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1. CLIMATIC DATA: RAINFALL VERSUS EVAPOTRANSPIRATION It is generally regarded that when precipitation exceeds evapotranspiration (Eto), deep percolation of surface rainwater may occur. Evapotranspiration refers to water evaporation that takes place both at the soil surface and transpiration by plants, and is defined as water loss from a large field of 4-to-7 inch tall, cool-season grass that is not water stressed. A number of factors determine deep percolation and ultimately groundwater recharge,primarily domestic irrigation volumes and stormwater subsurface storage. The.following chart illustrates rainfall and evapotranspiration, extrapolated for the Clements area. Precipitation data was obtained from the Soil Survey of San Joaquin County for the Lodi area, the closest station to Clements. The "2 years in 10" data was used and not the "average rainfall" data. Evapo- transpiration data was obtained from the California Irrigation Management Information System (CIMIS) for the Shenandoah Valley station, also the closest station to Clements: TABLE 3 ` PREGhPITATION AND,�Eto:DATA.EXTRAP�OL,ATED3F,OR CI,EMENTS, CA, Month 2 Years in 10 will have Eto Average Potential Deep more precipitation than: (inches/month) Percolation (in/mo) (in/mo) January 5.04 1.04 4.00 February 3.98 1.66 2.32 March 4.03 2.88 1.15 April 2.34 4.36 May 0.72 5.63 -- June 0.35 6.76 -- July 0.32 7.91 -- August 0.34 7.09 -- September 0.92 5.20 -- October 1.66 3.56 -- a_ November 3.98 1.69 2.29 �I December 4.43 1.00 3.43 TOTALS (in.) 23.00 48.78 13.19 Table 3 illustrates that the volume of evapotranspiration water is over twice the volume of precipitation water, even when using higher rainfall data that exceeds normal rainfall data by 25% (17.16 in. vs. 23.00 in.). Page -7- Chesney Consulting