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Depth -to- Groundwater Mans <br />Depth -to- groundwater maps for a representative area of each site were determined by <br />subtracting the groundwater - contour elevations from the surface- topographic elevations. Each <br />representative area was bounded by four wells, and the groundwater - contour elevations were <br />produced from the same data and dates as the groundwater contour maps. Surface- topographic <br />information was obtained from detailed topographic surveys conducted within each area using <br />standard engineering techniques. The detail of these surveys was not always adequate to <br />produce uniform topographic information, but the level of detail was sufficient to produce <br />better depth -to- groundwater maps than would have been produced using available topographic <br />maps. PROC G3GRID (SAS Institute Inc. 1990b) was used to generate a uniform matrix of <br />groundwater and surface elevations, and PROC GCONTOUR (SAS Institute Inc. 1990b) was <br />used to graph the data. <br />Evapotranspiration and the Effective Plant Rooting Depth <br />The effective plant rooting depth was determined by plotting the percent of potential <br />evapotranspiration (observed - potential x 100) versus the depth -to -water table for each <br />observation. White (1932) used a similar method to describe the effects of evaporation on the <br />water table by depth. The Committee on Irrigation Water Requirements of the Irrigation and <br />Drainage Division of the ASCE (1990) defines: <br />1) evaporation as the physical process by which a liquid or solid is transformed to the <br />gaseous state. <br />2) transpiration as the process by which water in plants is transferred as water vapor <br />to the atmosphere. <br />3) evapotranspiration (ET) as the combined processes by which water is transferred <br />from the earth surface to the atmosphere (evaporation plus transpiration). <br />4) potential evapotranspiration (PET) as the rate at which water if available would <br />be removed from wet soil and plant surfaces. <br />Water withdrawn from the water table by evapotranspiration (ETwT), plus water <br />withdrawn from the unsaturated soil profile by evapotranspiration (ETus), equals the total or <br />21 <br />