摘要：The availability of the water resource is important for hydrological planning, the availability depends of many causes such as climate variability and plant cover. The rainfall, temperature and evapotranspiration have changed in different parts of Spain. For the Jucar river basin and other water basins in Spain has been a change the surface runoff in 1980. This study will focus to analyze the climatic conditions and their effect in surface runoff moreover, and explain the degree of surface runoff reduction since 1980. In addition we considered the possibility like changes in plant cover could have influenced the reduction of surface runoff. The HBV model was used to determine the surface runoff in the Jucar river basin. The HBV model was calibrated for the years before of the 1980 (1950-1979) y re-calibrated for the years after of 1980 (1980-2007). The area more susceptible is the uplands of the Jucar river basin where the major amount of water is generated and major reduction in the surface runoff (1980-2007). In the middle basin of the Jucar less runoff was presented, however in the lower Jucar river basin an increase in surface runoff was determined. The hydrological changes can explain a part of decrease runoff. Other causes which justify the decrease of surface runoff are changes in plant cover, evapotranspiration and infiltration, therefore, this changes are able to explain the results in the Jucar river basin.
