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 an 870-m2 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 (laboratory 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.¿ 1997 American Geophysical Union