Spatial and inter-annual variability of specific sediment yield: case of hillside reservoirs in Central Tunisia

Water erosion is the major process responsible for soil loss in Central Tunisia. It represents a main risk of sedimentation which threatens the lifetime of water and soil conservation works in the region, such as hillside reservoirs. This paper focuses on the study of spatial and inter-annual variability of specific sediment yield (SSY) estimated from sediment yield (SY) of 23 small watersheds of hill reservoirs located in the Tunisian Dorsal and the Cap Bon regions. The calculated values of mean inter-annual specific sediment yield varied from 180 to 3298 t/km²/year, over a period of 12 years (from 1994 to 2006). The selected catchments were classified into three groups according to their SSY data. The SSY map, elaborated with a geographical information system (GIS), showed an important spatial variability between watersheds of the same group. Moreover, analyses of the coefficient of variation (CV) showed that 70% of the considered sites demonstrate a tendency to moderate and high inter-annual variability of SSY. In order to identify the principal factors governing variability of erosion and sedimentation, a multiple regression analysis was carried out using nine parameters characterizing morphology, topography, land cover, climate and protection works of the 23 considered sites. The generated model revealed that 85% of SSY variability is correlated to seven principal factors, which are drainage area (A), rainfall intensity recorded in 30 min (I ₃₀), percentage of water and soil conservation works, percentage of clay and carbonate in soils, vegetation land cover, overall slope index (I _G) and rainfall amount (R _a). The obtained SSY results were compared to others cited in a large bibliographical synthesis of studies carried out at the planet scale. We deduced that the SSY values obtained in this study are among the relatively high series in the world. The high variability of specific erosion rates in Central Tunisia is believed to be related to the semi-arid climate, characterized by the irregularity and the severity of storms and the spatial and temporal variability of other multiple factors affecting erosion processes.     

Effect of omitting one milking weekly on lactational performances and morphological udder changes in dairy cows

The response of dairy cows to omitting one milking weekly was investigated in two successive experiments conducted with Holstein cows milked twice daily. Experiment 1 compared the lactational performances and udder changes in eight cows (31.2 L/d, 201 d in milk) in the 5 wk before and the 5 wk after introducing the suppression of one milking weekly. Milk yield was recorded daily and milk composition twice weekly. Milk partitioning in the udder (alveolar and cisternal milk) and cisternal size (ultrasonography), 8 h after milking, were also measured at the start and the end of the experiment. Although daily milk yield decreased 32% during the experiment (10 wk), linear regression analysis revealed a loss of milk yield of 1.1 L/d (3.7%) as a consequence of the omission of one milking weekly. Milk composition, lactation persistency, and somatic cell count (SCC) were unaffected by milking omission. Milk partitioning in the udder decreased by 38% in alveolar milk volume and showed a tendency to decrease in cisternal milk volume (15%) and cisternal size (7%), as a result of milking omission and advancing lactation. Loss in total milk yield was negatively related with cisternal milk volume (r = -0.77) and cisternal size (r = -0.70) indicating smaller losses in the udders with large cisterns. In Experiment 2, five cows (21.0 L/d, 227 d in milk) previously adapted to the milking omission schedule were used to study the daily effects of milking omission on milk yield, milk composition and udder health during 10 wk. Milk yield and milk composition were approximately constant but SCC increased with lactation stage. The omission of one milking caused an important decrease in milk yield, fat content and SCC on the omission day and a compensatory increase over the following 2 d, but milk protein and lactose did not vary. All variables reached the average weekly value three days after the milking omission (six milkings). In conclusion, under the conditions used, omitting one milking weekly slightly reduced milk yield and did not affect milk composition when healthy cows were used. Milk losses by milking omission depend on udder cistern characteristics; evaluating cistern size by ultrasonography may be a useful tool for choosing cows that are better adapted to a reduced milking frequency.