Aspen tree loosing leaves branches by branch
If this condition is due to drought can this tree be saved by more frequent watering?
Denver County Colorado aspen defoliation
It's hard to know without seeing your tree and hearing more about your site and the management. Aspens are native to the mountains where conditions are very different than the Plains. It is common for them to struggle when grown in the metro area where temperatures are warmer, precipitation is less, soils are heavier, and there is more pollution. This summer has been very dry and hot; insufficient water well may be the cause of your tree's defoliation, however, Aspens also are susceptible to a number of foliar diseases, too, so that might be another cause. See more here: https://planttalk.colostate.edu/topics/trees-shrubs-vines/1701-aspen-trees/
At this time of year, you want to be encouraging your tree to go dormant rather than push new growth. Keep the area under the canopy moist (not wet) but gradually reduce the water as temperatures cool down and the days get shorter. Rake up and discard all the fallen leaves to prevent fungal spores from overwintering in the debris. Once the tree is dormant, give it a good soaking to better prepare it for weather events this winter. Ideally, it will leaf out next spring.