Staff Report and WELO Ordinance - StopWaste.Org

Hydrozones, Plant Factors, and Irrigation Efficiency"Hydrozones" are areas containing plants with similar water needs and irrigationequipment with similar efficiency. The formulas for estimating irrigation water use rely onnumeric figures assigned to hydrozones called "plant factors" and "irrigation efficiencies,"both of which are briefly explained below.Plant FactorsA plant factors is an estimate of a plant species' water need relative to other plants (e.g.,low, medium, and high water use plants). The numbers are on a scale of 0 to 1 (e.g., 0.1is very low water need and 0.9 is very high). Generally, a plant's water needs are lower ifthey are drought tolerant or are accustomed to temperate climates, such as theLivermore region and accordingly lower plant factor numbers are assigned to them.Plants are assigned a higher plant factor number if they are less drought tolerant andneed relatively more frequent applications of water to survive.The State model specifies using as a primary source for estimating relative water use, theWater Use Classification of Landscape Species published by the University of CaliforniaCooperative Extension (WUCOLS). (Proposed Section 13.25.060(B)(4)).Irrigation EfficiencyIrrigation efficiency is another important factor in calculating estimated water use, and is ameasure of a given irrigation equipment category's relative efficiency in delivering water.It is expressed on a scale of 0 to 1 with a higher number (e.g., 0.8) indicating a higherproportion of applied water is expected to actually reach the plant's root zones, with lowernumbers indicating less efficiency.Generally, equipment that targets lower volumes of water in a smaller area at or near theplants' base is more efficient (e.g., drip irrigation). Conversely, equipment that sprayshigher volumes and pressures of water over a larger more dispersed area is lessefficient.The 1992 City ordinance included charts of plant factors and irrigation efficiencies as anassist to staff in administering and designers in applying the ordinance. Staffrecommends retaining these charts.Staff recommends minor adjustments in plant factors to add temporarily irrigated areasas required by the State model. The recommended adjustments also recognizeLivermore's hotter climate (and higher plant water needs) compared to cooler morehumid microclimates, such as San Francisco's with which Livermore is regionallygrouped in the above-referenced WUCOLS book.105