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50 <br />(1942). One problem with fitting the equation was estimating fo. <br />• Obviously, there is never any runoff at t=o when rainfall begins, yet <br />the initial infiltration rate is not necessarily equal to the rainfall <br />rate. Each five minute measurement of runoff estimates what happened <br />during the five minute interval prior to the reading. Therefore, the <br />infiltration estimate made from the first runoff measurement is consi- <br />dered the initial infiltration rate. The ten minute runoff measurement <br />is used to estimate the five minute infiltration rate, the fifteen minute <br />measurement for the ten minute infiltration rate, and so on. Since <br />some of the rain during the first moments of rainfall goes to surface <br />retention storage instead of infiltration and runoff, the initial <br />infiltration rate is probably over estimated (assuming the rainfall rate <br />exceeds the infiltration rate). The final infiltration capacity, fc, <br />is estimated from the final runoff measurement. Calculated infiltration <br />curves are shown in Figures 2 - 15. The average final infiltration <br />rates (fc) are summarized in Table 10. Since infiltration rates are <br />determined by individual site characteristics and management practices <br />as well as soil and spoil characteristics, it is difficult and probably <br />not meaningful to compare infiltration rates by coal region. <br />Effects of Soil and Site Characteristics on Infiltration <br />Stepwise multiple regression was used to determine the extent of <br />influence of each of the plot characteristics on final infiltration <br />capacity. <br />Stepwise multiple regression is a version of forward regression <br />. which selects a best fitting regression equation for describing a <br />