Spatial and temporal variability of spontaneous grass cover and its influence on sediment losses in an extensive olive orchard catchment

Abstract

Vegetal cover might be a useful tool for soil protection under agricultural land-uses characterized by a high proportion of bare soil, susceptible to be eroded. Spontaneous grass covers are usually associated to high degrees of spatial and temporal irregularity, which limit its efficiency and management. However, they might be alternative measurements for soil erosion control in marginal farms due to its zero cost. Despite numerous studies have been addresses to compare the impact of different managements, soil con- ditions or species characteristics linked with cover crops in olive orchard due to its suitable experimental of the plot scale, there are no studies aimed to characterize spatial-temporal patterns of spontaneous grass cover in olive orchard catchments. Our work aimed to describe its distribution patterns and its hypothetical temporal stability as well as its role on soil protection. Firstly, temporal variability of spon- taneous grass cover was explored through of simple adjustments with meteorological features and related with the management. Secondly, Spatial Analysis by Distance Indices (SADIE) was applied to evaluate the existence of possible spatial patterns and its persistence and finally a descriptive analysis of events was applied to analyze the influence of the cover degree on hydrological response of the catchment. 12 surveys to measure the cover percentage of spontaneous grass through picture analyses, were carried out on a seasonal scale during 3 years (2011–2013). A permanent and regular grid of 37 points (6 samples/ha) was used to define each sample-location. In order to explore the relations between cover percentage and meteorological variables, multiple linear regressions were adjusted while the SADIE approach (Spatial analysis by distance indices) and correlation analysis were used to describe possible spatial aggregation patterns and its dependences on topographical and soil attributes such as aspect, slope, drainage area, height, richness and apparent electrical conductivity. The average annual cover percentage ranged from 23% to 36% with a coefficient of variation of 57% and 6%, respectively. On the seasonal scale, the cover varied between 0.2% and 50%. Accumulated precipitation during the precedent 15 days, the average of minimum temperature of the previous 60 days and the cumulated evapotranspitation of the previous 60 days showed a significant correlation on the mean cover percentage in the catchment. A simple multiple linear regression adjustment with a determination coefficient of 77% was adjusted based on the cumulated precipitation of 15 previous days and the number of months since the vegetation removal by herbicide or by summer conditions. Moreover, a permanent spatial pattern was observed for periods characterized by abundant preceding rainfall. Only the top-soil electrical conductivity (50 cm-depth) showed significant correlations with spatial patterns of spontaneous grass. Finally, similar rainfall events taking place under different degrees of soil cover presented very different hydrological responses, which underlined the role that spontaneous vegetation in the sediment dynamics control during humid periods.