SWCTs are tested. A detailed discussion of the data included in the database is given in chapter 2 and chapter 7.Several knowledge gaps with respect to the effect of land use type on Ra and SLa over the whole of Europe and the Mediterranean are addressed in chapter 3. The analysis confirmed the important control of vegetation cover on Ra and SLa rates, with marked differences in both Ra and SLa between cultivated land (i.e. cropland, fallow plots, vineyards, tree crops), and semi-natural vegetation (i.e. shrubland, rangeland, forest, post-fire and grassland) for the whole of Europe and the Mediterranean. Generally, there is a good correspondence between Ra and SLa for the different land use types, but at the regional scale, differences were found between Ra and SLa rates for different climatic zones. Mean SLa values were smaller in the Mediterranean than in temperate and cold climatic zones, and mean annual runoff coeffient (RCa) rates were generally higher in the cold climatic zone than in the temperate and Mediterranean zones for similar land use types. Nevertheless, each land use type also comprises a wide variability in plot-measured Ra and SLa, and only weak relations were found between Ra and SLa and other environmental factors that are generally considered important determinants of Ra and SLa at the local scale such as plot length and slope gradient, indicating that these factors explain only a small part of the large variability in Ra and SLa that is observed at the continental scale.Part of the large uncertainty associated with plot Ra and SLa rates is associated with temporal variability. To explore the inter-annual variability in plot Ra and SLa rates, an analysis of 234 Ra and 307 SLa time-series with measuring periods equal to or longer than five years is performed in chapter 4. Temporal variability of RCa and SLa rates were shown to be related, but temporal variability in RCa is generally smaller than temporal variability in SLa. This was confirmed by a Monte-Carlo analysis which indicated that compared to SLa, shorter measuring periods are needed for plot measurements of RCa to allow the expected long-term mean RCa to be estimated with a specified degree of certainty. Nevertheless, uncertainties with respect to the estimation of long-term mean RCa and SLa can be large even after long measuring periods (i.e. 30 years). Closer examination of several environmental factors (i.e. climatic zone, land use type, plot length, slope gradient and annual precipitation) showed that these factors explain little temporal variability, and indicate that a large portion of the observed variability may indeed be random. Furthermore, there are substantial differences between temporal variability in plot-measured SLa and catchment sediment yield, and a better understanding of these differences can improve our understanding of differences in erosion processes between these spatial scales.Plot-measured Ra (804 plots corresponding to 5 327 plot-years) has received much less attention than SLa (1 056 plots corresponding to 7 204 plot-years) in Europe and the Mediterranean, both with respect to the reported data, as with respect to the analysis of Ra data at a continental scale. Therefore, a closer analysis of the annual rainfall (Pa) - annual runoff (Ra) relation is presented in chapter 5. In this assessment, two simple models were used; a linear mixed effects model and a modified Curve Number Method, adapted for annual data. Fitting of the models showed the important controlling effect of both land use type and soil texture (as expressed by the Hydrologic Soil Group) on the Pa-Ra relation. Contrary to expectations, fine-textured soils generally did not show the highest runoff response, which was attributed to the cracking behaviour of some clayey soils. An important effect of intra-annual precipitation distribution was expected, but this could only be demonstrated through simulation and not in the plot-measured data.A confrontation of the plot SLa data with two models for the estimation of interrill and rill erosion at a continental scale (i.e. the process-based PESERA model and empirical SEM model) in chapter 6 showed that there is a large variability in the relation between predicted and observed SLa data. This variability is partly attributed to temporal variability due to the fact that these models predict long-term mean SLa. Both models tend to under-predict SLa values for the Continental climatic zone, which is attributed to the fact that important processes such as freeze-thaw cycles and snowmelt erosion are not accounted for. Furthermore, improvements to both models can be made by using a land cover classification that is specifically designed for erosion studies, rather than the more general CORINE land covers. While it was shown in chapter 3 that land use management can be a very effective means of controlling both Ra and SLa rates, this is not always possible. In these instances, specific soil and water conservation techniques (SWCTs) are used to reduce Ra and SLa. A review of the effectiveness of different SWCTs in reducing plot-scale Ra and SLa in Europe and the Mediterranean is presented in chapter 7. This analysis showed that most SWCTs are on average more effective in reducing SLa than in reducing Ra. Furthermore, the importance of vegetation cover as a factor controlling Ra and SLa was further confirmed by the finding that crop and vegetation management (i.e. buffer strips, mulching, cover crops) are more effective in reducing Ra and SLa than soil management techniques (i.e. no-tillage, reduced tillage, contour tillage). However, the effectiveness of individual SWCTs in reducing Ra and SLa was found to be highly variable, suggesting several controlling factors that are unaccounted for. An important effect of the Ra and SLa rate measured on control plots with conventional treatment was found, and especially for smaller Ra and SLa rates, effectiveness of the SWCTs was more variable. Effects of environmental factors such as plot length, slope gradient or Pa on SWCT effectiveness could not be clearly identified. Analysis of the temporal variability of SWCTs showed that there is considerable inter-annual variability in the effectiveness of conservation tillage techniques. With respect to runoff reduction, the effectiveness of no-tillage techniques tends to decrease over the years.Finally, chapter 9 gives a synthesis of this research, along with a discussion of possibilities for further research.Voorwoord Abstract Samenvatting Abbreviations and symbols Contents Chapter 1. Introduction Chapter 2. A field plot database of annual runoff and soil loss for Europe and the Mediterranean Chapter 3. Effects of land use type on annual runoff and soil loss in Europe and the Mediterranean: a meta-analysis of plot data Chapter 4. Inter-annual variability of plot scale annual runoff and soil loss Chapter 5. Application of the runoff curve number method to predict annual runoff Chapter 6. Confrontation of measured soil loss plot data with model predictions Chapter 7. How effective are soil conservation techniques in reducing plot runoff and soil loss in Europe and the Mediterranean? Chapter 8. Conclusions References List of publicationsnrpages: 276status: publishe

Assessment of the effectiveness of soil and water conservation measures in reducing runoff and soil loss: establishment of a European database

Soil erosion by water is recognised as a major soil degradation process that requires agiobal approach. Large regions all over the world are in need of integrated conservation strategies that sustainably prevent and remediate soil erosion. Therefore, quantitative and globally interpretable data are needed in support of models and decision making. The effects of various soil and water conservation techniques (SWCT) on runoff and soil loss in Europe have been extensively studied over the last 60 years. Runoff plots are the most widely used measurement technique to study the effects of SWCT on runoff and soil loss by water erosion. Hence, many data are available. However, the insights gained hereby remain mostly local and aften qualitative whereas the full potential of the available data is not exploited yet. This is mainly due to the fragmentation of knowledge and extrapolation difficulties inherently linked with this type of data. This study addresses the issue of the availability of quantitative runoff and soil loss data from plot measurements in Europe and the quantitative assessment of the effectiveness of various SWCT in reducing runoff and soil loss. In the framework of the EU-DESIRE project, a database is currently compiled. The database currently contains 3175 plot-year data on runoff and soil loss measurements from 108 study sites in Europe and the Mediterranean. The SWCT documented herein include crop rotations, conservation tillage, cover cropping, mulching, vegetated buffer strips and terraces. From this dataset, an evaluation can be made of the effectiveness of various SWCT in reducing runoff and soil loss by water for a range of environmental conditions in Europe. A first example, regarding the effectiveness of SWCT in olive orchards, is discussed in this paper.status: publishe

How effective are soil and water conservation techniques in reducing annual plot runoff and soil loss? A pan-European and Mediterranean review and analysis

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Plot runoff and soil loss rates under forest and shrubland in Europe and the Mediterranean. - A review -

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Effects of land use on annual runoff and soil loss -a meta-analysis of a European and Mediterranean plot database-

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How effective are soil conservation techniques in reducing plot runoff and soil loss in Europe and the Mediterranean?

The effects of soil and water conservation techniques (SWCTs) on annual runoff (Ra), runoff coefficients (RCa) and annual soil loss (SLa) at the plot scale have been extensively tested on field runoff plots in Europe and the Mediterranean. Nevertheless, a comprehensive overview of these effects and the factors controlling the effectiveness of SWCTs is lacking. Especially the effectiveness of SWCT in reducing Ra is poorly understood. Therefore, an extensive literature review is presented that compiles the results of 101 earlier studies. In each of these studies, Ra and SLa was measured on field runoff plots where various SWCTs were tested. In total, 353 runoff plots (corresponding to 2093 plot-years of data) for 103 plot-measuring stations throughout Europe and the Mediterranean were considered. SWCTs include (1) crop and vegetation management (i.e. cover crops, mulching, grass buffer strips, strip cropping and exclosure), (2) soil management (i.e. no-tillage, reduced tillage, contour tillage, deep tillage, drainage and soil amendment) and (3) mechanical methods (i.e. terraces, contour bunds and geotextiles). Comparison of the frequency distributions of SLa rates on cropland without and with the application of SWCTs shows that the exceedance probability of tolerable SLa rates is ca. 20% lower when SWCT are applied. However, no notable effect of SWCTs on the frequency distribution of RCa is observed. For 224 runoff plots (corresponding to 1567 plot-year data), SWCT effectiveness in reducing Ra and/or SLa could be directly calculated by comparing measured Ra and/or SLa with values measured on a reference plot with conventional management. Crop and vegetation management techniques (i.e. buffer strips, mulching and cover crops) and mechanical techniques (i.e. geotextiles, contour bunds and terraces) are generally more effective than soil management techniques (i.e. no-tillage, reduced tillage and contour tillage). Despite being generally less effective, no-tillage, reduced tillage and contour tillage have received substantially more attention in the literature than the other SWCTs. Soil and water conservation techniques are generally less effective in reducing Ra than in reducing SLa,which is an important consideration in areaswherewater is a key resource and in regions susceptible to flooding. Furthermore, all SWCTs show a more consistent and effective reduction of both Ra and SLa with increasing Ra and SLa magnitude, which is attributed to the reduced influence of measurement uncertainties. Although some significantly negative correlations between SWCT effectiveness and plot slope length, slope gradient or annual precipitation were found, the importance of these factors in explaining the observed variability in effectiveness seems limited. Time-series analyses of Ra during multiple years of SWCT application strongly indicate that no-tillage and conservation tillage become less effective in reducing Ra over time. Such an effect is not observed for SLa.status: publishe

Sediment yield as a desertification risk indicator

Soil erosion is often regarded as one of the main processes of desertification. This has led to the use of various desertification indicators that are related to soil erosion. Most of these indicators focus, however, on small spatial units, while little attention has been given to the amount of sediment exported at the catchment scale. Such a small spatial unit approach neglects the transfer of sediment through catchments as well as the scale-dependency of erosion processes. Furthermore, this approach does not consider important off-site impacts of soil erosion, such as sediment deposition in reservoirs, flooding as well as ecological impacts.status: publishe

Soil loss rates due to piping erosion

Compared with surface soil erosion by water, subsurface erosion (piping) is generally less studied and harder to quantify. However, wherever piping occurs, it is often a significant or even the main sediment source. In this study, the significance of soil loss due to piping is demonstrated through an estimation of soil volume lost from pipes and pipe collapses (n=560) in 137 parcels under pasture on loess-derived soils in a temperate humid climate (Belgium). Assuming a period of 5 to 10years for pipe collapse to occur, mean soil loss rates of 2.3 and 4.6t ha-1 yr-1 are obtained, which are at least one order of magnitude higher than surface erosion rates (0.01-0.29t ha-1 yr-1) by sheet and rill erosion under a similar land use. The results obtained for the study area in the Flemish Ardennes correspond well to other measurements in temperate environments; they are, however, considerably smaller than soil loss rates due to subsurface erosion in semi-arid environments. Although local slope gradient and drainage area largely control the location of collapsed pipes in the study area, these topographic parameters do not explain differences in eroded volumes by piping. Hence, incorporation of subsurface erosion in erosion models is not straightforward.JRC.H.7-Climate Risk Managemen

Effects of land use on annual runoff and soil loss in Europe and the Mediterranean: A meta-analysis of plot data

The largest currently compiled database of plot runoff and soil loss data in Europe and the Mediterranean was analysed to investigate effects of land use on annual soil loss (SL), annual runoff (R) and annual runoff coefficient (RC). This database comprises 227 plot-measuring sites in Europe and the Mediterranean, with SL for 1056 plots (PL) representing 7024 plot-years (PY) and R for 804 PL representing 5327 PY. Despite large data variability, continental-wide trends are observed. Construction sites have the highest mean annual RC (57%) and SL (325 Mg.ha-1.yr-1). Bare soil, vineyards and tree crops have high mean annual RC (5–10%) and SL (10–20 Mg.ha-1.yr-1). Cropland and fallow show similar mean annual RC (8.0 and 7.3%), but lower SL (6.5 and 5.8 Mg.ha-1.yr-1). Plots with (semi-)natural vegetation cover show lowest mean annual RC (<5%) and SL (<1 Mg.ha-1.yr-1). Plot length and slope gradient correlations with R and SL depend on land-use type and are not concurrent for R and SL. Most land-use types show positive correlations between annual R and SL. Plots in cold climates have higher annual RC than plots in temperate and pan-Mediterranean climates. Annual SL in the pan-Mediterranean is less than in temperate zones, due to stony or clayey soils having a low erodibility. Annual RC in the pan-Mediterranean was higher than in temperate zones. Annual R increases strongly with increasing annual precipitation (P) above 500 mm.yr-1, while annual SL was found to stabilize at P > 500 mm.yr-1. For shrubland, annual SL was found to decrease for P > 250–500 mm.yr-1, which is attributed to an accompanying increase in vegetation cover. However, no such trend was found for R. The results allow a rapid assessment of the impact of land-use changes on annual R, RC and SL, based on field-measured plot data.status: publishe