The present study is inspired by a current scientific debate around the economic valuation of soil ecosystem services. It details the estimation of the services identified in the watershed O.Lachbal and develops projections in order to analyse the total economic value trajectory in the absence and presence of water and soil conservation measures according to the evolution of water erosion, so as to reveal the cost of non-action related to soil degradation. This assessment help to clarify the picture of the benefits provided by each service. The total economic value of ecosystem services is estimated at TND 5.98 million (2022). The majority (60%) of this value is made up of direct use benefits (provisioning services), while indirect use benefits (regulating services [34%] and support services [6%]) constitute only 40% of the total value. The study of the evolution of the aggregate benefits made it possible to estimate the economic impact of combating degradation over 21 years and to identify the cost of non-action associated with soil water erosion. The results of the study show that progressive mitigation of soil water erosion leads to the stabilization of ecosystem functions and subsequent conservation of ES, while the opposite case of this scenario reveals an average cost of non-action 5 times higher than the total economic value of the current year (TND 30 million). In view of this problem, public intervention remains essential, and this study explored a concave relationship between the cost of non-action and the degree of degradation achieved or to be avoided, given that erosion is considered to be a natural phenomenon that can be controlled, but cannot be avoided, offering a decision key in relation to monetary availability.
