It is important to conserve natural resources, which requires identifying their location and distribution. To preserve natural resources, the Environmental Services Payment Program (EPS) was established in Mexico. The Thornthwaite method determines evapotranspiration based on temperature, latitude of the study area and monthly number of hours of sunshine per day. The objective of this analysis is to estimate the availability of water in the ANP Sierra de & Aacute;lvarez. The study area is located in the state of San Luis Potosi, Mexico. Thornthwaite's method was based on total monthly precipitation and average monthly temperature data from eight weather stations. The information used for the temperature and precipitation calculations is 34 years (1980-2014). In the case of evapotranspiration, up to 900 mm were obtained in the study area, representing 55 % related to precipitation recorded. A map was generated with interpolation of data using IDW, and it is observed that in the month of March there is an important water deficit, presenting an accentuation in the E part of the study area. Similarly, in part O a deficit of up to-57.6 mm/ha is reported. This study also underlines the fundamental role of the ANP Sierra de & Aacute;lvarez as a source of water recharge for the San Luis Potosi Valley, reinforcing its ecological importance.