which generally have little or no vegetation, produce runoff; sink areas, located downslope of the source ar-In many semiarid regions, runoff and erosion differ according to eas, receive and store the runoff and thereby become vegetation patch type. These differences, although hypothesized to fundamentally affect ecological processes, have been poorly quanti-enriched and relatively productive. The results of sevfied. In a semiarid piñ on-juniper woodland [Pinus edulis Engelm.eral theoretical studies suggest that the transfer of water and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico, and nutrients through this process is important both we measured runoff and erosion from the three patch types that ecologically and hydrologically (Mauchamp et al., 1994; compose these woodlands: Canopy patches (those beneath woody

Abstract. The mechanisms by which runoff is generated in semiarid forests have been little studied. Over the past 4 years we have been investigating runoff processes in semiarid regions by continuously monitoring runoff, both surface and lateral subsurface, from ari 870-m • ponderosa pine hillslope in northern New Mexico. We have found that runoff accounts for between 3 and 11% of the annual water budget. We have also found that lateral subsurface flow is a major mechanism of runoff generation, especially following periods of above-average fall and winter precipitation. In one winter, lateral subsurface flow was equivalent to about 20% of the snowpack (about 50 mm). When antecedent soil moisture was high, lateral subsurface flow was extremely responsive to snowmelt and rainfall events and was much more dynamic than would be suggested by the low (laborat6ry determined) hydraulic conductivity of the soil. The rapidity with which lateral subsurface flow follows these events suggests that macropore flow is occurring. In the case of surface runoff, the major generation mechanisms are intense summer thunderstorms, prolonged frontal storms, and snowmelt over frozen soils. Surface runoff at our site took the form of infiltration-excess overland flow; this type of surface runoff has not been found to dominate at other ponderosa pine sites studied. These detailed and continuous investigations are increasing our understanding of runoff processes in semiarid forests and are thereby laying the groundwork for improved predictions, not only of runoff, but also of the concomitant transport of sediment and contaminants within and from these zones.

Concentrations and distributions of Ba, a component of the high et al., 2001), constructed wetlands (Best et al., 1999), zeroexplosive baritol, and RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), valent iron (Comfort et al., 2003), and monitored natural a high explosive, were evaluated from 1996 to 2002 within a semiarid alluvial system in Cañ on de Valle, New Mexico. A high explosive attenuation (Pennington et al., 2001). machining facility discharged effluent containing these chemicals to Identifying the key surface and subsurface transport the canyon from 1951 to 1996. The connectivity between alluvial processes of high explosive contamination is critical to groundwater, surface water, and sediment was specifically addressed modeling transport and predicting contaminant concento understand the distributions and dynamics of Ba and RDX in trations in environmental media (Pennington and Branthe alluvial system. Surface water, groundwater, and sediment were non, 2002). However, we are unaware of any studies characterized by conducting hydrologic measurements, geomorphic that have focused on high explosives and Ba contaminamapping, and collecting samples. Barium and RDX in sediment preftion in alluvial and vadose zone canyon settings. In addierentially reside in fine-grained deposits that represent the suspended tion, studies that have examined contaminants in alluvial load redeposited on floodplains following channel scour. However, canyon settings do not typically consider all of the hydro-RDX and Ba show markedly different behaviors in surface water and geomorphic components (e.g., surface water, groundalluvial groundwater because of contrasting geochemical characteristics and transport mechanisms. Barium precipitates in sediments as water, unsaturated bedrock, and canyon bottom soils barite and witherite and readily sorbs to sediment minerals. Therefore, and sediments). Previous studies of contaminated minsediment transport is an important control on its distribution in the ing sites have partially focused on hydrologic conneccanyon. In contrast, RDX appears to occur predominantly in the tions between groundwater and surface water, as well dissolved phase, behaves conservatively, and is most significant in as the role of contaminant exchange between the two groundwater. There is a strong correlation between RDX concentra-(e.g., Fuller and Harvey, 2000; Winde and van der Walt, tions in water and the saturated thickness of the alluvial aquifer. 2004). However, these do not explicitly include interac-During prolonged wet periods, the alluvial aquifer enlarges, causing tions between the unsaturated zone, canyon bottom sedmore RDX to be mobilized within the alluvial system. Subsurface iments, and the surface water and groundwater systems. processes in the alluvial aquifer are therefore most important in con-A geomorphic approach has proven effective in identitrolling present RDX transport, whereas surface processes associated fying the sources and distribution of contaminants in with floods are most important in cont...

No abstract