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<br />fH11722 <br /> <br />The formula has not been tested thoroughly in the Western United <br />States. Until more work has been done to make it applicable to this <br />region and perhaps develop "E" values for the region; the Penman method <br />cannot be used with confidence. <br /> <br />Extrapolation of Evapotranspiration Data <br />to Growth Areas <br /> <br />In the transposition of evapotranspiration values from the place of <br />measurement to natural growth areas, many variables may be involved. <br />The two most important factors are differences in climate and in plant <br />growth. Other variables are differences in the texture, fertility, <br />salinity or alkalinity of the soil, depth to the water table, and <br />quality of the ground water. The effect of these factors or combination <br />of them is often reflected in the plant growth. Although not necessar- <br />ily a variable, consideration must be given also to the relative stage <br />of plant development at the two locations. Recent studies of the <br />relationship of water use and plant development indicate that after <br />the plant has become established and enters a period of active plant <br />growth--evapotranspiration is greater than after the plant reaches <br />maturity. Thus extrapolation of evapotranspiration values for immature <br />plants to areas of mature growth will err on the high side. <br /> <br />Realistic transposition of evapotranspiration values from the place <br />of measurement to natural stands of phreatophytes is equally as impor- <br />tant as determination of the values. The two methods in common use are <br />(1) the areal basis method, and (2) the volume of foliage basis method. <br /> <br />Early workers expressed evapotranspiration on an areal basis, that is, <br />depth over a unit area. As a result, most of the values in the litera- <br />ture are given in depth units, as acre-inches or acre-feet per acre. <br />When expressed on an areal basis, there is no indication of the growth <br />conditions for which the information was obtained. Stands of phreato- <br />phytes may range in size from seedlings to mature plants and from a few <br />percent to 100 percent in cover density. As a result, there may be an <br />appreciable error in computing the water use by a natural stand when <br />the conditions of growth for the measurement area values are not known. <br />In the past, some adjustments by judgment were made to correct for <br />marked differences in growth conditions on the assumption there was a <br />linear relationship between evapotranspiration and variation in cover <br />density. It is doubtful that the assumption is valid, for there are <br />fragmentary data suggesting a greater use in the range 80 to 90 percent <br />cover density than indicated by a dir~ct proportion. This may be due, <br />in part, to the "oasis effect.11 More information is needed on the <br />relation of ~vapotranspiration rates to variations in cover density in <br />order that the extrapolation may be r~alistic. <br /> <br />Within the past two decades, in order to avoid the difficulties and <br />uncertainties of the areal basis method, the volume of foliage method <br />was developed whereby water use is expressed as a unit quantity per <br /> <br />15 <br />