| Summary: | Water is essential to current and evolving ecosystem of plants and animals. It is a factor that interacts with climatic and soil conditions, so its quantity in the atmosphere and soil has a direct impact on the ecosystem. Water is the essential part of protoplasm plant and is primordial in the photosynthesis process. The amount, frequency and distribution of precipitation are relevant because of its relationship with the stages of vegetative growth. In the arid zone of Baja California Sur, the average annual rainfall is 200 mm, unevenly distributed throughout the year and the temperature ranges from 5 to 40 °C. Orange orchards face serious problems in adapting to the prevailing agro-climatic conditions, mainly to the evapotranspirative demand. The actual evapotranspiration (ETreal) is the process by which water is evaporated from the soil and transpired by plants. From the optical of water use, the ETreal is relevant for the application frequency and quantity of water to apply. The estimates of ETreal allow quantify the availability of the resource and determine the dimensions of the waterworks. The purpose of this study was to determine the water demand and to apply methods to estimate evapotranspiration of orange, in the arid citrus production zone of Baja California Sur, Mexico. For orange trees, water demand averaged 6.8 and 5.2 mm dÃa-1, with indirect and direct methods for estimating the ET, respectively, a difference of 30.8% was observed between them. The standard deviations were 0.05 and 0.03, and the variation coefficients 7.35 and 5.77% in the same order. The net requirement of water per ha was 25 and 19 m3 per day, 175 and 134 m3 per week, besides 751 and 574 m3 monthly, with indirect and direct methods, respectively. The methods which consider climatological parameters, the soil characteristics and the response of plants are models integrators, being at the same more representative of the agro-ecosystem. The use of water based on the implementation of methods to overestimate the ET implies a gradual deterioration and eventual depletion of the aquifer that sustains the production.
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