G Model AGEE-3918; No. of Pages 11 ARTICLE IN PRESS Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards Tom Vanwalleghem a,∗ , Juan Infante Amate b , Manuel González de Molina b , David Soto Fernández b , José Alfonso Gómez a a b Instituto de Agricultura Sostenible, CSIC (Córdoba), Apartado 4084, 14080 Cordoba, Spain Universidad Pablo de Olavide (Sevilla), Spain a r t i c l e i n f o Article history: Received 6 August 2010 Received in revised form 31 May 2011 Accepted 4 June 2011 Available online xxx Keywords: Environmental history Historical soil erosion RUSLE Tillage Soil management Olive orchard Spain a b s t r a c t Olive orchards are an important agro-ecosystem in the Mediterranean. Soil erosion is a widely recognized threat to their sustainability. However, the variability of short-term soil erosion measurements and the limited understanding of driving processes result in a considerable uncertainty over the long-term effects of soil erosion. This study aims at measuring and modelling soil erosion rates in olive orchards over a 250-year period, and relating these to changes in management practices and yield, as documented from historical sources. In three study areas in S-Spain, the height of relic tree mounds was measured in olive orchards dated between 153 and 291 years old to determine soil profile truncation. Measured average soil erosion rates were between 29 and 47 t ha−1 year−1 . Historical documents allowed characterizing land management since 1752 in eight distinct periods. This information was then used to calibrate a soil erosion model, combining water and tillage erosion. The model reproduced the temporal patterns in soil erosion rates and showed considerable historical variation: between 8 and 124 t ha−1 year−1 for water and between 3 and 42 t ha−1 year−1 for tillage. Mainly due to improved agronomic practices, yield was not affected by soil erosion and has continuously increased over time. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The olive tree is a dominant crop in the Mediterranean basin. Economic relevance of olive cultivation has increased considerably since the second half of the 20th century. This coincided with serious concern about soil degradation due to severe erosion problems in many Mediterranean countries (Beaufoy, 2001). In response to this growing environmental threat, a number of studies started measuring actual soil erosion rates in olive orchards. Although the actual extension of severe erosion in olive growing areas is still under debate (Fleskens and Stroosnijder, 2007; Gómez et al., 2008, 2009a), available information from these studies has demonstrated that the current management, with regular tillage and the absence of vegetative cover among the trees, is highly unsustainable and results in an average soil erosion rate of 23.2 t ha−1 year−1 . However, very little is known about the long-term trends of soil erosion and its effects on the long-term sustainability of olive growing areas. In the following analysis, agricultural sustainabil- ∗ Corresponding author at: Institute for Sustainable Agriculture – CSIC, P.O. Box 4048, Avd. Menendez Pidal s/n, 14004 Cordoba, Spain. Tel.: +34 957 499 169. E-mail addresses: [email protected], [email protected] (T. Vanwalleghem). ity is considered in terms of the balance between soil erosion and soil production, following the approach by Montgomery (2007a). Unsustainable systems are then characterized by a net loss of soil profile thickness. It is generally recognized that in the long run, soil erosion rates over 0.001 m year−1 or about 12 t ha−1 year−1 will result in decreased agricultural potential, although yields might not be affected in the short run due to changes in management practices (Montgomery, 2007a). A long-term evaluation of sustainability can, however, not be based directly on field estimates of contemporary soil erosion rates. These extend to a few years at best (Gómez et al., 2008, 2009b,c; Taguas et al., 2010) so they probably do not account for the effect of the largest events that are responsible for most of the soil erosion under Mediterranean conditions (González-Hidalgo et al., 2009). Measurements are also limited to water erosion processes, even though it is well known that frequent plowing leads to important translocation of soil by tillage downslope. Experimental studies available for other land use types indicate that tillage translocation rates for mechanized agriculture, even for areas with gentle slopes, are typically in the order of 3–70 t ha−1 year−1 (Van Oost et al., 2006). Most importantly, however, soil management practices in olive orchards have changed significantly over the last centuries, which makes it particularly challenging to establish such long-term trends. Especially since 18th century, changes followed 0167-8809/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2011.06.003 Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 2 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx each other fast: improvement in tillage implements, changes of soil use in the area between the tree lanes, mechanization and introduction of herbicides among others (Guzmán, 1999). It can then be expected that also soil erosion rates varied significantly over time as a result of these changes. In general, it is accepted that the intensification of olive cultivation has led to severe erosion rates in many olive growing areas on sloping land (Beaufoy, 2001), especially in the last decades. It is implicit in the analysis of many experts that traditional olive orchards were sustainable before mechanization (e.g. Loumou and Giourga, 2003). However, at present, this assumption is not justified by any data. In fact, there are several indications that traditional olive orchards were far from sustainable. Existing studies point to the fact that cultivated Mediterranean areas have historically been affected by severe water erosion (e.g. Montgomery, 2007b). One important Mediterranean crop being olives, it is probable that these orchards were equally affected. In addition, technical reports made before the intensification of olive cultivation also reported relevant erosion problems, for instance Bennett (1960) in Spain. In order to clarify the issue of the long-term environmental sustainability of olive orchards, it is necessary to determine soil erosion trends over a long period of time and under changing soil management conditions. While several methods exist to do this, most of these are unsuitable to unravel variations in soil erosion rates with a high temporal and spatial resolution. Long term erosion rates obtained through analysis of sediment cores taken from depositional areas (e.g. Valero-Garcés et al., 2006) tend to lump the effect of different areas and land uses. In situ studies of soil truncation could offer a valuable alternative as the uncertain relation between sediment and source is eliminated, but do not have the temporal resolution to evaluate the impact of changing soil management. The only long-term estimates of soil erosion rates in olive orchards are by Vanwalleghem et al. (2010), who studied soil profile truncation by means of tree root exposure. They reported an average soil loss of 95 t ha−1 year−1 over a period of 55–100 years. However, these data were gathered in very specific conditions, with steep slopes (between 10% and 33%) and erodible soils, and cannot be extrapolated to other areas. More importantly, however, this method of reconstructing soil profile truncation only yields a single soil loss rate, averaged over the entire period under study. Therefore, in order obtain insight about the temporal variation of historical erosion rates in olive orchards, the estimation of soil truncation needs to be combined with a modelling approach. Application of soil erosion models on historical time scales is often hampered by the difficulties of correctly calibrating the input parameters. Because of the absence of historical documents, input parameters are usually not derived from existing data on soil management. For instance, Peeters et al. (2006) use a single input parameter set for a period over nearly 2500 years when modelling soil erosion in the Belgian loess belt. This set was determined through parameter optimization techniques, minimizing the difference between modelled and observed total soil loss. A detailed historical reconstruction of soil management is therefore paramount for the application of soil erosion models over longer time periods. In summary, at present, the lack of available field data on the one hand and the limited historical knowledge on input factors for erosion models prevent evaluating long-term soil erosion rates in olive-growing areas and therefore difficult making well-founded recommendations towards soil management. Furthermore, in order to understand the evolution of historical soil management, it is necessary to analyze whether the levels of production show a signal of these high soil erosion rates. An absence of such a warning signal could explain why unsustainable soil management was prolonged over time in these systems without taking into account soil conservation measures. Fig. 1. Location of the study area within SW Spain. Therefore, the general objective of this study is to assess the long-term sustainability of olive cropping in mountainous areas of the Mediterranean by reconstructing the temporal variation of soil management and the resulting soil erosion rates. The specific objectives of this study are then (i) to document and evaluate the evolution of soil management and yield in olive orchards in a representative area in Spain over a 250-year period from historical documents; (ii) to quantify the total historical soil erosion rate over the same period by field measurement; (iii) to calibrate a soil erosion model that includes water and tillage erosion processes and is based on the evolution of soil management documented in (i); (iv) to analyze the contribution of both erosion processes and their historical evolution during the 250 year period and (v) to assess whether the temporal variation in soil erosion rates caused a signal in olive yield. 2. Materials and methods 2.1. Study area The study area is located in Andalusia, SW Spain in the municipality of Montefrío, about 40 km NW of the city of Granada (Fig. 1). The climate is Continental Mediterranean with an average annual rainfall of 550 mm and an average annual temperature of 15.2 ◦ C. The region is hilly, with altitudes ranging from 714 to 1100 m, and a long tradition of olive farming. The study area was selected because of the availability and quality of the historical sources required to document the management and age of the olive plantations. Historical documents were available since 1752, hereby covering a period of around 250 years. Based on the quality of the information available about the farm sites, three representative sites were selected. Field measurements at these sites were done in 2009. The soils at these sites are classified as Calcic Cambisol according to the FAO classification. The basic soil properties at each site are described in Table 1 Table 1 Basic soil properties at the three study sites. Site Name Soil properties by horizon Depth (cm) Texture USDA OM (%) 1 Juan Ángel 0–66 66–128 >128 Silty-clay-loam Silty-clay-loam Silty-clay-loam 1.87 0.91 0.42 2 Hundideros 0–35 35–70 >70 Clay Clay Clay-loam 0.71 0.18 0.27 3 Silveria 0–17 17–78 Clay-loam Clay 1.05 0.42 Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx 3 2.2. Historical sources The earliest historical documents that could be found date from 1752, which sets the time frame under study to 257 years (1752–2009). This period was then subdivided in different subperiods, based on the information found in later historical sources. Each useful historical source that could be found defines in this way a new subperiod. This assures the maximum temporal resolution in the analysis since the characterization of soil use and management of each of these subperiods was later used as an input in the calibration of the erosion models. Since 19th century many agronomic reports give clues about the management of olive orchards (Rojo Payo, 1840; Esponera, 1851; Dirección General de Agricultura, Industria y Comercio, 1891; Hidalgo Tablada, 1899). Based on these, historiography has rebuilt the management related to the history of the olive groves. A systematic summary on the subject can be found in Zambrana (1987). The main problem is that these all bring a general, national vision of management. It is well know that soil management varies locally according to factors such as soil type, slopes or variety. Therefore, existing generalist sources are not appropriate and the focus of the historical documentation work was on the documentary sources that exist at the local level. Even though this implies more complex documentary work, only these sources allow reconstructing local soil management of the olive over the past 257 years. The following summary includes all local sources that were found and used: (i) Notarial Archives of Granada, account partition of María Josefa López Soto, clerkship Don Francisco Diaz de Otazu, single volume, Montefrio, 1752. (ii) Montefrío Municipal Historical Archive, “Amillaramientos”, several bundles of papers, 1856–1895. (iii) Montefrío Municipal Historical Archive, “Trabajos Agronómicos de la Junta Consultiva Agronómica”, bundle of paper, 1896. (iv) IRYDA Archive in San Fernando de Henares. Bundle for the province of Granada. In this case, data from a farm in Moclín was used. In Montefrío, documentation of expropriation could not be matched and Moclín is the nearest village and similar to the study area. Reference: Servicio Provincial de la Reforma Agraria. Estudio agrosocial del término de Moclín. Granada, February 1935. Before 1850 the only way to know what was happening at the municipal level is to analyze notarial archives (i). This source includes documents such as “postmortem inventories” or “partitions accounts” in which some owners were collecting crop expenses. Files from the mid-18th century to the mid-19th century were reviewed. Although documentation is scarce, it is a more reliable source than tax documents because here there was no incentive for concealment. Between 1850 and 1905 systematic tax documents were found (ii). These so-called “Cartillas evaluatorias” documented, year by year, expenditures for each crop and, therefore, the olive grove in each municipality. In subsequent years, the “Junta Consultiva Agronómica” continued the same reports with so-called “Trabajos agronómicos” (iii). For 1935 there is another important source derived from the agrarian reform of the Second Republic. From this source, documents of expropriation of farms can be found, detailing the farm expenses of the same locality (iv). Finally, from 1950 to 2009, the main source was the oral history. This included interviews with various farmers and reviewing Montefrío agronomic reports as “Hojas Divulgativas” of the Ministry of Agriculture, Fisheries and Food. Fig. 2. Mound formed around a tree at site 2 – Hundideros. The actual soil surface is indicated by the continuous black line, while the dashed line indicates the position of the original, eroded soil surface. The difference between both surfaces corresponds to the eroded soil profile (h). 2.3. Field measurements The total soil profile truncation attributed to soil erosion (water and tillage) since the implantation of the orchards was determined using the methodology proposed by Vanwalleghem et al. (2010). This method uses the height of the mounds that form around the tree trunks in old plantations as a proxy for the position of the original soil surface (Fig. 2). The survey was done in Spring 2009. At each of the three sites, between 14 and 21 tree mounds were measured, covering a hillslope section of around 2000 m2 within each orchard. At each site, the objective was to cover a hillslope section from the top drainage divide to the bottom drainage line, which could be considered equivalent to a USLE-type plot. This was to allow direct validation of modelling results with field measurements. Both the actual soil surface and the height of these tree mounds were measured in detail with a total station. After interpolation of both surfaces, the difference between both surfaces was used to calculate the total volume of eroded soil. The interpolation method that resulted in the best fit (lowest RMSE and no artefacts) in this case was the fit of quadratic surfaces. The age of the plantation, that sets the starting time for tree mound development by erosion, was determined from historical documents and interviews. Using the eroded volume, tree age and bulk density, the total average soil erosion rate for each site is calculated. A minimum and maximum value was calculated to cover the uncertainty related to the age of the plantation. Additionally, at each site, a soil profile pit was dug to classify the soil type and soil samples taken from each horizon to determine soil properties. These were also used in the calibration of the erosion model. 2.4. Calibration of erosion models The used erosion model needs to account for the two main processes of soil redistribution: water and tillage erosion. 2.4.1. Water erosion Estimates of the long-term water erosion losses (Ewater , t year−1 ) were made using the Revised Universal Soil Loss Equation (RUSLE, Renard et al., 1997). RUSLE is an erosion model designed to predict the long-time average annual soil loss, A, carried by runoff Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 ARTICLE IN PRESS G Model AGEE-3918; No. of Pages 11 T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx 4 Table 2 Data used to calculate mean soil displacement by tillage in function of slope for a variety of tillage implements. Tillage operation b Roman plow, animal traction Mouldboard plow, animal traction Harrow, animal traction Mouldboard plow, mechanical traction (Disc) harrow, mechanical traction a b Tillage depth (m) aa bb Source Comment 0.1 0.15 0.05 0.25 0.1 0.5 0.58 0.58 0.99 0.99 −0.0006 0.24 0.24 0.3 0.3 Dercon et al. (2007) Gerontidis et al. (2001) Gerontidis et al. (2001) Gerontidis et al. (2001) Gerontidis et al. (2001) Parallel plowing Plowing direction: contour; depth 0.2 m Plowing direction: up/down; depth 0.3 m Regression coefficients in Eq. (5). A roman plow is a non-inverting plow with an iron share (Lal et al., 2007). from specific field slopes in specific crop and management systems, computed as Eq. (1): A=R K L S C P (t ha−1 (1) year−1 ) where A is the average annual soil loss, R (MJ mm ha−1 h−1 year−1 ) is a rainfall erosivity factor; K (t ha h ha−1 MJ−1 mm−1 ) is soil erodibility; L, is the slope length factor; S, is a slope gradient factor; C, is a cover-management factor; and P, is a support practice factor. This erosion prediction technology was chosen because it is appropriate to compare the relative differences in soil losses between different soil managements when limited information available (Wischmeier, 1976), and also because a methodology for calibration had been previously published for olive orchards under different soil managements (Gómez et al., 2003). The factors C, P, K and R were derived for each of the management periods that were identified in the analysis of the historical soil management, and LS from topographical information. The R factor was derived from a combination of historical yearly precipitation data calibrated against yearly and daily rainfall data for short term periods in Montefrío. Available historical precipitation data was expressed on a hydrological year basis (September–August). In the following discussion, yearly precipitation data therefore refer to hydrological years. For the study area instrumental, albeit low-quality, daily rainfall data is available since 1918. After quality checking of this data, 10 years were retained for analysis. These were the years that contained complete daily rainfall data without missing periods. Afterwards, daily R values were calculated for this dataset using the following regression relation between EI30 (MJ ha−1 mm h−1 ) and daily rainfall (RFdaily , mm), established by Ayuso Ruiz (2010) for Granada, Eq. (2): 1.8343 EI30 = 0.0893 RFdaily R2 = 0.61; n = 283 (2) Aggregating daily into yearly records, a regression relation was then established between yearly rainfall (RFanual , mm) and yearly erosivity R (MJ ha−1 mm h−1 year−1 ), Eq. (3): R = 1.19 RFannual − 128.11 R2 = 0.82; n = 10 (3) This relation was then used to reconstruct R for the entire 257-year period used for the simulation using annual rainfall. Between 1752 and 1837, annual rainfall was derived from the time series reconstructed by Rodrigo et al. (1999), based on documentary sources. After 1837, rainfall was derived from instrumental records in the station of San Fernando, which holds one of the longest rainfall records in the Mediterranean. Annual rainfall at this station (RFannual, SF , mm), at a distance of about 250 km, correlated well with the annual rainfall in the study area (RFannual, MF , mm), so Eq. (4) was used to convert RFannual, SF to RFannual, MF. RFannual, MF = 1.07 RFannual, SF + 50.1 (R2 = 0.77, n = 10) (4) The LS factor was calculated based on the present-day measured topography and was considered constant throughout the simulated period using the expressions of McCool et al. (1989) indicated in the RUSLE documentation. The choice for using the actual topography rather than the original one was made to assure that future studies could use the model calibration of this study. The effect of using the actual instead of the original topography, as reconstructed from the tree mounds, appeared to have a limited impact on the model simulations, as both resulted in comparable LS factors. Final modelling results were affected only slightly: simulated cumulative soil truncation values differed on average 6% (for tillage erosion) and 14% (for water erosion). Calibration of the K factor was based on soil properties that were either measured directly or derived from comparison with present-day orchards under soil management comparable with those existing in each of the subperiods. Such values were derived from existing studies in present-day organic olive groves (Gómez et al., 2003, 2009b,c; Álvarez et al., 2007). It is assumed that the closest match for the historical soil management in each of the subperiods is provided by these organic groves. Texture was assumed constant during the entire period that was simulated. The actual stoniness of the sites was taken into account to calculate K. As the depth-distribution of stoniness was not known for the eroded profiles, it was assumed constant over the simulated period. Temporal changes in K with changing soil management and tillage intensity were then the result of variations in the input of soil organic carbon, soil structure and permeability according to the most probable values from studies in low-intensity orchards in the area (Álvarez et al., 2007). Table 3 summarizes the values used for each management period. Table 3 Summary of the soil properties used for the calibration of the K factor in RUSLE during each of the subperiods. Period Duration Equivalent systema , b Permeability classc Structure classc Organic matter (%)b Textureb 1 2 3 4 5 6 7 8 1752–1856 1856–1888 1888–1896 1896–1935 1935–1950 1950–1970 1970–1990 1990–2009 VG OT OT OT VG VG OT av 2 3 3 3 2 2 3 4 3 3 3 3 3 3 3 4 2.01 1.86 1.86 1.86 2.01 2.01 1.86 av av av av av av av av av a Present-day equivalent system, based on analysis of 13 organic olive groves in Álvarez et al. (2007). VG: organic olive orchard with vegatative cover among the trees, seasonally extensively grazed. OT: organic olive orchard under tillage. b av = measured actual value. c Following the definition in Renard et al. (1997). Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 ARTICLE IN PRESS G Model AGEE-3918; No. of Pages 11 T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx Table 4 Temporal variability of the half-month variablesa used in the calibration of the subfactors of the C factor in RUSLE for two contrasting soil management situations, with maximumb and minimumc vegetation cover, respectively. Timed J-1 J-2 F-1 F-2 M-1 M-2 A-1 A-2 M-1 M-2 J-1 J-2 JL-1 JL-2 A-1 A-2 S-1 S-2 O-1 O-2 N-1 N-2 D-1 D-2 VM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 RU VC GC Max Min Max Min Max Min Max Min 0.37 0.34 0.32 0.34 0.37 0.32 0.29 0.28 0.27 0.29 0.33 0.38 0.35 0.33 0.31 0.30 0.24 0.23 0.23 0.26 0.30 0.34 0.38 0.40 0.38 0.36 0.36 0.39 0.39 0.38 0.37 0.35 0.34 0.34 0.38 0.38 0.35 0.33 0.31 0.30 0.24 0.23 0.23 0.26 0.30 0.34 0.38 0.40 1.02 0.99 1.06 0.99 0.93 0.84 0.78 0.74 0.72 0.65 0.60 0.58 0.55 0.55 0.55 0.55 0.55 0.55 1.09 1.06 1.06 1.06 1.06 1.06 0.59 0.57 0.55 0.52 0.50 0.47 0.44 0.43 0.42 0.39 0.38 0.99 0.91 0.85 0.82 1.01 0.98 0.92 0.89 0.59 0.59 0.59 0.59 0.60 0.83 0.93 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.18 0.13 0.13 0.13 0.13 0.13 0.18 0.23 0.33 0.43 0.53 0.63 0.73 0.23 0.25 0.28 0.30 0.13 0.15 0.18 0.20 0.23 0.28 0.23 0.20 0.13 0.13 0.13 0.13 0.13 0.18 0.20 0.23 0.13 0.15 0.18 0.20 0.11 0.08 0.06 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.43 0.41 0.38 0.36 0.33 0.31 0.28 0.26 0.23 0.21 0.18 0.16 0.13 0.03 0.03 0.03 0.03 0.13 0.08 0.06 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 a VM: average volumetric moisture content, RU: surface roughness (maximum value of tillage-induced and vegetation-induced roughness), VC: vegetative cover (sum of fixed tree canopy cover and variable vegetative ground cover), GC: ground cover (sum of variable plant residue and stone cover). For a complete description of the calculus of the subfactors Sm , Sr , Cc and Sc , see Renard et al. (1997). b Max: maximum vegetation cover corresponds to subperiod 5, with cereal intercropping between the olive trees. c Min: minimum vegetation cover corresponds to subperiod 7, with intensive mouldboard tillage. d 1 = first half and 2 = second half of month from January to December. The C and P factors were calibrated based on the description of the orchards and soil management obtained from the historical sources and field visits. Each of the subfactors was calibrated according to the RUSLE documentation, following the methodology proposed by Gómez et al. (2003). Table 4 illustrates the calculation of the half-month variables necessary to calculate the subfactors. Two soil management situations were compared: the first corresponds to the management with the highest vegetation cover (max: see below, subperiod 5 with cereal cropping among the trees) and the latter to the management with the lowest vegetation cover (min: see below, subperiod 7 with intensive tillage). The surface moisture subfactor (Sm ) was calculated based on the typical soil moisture for each management system corresponding to each of the identified subperiods. To do this, the soil water balance model OLIVCROP (Castro et al., 2006) was executed for an average year for each subperiod in the study area. Sm was then linearly related to the simulated soil moisture, with a value of 1 at field capacity and a value of 0 at permanent wilting point. As can be seen from Table 4, the annual soil moisture reflects the typical Mediterranean climate, with lowest values in summer and higher values during winter, and it reflects the increased transpiration with increasing vegetation cover. This is why soil moisture is depleted more in spring in the management scenario with a high vegetation cover. The subfactor surface roughness (Sr ) was calculated based on the implement, number and timing used for tillage, and the vegetation that could develop between operations. The latter was based on observations in current orchards under similar soil management. The values reported in Table 4 are then the maximum of the tillage-induced roughness, which is set to a maximum value after each tillage operation and then gradually declines, and the vegetation-induced 5 roughness, which gradually increases after each tillage operation as vegetation grows. The latter only plays a role in the scenario with highest vegetation cover where vegetation is allowed to grow. The canopy cover (Cc ) subfactor was calculated based on the type of cover and siege date applied in each management period, using a ground cover by olive trees of 13%. This number was based on measurements made during the field visits. It was assumed that cover crops yielded complete coverage. The vegetative cover reported in Table 4 is then the sum of the fixed olive tree cover and the variable cover crop cover. In the scenario with minimum vegetation cover, the frequent tillage again prevents the development of the crop cover and the vegetative cover is equal to the olive tree cover. In the scenario with maximum vegetation cover, the cover crop is allowed to grow linearly up to a full coverage. The surface cover factor (Sc ) is calculated from the ground cover, which is the sum of a variable crop residue fraction and the stone cover (Table 4). Again, for the minimum vegetation scenario, the crop residue fraction is zero during most of the year, so the ground cover is equal to the stone cover. For the maximum vegetation scenario, the crop residue fraction reaches a maximum of 0.40 in the second half of June, after the cereal crop is harvested and harvest residue is tilled in. Based on field observations, a fixed 3% ground cover by rocks was considered. Factor P was considered constant through time at each site. We assigned a P value of 0.9 to sites 1 and 2, since it was reasoned that the high slope gradient of these sites made contour cropping imperative. As site 3 has a lower slope gradient, we did not impose the same condition of contour tillage and assigned a P value of 0.7. 2.4.2. Tillage erosion The main factor controlling the translocation of soil by tillage is slope gradient, as it is a gravity-driven process (Van Oost et al., 2006). Although a net movement occurs on sloping land, if the slope is constant, as long as the influx of soil material at each point equals the outflux, no net soil loss occurs. However, as the displaced soil at the upper divide of each hillslope section is not replaced, each tillage pass will result in a net soil loss which is directly proportional to the mean soil displacement distance in the tillage direction. Therefore, to calculate the net soil loss by tillage, each of the three study sites was approximated by a rectangular plane of length L and width W, which were determined through field measurements. The mean displacement distance di for a specific tillage implement was derived from field experiments, measuring tracer displacement, available in the literature. Field experiments usually measure tracer displacement as a function of slope (S) and report regression coefficients of the relation between both for each implement, Eq. (5). di = ai S + bi (5) The coefficients, a and b, that were used, including animal traction and mechanized agriculture, are summarized in Table 2. Using the historical data on the type of implement used during each management period in the study area, tillage depth (Di ) and number of passes per year (ni ) with each implement, the total yearly soil loss (Etillage in t year−1 ) for each of the 8 subperiods was then calculated according to: Etillage = ni W Di di (6) i where i represents the number of different implements used in each agricultural year and is the soil bulk density (t m−3 ). Note that some studies (e.g. Li et al., 2007) have indicated feedback mechanisms between tillage and water erosion where one process works as a delivery mechanism for the other. As a result, both are not always additive. However, as the areas that are Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx 6 Fig. 3. Evolution of olive orchard yields (fruit and wood) during the study period. simulated are relatively small, similar to rectangular planes and therefore equivalent to USLE-type plots, there are no areas of colluviation considered. Such interactions are therefore not taken into account. 3. Results 3.1. Historical evolution of soil management and yield Table 5 summarizes the evolution of soil management in the study areas since 1752 according to historical sources. The historical sources allowed distinguishing eight different periods, which correspond in most cases to significant changes in soil management. Fig. 3 summarizes the average annual yield from the olive orchards, which can be understood to a large extent from the soil management changes. Olive orchards generally yield three products: olives, firewood and small wood. The latter refers to twigs and small branches, for example to make baskets. Period 1, from 1752 to 1856, was a period of limited intensification in which the olive trees’ main yield was wood in addition to a low yield of olive fruits used for oil production. This low intensification resulted in one yearly tillage pass with an animal-drawn, roman plow, aimed to maintain the growth of vegetation among the trees under control. During summer, vegetation was manually reaped. Overall, this system allowed considerable vegetation cover and relatively large surface roughness during most of the year. As from Period 2, from 1856 to 1888, intensification of olive cultivation and specialization in olive fruit (rather that wood and fruit combined) starts. This is apparent in the increase in olive yield, with annual yields increasing from around 200 to about 1000 kg ha−1 . This can be explained by an intensification in orchard management (pruning and tillage), related to the expansion of the local and national markets after the gradual abolition of Feudalism (Guzmán, 2005; Zambrana, 1987). The main change in soil management is more frequent tillage with two passes per year and one additional pass with a harrow. In Period 3, between 1888 and 1896, the roman plow is replaced by a mouldboard plow. The number of operations remains the same. Although this seems to have a minor effect on orchard yields, it should have resulted in a better incorporation of vegetation stubble into the soil and increased surface roughness. Also, during this period, with the agrarian economic crisis of the 19th century, Spanish farmers needed to reduce costs in order to be more competitive with respect to cheaper imported oils (Ramón-Muñoz, 2000; Zambrana, 1987). During Period 4, from 1896 to 1935, the soil management remains unaltered, and olive yield remained almost constant compared to the previous period. The small yield decrease in 1896 is possibly due to the increase of olive plantations in the area. A larger proportion of young trees are probably what lowered the area average for olive and wood production. Finally, after Period 4, wood from olive orchards looses its commercial value. Therefore, after 1935, no reliable numbers can be quoted in Fig. 3. From 1935 to 1970, the need for cereals in the Spanish market, resulted in a major change in the management of the olive orchards in the study area. Cereals (wheat or barley) were cultivated in the interrow areas between the trees. This change was probably driven by an increased demand for food crops since the mid-1930s, especially during and after the Spanish Civil War, and due to the autarkic policy of the Franco regime that crippled trade relations and caused flourishing of black markets in agricultural production (Barciela, 2003). Cereal cropping implies a significant increase in ground cover from spring to summer. This time period can be further subdivided from 1935 to 1950 (Period 5), where still two tillage passes were made, and from 1950 to 1970 (Period 6), where only one pass with the animal-drawn mouldboard plow was made. Despite this intercropping, increased inputs and more intensive management (pruning, pest control, fertilization) resulted in a significant increase in olive yield. After 1970, major changes occurred again. During Period 7, from 1970 to 1990, the interrow cultivation of cereal is abandoned and mechanized farming is introduced in the area. This resulted in more frequent plowing, with at least two passes per year with the mouldboard plow and between two and four – more shallow – passes with the harrow. This intensification of tillage resulted in an effective control of vegetation growth among the trees and subsequently very low ground cover during the whole year. Increased intensification and lack of competition with the cereal intercrop resulted in a further increase in olive yield compared to the previous period (Fig. 3). From 1990 to date, Period 8, the mouldboard plow operations were replaced by weed control through herbicides, although harrowing during summer months is maintained. These changes allowed maintaining a high control of vegetation growth and low ground cover but resulted in a higher soil consolidation and reduced surface roughness compared to the previous period. At present, annual olive yields in the study area are around 2500 kg ha−1 . 3.2. Cumulative soil losses during the study period Fig. 4 depicts the actual and reconstructed soil surface of the olive orchards in the three study sites, and the differences in ele- Table 5 Summary of the 8 main soil management phases. Numbers indicate the number of yearly tillage passes. Period Year Animal roman plow & harrow Animal mouldboard plow & harrow Mechanized mouldboard plow & harrow Roller Herbicides Cereal associated (inter-row) Harrowing Manual reaping 1 1752 1 2 1856 3 1888 4 1896 5 1935 6 1950 7 1970 8 1990 2009 2 2 2 2 1 2 1 1 1 1 1 1 3 3 Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx 7 Fig. 4. Digital elevation models of the actual soil surface, the reconstructed soil surface and the resulting soil loss map for the tree study sites. The measured mounds are indicated by white dots in the latter. Table 6 Summary of the site characteristics and measured historical soil erosion rates. Study site Site 1. Juan Ángel Site 2. Hundideros Site 3. Silveria Average Tree age (years)a Bulk density (ton m−3 ) Number of measured mounds Average slope (m m−1 ) Surface area measured (m2 ) Total mean soil profile truncation (m) SD (m)b Min soil profile truncation (m) Max soil profile truncation (m) RMSE (m)c Mean soil erosion rate (t ha−1 year−1 )d 257–291 1.44 14 0.24 1834 0.48 0.18 0.25 1.20 0.03 23–34 153–189 1.73 16 0.38 1897 0.52 0.25 −0.02 1.64 0.04 38–63 153–189 1.70 21 0.15 2161 0.35 0.12 0.17 0.83 0.03 25–43 188–223 1.62 17 0.26 1964 0.45 0.18 0.13 1.22 0.03 29–47 a Range reported is minimum and maximum tree age (see text for details). Standard deviation on the measured total soil profile truncation. c Root mean squared error for the interpolation of the former soil surface. d Minimum mean soil erosion rate = [(total mean soil profile truncation − 1.96RMSE)/maximum tree age] × bulk density × 10,000; maximum mean soil erosion rate = [(total mean soil profile truncation + 1.96RMSE)/minimum tree age] × bulk density × 10,000. b Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx 8 Fig. 6. Reconstructed historical rainfall in the study area and calculated rainfall erosivity R, averaged per management period. Fig. 5. Average annual erosion rates by water erosion as calculated by RUSLE during the study period. vation between both. They allow the determination of an average value of the total soil profile truncation at these sites: 0.48, 0.52 and 0.35 m for sites 1 – Juan Angel, 2 – Hundideros, and 3 – Silveria, respectively (Table 6). The spatial variation of the profile truncation within each field is considerable and ranges between −0.02 m and 1.64 m for all sites. The negative value indicates deposition. The age of the olive trees is at site 1 is between 257 and 291 years. The minimum age corresponds with the first appearance of the olive orchard in the historical documents of 1752. Although no older documents are available, the maximum age is set based on information in the same historical documents that mentions most orchards in the area to be newly planted and based on additional interviews with farmers that report the planting date to be 1718. The olive orchards at sites 2 and 3 are between 153 and 189 years old. They are mentioned for the first time in 1856 and, based on local documentary sources, the maximum planting date is 1820, which is corroborated by an important area-wide growth of olive plantations. Field measurements showed that soil bulk density ranged from 1.44 to 1.74 t m−3 depending on the site. Five samples were taken per site and corrected for rock fragment content. However, these differences result probably from current farming operations and there was no reason to suppose that they should be significantly different among the sites during the study period. Therefore, an average bulk density of 1.62 t m−3 for the three sites was used in all calculations. Taking into account the uncertainty related to tree age and the uncertainty related to the interpolation of the former soil surface, as expressed by the root mean squared error (Table 6), a minimum and maximum value was calculated for the mean soil erosion rate. The long-term average mean soil erosion rate was between 23–34, 38–63 and 25–43 t ha−1 year−1 for Juan Angel, Hundideros, and Silveria, respectively. Based on all three sites, the average soil erosion rate between the second half of the 18th century and today would be between 29 and 47 t ha−1 year−1 (Table 6). servation perspective are when the areas between the olive trees was used to grow cereal during periods 5 and 6 (from 1935 to 1970). Nevertheless, even during these periods, erosion was still considerable: between 8 and 20 t ha−1 year−1 . This is due to the high slopes in this mountainous area. It is also remarkable that during the starting periods of olive cultivation in the region with less intensive management methods, periods 1–4 up to 1935, predicted erosion rates still were relatively high, from 12 to 55 t ha−1 year−1 . It is clear that these observed variations are mainly due to management. Some differences in rainfall erosivity were observed among the several periods considered in our study (Fig. 6). In particular, in periods 2 (1856–1888) and 3 (1888–1896), the R-value is higher compared to the long-term average. However, as can be seen in Figs. 5 and 6, as C, P and K are low during these periods, the resulting soil erosion rates remain low. The general trend of the overall water erosion rates follows the changes of the factors considering the impact of soil management on soil properties: soil erodibility (K factor) and surface cover and management (C and P factor). Fig. 7 shows the values of the C and P factors for the three sites and periods, depicting large differences in the erosion risk due to changes in cover and management for each site. Fig. 8 presents a similar graph comparing the values of the K factor for the different sites and periods. Differences in soil erodibility are observed among the different sites and are partially responsible of the differences in predicted erosion rates among sites. So site 3 (Silveria), which has a LS factor similar to the one from site 1 (Juan Angel), 5.6 and 6.2, respectively, presents a lower erosion rate (Fig. 5), due to its lower soil erodibility. On the other hand, site 2 (Hundideros), despite having the lowest soil erodibility, is characterized by the highest erosion rates due to the higher slope steepness (Table 6), 3.3. Modelled soil erosion The historical distribution of water erosion can be temporally disaggregated based on the RUSLE analysis. The reconstructed historic erosion rates per site and period are presented in Fig. 5. The periods with the highest erosion rates have been the two most recent periods (7 and 8) in which intensive soil management has been oriented to maintain soil bare of vegetation among the trees during most of the year. This has been obtained by herbicide application and plowing in summer since 1990 and by all-year round plowing between 1970 and 1990. Present-day modelled soil losses are between 43 and 124 t ha−1 year−1 . From this modelling analysis it becomes clear that the best management periods from a soil con- Fig. 7. Product of the calibrated crop-management (C) and support practice (P) factors, as derived from the historical reconstruction of soil management. Note that after 1856 lines for site 2 and site 1 overlap. Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx 9 Fig. 8. Calibrated soil erodibility (K) factor as derived from the historical reconstruction of soil management. which results in a much higher LS factor (17.6). There is also a significant temporal variation of the K factor at each site during the study period. When this is coupled with a decrease in ground cover due to more intense tillage, for example during Period 7, it results in an increase in the erosion rate. Fig. 9 shows the temporal pattern of tillage erosion rates in the three study sites since 1752. It also presents a large variability among the three sites, due to the different slope steepness (Table 6), and among the different periods at each site. Current soil losses by tillage are low because of the replacement of the spring tillage operations by herbicide application since the start of Period 8 in 2000. Only superficial harrowing is continued during summer. These superficial operations result in a lower movement of soil. In contrast, the highest soil erosion rates by tillage are found right after the introduction of mechanized agriculture, between 1970 and 1990 (Period 7), mainly because of deep mouldboard plowing, which is done at least twice a year. Also, mechanized agriculture permitted plowing in the direction of the steepest slope. This tillage direction corresponds to the largest displacement of soil particles. Since animal plowing is always done perpendicular to the contour lines, especially in steep environments, soil displacement is typically lower. Nevertheless, between 1888 and 1950, soil losses by tillage erosion remain considerable. In 1888, the typical roman plow is replaced by the mouldboard plow which results in much higher displacement of soil with each operation. Between 1950 and 1970, tillage intensity was temporarily reduced, which explains the low during Period 5. Only before 1888 was the impact of plowing operations nearly negligible on the movement of soil in these olive orchards. Fig. 9. Modelled annual tillage erosion rates. Fig. 10. Comparison of modelled and measured erosion rates. 3.4. Evaluation of cumulative model predictions To evaluate the model predictions, total water and tillage erosion rates were calculated for the entire period since the orchard was planted at each of the sites. This was then compared with the total soil truncation measured in the field, based on mound height. Although not a rigorous validation, it provides insight weather the models used have been able to capture reasonably the magnitude of the cumulative soil losses. Fig. 10 depicts that the comparison with the simulated and observed values is not far off the 1:1 line, although Site 1 (Juan Angel) departs from that line. The field measurements include the uncertainty related to the interpolation of the reconstructed soil surface while the modelled profile truncation includes the uncertainty related to the age of the olive orchards. It is worth indicating that no attempt was made during the calibration of the models to match against the observation. Results of Fig. 10 suggests that the models used have been able to capture the magnitude of cumulative soil losses based on the two main erosive agents, water and tillage, observed and described in the region. 4. Discussion The average annual soil loss between 29 and 47 t ha−1 year−1 that was derived from field measurement of historical soil truncation around the olive trees is relatively high with respect to measurements of contemporary erosion rates. It is in the upper range of the erosion rates measured in plots experiments in olive orchards, summarized by Gómez et al. (2008). However, it is lower than the rate of 95 t ha−1 year−1 estimated by Vanwalleghem et al. (2010) as the long-term erosion rate in several orchards in a different area of SW Spain. While slopes were similar with this study, the differences in erosion rate can probably be attributed to lower rainfall erosivity and soil erodibility in the Montefrio area. Also, possible changes in management were not documented in the Vanwalleghem et al. (2010) study. Agricultural activity on areas previously covered by Mediterranean forest, lasting only between 153 and 291 years, has thus resulted in a loss of a large fraction of the original soil profile. Compared to the total average soil depth of the current soils profiles, as determined from field surveys (Table 1), between 29% and 40% of the total soil depth (A + B horizons) has been lost. The lack of decrease in yield (Fig. 3), in spite of these large Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003 G Model AGEE-3918; No. of Pages 11 10 ARTICLE IN PRESS T. Vanwalleghem et al. / Agriculture, Ecosystems and Environment xxx (2011) xxx–xxx soil losses and the resulting degradation and reduction of available soil depth, can then be understood from the relatively large depth of the still remaining soils and the positive effects of more intensive olive orchard management with higher inputs (fertilization, pruning, pest and disease control). However, the average soil loss rates in the study area are nearly two orders of magnitude higher than natural soil production rates (Montgomery, 2007a) and are therefore highly unsustainable. Additionally, they are well beyond the upper limit to prevent degradation of water quality, e.g. 1 t ha−1 year−1 quality (Jürgens and Fander, 1993). It has to be noted that regionally averaged sediment yield will be lower as this method of measuring historical soil erosion rates does not take into account deposition occurring on larger scales. The modelling of water and tillage erosion allows exploring the historical variation of soil losses rates. Both showed considerable variation in the 250-year time frame that was considered. Important changes in soil management could be designated as the main driver for this variation. As expected, the current soil management proved to be the most unfavourable from a soil conservation perspective. Remarkably, however, the modelling analysis revealed that historical soil management was also far from sustainable. Already 250 years ago, farmers were losing their fertile soils at an alarming rate, with total soil losses between 13 and 31 t ha−1 year−1 . This finding is in sharp contrast with the common perception that traditional olive orchards are or were environmentally sustainable (Loumou and Giourga, 2003). The absence of an early warning signal in terms of yield could explain why unsustainable soil management was prolonged over such a long time. Since there was no correlation between soil erosion rates (Figs. 5 and 9) and olive yield (Fig. 3), even in periods of intensive soil erosion, there was never a direct incentive for adapting soil management towards soil conservation. The modelling also allows evaluating the relative importance of water and tillage erosion during the study period. Comparison of the erosion rates presented in Figs. 5 and 9 show clearly how water erosion has been the main agent over the entire time span considered. The average contribution of tillage to the total soil loss is 26% for the three sites, taken over the entire study period. Its contribution shifts frequently throughout history however. Interestingly, the contribution of tillage was lowest during the beginning and end of the 250 year period (10% and 7% for periods 1 and 8, respectively). At the start of the study period, tillage was infrequent, while recently, the main tillage operations have been replaced by herbicide control of weeds. Since the latter also reduces surface cover and roughness, however, water erosion reaches a maximum value. The balance was particularly shifted towards tillage erosion, between 47% and 62% of the total soil loss, during Period 6 (1950–1970) because of the frequent tilling with mouldboard plows. Future studies will try to validate if the disaggregation of soil losses by periods and processes (tillage vs. water) using sediment source tracking and radioisotopes match the ones presented in this manuscript based entirely on model predictions. 5. Conclusions A combination of field survey and modelling analysis has provided an analytical framework to describe and discuss the historical evolution of olive cultivation techniques, yield and environmental impact on soil erosion rates in an area representative of sloping areas in Southern Spain that have been transformed from forest to olive cultivation in the last 250 years. Our results indicate a constant trend towards more intensive management and, with the exception of the period from 1935 to 1970, concentration of the orchard in the production of a single product, olive fruit. This has allowed incorporating technological advances in the agronomical management of the orchards which resulted in a progressive increase in olive yield, especially since 1930s. During the same period, soil degradation by erosion in the orchards has been intense, with the loss of at least 30% of the total soil depth. The fact that no negative effect on yield was found so far can only be explained by the relatively large soil depth in the area and the positive agronomic effects of management. It is clear, however, that the situation is unsustainable. Management effects are not expected to generate additional yield increases and it can be expected that the continuing loss of fertile soil will result in a collapse in a few generations if effective soil conservation measures remain absent as they have been in the study period. Model analysis has identified water erosion as the main erosion process, although tillage has been responsible for a significant fraction of the soil losses, around 26% in average. Most of the tillage erosion occurred during periods of intense tillage, like from 1970 to 1990. To date, tillage is of minor importance due to the preference for herbicide use for controlling weeds. Model analysis has also identified the period since 1970 to date as the period of most intense water erosion due to a decrease in ground cover and increase in soil erodibility. Our analysis also suggests that during historical periods under less intensive management, basically before the 1930s, soil loss rates were already relatively high. It has also pinpointed a period, 1935–1970, in which intercropping with cereals provided the lowest soil loss rates estimated for the whole period. Acknowledgements The authors wish to thank Dr. J.L. 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Please cite this article in press as: Vanwalleghem, T., et al., Quantifying the effect of historical soil management on soil erosion rates in Mediterranean olive orchards. Agric. Ecosyst. Environ. (2011), doi:10.1016/j.agee.2011.06.003
