Soil Figure 7.3 Effects of plants on evaporation rates. would be evaporated from a pan of water. If you plan only summer use of the ET bed, go ahead and increase the pan evaporation data for the summer months or take credit for plant transpiration. Figure 7.3 shows percent increases measured for vegetation cover on an experimental ET bed in Bexar County, Texas, as reported in the Edwards Underground Water District's ET Design Manual. Now we are going to construct a water budget in order to see how the effluent in our ET bed will behave over a year. To see how this is done, we will develop an example budget using data from an area in New Mexico. The water budget shown in Table 7.1 is calculated using the fol- lowing steps. The first step is to list the months of the year and the pan evaporation rates (Ep). Then list the precipitation rates (P) and subtract the precipitation from the evaporation to find net evapora- Table 7.1 ET Water Budget (Dimensions in Inches) Loading Rate Loading Rate (0.04 gpd/ft2 (0.05 gpd/ft2 Evaporation Precipitation Evaporation Input Storage Input Storage -0.25 -1.32 -1.89 -1.85 -2.28 11.26 -2.31 17.04 -1.43 -2.13 21.77 -0.78 -1.21 25.00 24.25 18.30 LOW-IMPACT SYSTEMS tion (En). Now select a loading rate. As a guide for your initial selection, if your lowest monthly net evaporation is about 0.1 in., start with a loading rate of 0.04 gal/ft2/day; if the lowest evaporation rate is about 1.0 in./month, use 0.6 gal/ft2/day; if the lowest rate is 2.0 in./month, use 0.7 gal/ft2/day; and if the lowest rate is 3.0 in./ month, use 0.8 gal/ft2dday to start with. Multiply the loading rate by 1.604 to convert gal/day/ft2 to in. 0.04 gpd/ft2 1.604 = 0.064 in./day Then multiply the loading rate in inches per day by the number of days in each month. For example, 0.064 in./day X 31 in July = 1.98 in. in July Complete the input column (I), and then calculate the monthly differences (D) between the input and loss by subtracting the input from the net evaporation. Some of the numbers will be negative, indic