1. The water climate balance of the soil is the sum of the amount of water that gets in and out of a given area in a given time period. The input of water is represented by the rainfall and the output by the evapotranspiration. The result is the liquid amount of water that remains in the soil and is available to the plants (TUBELIS, 2001; TOMASELLA e ROSSATO, 2005). The knowledge of water availability in a watershed is essential to water resources management, becoming vital for the appropriate management of those resources (SANTOS et al., 2009). The present study was carried out to compute the monthly water climate balance and the crop water balance for irrigation control in the Boi Branco watershed, in SP, Brazil, taking into account the available rainfall and temperature data. The Boi Branco watershed is located in the southwestern region of São Paulo State, Brazil, with an area of 80,71 km². The soil of this region has clay texture, and it is classified as Red Latosol. The climate of the region, according to Köppen is Cwa, is described as humid temperate with dry winters (temperature under 18ºC) and wet summers (temperature above 22ºC). The water climate balance was calculated by Thornthwaite´s equation and Mather´s method using the following data: -Precipitation; -Evapotranspiration; -Available water capacity (AWC). For each crop irrigation water balance the same procedure used in the water climate balance was followed. Thus, it was used the crop evapotranspiration instead of the potential evapotranspiration and the maximum soil water capacity (MWC). MWC is only a percentage of the total available water in the soil (water between FC and WP), and it was also considered a water soil depletion factor of 0.4. The soil of the watershed has low available water storage capacity; thus, it is recommended an irrigation management with shallow depths and high frequency, seeking to improve the irrigation efficiency. The volume of annual rainfall is higher than the annual evapotranspiration, except when it is assessed as effective precipitation with 75% of probability. The crop water balance for irrigation control showed a deficit in all the crops studied and in all the months. For the studied region, it is recommended Thorthwaite´s equation which presents more satisfactory results for the region. SALES, M. A. de. L.; SÁNCHEZ-ROMÁN, R. M.; SINOBAS, L. R.; SOUZA, J. V. R. da S.; MONTEIRO, R. N. F. Results and Discussion Material and Methods Conclusions ASSESSMENT OF WATER AVAILABILITY AT BOI BRANCO WATERSHED USING THE WATER CLIMATE BALANCE Table 1. Water climate balance in the Boi Branco watershed using effective precipitation with probability of 75% of occurence. Months ETPPeP-ETPNEG.ARMALTETRDEFEXC mm Jan111.870.9-41.0 3.1-18.289.022.80.0 Feb108.6149.540.90.021.318.2108.60.022.7 Mar108.665.0-43.6 2.8-18.583.625.00.0 Apr91.722.8-68.9-112.50.1-2.625.466.30.0 May79.930.6-49.2-161.70.0-0.130.749.10.0 Jun56.031.3-24.7-186.40.0 31.324.70.0 Jul56.861.95.1-30.45.1 56.80.0 Aug65.09.5-55.6-86.00.4-4.714.250.80.0 Sep70.872.92.1-45.72.52.170.80.0 Oct102.3103.91.6-35.14.11.6102.30.0 Nov100.2126.726.50.021.317.2100.20.09.3 Dec118.8143.524.80.021.30.0118.80.024.8 Total1,070.4888.4-181.9 -- 0.0831.6238.856.corn8 Figure 1. Water depth (mm) to be used during the crops cycle.