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thus resulting in what appear to be relatively young stands. Secorid, we <br />assumed that as we subtracted 20-years from the current age of each st:and, it ?.:'. <br />remained in the same size cla:;s (and hydrologic condition) unless tYne new <br />stand age fell below the mi.dpoint in the range in years between the c;urrent <br />size class and the next yourigei• size class. Once past the mid-point be;tween <br />two age classes, the stand was moved to the next lower age class. If' a size <br />class was non-stocked, it was assumed that 20-years earlier it was a <br />sawtimber stand and the pattern repeated. In this way, the current size <br />classes were proj ected back in time, at 20-year inteivals, to the year 1860. <br />We did not account for stand evolution or changes in specie composition <br />that might also have occurr-ed over time. We also had to assume there was <br />compatibility between the field survey derived esrimates of stand age for the <br />4 classes and the GIS baseci estimates of area in each size class. Finally, it <br />was assumed that the hydrologic model, WRENSS, could be used to <br />characterize the annual water yield for the forest conditions characterized at <br />20-year increments from 1860 to 2000. <br />Any trends in the water yie:lds simulated for the forest condition assumed to <br />exist at successive increm?;nts of time is assumed to reflect the effect of <br />forest cover changes on ?;?at??r yield. This phase of the analysis was <br />(?.? . <br />performed on all national forest forested land in the North Platte River <br />Basin. The estimate of total acres in each of the 4 size classes for the period <br />1860 to 2000 is presented in table 3. <br />At first glance, the projections in table 3 seem erratic. For example, we can <br />pretty well assume there were more than zero non-stocked acres in 1940 and <br />that some sawtimber stands were present in 1920; contradicting the numbers <br />presented in table 3. However, these discontinuities or discrepancies are a <br />result of working with age class means, size class means, perhaps with the <br />choice of a time increment, arid with the assumption that one size class is <br />reduced to its preceding size class at the mid-point in years between the two <br />size classes. Discontinuity 1;3 a problem that occurs whenever discrete <br />values are used to describe continuous, and often non-linear functions. The <br />value of the information in. table 3 is in the trends it expresses. Currently <br />(year 2000) over 50 percent of the forested area is occupied by relatively <br />young sawtimber stands tha.t were quite young in 1860. <br />29