ASSESSING THE HYDROLOGIC IMPACT OF LAND-USE CHANGE IN UPPER GORDON GULCH, A SMALL UPLAND CATCHMENT IN THE ARAPAHO NATIONAL FOREST, COLORADO

Assessing the hydrologic impacts of future land management on National Forest land is important because increases in low-permeability areas (roads, trails, logged areas) produce higher peak discharges during high intensity precipitation events. The cumulative impact of such land-use changes is significant in areas such as the upland catchments of the Front Range, where forested land helps to buffer the effects of intense summer thunderstorms. We used the upper Gordon Gulch catchment (0.94 km²), part of the Boulder Creek Critical Zone Observatory in Arapaho National Forest, to measure and model the effect of precipitation intensity and land-use change on peak discharge.

We measured soil infiltration rates and soil compaction (dynamic cone penetrometer), analyzed soil samples, and estimated 1-hr precipitation intensities for recurrence intervals ranging from 0.5 to 500 years using local precipitation data. Infiltration rates reflect regolith properties and recent human impacts. Our measurements show that rates are correlated (p-value <0.05) with field values for soil compaction and with laboratory values for soil texture and loss-on-ignition (LOI). Estimated precipitation intensities and measured infiltration rates show that precipitation in most areas of the catchment will flow through soil layers and sandy regolith and as groundwater toward surface channels.

We modeled peak flows for upper Gordon Gulch using Kineros 2, a GIS-based, distributed property runoff model (Semmens et al., 2008) and the Rational Runoff Technique. Model runs suggest that a substantial fraction of peak discharge originates from roads and trails, logged areas and rock outcrops, which collectively comprise < 10% of the catchment. Peak discharge values modeled using Kineros 2 exceed 1 m³s^-1km^-2 for the 100-year storm only if basin impermeable area increases to >10% or if large portions of the basin burn. Our results suggest that planners should carefully consider and model possible hydrologic effects from cumulative land-use change in the National Forest. The local variation and range in soil properties also indicate that catchment-scale hydrologic models require detailed, locally measured data to contribute useful information to planners of National Forest land.