Near-surface hydrologic response for steep, unchanneled catchment near Coos Bay, Oregon: 1. Sprinkling experiments

Abstract

Sprinkling systems are frequently used to simulate rainfall for processbased investigations of near-surface hydrologic response without measuring or accounting for spatial variability. Data analyses from three sprinkling experiments at the Coos Bay 1 experimental catchment (CB1) demonstrate considerable spatial variability in sprinkling. Furthermore, simulated rainfall from sprinklers was found to be more heterogeneous than natural storms at CB1. Water balance calculations and evapotranspiration estimates indicate that evaporation of airborne droplets is a significant portion of applied sprinkling rates, although still less than the amount blown off the field site by strong winds. Incorporation of spatial variability in sprinkling input and soil-water storage did not significantly change water balance calculations. Saturation patterns within the near-surface sod profile and the timing of tensiometric response are affected by sprinkling heterogeneity. Pore-water pressure and saturation development at the soil-saprolite interface are primarily controlled by convergent surface / subsurface topography and bedrock fracture flow, but are also sensitive to sprinkling spatial variations. The analyses presented herein suggest that incorporating spatial variability in sprinkling rates is important when conducting hydrologic-response modeling of sprinkler experiments. This paper is the first-part of a two-part series focused on CB1. The data analyses in this paper are used to parameterize comprehensive physics-based hydrologic-response simulations of three CB1 sprinkling experiments reported in the companion papers.