in a day or two, the water level in the tube should rise to a height above the water level in the bowl equal to the depth of your bed. Siting of the ET bed is of critical importance. The bed should not be shaded in the winter by buildings or trees, nor should it be screened from the wind. The site should be flat or should slope to the south at no more than about a 15% grade. The next consideration of critical importance is the climate. Cli- matic records can be obtained from your state geological survey, the United States Geological Survey, the United States Weather Ser- vice, the United States Soil Conservation Service, the United States Department of Agriculture, and/or your county extension service. What you need is the monthly pan evaporation rate for each month over a 10-year period for your area or for a very similar area. Be sure that the data is reported as Class A pan evaporation. Also, you will need the monthly precipitation data for that same time so that you can subtract it from the pan evaporation figures to get a net evapora- tion rate. Once you have 10 years of mean monthly net evaporation rates, use the year with the lowest net evaporation during the winter if you want your system to work for 10 years or SO without overflow- ing. If you are willing to put up with more frequent overflows, and if your neighbors are far enough away, you can select the data for a dry year. The next assumption, and a very important assumption, is that this net evaporation rate is equal to the rate at which effluent will evaporate from the surface of your ET bed. There is still much dis- pute about the relationship between pan evaporation and evapo- transpiration potential. By assuming a 1:1 relationship, we neglect the effects of plants (which may have a negative impact in the win- ter), and we disregard the possibility that evaporation from an ET bed (which is specifically designed to evaporate water) is less than LOW-IMPACT SYSTEMS Winter Rye Coastal Burmuda Native Percentage Increase in Evapotranspiration over Bare 