Core Ideas Water quality and stream flow have temporal and spatial trends in response to variable climate. Our work reveals how Sierra Nevada forests responded to and recovered from multiyear drought. Regolith thickness trends reveal water storage capacity differences with elevation. Monitoring shows deep‐water changes via plant utilization or capillary flow during drought. Sensor networks within the Southern Sierra Critical Zone Observatory (SSCZO) and Kings River Experimental Watersheds (KREW) document changes in the water cycle spanning the west slope of the southern Sierra Nevada in California. The networks were established to document water dynamics throughout the critical zone spanning profile, hillslope, catchment, and watershed scales at key locations that reflect systematic differences in bioclimatic conditions imposed by a strong elevation gradient. The critical zone observatory attempts to constrain the hydrologic budget via representative measurements of streamflow, eddy flux covariance, snow depth, meteorological conditions, and water content and water potential in soil and deep regolith. These measurements reveal the complexity of interactions among all aspects of the water balance (runoff, storage, evapotranspiration [ET], and precipitation) through daily, seasonal, and annual timescales. Multiyear drought, catastrophic wildfires, insect outbreaks, and disease have caused widespread tree mortality in the Sierra Nevada. These disturbances offer a window into the future for this region, which is expected to undergo significant change in response to global warming. This hydrological observatory provides valuable hydrometric attributes and fluxes across the stream–groundwater–vadose zone–soil–vegetation–atmosphere continuum.

Abstract. Lateral movement of organic matter (OM) due to erosion is now considered an important flux term in terrestrial carbon (C) and nitrogen (N) budgets, yet most published studies on the role of erosion focus on agricultural or grassland ecosystems. To date, little information is available on the rate and nature of OM eroded from forest ecosystems. We present annual sediment composition and yield, for water years 2005-2011, from eight catchments in the southern part of the Sierra Nevada, California. Sediment was compared to soil at three different landform positions from the source slopes to determine if there is selective transport of organic matter or different mineral particle size classes. Sediment export varied from 0.4 to 177 kg ha −1 , while export of C in sediment was between 0.025 and 4.2 kg C ha −1 and export of N in sediment was between 0.001 and 0.04 kg N ha −1 . Sediment yield and composition showed high interannual variation. In our study catchments, erosion laterally mobilized OM-rich litter material and topsoil, some of which enters streams owing to the catchment topography where steep slopes border stream channels. Annual lateral sediment export was positively and strongly correlated with stream discharge, while C and N concentrations were both negatively correlated with stream discharge; hence, C : N ratios were not strongly correlated to sediment yield. Our results suggest that stream discharge, more than sediment source, is a primary factor controlling the magnitude of C and N export from upland forest catchments. The OM-rich nature of eroded sediment raises important questions about the fate of the eroded OM. If a large fraction of the soil organic matter (SOM) eroded from forest ecosystems is lost during transport or after deposition, the contribution of forest ecosystems to the erosion-induced C sink is likely to be small (compared to croplands and grasslands).

Using 6 yr (Water Year [WY] 2009-WY 2014) of hourly in situ measurements from a spatially distributed water-balance cluster, we quantified the long-term accuracy of an algorithm used to predict spatial patterns of depth-integrated soil-water storage within the rain-snow transition zone of the southern Sierra Nevada. The algorithm-the multivariate, non-parametric regression-tree estimator Random Forest-was used to predict soil-water storage based on a combination of attributes at each instrument cluster (soil texture, topographic wetness index, elevation, northness, and canopy cover). Out-of-bag R 2 (similar to cross-validation for Random Forest) was used to quantify the accuracy of the estimator for unobserved data. Accuracy was consistently high during the wet-up, snow-cover, and early recession periods of average and wet years. The accuracy declined at the end of a 3-yr dry period, and the relative rank of the independent variables in the model shifted. Soil texture was the highest-ranked independent variable across all years, followed by elevation and northness. Topographic wetness increased in importance during dry periods. Northness exhibited high importance during the wet-up and early recession periods of most water years. During dry years, the importance of elevation declined. In dry years, notable differences in soil-water storage at each depth include lower-than-average storage in the deeper regolith at the beginning of the water year and lower storage in near-surface layers during the winter resulting from transient snow cover.

Abstract. Soil erosion plays important roles in organic matter (OM) storage and persistence in dynamic landscapes. The biogeochemical implication of soil erosion has been a focus of a growing number of studies over the last two decades. However, most of the available studies are conducted in agricultural systems or grasslands, and hence very little information is available on rate and nature of soil organic matter (SOM) eroded from forested upland ecosystems. In the southern parts of the Sierra Nevada Mountains in California, we determined the rate of carbon (C) and nitrogen (N) eroded from two sets of catchments under different climatic conditions to determine how the amount and distribution of precipitation affects lateral distribution of topsoil and associated SOM. We quantified sediment and SOM exported annually (for water years 2005–2011) from four low-order, snow-dominated catchments, and four low-order catchments that receive a mix of rain, and snow and compared it to soil at three different landform positions from the source slopes to determine if there is selective transport of some soil OM components. We found that the amount of sediment exported varied from 0.4 to 177 kg N ha-1, while export of particulate C was between 0.025 and 4.2 kg C ha-1, compared to export of particulate N that was between 0.001 and 0.04 kg ha-1. Sediment yield and composition showed high interannual variation, with higher C and N concentrations in sediment collected in drier years. In our study catchments, erosion laterally mobilized OM-rich topsoil and litter material, some of which readily enters streams owing to the topography in these catchments that includes steep slopes adjacent to stream channels. Annual lateral sediment mass, C, and N fluxes were positively and strongly correlated with stream flows. Our results suggest that variability in climate, represented by stream discharge, is a primary factor controlling the magnitude of C and N eroded from upland temperature forest catchments.

Abstract. We strategically placed spatially distributed sensors to provide representative measures of changes in snowpack and subsurface water storage, plus the fluxes affecting these stores, in a set of nested headwater catchments. The high temporal frequency and distributed coverage make the resulting data appropriate for process studies of snow accumulation and melt, infiltration, evapotranspiration, catchment water balance, (bio)geochemistry, and other critical-zone processes. We present 8 years of hourly snow-depth, soil-moisture, and soil-temperature data, as well as 14 years of quarter-hourly streamflow and meteorological data that detail water-balance processes at Providence Creek, the upper part of which is at the current 50 % rain versus snow transition of the southern Sierra Nevada, California. Providence Creek is the long-term study cooperatively run by the Southern Sierra Critical Zone Observatory (SSCZO) and the USDA Forest Service Pacific Southwest Research Station's Kings River Experimental Watersheds (KREW). The 4.6 km2 montane Providence Creek catchment spans the current lower rain–snow transition elevation of 1500–2100 m. Two meteorological stations bracket the high and low elevations of the catchment, measuring air temperature, relative humidity, solar radiation, precipitation, wind speed and direction, and snow depth, and at the higher station, snow water equivalent. Paired flumes at three subcatchments and a V-notch weir at the integrating catchment measure quarter-hourly streamflow. Measurements of meteorological and streamflow data began in 2002. Between 2008 and 2010, 50 sensor nodes were added to measure distributed snow depth, air temperature, soil temperature, and soil moisture within the top 1 m below the surface. These sensor nodes were installed to capture the lateral differences of aspect and canopy coverage. Data are available at hourly and daily intervals by water year (1 October–30 September) in nonproprietary formats from online data repositories. Data for the Southern Sierra Critical Zone Observatory distributed snow and soil datasets are at https://doi.org/10.6071/Z7WC73. Kings River Experimental Watersheds meteorological data are available from https://doi.org/10.2737/RDS-2018-0028 and stream-discharge data are available from https://doi.org/10.2737/RDS-2017-0037.