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<br />49 <br />1 <br />• Surface Water budget _ The surface water balance is stated as <br />1 = P + M - Q <br />where I = infiltration, P = precipitation, M = snowmelt, and Q = over- <br />land runoff. Accurate estimates of mean monthly runoff are difficult to <br />obtain. Although it is obvious that runoff from the refuse dump occurs, <br />for the purposes of this calculation, runoff was assumed to be zero. Such <br />a procedure will result in an overestimate of infiltration and, consequently, <br />a "worst case" estimate of deep percolation or subsurface runoff. There- <br />fore, <br />I = P + M <br />and I is given in the lefthand column of Table 12 <br />Subsurface Water Balance - The equation used for the monthly accounting of <br />infiltrated water is <br />• <br />W= I -Eta-eS <br />in which W is deep percolation, I is infiltration Eta is actual evapo- <br />ration and oS is the change in soil moisture storage. The maximum volume <br />per unit area that can be stored in the soil fora prolonged period is the <br />available water capacity, AW. The water in the soil, at any particular <br />time, cannot exceed the value of AW. If, in any particular month, the sum <br />of infiltrated water and stored water exceeds the evaporation requirement, <br />then deep percolation occurs. If the sum of infiltration and stored water <br />is less than the evaporation requirement, then evaporation is limited by <br />this sum. <br />Because there is no crop cover on the refuse dump at the present time, <br />the value of stored water, 05, will not change with time (other than the <br />top few centimeters in which evaporation will occur). Therefore, <br />•_ W= I-Eta <br />