Using hydrochemical and isotopic compositions of springs and wells, we trace carbon from critical zone carbon dioxide (CO2) into groundwater of the semi-arid Reynolds Creek Experimental Watershed - Critical Zone Observatory, southwestern Idaho, USA. Dissolved inorganic carbon (DIC) concentrations, pH and stable isotope tracers of carbon for DIC (δ13CDIC), are used to show that most groundwater evolves under open system conditions, moving carbon into the groundwater and acting as a carbon sink. However, one sample (−10.94‰ δ13CDIC, 6,350 14C years before present (yrs. BP)) may have evolved under closed system conditions with a higher partial pressure of critical zone CO2 than present-day soils. By characterizing the carbon cycle, we show that (1) carbon evolution is primarily under open-system conditions, (2) shallow groundwater samples are generally less mixed and more recent (10 to 70 3H yrs. BP) than deeper groundwater samples (1,469 to 6,350 14C yrs. BP), and (3) the older portion of the groundwater may be even older than the calculated 14C ages, as indicated by the mixing of age tracers in intermediate wells. Our global conception of the deep critical zone should include carbon cycling of critical zone CO2 in old groundwater. Characterizing the deep critical zone in a semi-arid weathered silicate watershed improves our global understanding of carbon, nutrient and water cycling.

Silicate weathering can induce calcite precipitation from groundwater, enabling carbon dioxide (CO2) sequestration in the critical zone (CZ), which acts as a net carbon sink with significant implications for the global carbon budget. In weathered silicates, secondary calcite dissolution accompanies precipitation-dissolution reactions, and it is unclear how calcite dissolution affects CO2 consumption in natural settings. At the Reynolds Creek Experimental Watershed- Critical Zone Observatory (RCEW-CZO), southwestern Idaho, USA, we estimate in-situ carbon sequestration rates in a semi-arid weathered silicate aquifer using hydrochemical compositions and age tracers from 6 springs and 10 wells. We delineate water-rock interactions by using observed groundwater chemistry to model open system carbon evolution, evapoconcentration in wells, silicate weathering, and formation of clays along groundwater flowpaths. We suggest carbonate precipitation under closed system conditions in deep groundwater, as calcite saturation is reached and CZ CO2 drops to just 41 % of initial concentrations in older waters. In a closed system, we estimate approximately 9 % of CZ CO2 would precipitate, indicating that on-going water-rock interactions in our weathered silicate system appear to drive continued carbon sequestration. Carbon sequestration rates via silicate weathering may help to explain a missing C sink observed at the RCEW-CZO and in other weathered silicate basins.

Long‐term (>20 y) suspended sediment (SS) and particulate organic carbon (POC) records are relatively rare and yet are necessary for understanding linkages between climate, erosion and carbon export. We estimated long‐term (>23 y) SS and POC yields from four nested catchments that ranged from <1 to 54 km2 in area across the Reynolds Creek Experimental Watershed and Critical Zone Observatory (RCEW‐CZO) in southwestern Idaho, USA. We found strong relationships between log10SS and log10POC (R2 = 0.38–0.86) that varied across catchments but remained robust across years, one dry and one of the wettest water years on record. Mean annual SS yields varied from 18 to 89 g SS m−2 y−1 and POC from 0.6 to 11.0 g C m−2 y−1 across the four catchments. Water yield explained much of the temporal variation (72%–85%) in SS and POC yields except in a small, snow‐dominated headwater catchment where it explained 15%–51%. The largest five water years accounted for 69%–84% of the total SS and POC yields in catchments with 24 y records. All catchments had positive slopes (>0) for SS and POC concentration‐discharge (C‐Q) relationships, with large catchments exhibiting greater slopes (0.66–0.97) than smaller ones (0.14–0.16). In addition, most catchments were dominated (80%) by clockwise hysteretic curves. Lack of seasonal exhaustion in the SS‐POC relationships, positive C‐Q and clockwise relations indicated that these systems were transport‐rather than supply limited, and that sediment and POC appeared to be sourced from channel/bank erosion and remobilization. POC yields represent 1%–10% of mean water year net ecosystem exchange depending on elevation; lower elevation catchments may shift from being carbon sinks to sources after accounting for fluvial POC export associated with changes in climate.

Long-term datasets of suspended sediment (SS) and particulate organic carbon (POC) are valuable to understand the role the fluvial export plays in the landscape responses to changing climate and disturbance regimes such as fire. Here we report a dataset that includes raw SS and POC concentrations and estimated long-term (> 23 years) SS and POC yields from 4 nested catchments that ranged from < 1 to 54 km2 in area across the Reynolds Creek Experimental Watershed and Critical Zone Observatory (RCEW-CZO) in southwestern Idaho, USA. We also estimated SS and POC yields from a burned catchment in the first two years following fire.

Meteorological, snow, streamflow, topographic, and vegetation height data are presented from the South Mountain experimental catchments. This study site was established in 2007 as a collaborative, long-term research laboratory to address the impacts of western juniper encroachment and woodland treatments in the interior Great Basin region of the western USA. The data provide detailed information on the weather and hydrologic response from four highly instrumented catchments in the late stages of woodland encroachment in a sagebrush steppe landscape. Hourly data from six meteorologic stations and four weirs have been carefully processed, quality-checked, and are serially complete. These data are ideal for hydrologic, ecosystem, and biogeochemical modeling. Data presented are publicly available from the USDA National Agricultural Library administered by the Agricultural Research Service (https://data.nal.usda.gov/dataset/data-weather-snow-and-streamflow-data-four-western-juniper-dominated-experimental-catchments, doi:10.15482/USDA.ADC/1254010).

Abstract. Weather, snow, stream, topographic, and vegetation data are presented from the South Mountain Experimental Catchments. This study site was established in 2007 as a collaborative, long-term research laboratory to address the impacts of western juniper encroachment and woodland treatments in the interior Great Basin region of the western USA. The data provide detailed information on the weather and hydrologic response from four highly instrumented catchments in the late stages of woodland encroachment in a sagebrush steppe landscape. Hourly data from six meteorologic stations and four weirs have been carefully processed, quality checked, and are serially complete. These data are ideal for hydrologic, ecosystem, and biogeochemical modeling. Data presented are publicly available from the USDA National Agricultural Library administered by the Agricultural Research Service (https://data.nal.usda.gov/dataset/data-weather-snow-and-streamflow-data-four-western-juniper-dominated-experimental-catchments, doi:10.15482/USDA.ADC/1254010).

The U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center initiated a stream discharge and suspended‐sediment research program at Reynolds Creek Experimental Watershed in the early 1960s. Continuous discharge measurements began at two sites in 1963, at three additional sites in 1964, and at eight additional sites in subsequent years. Contributing areas to these gauging stations range from 1.03 to >23,822 ha, selected to represent the broad range of environmental settings found across northwestern rangelands. Quality‐controlled, validated breakpoint and hourly stream discharge data sets are available for these 13 sites for the period 1963 through 1996 (or for a subset of that time for some sites). Suspended‐sediment data are available for three gauging stations (high elevation, middle elevation, and low elevation). All data are available on the Northwest Watershed Research Center anonymous ftp site (ftp.nwrc.ars.usda.gov).

The Northwest Watershed Research Center initiated a streamflow and suspended sediment research program at Reynolds Creek Experimental Watershed (RCEW) in the early 1960's. Continuous streamflow measurement began at two sites in 1963, at three additional sites in 1964, and at eight additional sites in subsequent years. Measurements were later discontinued at five sites. Data were or are currently acquired for basins ranging in contributing area from 1.03 ha to 23,866 ha, selected to represent the broad range of environmental settings found in northwestern rangelands and in RCEW. Quality-controlled, validated hourly streamflow data sets are available for these 13 sites for the period 1963 through 1996 (or for a subset of that time for some sites). Suspended sediment data were acquired from a restricted set of streamflow stations. Suspended sediment data are available for three streamflow measurement sites (high elevation, mid-elevation, and low elevation). All data are available on the Northwest Watershed Research Center anonymous FTP site (ftp.nwrc.ars.usda.gov).