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Climatic Data: Rainfall versus Evapotranspiration It is generally and scientifically accepted 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. The following Table illustrates rainfall and evapotranspiration for the Tracy area. Precipitation data was obtained from the Soil Survey of San Joaquin County. The conservative"2 years in 10"data was used and not the"average rainfall"data. Evapotrans- piration data was obtained from the California Irrigation Management Information System (CIMIS): TABLE 2 PRECIPITATION AND Eto DATA FOR TRACY,CA Month 2 Years in 10 will have more Eto Average Potential Deep Percolation precipitation than:(in/mo) (in/mo) (in/mo) (in/day) January 2.93 1.55 0.05 1.38 February 2.21 2.24 0.08 -- March 2.38 3.72 0.12 -- April 1.51 5.10 0.17 May 0.71 6.82 0.22 -- June 2.09 7.80 0.26 -- July 0.28 8.68 0.28 -- August 1.05 7.75 0.25 -- September 0.84 5.70 0.19 -- October 1.06 4.03 0.13 -- November 2.10 2.10 0.07 -- December 2.38 1.55 0.05 0.83 TOTALS(in.) 13.18 57.0 ------ 2.21 The Table illustrates that evapotranspiration far exceeds precipitation, even when using higher rainfall data that exceeds normal rainfall data by 28%(10.33 in. vs. 13.18 in.) Only in the months of December and January,and possibly November and February would deep percolation potentially occur. This data illustrates a relatively high potential for there to be significant evapotranspiration(or bare soil surface evaporation)from the mound system disposal area. 6