Loss of the site usually means loss of all future recreational opportunities, not just the current annual value. The entire future stream of annual recreational values must therefore be included. Economic theory suggests this stream of benefits, because they happen in the future, should be discounted to make them comparable with the present. Assuming that the annual value of recreation were constant over time, the "present value" of the stream of future benefits can be calculated simply by dividing the annual consumer surplus by the real interest rate