The use of groundwater in irrigated agriculture is often regarded as an effective way of increasing farm productivity. However, the effects of differential access to groundwater by farmers have rarely been studied at the microeconomic scale, particularly the possibly negative effects of marginalization of small farmers. During the hydraulic crisis that has affected Algerian irrigation schemes since the 1980s, groundwater has become a major source of irrigation for farmers. A process of farm differentiation occurred linked to farmers' access to groundwater, causing social inequity. To understand the differentiation of farms in the Ouarizane irrigation scheme, we first undertook a historical inquiry. Second, we used a participatory prospective approach to analyse possible changes in farming systems after the adoption of a new land reform. Our results confirmed that access to groundwater has become a structural factor in the differentiation of farms and in land concentration in Algerian irrigation schemes. Land concentration and the marginalization of small farmers are likely to worsen when the new land law is applied.