334 CHAPTER TWELVE
whereKandbare dependent on the storm frequency and region of the United
States (Fig. 12.26 and Table 12.4).
The Steel formula gives the average maximum precipitation rates for dura-
tions up to 2 h.
GROUNDWATER
Groundwater is subsurface water in porous strata within a zone of saturation. It
supplies about 20 percent of the United States water demand.
Aquifersare groundwater formations capable of furnishing an economical
water supply. Those formations from which extractions cannot be made eco-
nomically are called aquicludes.
7
5
3
6 2
1
3
4 4
2
FIGURE 12.26 Regions of the United States for use
with the Steel formula.TABLE 12.4 Coefficients for Steel Formula
Region
Frequency,
years Coefficients 1 2 3 4 5 6 7
2 K 206 140 106 70 70 68 32
b 30 21 17 13 16 14 11
4 K 247 190 131 97 81 75 48
b 29 25 19 16 13 12 12
10 K 300 230 170 111 111 122 60
b 36 29 23 16 17 23 13
25 K 327 260 230 170 130 155 67
b 33 32 30 27 17 26 10