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Storm events dominate riverine loads of dissolved organic carbon (DOC) and nitrate and are expected to increase in frequency and intensity in many regions due to climate change. We deployed three high‐frequency (15 min) in situ absorbance spectrophotometers to monitor DOC and nitrate concentration for 126 storms in three watersheds with agricultural, urban, and forested land use/land cover. We examined intrastorm hysteresis and the influences of seasonality, storm size, and dominant land use/land cover on storm DOC and nitrate loads. DOC hysteresis was generally anticlockwise at all sites, indicating distal and plentiful sources for all three streams despite varied DOC character and sources. Nitrate hysteresis was generally clockwise for urban and forested sites, but anticlockwise for the agricultural site, indicating an exhaustible, proximal source of nitrate in the urban and forested sites, and more distal and plentiful sources of nitrate in the agricultural site. The agricultural site had significantly higher storm nitrate yield per water yield and higher storm DOC yield per water yield than the urban or forested sites. Seasonal effects were important for storm nitrate yield in all three watersheds and farm management practices likely caused complex interactions with seasonality at the agricultural site. Hysteresis indices did not improve predictions of storm nitrate yields at any site. We discuss key lessons from using high‐frequency in situ optical sensors.

Many geoscientists now recognize stemflow as an important phenomenon which can exert considerable effects on the hydrology, biogeochemistry, and ecology of wooded ecosystems and shrublands. Despite the explosive growth of stemflow research, until this review there has been no comprehensive attempt to summarize and synthesize this literature since 2003. Topical areas of substantive new knowledge in stemflow research include the following: (1) the interrelationships among stemflow and meteorological conditions, especially within individual rain events; (2) the dynamic interplay between stemflow and canopy structure; (3) stemflow and the cycling of solutes and transport of particulate matter; (4) stemflow and its interactions with canopy fungi and corticolous lichens; and (5) stemflow-soil interactions. Each of these five topical areas of substantive new stemflow research is summarized and synthesized, with areas of future research opportunities discussed. In addition, we have reviewed the parameters which can be used to describe stemflow and critically evaluate their utility for different purposes. This review makes a call for scientists studying stemflow to utilize common metrics in an effort to increase the cross-site comparability of stemflow studies. Capitalizing on the insights of prior research, exciting research opportunities await hydrologists, biogeoscientists, and forest ecologists who will conduct studies to deepen our knowledge of stemflow which will enable a better and more accurate framing of stemflow in the larger context of watershed hydrology and biogeochemistry.

Throughfall is a critical component of the hydrological and biogeochemical cycles of wooded ecosystems with a characteristically large degree of temporal and spatial variability. The highly variable nature of throughfall is of importance to scientists and natural resource managers concerned with the effects of water and solute inputs to the subcanopy, including understory vegetation, soil moisture, soil solution chemistry, and the fate of atmospheric dryfall. The purpose of this study is to critically review and evaluate the present state of knowledge pertaining to the temporal and spatial variability of throughfall volume and solute inputs in wooded ecosystems. The authors are optimistic that this review will facilitate the advancement of science by exposing gaps in our current understanding and mitigating the duplication of unwarranted research efforts. Several key areas where current knowledge is weak are: (1) the effect of meteorological conditions on the variability of throughfall volume; a data gap exists concerning the effects of precipitation type (eg, rain, snow, snow-to-rain, rain-to-snow), incident rain drop size, intensity, duration, wind speed and direction, and wind run on the throughfall variability; (2) the effect of meteorological conditions on the variability of throughfall solute inputs; (3) the role of canopy structure on precipitation partitioning into throughfall and stemflow and the variability of throughfall volume and solute inputs; (4) effects of epiphytes on the spatial variation of throughfall volume and solute inputs; (5) the physics and fluid dynamics of water flow over vegetative surfaces and its impact on throughfall yield and chemical enrichment; and (6) intraspecific variation of throughfall water and solute inputs. Future research projects undertaken with the specific aim of addressing the deficiencies identified will improve our understanding of interactions among the biosphere-atmosphere-lithosphere and promote better stewardship of forest and water resources.

Soil moisture is a critical component of the earth system and plays an integrative role among the various subfields of physical geography. This paper highlights not just how soil moisture affects atmospheric, geomorphic, hydrologic, and biologic processes but that it lies at the intersection of these areas of scientific inquiry. Soil moisture impacts earth surface processes in such a way that it creates an obvious synergistic relationship among the various subfields of physical geography. The dispersive and cohesive properties of soil moisture also make it an important variable in regional and microclimatic analyses, landscape denudation and change through weathering, runoff generation and partitioning, mass wasting, and sediment transport. Thus, this paper serves as a call to use research in soil moisture as an integrative and unifying theme in physical geography.

Understanding the quantity and quality of dissolved organic matter (DOM) in potential watershed sources is critical for explaining and quantifying the exports of DOM in stream runoff. Here, we examined the concentration and quality of DOM for ten watershed sources in a 12 ha forested catchment over a two-year period. DOM composition was evaluated for: throughfall, litter leachate, soil water (zero and tension), shallow and deep groundwater, stream water, hyporheic zone, and groundwater seeps. DOM quality was measured using a suite of optical indices including UV-visible absorbance and PARAFAC modeling of fluorescence excitationemission matrices (EEMs). DOM concentrations and quality displayed a pronounced trend across watershed sources. Surficial watershed sources had higher DOM concentrations and more humic-like DOM with higher molecular weight whereas deeper groundwater sources were rich in % protein-like fluorescence. The greater % contribution of proteinlike fluorescence in groundwater suggested that a larger fraction of groundwater DOM may be bioavailable. DOM for wetland groundwater was more aromatic and humic-like than that at the well-drained riparian location. Principal component analyses (PCA) revealed that the differences in surficial watershed compartments were dictated by humic-like components while groundwater sources separated out by % protein-like fluorescence. Observations from optical indices did not provide any conclusive evidence for preferential association of dissolved organic carbon (DOC) or dissolved organic nitrogen (DON) with any particular DOM quality pools.

[1] The concentrations and quality of dissolved organic matter (DOM) and their sources were studied for multiple storm events collected over a three-year period in a forested headwater (12 ha) catchment in the mid-Atlantic Piedmont region of the USA. DOM constituents were characterized using a suite of indices derived from ultraviolet absorbance and PARAFAC modeling of fluorescence excitation emission matrices. Runoff sources and hydrologic flow paths were identified using an end-member mixing model, stable isotope data, and groundwater elevations from valley-bottom saturated areas. DOM constituents and their sources differed dramatically between base flow and storm-event conditions. The aromatic and humic DOM constituents in stream water increased significantly during storm events and were attributed to the contributions from surficial sources such as throughfall, litter leachate and soil water. Groundwater sources contributed a large fraction of the DOM constituents during base flow and were responsible for the high % protein-like fluorescence observed in base flow. Hydrologic flow paths and runoff sources were critical for explaining the differences in DOM among the storm events. This study underscored the value of studying multiple storm events across a range of hydrologic and seasonal conditions. Summer events produced the highest concentrations for humic and aromatic DOM while the corresponding response for winter events was muted. A large event following summer drought produced a complex DOM response which was not observed for the other events. These extreme events provided important insights into how DOM quality may change for future changes in climate and water quality implications for sensitive coastal ecosystems.

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