cs of the distribution system must be calculated. The process is the same as that for the sand mound distribution system. First, select the pressure desired in the distribution laterals - usually 3 ft of hydraulic head. A pressure of more than 5 ft might cause erosion, while a pressure of less than 2 ft could result in low flows at the far ends of the laterals. If, on a sloping site with eleva- tion differences of more than 4 ft, pressures would exist outside this range, redesign with multiple manifolds and valves is necessary. Next, a hole diameter should be selected. Holes as small as one- eighth inch can be used, but a larger diameter is recommended to reduce the chance of the holes becoming clogged. Three-sixteenth inch holes are fine as a starting value. Holes in low-elevation, high- pressure laterals may be made smaller (down to three-thirty- seconds of an inch diameter), and holes in high-elevation, low- pressure laterals may be made larger in order to balance the flow rates among laterals. Now the hole spacing along the laterals must be selected. Spacing can vary, depending on necessities imposed by slopes, but 5-ft spac- ing should be chosen as a starting value. Spacing is increased for high-pressure, low-elevation laterals and decreased for low- pressure, high-elevation laterals. Again, the objective is to balance flow rate among laterals. To determine the hole spacing in laterals at different elevations, multiply the 5-ft spacing times the flow rate per hole (see below and Table 6.5) at the elevation head for the particu- lar lateral, and divide the product by the average flow rate desired. Initial spacing (5 ft) X flow per hole (Table 6.5) Desired average flow rate (Table 6.5) CONVENTIONAL SYSTEM MODIFICATIONS Figure 6.19 Cross section of a shallow subsurface trench. For example, if you want an average of a 3-ft pressure head in the distribution pipes, and an upper lateral is 1 ft higher than the center lateral in which you will have a 3-ft head, and you are using three- sixteenth inch holes, then the