In Mexico, water resources, apart from being limited, are badly distributed; on the coastal plain, with the exception of the Peninsula of Baja California and Sonora, there is an abundance of water, while in the rest of the country, it is scarce. With the transfer of the large irrigation districts to user organizations, it was intended that these districts become operationally and economically efficient organizations. In this way, decision-making passed from global planning, which covered an entire irrigation district, to fractioned planning, in which a small irrigation module is responsible for the administration of the water resource. Undoubtedly, the scarceness of this resource, together with the high demand of the agricultural sector and the low fees, is producing pressure for a change in the policies for management of this resource. In the management of the sources, it is not convenient to extract more water from the reservoirs than the average available volume; an additional volume generates less benefit, both economically and operationally, than what would be generated if it were stored for a dry year. In the case of the network, the deteriorated state of the infrastructure propitiates losses by leakage, infiltration in unlined ditches and inefficient water control in the channels. In the case of water management in the field delivery network, it is necessary to define actions to encourage the volumetric quotas and, in this aspect, the support of the government is essential; this means "political will," which could permit a considerable increase in the availability of water and, as a consequence, higher productivity of soil and water resources, as well as increased agricultural production and better incomes for the producers.

Visando a definir critérios para o manejo da irrigação para alface tipo americana, cv. Raider, esse trabalho foi desenvolvido para avaliar o efeito de diferentes tensões da água no solo sobre o comportamento produtivo da cultura, em ambiente protegido, na região de Lavras - MG. O experimento foi instalado em casa de vegetação com delineamento em blocos casualizados, tendo quatro repetições. Os tratamentos foram constituídos de seis tensões de água no solo como indicativas do momento de irrigar. As tensões preestabelecidas foram 15; 30; 45; 60; 75 e 90 kPa, à profundidade de 0,15 m. Os resultados permitiram concluir que, no emprego de tensões em torno de 15 kPa, há tendência em obter plantas mais altas, com maior peso de matéria fresca comercial e menor teor de matéria seca da parte comercial. Também foi possível observar melhor eficiência no uso da água (469,22 kg ha-1 mm-1) sob tensão de 15 kPa.<br>Seeking to define a criteria for irrigation scheduling of lettuce cultivated in a protected environment in the region of Lavras - MG, Brazil, the purpose of this work was to evaluate the effects of different soil water tensions in the crisphead lettuce, Raider cv., on productive behavior. The experiment was carried out in a greenhouse following the completely randomized blocks design with four replications. The treatments consisted of six water tensions as indicatives of the irrigation schedule criteria. Preset tensions were 15; 30; 45; 60; 75 and 90 kPa. Results allowed concluding that under tensions around 15 kPa at a 0.15 m depth, there is a tendency in obtaining taller plants, featuring more fresh matter weight of marketable head, and lower dry matter contents in the marketable part. It was also possible to obtain a better efficiency water use (469.22 kg ha-1 mm-1) under tensions around 15 kPa.