by Zoe Dilles
The already scarce water resources of the semi-arid western United States will become increasingly precarious with the progression of warming and drying climate change. When compounded with growing demand for water, this issue mandates a balanced management practice incorporating sustainable water budgets and land use. Forest restoration efforts are currently slated for an area of some 600,000 acres of National Forest in north-central Arizona, comprising nearly 1% of the state’s footprint. These treatments, anticipated to last the duration of the coming decade, will consist of selective thinning and burning of high-density conifer forest to mitigate wildfire potential and increase the health of regional forested watersheds. The impact of tree removal on surface water has been the subject of previous study but is rarely quantified in regards to groundwater resources. Especially in such a dry region, the future of water availability lies in the relationship between rainfall and recharge of deep aquifers, reservoirs that are permanently diminished through over-pumping.
Wyatt et al. (2015) apply novel modeling of the regional hydrology to understand how forest restoration affects groundwater recharge in aquifers. Previous work has shown that tree planting decreases groundwater recharge; conversely Wyatt et al. suggest that tree removal might increase groundwater recharge. In crafting an interpretive groundwater model that takes into account the landscape scale forest restoration, the authors had to deal with the fact that there are no published estimates of how forest restoration influences the factors in question for a semi-arid climate. The regional water budget balances a single input, precipitation, with all other outputs consisting of proportional values for evapotranspiration of plant cover, runoff, and groundwater recharge. Several recharge scenarios were explored in hydrologic models based on field measurements from regional monitoring wells. These interpretive models take into account precipitation values from the past 50 years and project recharge values through 2099 for a range of possible climate change scenarios. Although these account for future precipitation trends, changing human demands and groundwater pumping were not explicitly accounted for. The model suggests a slight, temporary increase in recharge after forest restoration efforts but, overall, recharge and groundwater storage in all the watersheds studied are predicted to decline in the long-term for all precipitation scenarios. Assuming that changes in groundwater flow may actually be impacted by forest restoration efforts and rainfall due to climate change, there is a clear disconnect in supply and demand that can serve to inform constructive water management policy.
Wyatt, C. J. W., O’Donnell, F. C., Spring, A. E. 2015. Semi-Arid Aquifer Responses to Forest Restoration Treatments and Climate Change. Groundwater, 53, 207-216.