Crop evapotranspiration (ETc) is the sum of transpiration from leaves, and evaporation from soil and is referred to as the crop water requirement, or orchard water requirement. In Dr. David Goldhamer’s irrigation scheduling chapter, in our UC Almond Production Manual #3364, ETo from April 1–15 is 1.4 inches (31.2 gallons/tree/day) and ETo from April 16–30 is 1.6 inches (34.2 gallons/tree/day) while ETc from April 30 is 4.8 total inches. The amount of water stored in the soil
depends on rainfall intensity, duration, and soil water holding capacity, but generally it is between 50–70% of total winter rainfall. Most of the soils that I have observed this winter are dry, although in some cases quite a bit of water has been applied for frost protection. Because of the prolonged drought, most of our reservoirs are below average, and several districts may reduce allocations this year. Research has shown that season long limited irrigation is better than fully irrigating at the start of the season and then reducing water later in the season. If only 60% of your annual irrigation water is available, then you should apply 60% of your normally applied water at each irrigation throughout the year. Removing crop or heavy pruning does not reduce water use, and heavy pruning can increase shoot growth and leaf area and actually increase water use later. The University has produced a useful publication for “Drought management for California Almonds” at anrcatalog. ucanr.edu/pdf/8515.pdf. One year of severe water stress can result in two years of yield reduction, even if full irrigation is restored in the second year. This is because flower bud differentiation for next year’s bloom occurs around harvest time, and fewer flower buds will differentiate under water stress. As a result, crop
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