Drainage from the critical zone: lithologic, aspect, and vegetation controls on the spatial extent of wetted channels during the summer dry seasons.

Abstract:

In seasonally dry environments the base flow that sustains river ecosystems is supplied by drainage from the critical zone. The extent of wetted channels and the magnitude of flow in these channels are rarely documented; and as yet, no general theory exists that enables the prediction of these key ecosystem properties. In the Eel River Critical Zone Observatory, contrasting lithology, vegetation and aspect produce watersheds with diverse ecology and hydrology. In this study, channel surveys were conducted in five headwater drainage networks (1 to 17 km²) in Mendocino County, California in the early and late summers of 2012, 2014 and 2015. Channel width, depth, velocity, water temperature, air temperature, humidity and water isotope data were collected and revisited at all flow sources and select channel locations. In the Coastal Belt (argillite and inter-bedded sandstone) of the Franciscan Complex, springs, mostly fixed in location, controlled the extent of flow. Identical wetted channel drainage densities (or WCDDs, about 1.94 km/km² in early summer and 1.44 km/km² in late summer) were found in adjacent watersheds of different sizes for entire watersheds. These surveys were conducted during a period of sustained, multi-year drought, and, despite this, the drainage from groundwater, perched in the critical zone, sustained stream flow. Aspect, however, significantly influenced WCDDs within a watershed, with south-facing slope WCDDs decreasing 3-fold during the summer, and north-facing WCDDs decreasing by only 23%. Sap-flow data suggest that different tree types on the north (primarily Douglas Firs) and south-facing (hardwood evergreens) slopes likely play a role in this dichotomy. WCDDs in nearby watersheds, underlain by mélange, were much lower than in the argillite; and, by late summer the channel system was nearly entirely dry, suggesting much less seasonal groundwater storage is in this bedrock. Our data suggest that lithology can have a primary control on the extent of wetted channels, but that aspect and associated vegetation can locally strongly influence water delivery to channels.