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River Islands Development at Lathrop January 2021 FINAL River Islands Non-Potable Water Technical Report Rev 2 #6520 3 Non-Potable Water Supply Requirements 3.1 Non-Potable Water Supply Requirements The River Islands NPW Distribution will be designed to adequately and dependably provide NPW during peak conditions. Peak conditions shall refer to the peak month's irrigation demands (PMID)occurring over a daily 8-hour irrigation window. The following sections will describe the NPW PMID factors, demands, and the subsequent improvements required to meet peak operating conditions. 3.2 Historical Average Year Precipitation and Evapotranspiration Data—California Irrigation Management Information System The California Irrigation Management Information System (CIMIS) is a unit in the Water Use and Efficiency Branch within the California Department of Water Resources (DWR) that manages a network of over 145 automated weather stations in California. The data provided by CIMIS is intended to assist irrigators in managing their water resources efficiently. Historical average precipitation and evapotranspiration (ET.) data from CIMIS Station 70, located near the City of Manteca, was used to determine the peak irrigation month. The peak month is defined as the month with the lowest estimated precipitation and the highest reference evapotranspiration as this will result in the highest irrigation demand. Based on historical data from CIMIS Station 70, the peak irrigation month is July which has an average year precipitation and reference evapotranspiration values of 0.02 inches and 7.98 inches respectively. Table 7-2 in the City's Standards defines common crop coefficients (Ko) which should be used in determining the actual evapotranspiration value for a given crop (ETo) such as rye grass or turf. The evapotranspiration for a given crop is estimated by multiplying the crop's crop coefficient (Kc) by the reference evaporation (ETo). For determining the NPW irrigation demands within the River Islands Development, crop coefficients for turf were utilized. 3.3 Non-Potable Water Peak Month Irrigation Demand Factors For the peak month of July, the irrigation demand factor was determined through the following equation as defined in Section 7-5 J in the City's Standards. July Peak Month Irrigation Demand Factor = ETA —Precipitation Irrigation Ef ficieny x (1 —Leaching Requirement) Where July Peak Month Irrigation Demand Factor is in units of inches per month ETo= Evapotranspiration for turf in July, in inches per month Precipitation = Month of July precipitation data in Table 7-1 of the City Standards, in inches per month Irrigation Efficiency= Per Section 492.13 in the California Code of Regulations, drip and spray irrigation systems should have an average irrigation efficiency of 81 and 75 percent respectively. Leaching Requirement= 10% leaching requirement assumed per Section 7-5 1 in the City Standards. Per the equation above, the July peak month irrigation demand factors for drip and spray type irrigations systems is 10.27 and 11.09 inches per month respectively. 10