PhD thesis in environment, by Slaheddine KHLIFI

Analysis of soil and water conservation measures: case of contour bench ridge in Central Tunisia and small hill dams in the upper valley of Medjerda (Abstract)

PhD thesis in environment, by Slaheddine KHLIFI

The objective of this work is the assessment of some soil and water conservation management, the soil contour ridges and the hill dam reservoir. The evaluation of the soil contour ridges is based on their characterization, their state, their incidence on the development and the growth of the barley culture and their impact on some soil physicochemical parameters. The reconstitution of the water supply, the estimation of the taking away for the irrigation and the determination of the mean and exceptional rates of silting up are used as indicators for the assessment of the hill dam reservoir.

The diachronic analysis highlights the importance of the annual cultures, in the area equipped with soil contour ridges, and the decline of the rangeland surface along time. The results show that some soil contour ridges characteristics are oversized such as spacing, controlled surface of the soil contour ridge and their length. Their survey shows that a high proportion of the structures is affected by various forms of degradation, attributed to the exceptional rainy event of the 2003 winter. The evaluation of the effects of the benching on the cereal production shows that the growth, the vegetative development and the production of dry matter of the barley culture in the vicinity of the soil contour ridges in the treated plots is significantly improved compared to the control plot. After more than three decades of the soil contour ridges’ construction, the increase in the yield, assigned to the implementation of these structures, is estimated at 33% near the ridges and around 10% considering the entire treated area. The soil contour ridges may increase the soil depth, the content of organic matter, the water holding capacity, the fine fraction of the soil content and reduce the rock fragment rate. The calcareous rate, the pH and the electric conductivity seem to be not affected by the implementation of the soil contour ridges. The evaluation of the annual used water, for irrigation of crops on average during the period of the survey, from the hill reservoir at Oued Fathiza I, Oued Rmal and Oued Sned are less than the effective available water indicating an underutilization from these reservoirs. The average of the effective water supply varies between 36% and 195% of the estimated supplies. The expected amounts of the silting up for each of the three reservoirs are underestimated. The high sedimentation rates will compromise the hill dam reservoir’s life. The capacity losses, related to the exceptional precipitations of 2003 winter, vary from 3,5% to 7,3% of the initial capacity, depending on hill dam. These sedimentation rates show the impact of the exceptional events on erosion of the hill reservoir watershed. The spacialization of the sedimentation through the dam, carried out by transversal sections, show that the most of the silting materials will take place in the upstream part of the reservoir.

Keywords: Contour bench ridge, hill dam, assessment, Tunisia, erosion