Water use by perennial crops in the lower Sonora watershed

The North American Monsoon is the principal source of fresh water in northwest Mexico, accounting for almost 70% of annual precipitation and around 80% of surface runoff. In the northern regions above ∼28°N, there is insufficient surface runoff to satisfy urban, industrial and agricultural users and large coastal aquifers are used to supply water for irrigation. The lower part of the Río Sonora watershed is devoted to irrigated agriculture, where perennial cash crops account for 40% of the area. The major crops are table grapes and pecan with cultivated areas of 100 km² and 50 km², respectively. Although efficient drip irrigation systems have been installed for these crops, annual water use remains very high: ∼1150 mm for grapes and ∼1700 mm for pecan. During the 2005-2006 growing seasons, eddy correlation measurements of evapotranspiration (ET) were performed for two different grape varieties, obtaining 680 mm for Perlette and 776 mm for Superior. During theses measurement periods, the applied irrigation depths were 1121 mm and 1088 mm, respectively. For pecans, the ET measurement was 1297 mm and the applied irrigation was 1459 mm. In other words, the applied irrigation is in excess of the water consumed by the plants: about 50% for grapes and 10% for pecan. This conclusion is supported by soil humidity (θ_v) measurements for grapes, where the values of θ_v between depths of 10 cm and 120 cm are larger than field capacity during most of the annual cycle. Therefore, it is proposed that the irrigation depth should be gradually reduced to 800-900 mm, which would save ∼30 hm³ of fresh water for other uses each year.