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This paper analyses the Pisano model, commonly used to estimate sediment load in combined sewer systems, and it is discussed its validity in semi-arid areas, which are characterized by irregular and scarce rainfall with short and intense precipitations. The model was applied to the sewer systems of eight watersheds of the city of Murcia (Spain) and it was verified that the value of the sediment load was much lower than the actual results obtained. Upon adjusting the original model by introducing typical semi-arid climatic factors, mainly periods of dry weather, it has been found that the real and theoretical values virtually coincide. The results obtained show that the sediment load is strongly influenced by periods of dry weather, which may be extremely prolonged in semi-arid areas, sometimes extended over 80 days. The results allow the planning of sewer system cleaning in order to prevent potential environmental impact caused by sewer system overflow into water channels during periods of intense rainfall.