It has been shown by nitrogen adsorption that the removal of lignin and hemicellulose from the middle lamella and cell wall of spruce wood produces pores with a median size in the 20–40 A. range throughout the dissolution process. During both kraft and sulfite pulping, these pores have a total volume which from 90–100% yield equals, from 55–90% yield exceeds, and below 55% yield is less than, the volume of material removed from the wall. The results therefore indicate a swelling of the wall at intermediate yields followed by a contraction below the initial dimensions towards the end of pulping. In accordance with a previously postulated theory of cell‐wall structure based on a multilamellur model, the pores produced by the dissolution of wood components are the slit‐like spaces between adjacent lamellae. Specific surface measurements show that, on this model, the cell wall of water‐swollen wood consists of about 5 lamellae, which subdivide into a total of about 350 lamellae each 35 A. thick at the point of maximum swelling (65–70% yield). Thereafter, towards lower yields, the lamellae start to reaggregate. This technique applied to groundwood pulp shows that the process of splitting the wall into layers may be accomplished to some extent by purely mechanical action.

Soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows. This study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment. A field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface. The method allowed for measurements in this study of flow erosion at a much wider range of slopes (2Á6 to 30Á1 per cent) and unit discharge rates (0Á0007 to 0Á007 m 2 s À1 ) than have been previously feasible. Flow velocities were correlated to discharge and hydraulic radius, but not to slope. The lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity. The detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected. Flow detachment rates were better correlated to a power function of either shear stress (r 2 = 0Á51) or stream power (r 2 = 0Á59).

[1] This study reports sediment yields from seven small (0.18-5.42 ha) watersheds in Southern Arizona measured from 1995 to 2005. Sediment concentrations and total event sediment yields were related to storm-runoff characteristics, and statistical relationships were developed to estimate sediment yields for events with missing data. Precipitation ranged from 263 to 298 mm yr À1 , runoff ranged from 8.2 to 26.4 mm yr À1, and sediment yields ranged from 0.07 to 5.7 t ha À1 yr À1 , with an areal average of 2.2 t ha À1 yr À1 . For six of the seven watersheds, between 6 and 10 events produced 50% of the total sediment yields over the 11-year period. On the seventh watershed, two storms produced 66% of the sediment because of differences in the geomorphology and vegetation characteristics of that area. Differences between sediment yields from all watersheds were attributable to instrumentation, watershed morphology, degree of channel incision, and vegetation.

Millions of hectares of rangeland in the western United States have been invaded by annual and woody plants that have increased the role of wildland fire. Altered fire regimes pose significant implications for runoff and erosion. In this paper we synthesize what is known about fire impacts on rangeland hydrology and erosion, and how that knowledge advances understanding of hydrologic risks associated with landscape scale plant community transitions and altered fire regimes. The increased role of wildland fire on western rangeland exposes landscapes to amplified runoff and erosion over short-and long-term windows of time and increases the risk of damage to soil and water resources, property, and human lives during extreme events. Amplified runoff and erosion postfire are a function of storm characteristics and fire-induced changes in site conditions (i.e., ground cover, soil water repellency, aggregate stability, and surface roughness) that define site susceptibility. We suggest that overall postfire hydrologic vulnerability be considered in a probabilistic framework that predicts hydrologic response for a range of potential storms and site susceptibilities and that identifies the hydrologic response magnitudes at which damage to values-at-risk are likely to occur. We identify key knowledge gaps that limit advancement of predictive technologies to address the increased role of wildland fire across rangeland landscapes. Our review of literature suggests quantifying interactions of varying rainfall intensity and key measures of site susceptibility, temporal variability in strength/influence of soil water repellency, and spatial scaling of postfire runoff and erosion remain paramount areas for future research to address hydrologic effects associated with the increased role of wildland fire on western rangelands. Resumen Millones de hectáreas de pastizales en el oeste de Estados Unidos han sido invadidos por plantas arbustivas y anuales que han aumentado la función de los incendios forestales. La modificación de los regímenes de fuego implica cambios significativos para el escurrimiento y la erosión. En este documento resumimos lo que se conoce sobre los impactos de fuego sobre hidrología y la erosión en pastizales, y cómo ese conocimiento nos ayuda a comprender mejor los riesgos hidrológicos asociados con la transición en la comunidad de plantas y con el cambio en los regímenes de fuego. El aumento en la incidencia de los incendios forestales en los pastizales occidentales expone al paisaje a un aumento en el escurrimiento sobre un periodo a corto y largo plazo y a un incremento en el riesgo de dañ o a los recursos del suelo, agua, bienes y vidas humanas durante eventos extremos. Un aumento en el escurrimiento y la erosión despue´s del fuego están en función de los cambios inducidos por el fuego en las características del sitio (es decir la cubierta del suelo, repelencia del agua del suelo, estabilidad de los agregados, y la rugosidad de la superficie) que definen la susceptibilidad del sitio. Sugerimos que se consid...