Brent D. Newman

ECOHYDROLOGICAL CONTROL OF DEEP DRAINAGE IN ARID AND SEMIARID REGIONS

The amount and spatial distribution of deep drainage (downward movement of water across the bottom of the root zone) and groundwater recharge affect the quantity and quality of increasingly limited groundwater in arid and semiarid regions. We synthesize research from the fields of ecology and hydrology to address the issue of deep drainage in arid and semiarid regions. We start with a recently developed hydrological model that accurately simulates soil water potential and geochemical profiles measured in thick (.50 m), unconsolidated vadose zones. Model results indicate that, since the climate change that marked the onset of the Holocene period 10 000-15 000 years ago, there has been no deep drainage in vegetated interdrainage areas and that continuous, relatively low ( ,21 MPa) soil water potentials have been maintained at depths of 2-3 m. A conceptual model con- sistent with these results proposes that the native, xeric-shrub-dominated, plant communities that gained dominance during the Holocene generated and maintained these conditions. We present three lines of ecological evidence that support the conceptual model. First, xeric shrubs have sufficiently deep rooting systems with low extraction limits to generate the modeled conditions. Second, the characteristic deep-rooted soil-plant systems store suffi- cient water to effectively buffer deep soil from climatic fluctuations in these dry environ- ments, allowing stable conditions to persist for long periods of time. And third, adaptations resulting in deep, low-extraction-limit rooting systems confer significant advantages to xeric shrubs in arid and semiarid environments. We then consider conditions in arid and semiarid regions in which the conceptual model may not apply, leading to the expectation that portions of many arid and semiarid watersheds supply some deep drainage. Further ecohy- drologic research is required to elucidate critical climatic and edaphic thresholds, evaluate the role of important physiological processes (such as hydraulic redistribution), and evaluate the role of deep roots in terms of carbon costs, nutrient uptake, and whole-plant devel-

Runoff from a semiarid Ponderosa pine hillslope in New Mexico

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.

Tracer-based studies of soil water movement in semi-arid forests of New Mexico

Abstract The related issues of water movement and contaminant transport in arid and semi-arid environments have generated considerable interest and concern in the last few decades. Essential to understanding these issues is knowledge of how water moves through the soils that form the uppermost part of the vadose zone. The use of tracers, both natural and artificially introduced, is proving to be an effective method for gaining such knowledge in dry regions, where investigation by other means is difficult. In this study, natural stable-isotope and chloride tracers were used to investigate water movement in the soils of a pinon–juniper woodland and of a ponderosa pine forest on the Pajarito Plateau in northern New Mexico. The objectives were to (1) estimate and compare near-surface flux rates and evaluate the importance of evaporation in the two communities, and (2) determine to what extent differences in flux rates and evaporation are due to differences in plant cover and/or soil hydraulic properties. The results of this study will aid in evaluating the potential for contaminant mobility in semi-arid systems such as the Pajarito Plateau and, in addition, will increase understanding of nutrient distributions and plant water use in semi-arid environments. The stable-isotope data indicate a similarity between the pinon–juniper and ponderosa communities with respect to evaporation: in both, it is restricted mainly to the upper 10 cm of soil. Chloride profiles from the two communities, on the other hand, show a distinct difference with respect to downward fluxes: in the ponderosa pine forest, these fluxes (≈0.02 cm year −1 ) are an order of magnitude lower than those in the pinon–juniper woodland (≈0.2 cm year −1 ), even though total precipitation is about 4 cm year −1 higher in the ponderosa pine forest. This difference, however, appears to be related not to plant cover, but to differences in soil hydraulic properties. The soils of the ponderosa pine forest contain clay-rich B horizons that appear to restrict downward movement of water through the soil matrix, whereas the soils of the pinon–juniper community have B horizons much lower in clay content. The effect of differing soil properties on water movement suggests that contaminant distributions will vary across the Pajarito Plateau. The data on soil water ages support this hypothesis: they indicate that water (and, thus, contaminants) moves through the soil matrix in less than a decade in some areas, whereas in other areas, water takes hundreds of years to pass through the entire soil profile.

Dating of 'young' groundwaters using environmental tracers: advantages, applications, and research needs.

Many problems related to groundwater supply and quality, as well as groundwater-dependent ecosystems require some understanding of the timescales of flow and transport. For example, increased concern about the vulnerabilities of ‘young’ groundwaters (less than ∼ 1000 years) to overexploitation, contamination, and land use/climate change effects are driving the need to understand flow and transport processes that occur over decadal, annual, or shorter timescales. Over the last few decades, a powerful suite of environmental tracers has emerged that can be used to interrogate a wide variety of young groundwater systems and provide information about groundwater ages/residence times appropriate to the timescales over which these systems respond. These tracer methods have distinct advantages over traditional approaches providing information about groundwater systems that would likely not be obtainable otherwise. The objective of this paper is to discuss how environmental tracers are used to characterise young groundwater systems so that more researchers, water managers, and policy-makers are aware of the value of environmental tracer approaches and can apply them in appropriate ways. We also discuss areas where additional research is required to improve ease of use and extend quantitative interpretations of tracer results.

Microtopographic and depth controls on active layer chemistry in Arctic polygonal ground

Polygonal ground is a signature characteristic of Arctic lowlands, and carbon release from permafrost thaw can alter feedbacks to Arctic ecosystems and climate. This study describes the first comprehensive spatial examination of active layer biogeochemistry that extends across high- and low-centered, ice wedge polygons, their features, and with depth. Water chemistry measurements of 54 analytes were made on surface and active layer pore waters collected near Barrow, Alaska, USA. Significant differences were observed between high- and low-centered polygons suggesting that polygon types may be useful for landscape-scale geochemical classification. However, differences were found for polygon features (centers and troughs) for analytes that were not significant for polygon type, suggesting that finer-scale features affect biogeochemistry differently from polygon types. Depth variations were also significant, demonstrating important multidimensional aspects of polygonal ground biogeochemistry. These results have major implications for understanding how polygonal ground ecosystems function, and how they may respond to future change.

Regolith Water in Zero-Order Chaparral and Perennial Grass Watersheds Four Decades after Vegetation Conversion

In 1960, areas of chaparral were converted to perennial grass after a fire burned most of the San Dimas Experimental Forest in southern California. This conversion provided an opportunity to compare regolith moisture patterns of zero-order watersheds under native chaparral with those under nonnative veldt grass ( Ehrharta calycina Sm.). We collected data as a function of vegetation type and watershed element to test the hypothesis that conversion from chaparral to grass altered water distribution in the vadose zone as a result of changes in the physical environment, including rooting depth and soil horizonation. Patterns in vadose zone water distribution during the dry season, including soil water potential and residual flux, were significantly different in converted areas, reflecting the different rooting habits of the two vegetation types. In chaparral areas, there was no significant change in soil water potential between the surface and the 150-cm depth; soil water potential was consistently below −1.5 MPa, reflecting the extensive root system. In grass areas, soil water potential was most negative close to the surface, where grass roots were most abundant. Plant available water was present below the 100-cm depth, suggesting that recharge to groundwater may occur under grass in average or wetter years. Under both vegetation types, the largest differences in residual water fluxes were near the soil–weathered rock contact. However, there was a significant relation between minor differences in fluxes and soil horizon boundaries, confirming the effects of vegetation conversion on soil properties and vadose zone soil water.

New capabilities for studies using isotopes in the water cycle

The characterization and quantification of hydrological fluxes within components of the water cycle and across interfaces (e.g., atmosphere/land surface, aquifer/river, soil/plant) are critical for assessing and managing water resources and for understanding the impacts of climate change and variability on the hydrological cycle. Stable isotopes of oxygen and hydrogen, and radioactive isotopes such as tritium and carbon-14, provide unique insights into hydrological and climatic processes at local, regional, and global scales, including the role of groundwater in rivers and lakes, groundwater recharge rates, and sources and recycling rates of atmospheric moisture [Aggarwal et al., 2005; Gat, 1996; Kendall and McDonnell, 1998]. Isotopes also provide critical insights into understanding feedbacks and interactions between physical and biological processes (e.g., ecohydrology).

Global Hydrological Isotope Data and Data Networks

Isotopes of light elements constitute a set of powerful and widely used environmental tracers that often provide unique information about hydrological, climatological, and ecological processes. Environmental isotopes are extensively used in groundwater and surface water hydrology, palaeoclimatic reconstructions, atmospheric circulation processes, ocean dynamics, archaeology, palaeontology, anthropology, ecology, food webs, forensics and food authentication. Basic data on spatial and temporal distribution of isotopes at varying scales in the different components of the water cycle are required for a meaningful application of these tracers. A major source of isotope data on a global scale has been provided since the 1960s by the International Atomic Energy Agency (IAEA), which collects and disseminates isotope data and related hydrological information obtained as part of global or regional monitoring programmes and isotope hydrology studies. Available isotope data are gathered and compiled through global networks such as the global network of isotopes in precipitation (GNIP); global network of isotopes in rivers (GNIR); and moisture isotopes in biosphere and atmosphere (MIBA) network. In addition, global isotope data from surface waters and groundwaters are also being compiled. Other important hydrological isotope databases not covered by these networks are the Global Seawater Oxygen-18 Database; and GNIP-Antarctica, an extensive data set containing isotope composition of samples collected in Antarctic snow pits and ice cores. This chapter reviews the current status of and the basic information provided by global isotope networks and databases, and includes some examples of how such data are used to understand regional- to global-scale processes.

Quantifying uncertainty in stable isotope mixing models

Mixing models are powerful tools for identifying biogeochemical sources and determining mixing fractions in a sample. However, identification of actual source contributors is often not simple, and source compositions typically vary or even overlap, significantly increasing model uncertainty in calculated mixing fractions. This study compares three probabilistic methods, Stable Isotope Analysis in R (SIAR), a pure Monte Carlo technique (PMC), and Stable Isotope Reference Source (SIRS) mixing model, a new technique that estimates mixing in systems with more than three sources and/or uncertain source compositions. In this paper, we use nitrate stable isotope examples (δ15N and δ18O) but all methods tested are applicable to other tracers. In Phase I of a three-phase blind test, we compared methods for a set of six-source nitrate problems. PMC was unable to find solutions for two of the target water samples. The Bayesian method, SIAR, experienced anchoring problems, and SIRS calculated mixing fractions that most closely approximated the known mixing fractions. For that reason, SIRS was the only approach used in the next phase of testing. In Phase II, the problem was broadened where any subset of the six sources could be a possible solution to the mixing problem. Results showed a high rate of Type I errors where solutions included sources that were not contributing to the sample. In Phase III some sources were eliminated based on assumed site knowledge and assumed nitrate concentrations, substantially reduced mixing fraction uncertainties and lowered the Type I error rate. These results demonstrate that valuable insights into stable isotope mixing problems result from probabilistic mixing model approaches like SIRS. The results also emphasize the importance of identifying a minimal set of potential sources and quantifying uncertainties in source isotopic composition as well as demonstrating the value of additional information in reducing the uncertainty in calculated mixing fractions.

Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes

Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem-scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) toward hydrogenotropic (September) methanogenesis at sites located toward the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape-scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly, CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to <0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e- acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH4 and CO2 production.