A 3-year experiment compared in an olive orchard the effect of different cover crops’ composition on runoff, water erosion, diversity of annual plants, and arthropod communities which could provide ...an alternative to conventional management based on tillage (CT). The cover crops evaluated were a seeded homogeneous grass (GC), a seeded mix of ten different species (MC
seeded
), and a non-seeded cover by vegetation naturally present at the farm after 20 years of mowing (MC
natural
). The results suggest that heterogeneous cover crops can provide a viable alternative to homogeneous ones in olives, providing similar benefits in reducing runoff and soil losses compared to management based on bare soil. The reduction in soil loss was particularly large: 46.7 in CT to 6.5 and 7.9 t ha
−1
year
−1
in GC and MC
seeded
, respectively. The heterogeneous cover crops resulted in greater diversity of plant species and a modification of the arthropod communities with an increased number of predators for pests. The reduction of the cost of implanting heterogeneous cover crops, improvement of the seeding techniques, and selection of species included in the mixes require additional research to promote the use of this practice which can deliver enhanced environmental benefits.
Is the cover crop practice suitable for soil and water conservation in olive tree cropping? Rainfall, runoff, sediments, nutrient and organic carbon losses from 8
×
60
m plots were measured during ...four hydrological years (2002–2007) in a field trial, in which two different soil management systems were used to confirm this hypothesis: a cover crop (CC), and conventional tillage (CT). The plots were located in a private olive tree farm on a sandy-loam soil, near Seville, southern Spain. The cover crop, as compared to conventional tillage, efficiently reduced runoff and sediment yield down to tolerable levels, 5.68% of the rainfall being converted to runoff, and the soil loss reaching 0.04
kg
m
−2
year
−1, as the average of four years. Additionally, in the cover crop treatment, the values of the nutrient export either dissolved in the runoff water or adsorbed in the sediment, were lower than the analogous values of the conventional tillage treatment: 0.631 and 0.065
kg
m
−2
year
−1 of organic carbon and nitrogen, respectively, 0.175 and 0.0333
kg
m
−2
year
−1 of soluble K and P, respectively, and 0.010 and 0.002
kg
m
−2
year
−1 of available K and P, respectively. The adoption of a cover crop as a soil management practice can be a feasible way to reach sustainability in many olive-cropped soils of southern Spain, although this method is not always easy to implement due to technical problems such as seed selection, its maintenance, and the choice of the correct killing date to avoid water competition. These difficulties could explain the slow rate of its adoption by many farmers. Further exploration of these aspects is required, as well as a specific agricultural extension campaign.
Rainfall, runoff and soil loss from 6
m
×
12
m plots were recorded during 7 years (2000–2006) in an experiment in which three different soil management systems were compared in a young olive grove ...installed on a heavy clay soil, near Cordoba, Southern Spain. The no-tillage (NT) system, kept weed-free with herbicides, had both the largest soil loss (6.9
t
ha
−1
year
−1) and the highest average annual runoff coefficient (11.9%). By contrast, a cover crop (CC) of barley, reduced the soil losses to 0.8
t
ha
−1
year
−1 and the average annual runoff coefficient to 1.2%. Conventional tillage (CT), had intermediate values of soil loss (2.9
t
ha
−1
year
−1) and an average runoff coefficient of 3.1%. The different treatments were established 4 years after planting the olive trees, and a significant decrease in soil and runoff losses was observed with time as the olive trees grew and their canopies developed. Measurements at the end of the experiment showed a significant improvement in the topsoil properties of the CC treatment as compared to CT and NT. The soil under NT presented a significant degradation with respect to traditional CT management. Organic matter values were 2.0, 1.4 and 1.0%, and stability in water of macroaggregates was 0.452, 0.418 and 0.258
kg
kg
−1 for CC, CT and NT, respectively. These results indicate that the use of a cover crop can be a simple, feasible soil and water conservation practice in olive groves on rolling lands in the region. A key factor in its practical use is to establish it early enough to protect the soil in the critical initial years of the grove, when most of the soil is unprotected by the small olive canopy.
Soil erosion constitutes a major problem in the dry‐farming agriculture of Mediterranean areas. The coincidence of fall showers falling over bare soils after a long, hot, dry summer, steep slopes, ...and intensive tillage, or the occurrence of large uncultivated patches like in tree‐cropping, aggravate the problem. Among several soil conservation practices, cover crops are being adopted by olive farmers as a promising method to reduce soil and water losses. This report summarizes the results of 2 yr from a network of microplots installed in olive orchards in the olive‐growing area of Southern Spain to improve the technique of soil management and extend it to farmers. The cover crop diminished soil losses in all the experimental plots with an average of 76%. Water loss was also reduced, although to a lesser extent, in six of the eight fields, with an average of 22%. Additionally, the results showed the great influence of the cover percentage in the decrease in soil loss and in the concentration of sediments in the runoff. However, it should be taken into account that plant cover consumes water, and that the advantages of the decrease in runoff in the water balance with the cover could be eliminated if it is not managed appropriately. Therefore, more years of experimentation covering different climate conditions are necessary.
Is the von Kármán constant affected by sediment suspension? The presence of suspended sediment in channels and fluvial streams has been known for decades to affect turbulence transfer mechanism in ...sediment‐laden flows, and, therefore, the transport and fate of sediments that determine the bathymetry of natural water courses. This study explores the density stratification effects on the turbulent velocity profile and its impact on the transport of sediment. There is as yet no consensus in the scientific community on the effect of sediment suspension on the von Kármán parameter,κ. Two different theories based on the empirical log‐wake velocity profile are currently under debate: One supports a universal value ofκ = 0.41 and a strength of the wake, Π, that is affected by suspended sediment. The other suggests that both κ and Π could vary with suspended sediment. These different theories result in a conceptual problem regarding the effect of suspended sediment on κ, which has divided the research area. In this study, a new mixing length theory is proposed to describe theoretically the turbulent velocity profile. The analytical approach provides added insight defining κas a turbulent parameter which varies with the distance to the bed in sediment‐laden flows. The theory is compared with previous experimental data and simulations using ak‐εturbulence closure to the Reynolds averaged Navier Stokes equations model. The mixing length model indicates that the two contradictory theories incorporate the stratified flow effect into a different component of the log‐wake law. The results of this work show that the log‐wake fit with a reducedκ is the physically coherent approximation.
Key Points
Turbulent velocity profile allowing for sediment suspension
Variable mixing length depending on sediment concentration
Behavior of the von Karman constant in flows with sediment suspensions
Soil erosion is intense on steep slopes, where many olive orchards are located in Mediterranean areas. The adoption of cover crops is a promising soil and water conservation practice for these areas. ...Nevertheless, there has not been enough information to be able to advise farmers on the selection of plant species. The purpose of this report is to assess different plant species as cover crops to reduce erosion and soil organic carbon loss through sediments. Twenty‐four tests were performed in 6 plots on a 20% slope in southern Spain. A gramineous plant (Brachypodium distachyon), 2 leguminous species (Vicia sativa and Vicia ervilia), and a cruciferous plant (Sinapis alba) were sown and compared with spontaneous vegetation and conventional tillage. Simulated rainfall with intensities of 18.1 (±1.6) and 38.8 (±2.3) mm hr−1 was applied during 2 years. All cover crop treatments, in comparison with tillage, significantly reduced soil and soil organic carbon losses by more than 92%, S. alba being the species with the lowest runoff values. The high rate of soil and water losses observed in the tillage treatment emphasizes the need to protect the soil and its fertility. A kinematic wave model considering variable soil infiltration rates was fitted to the runoff data to evaluate relevant soil and surface characteristics. The estimated saturated hydraulic conductivity and length of the capillary scale were not affected by the treatments, but the surface resistance to water flow indicated the efficiency of S. alba, B. distachyon, and V. ervilia in reducing the runoff velocity.
The snow pack of Sierra Nevada, southern Spain, melts during spring and early summer, feeding ancestral irrigation channels in the region known as the Alpujarra. This simulation study compares the ...traditional irrigation water supply, based on proportional division, with the actual supply, based on up-downstream priority, in one watershed in the Alpujarra. A combination of three models was used for the analysis. Water supply was simulated using the snowmelt-runoff model. The snow cover required by that model was determined using satellite images. A second model simulated the distribution of water based on proportional division and on up-downstream priority. Irrigation requirements and return flows were simulated using a soil–water balance. Proportional division is an inherently equitable mechanism for distributing water, but can lead to water deficits for different crops in a command area. The analysis demonstrated this premise not be true. The superiority of proportional division was magnified in dry years. Because of the internal reuse of return flows, irrigation consumptive use coefficient (the ratio of irrigation water that is evapotranspirated by the crops to the total amount of irrigation water that leaves the area of concern during the period of analysis) at the watershed scale was significantly greater than at the irrigation-channel scale. This result illustrated, based on tradition in ancestral irrigation community, the notion of integrated water resources management.
The combination of runoff plot studies and soil tracking using a silt size magnetic sediment tracer, allowed a better understanding of the relative contribution of different zones in olive (Olea ...europaea L.) orchard plots to total soil losses. The average erosion rates were different in the tree rows (tr) (0.8 kg m−2 mo−1) and in the inter‐tree rows (itr) (1.4 kg m−2 mo−1), inter‐tree rills (r) being the most eroded areas (4 kg m−2 mo−1) from October 2008 to April 2010. Since soil under the olive canopies has a high infiltration capacity, splash is a major erosion process in this area, while, in the itr, sheet and rill erosion are the dominant processes. The proposed magnetic tracer technology was an effective tool for determining the cumulative soil losses at the plots for a 17 mo period (average 141 Mg ha−1) with an accuracy of 7.2 Mg ha−1. To achieve this accuracy, determination of bulk density, selectiveness in the transport process, tracer distribution in the soil profile and field calibration of the magnetic susceptibility probe were required. The evolution of tracer distribution provided insight into soil displacement within the runoff plots due to erosion processes. The tracer distribution maps also indicate a high coefficient of variability of the incorporation of tagged soil into the plot, which should be improved in future research to increase the usefulness of this approach in water erosion studies.
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