Subsurface Drainage Processes and Management Impacts 1

Storm-induced streamflow in forested upland watersheds is linked to rainfall by transient, variably saturated flow through several different flow paths. In the absence of exposed bedrock, shallow flow-restrictive layers, or compacted soil surfaces, virtually all of the infiltrated rainfall reaches the stream as subsurface flow. Subsurface runoff can occur within micropores (voids between soil grains), various types of macropores (structural voids between aggregates, plant and animal-induced biopores), and through fractures in weathered and consolidated bedrock. In addition to generating flow through the subsurface, transient rain events can also cause large increases in fluid pressures within a hillslope. If pore pressures exceed stability limits of soils and shallow geologic materials, landslides and debris flows may result. Subsurface monitoring of pipeflows and pore pressures in unchanneled swales at North Fork Caspar Creek in the Jackson Demonstration State Forest began in 1985. Four sites have been established to investigate the effects of timber harvest (K1 and K2) and road building (E-road) for comparison with an unmanaged control drainage (M1). Flow through large soil pipes at these sites is highly transient in response to storm events, reaching peak discharges on the order of 100 to 1,000 L min-1. Pore pressures at these sites also respond dynamically to transient rain events, but to date have not exceeded slope stability limits. Most soil pipes cease flowing in the dry summer period and hillslope soil moisture declines to far below saturation. The clearcut logging and skyline-cable yarding of the K2 site resulted in dramatic increases in soil pipeflow and subsurface pore pressures. During the first 4 years after timber harvest, pore pressures increased 9 to 35 percent for the mean peak storm event in the control M1 site. Peak soil pipeflow response was far greater, increasing 400 percent in the 4-year postlogging period. These results suggest that the soil pipes are a critical component of subsurface hillslope drainage, acting to moderate the pore pressure response. As the subsoil matrix becomes saturated and pore pressures build, soil pipes efficiently capture excess water and route it to the stream channel. This logging does not appear to have impaired the hillslope drainage function. Methods and results at the E-road site are quite different. Here, the mid-swale road construction and tractor yarding have resulted in large changes in the pore pressure response. Positive pore pressures were negligible in the upper portion of this instrumented swale before disturbance. Subsequent to the road construction in May 1990, there was little indication of immediate impacts. But, after the completion of felling and tractor yarding in late summer 1991, dramatic changes in pore pressure response were observed beginning in hydrologic year 1993 and continuing to date (1998). Largest pore pressure increases have occurred at sensor locations in and up-slope of the road prism. Below the road, the response is muted. These data support previous studies documenting the profound effects of roading and tractor logging on watersheds and provide special insight into these effects for this region. The hydrologic response of forested watersheds to rain events occurs through several interrelated flow processes. Soil surface conditions determine whether rainfall will run off as surface flow or whether it will infiltrate and travel through the subsurface. Infiltration capacities for soils in the coastal redwood region exceed maximum rainfall intensities common in the region. Exceptions occur in isolated areas where bedrock is exposed at the land surface. More widespread are infiltration limitations resulting from soil compaction associated with road building, landings, and other constructed surfaces. Over the great majority of forested landscapes, rainfall infiltrates into the soil and flows through the subsurface to streams, rivers, and lakes. Subsurface flow may occur within soil horizons, regolith (weathered bedrock), or bedrock (fig. 1). The conductive and storage properties of a given earth material as well as the spatial relations of adjoining materials strongly influence the actual flow path through the subsurface. For example, water may flow within soil horizons through the matrix, a porous medium of individual grains. Pores on the individual grain scale transmit water very slowly, several orders of magnitude less than surface water flows. Larger pores (on the order of 1 mm in diameter or larger) are commonly referred to as macropores, and can conduct substantial quantities of water at rates approaching surface flow velocities. By virtue of their geometry, macropores can be shown to conduct water more rapidly under high moisture conditions than the “micropores” of the soil matrix. Macropore geometry and type varies with depth below the land surface arising from various biologic and soilforming processes (fig. 1). Interconnected large macropores (on the order of 2 cm in diameter or larger) are often referred to as “soil pipes. ” These features are erosion pathways that extend within the shallow subsurface horizons as continuous or interconnected conduits forming complex branching networks (Albright 1992). An important hydrologic attribute of macropores is that the surrounding soils must be saturated before water can flow into these large pores. Thus, the antecedent moisture conditions in forest soils strongly control the importance of flow through macropores; and hence, the hydrologic response of a watershed to a precipitation event. Similarly, fractures in regolith or bedrock may dominate the flow response under saturated conditions, and thus define a significant flow path distinct from the soil matrix or macropores. The movement of water into and through these flow paths has two consequences of both theoretical interest and practical application to the management of forestlands. First, surface runoff in streams is generated on two widely different time scales: (1) on a seasonal basis and (2) during individual precipitation events. Runoff volume, timing, and duration affect both water supply and flood propagation. Seasonal effects of subsurface flows are manifest in the storage properties of forest soils. During the summer, water 1 An abbreviated version of this paper was presented at the Conference on Coastal Watersheds: The Caspar Creek Story, May 6, 1998, Ukiah, California 2 Hydrologist, Redwood Sciences Laboratory, Pacific Southwest Research Station, USDA Forest Service, 802 N. Main St., Fort Bragg, CA 95437 (ekeppeler@rsl.psw.fs.fed.us) 3 Assistant Professor, Department of Geosciences, California State University, Chico, CA 95929 (dbrown@shasta.csuchico.edu) Coastal Watersheds: The Caspar Creek Story Subsurface Drainage Processes and Management Impacts Keppeler and Brown USDA Forest Service Gen. Tech. Rep. PSW-GTR-168. 1998. 26 drains from soils and supports perennial streamflow (baseflow). This drainage creates a water deficit in the soil that must be replenished before maximum flow through a hillslope can occur. The second consequence of transient subsurface flow is directly related to the storm-driven evolution of pore pressures at the hillslope scale. Gravity is the primary force driving the flow of water in upland forested watersheds. However, if soil compaction closes pore spaces and prevents or reduces drainage through macropores, water pressure may increase such that the strength of the hillslope is lost and shallow landslides or debris flows may occur. Mass failures are a significant source of sediment reaching streams and are generated from background earth surface processes and from human activities such as road building. Dynamic interactions between pore pressures, drainage geometry, and the material properties of soil and bedrock can significantly influence the stability of slopes and channel heads, as well as sediment releases to streams (Dietrich and others 1986). Research investigations at Caspar Creek have explored these hillslope and subsurface drainage processes with the dual objectives of identifying impacts associated with logging and road building and reducing the risk of mass failures associated with timber harvest activities in the redwood region.