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49 <br />• ~ Surface Water Budget _ The surface water balance is stated as <br />Ip+M-Q <br />l where I infiltration,, P • precipitation, M snawmelt, 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 />I 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 />J and I is given in the lef thand column of Table 12.• <br />Subsurface Water Balance - The equation used for the monthly accounting of <br />• infiltrated water is <br />T <br />W• I-Eta-o5 <br />' in which W is deep percolation, I is infiltration Eta is actual evapo- <br />ration and a5 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 />I <br />• `i W ' I - EC a <br />,i <br />