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Traditionally, oasis farmers in hyperarid northern Oman have adapted to the interannual variation of irrigation water supply by dedicating an often sizable proportion of the agricultural area to the production of annual crops and leaving this area uncultivated in drought years. We hypothesized that increases in the share of perennial crops may put long-term hydrological sustainability at risk. To test this hypothesis, we compared agricultural water demand patterns of five oases in the mountain region of Al Jabal al Akhdar for 1978 and 2005. We analyzed land use changes by classifying aerial photographs taken in 2005 and 1978 into five land use types. Water demand in 2005 was estimated based on two GIS-based detailed crop inventories during the hot and cool season and on measurements of temperature, solar radiation and wind speed, from which crop evapotranspiration was calculated using the Penman-Monteith equation. Radiation and wind speed measurements were improved by topographic modeling. Water demand per area was summarized for each land use type and the results used to estimate water demand in 1978. Water supply estimates were based on spring flow measurements. Between 1978 and 2005, agricultural water demand in the study area rose from 218,800 to 256,377 m(3) a(-1). The most prominent land use changes were the disappearance of non-palm orchards at Masayrat ar Ruwajah and expansions of the area under perennial crops at Al ’Aqr, Ash Sharayjah and Qasha’ at the expense of field crops. As a consequence, the summer peak of crop water demand decreased at Masayrat ar Ruwajah, where all perennial crops could easily be irrigated with the minimum water supply observed during our study, but this summer peak increased at the other oases. Meeting the water demand of all perennial crops at Qasha’ required average spring flow conditions in 2005, indicating severe water deficiency in drought years. In addition to increased water demand, the oases’ water balance might also be under pressure from reductions in water supply caused by increased extraction of water from the same aquifer by a rapidly growing new town in the area.