, but use of these systems is limited in general practice. A rule of thumb is that evapotranspiration systems used year-round are feasi- ble where evaporation exceeds precipitation during every month of the year. Some designers have tried to overcome this limitation by designing storage capacity into the system to accommodate periods of low net evaporation, but these systems have met with a varying degree of success throughout the country. In the humid Southeast, they have been unsuccessful for the most part, but they have been used with success in the Southwest and West. Other areas of the country have had mixed success. Because net evaporation rates are usually higher in the summer, the area where these systems can be used for summer homes is greater than the area practical for year- round use. Figure 7.2 shows the U.S. Environmental Protection 176 ONSITE WASTEWATER DISPOSAL Septic Evapotranspiration Figure 7.1 Evapotranspiration system. Agency's prediction of the suitability of ET systems nationally. Keep in mind, however, that local conditions dictate whether or not an ET or ETA system will work. A 14,000-ft mountain peak in the South- west may be a less desirable location than a Montana plain. Check with the local regulatory authorities to find out the status of use of the ET system in your area before you start designing one for your backyard. Evapotranspiration systems can be designed to work under many difficult circumstances, but another rule of thumb is that the more difficult the situation, the more expansive and expensive the sys- tem. ET beds require a lot of area and cost a lot of money (around one to two dollars per sq ft; price varies tremendously). So given the limitations imposed by climate, the need for land area, and rela- tively high cost, ET systems should be carefully compared with other possible alternative disposal systems before a final choice is made. Process Evapotranspiration is a combination of evaporation and transpira- tion. Evaporation is the direct passage of water to the atmosph