No abstract

Despite the high risk of erosion in olive orchards located in mountainous areas in Spain, little research has been carried out to account for the complexity and interaction of the natural processes of runoff and soil erosion on the catchment scale or small catchment scale. In this study, a microcatchment of 6·7 ha in a mountainous area under no-tillage farming with bare soil was set up to record runoff and sediment. Soil erosion and runoff patterns were monitored over a two-year period. Totally, 22 events were observed. The data were analysed, and then used to calibrate the AnnAGNPS model, which allowed us to complete the data period and describe the hydrological and erosive behaviour on a monthly and annual basis. A high variability in catchment responses was observed, due to differences in the storms and to the effect of the surface soil moisture content. Maximum intensities of 10 and 30 min determined the final runoff values while the total sediment loads were dependent on the rainfall depth. The impact of management on the reduction of porosity can explain the relationship between runoff and intensity in the microcatchment. However, the impact of the spatial scale meant that the transport of sediment required substantial rainfall depths to ensure a continuous flow from the hillslopes. The results of the calibration (E > 0·60 and r > 0·75) on the event and monthly scale confirmed the applicability of AnnAGNPS to predict runoff and erosion in the microcatchment. The predicted average runoff coefficient was 3·3% for the study period and the total average sediment loads, 1·3 Mg/ha/yr. Despite these low values, the model simulation showed that much larger runoff coefficients and soil losses can be expected for periods with several consecutive years in which the annual rainfall depth was over 500 mm. The use of cover is recommended to prevent the high levels of erosion associated with these conditions.

Most studies on runoff and soil loss from olive orchards were performed on plots, despite the fact that measurements that examine a range of erosive processes on different scales are essential to evaluate the suitability of the use and soil management of this type of land. The main environmental limitations of much of the land used for olive orchards in the Mediterranean are the steep slopes and the shallow soil depth -and this was the case in the study area. Soil erosion and runoff over two hydrological years (2005-2006 and 2006-2007) were monitored in an olive orchard microcatchment of 6·1 ha under no-tillage with spontaneous grass in order to evaluate its hydrological and erosive behaviour. Moreover, soil parameters such as organic matter (%OM), bulk density (BD) and hydraulic saturated conductivity (Ks) were also examined in the microcatchment to describe management effects on hydrological balance and on erosive processes.In the study period, the results showed runoff coeffi cients of 6·0% in the fi rst year and 0·9% in the second. The differences respond to the impact of two or three yearly maximum events which were decisive in the annual balances. On the event scale, although maximum rainfall intensity values had a big infl uence on peak fl ows and runoff, its importance on mean sediment concentrations and sediment discharges was diffi cult to interpret due to the likely control of grass cover on volume runoff and on soil protection. In the case of annual soil erosion, they were measured as 1·0 Mg ha −1 yr −1 and 0·3 Mg ha −1 yr −1 . Both are lower than the tolerance values evaluated in Andalusia (Spain). These results support the implementation of no-tillage with spontaneous grass cover for sloping land, although the reduced infi ltration conditions determined by Ks in the fi rst horizon suggest grass should be allowed to grow not only in spring but also in autumn. In addition, specifi c measurements to control gullies, which have formed in the terraced area in the catchment, should be included since it is expected that they could be the main sources of sediments.

Despite the high variability of the precipitation regime characterizing the Mediterranean area, the records of rainfall depth are usually not appropriate for long-term calculations of erosivity and soil losses, because they do not reveal details of short lengths or long durations (daily, monthly). In this work, we present a simple approach to calculate annual erosivity through monthly precipitation records. The study area (olive groves on steep slopes) has a high erosion risk associated to the main soil land use, combined with an irregular and erosive rainfall regime. The relationships between rainfall data at intervals of 10 min for a period of 3 years, daily rainfall records over 10 years and a long-term monthly dataset of 60 years were checked to calculate the annual erosivity values through daily data, Fourier's index and modified Fourier's index values. A good, adjusted linear relationship between modified Fourier's index and the erosivity was found, which allowed us to optimize the use of the 60-year monthly data series and to carry out a long-term analysis of the erosivity quantiles in the study area. The estimated mean erosivity showed a return period of between 2 and 5 years and a variation coefficient of over 50 per cent, which illustrate its high variability and frequency. This approach to calculate erosivity and the use of quantiles could be applied in other areas with month-long data series in order to study and model the erosion risk using suitable temporal periods. Correlation: r = 0 -999; r = 0 -998 Correlation: r = 0 -781; r = 0 -661 P -meas (mm) P -AEMET (mm) -AEMET (mm) Figure 3. Analysis of daily precipitation (P d ) at the Puente Genil and AEMET-Santaella stations: (a) correlation analysis of the accumulated daily values of rainfall depth; (b) correlation analysis of the daily rainfall depth. r P , Pearson correlation coefficient; r Sp , Spearman correlation coefficient. 182 E. V. TAGUAS ET AL.

Abstract:A multifractal analysis of hourly and daily rainfall data recorded at four locations of Andalusia (southern Spain) was carried out in order to study the temporal structure of rainfall and to find differences between both time resolutions. The results show that an algebraic tail is required to fit the probability distribution of the extreme rain events for all the cases. The presence of a multifractal phase transition associated with a critical moment in the empirical moments scaling exponent function was also detected. Both facts indicate that the rainfall process is a case of self-organized criticality (SOC) dynamics, although the results differ for each place according to the time resolution and the nature of the rainfall, either convective or frontal. This SOC behaviour is related to a statistically steady state that implies the presence of clusterization in the time-occurrence sequence of rain events. Such fluctuations have been shown by performing the analysis of the Fano and Allan factors and the count-based periodogram. The values for the "synoptic maximum", the typical lifetime of planetary scale atmospheric structures, have been obtained for each place and some important periodicities have been detected when dealing with extremes.