Journal of Irrigation and Drainage Engineering, 2009
Riparian buffers are commonly promoted to protect stream water quality. A common conceptual assum... more Riparian buffers are commonly promoted to protect stream water quality. A common conceptual assumption is that buffers "intercept" and treat upland runoff. As a shift in paradigm, it is proposed instead that riparian buffers should be recognized as the parts of the landscape that most frequently generate storm runoff. Thus, water quality can be protected from contaminated storm runoff by disassociating riparian buffers from potentially polluting activities. This paper reviews and synthesizes some simple engineering approaches that can be used to delineate riparian buffers for rural watersheds based on risk of generating runoff. Although reference is made to specific future research that may improve the proposed methods for delineating riparian buffers, the approaches described here provide planners and engineers with a set of currently available scientifically defensible tools. It is recommended that planners and engineers use available rainfall and stream discharge data to parameterize the buffer-sizing equations and use variable-width buffers, based on a topographic index, to achieve a realistic representation of runoff generating areas.
Balancing ecological and human water needs often requires characterizing key aspects of the natur... more Balancing ecological and human water needs often requires characterizing key aspects of the natural flow regime and then predicting ecological response to flow alterations. Flow metrics are generally relied upon to characterize long-term average statistical properties of the natural flow regime (hydrologic baseline conditions). However, some key aspects of hydrologic baseline conditions may be better understood through more complete consideration of continuous patterns of daily, seasonal, and inter-annual variability than through summary metrics. Here we propose the additional use of high-resolution dimensionless archetypes of regional stream classes to improve understanding of baseline hydrologic conditions and inform regional environmental flows assessments. In an application to California, we describe the development and analysis of hydrologic baseline archetypes to characterize patterns of flow variability within and between stream classes. We then assess the utility of archetyp...
JAWRA Journal of the American Water Resources Association, 2017
Alterations to flow regimes for water management objectives have degraded river ecosystems worldw... more Alterations to flow regimes for water management objectives have degraded river ecosystems worldwide. These alterations are particularly profound in Mediterranean climate regions such as California with strong climatic variability and riverine species highly adapted to the resulting flooding and drought disturbances. However, defining environmental flow targets for Mediterranean rivers is complicated by extreme hydrologic variability and often intensive water management legacies. Improved understanding of the diversity of natural streamflow patterns and their spatial arrangement across Mediterranean regions is needed to support the future development of effective flow targets at appropriate scales for management applications with minimal resource and data requirements. Our study addresses this need through the development of a spatially explicit reach-scale hydrologic classification for California. Dominant hydrologic regimes and their physio-climatic controls are revealed using available unimpaired and naturalized streamflow time-series and generally available geospatial datasets. This methodology identifies eight natural flow classes representing distinct flow sources, hydrologic characteristics, and catchment controls over rainfall-runoff response. The study provides a broad-scale hydrologic framework upon which flow-ecology relationships could subsequently be established towards reach-scale environmental flows applications in a complex, highly altered Mediterranean region. 26
Over the past five decades, gullying has become more severe in the Ethiopian highlands. Besides n... more Over the past five decades, gullying has become more severe in the Ethiopian highlands. Besides negatively affecting soil resources, lowering crop yields in areas between the gullies and reducing grazing land available for livestock, gully erosion is one of the major causes of silting of reservoirs. Assessing the rate of gully development and the controlling factors of gullying will help to explain the causes for current land degradation and to design reliable conservation measures for already existed gullies and preventing strategies for those areas susceptible to further gullying. The study was conducted in the 523 ha of Debre-Mewi watershed south of Bahir Dar, Amhara region, Ethiopia, where active gullies were retreating upslope. Semi structured group interview and monitoring of gully development through time was made with profile measurements of contemporary gully volumes. Gullying started in the beginning of the 1980`s following the clearance of indigenous vegetation, leading to an increase of surface and subsurface runoff from the hillside to the valley bottoms. A comparison of the gully area estimated from 0.58 m resolution quick bird image with current gully area walked with a Garmin GPS, indicated that the total eroded area of gully was increased from 0.65 ha in 2005 to 1.0 ha in 2007 and 1.43 ha in 2008. The water levels measured with piezometers showed that in the actively eroding sections the water table was in general above the gully bottom and below it in stabilized sections. The elevated water table facilitates the slumping of the gully wall and their retreat. Water table height is decreasing after the gully has been formed. The gully erosion rate between 2007 and 2008 was 530 t ha-1 yr-1 in the 17.4 ha watershed, equivalent to almost 5 cm soil loss in the contributing area. Gully erosion rate was approximately 20 times the measured upland soil losses.
This study uses an instrumented (trenched) 0.5 ha hillslope in the southern tier of New York Stat... more This study uses an instrumented (trenched) 0.5 ha hillslope in the southern tier of New York State, USA, to provide new data and insights on how variable source areas and associated flow pathways form and combine to connect rainfall with downstream water flows across a hillslope. Measurements of water fluxes in the trench, upslope water table dynamics, surface and bedrock topography, and isotopic and geochemical tracers have been combined for a four-dimensional (space-time) characterization of subsurface storm flow responses. During events with dry antecedent conditions infiltrating rainwater was found to percolate through a prevailing fragipan layer to deeper soil layers, with much (33-71%) of the total discharge of the hillslope originating from deeper water flow below the fragipan. During storm events with wet antecedent conditions and large rainfall amounts, shallow lateral flow of event and pre-event water above the fragipan occurred and was one magnitude greater than the deeper water flow contribution. Spatial surface and subsurface water quality observations indicate that water from a distance of up to 56 m contributed runoff from the hillslope during storm events. In addition, mobilization of total dissolved phosphorus (TDP) with subsurface flow played a greater role than with overland or near-surface flow. During all events TDP loads were highest in the total discharge during peak flows (8-11.5 kg ha À1 d À1), except during the largest storm event, when TDP concentrations were highly diluted. These results have implications for strategies to protect streams and other downstream water recipients from waterborne nutrient and pollutant loading.
Precipitation occurs in two basic forms defined as liquid state and solid state. Different from r... more Precipitation occurs in two basic forms defined as liquid state and solid state. Different from rain-fed watershed, modeling snow processes is of vital importance in snow-dominated watersheds. The seasonal snowpack is a natural water reservoir, which stores snow water in winter and releases it in spring and summer. The warmer climate in recent decades has led to earlier snowmelt, a decline in snowpack, and change in the seasonality of river flows. The Soil and Water Assessment Tool (SWAT) could be applied in the snow-influenced watershed because of its ability to simultaneously predict the streamflow generated from rainfall and from the melting of snow. The choice of parameters, reference data, and calibration strategy could significantly affect the SWAT model calibration outcome and further affect the prediction accuracy. In this study, SWAT models are implemented in four upland watersheds in the Tulare Lake Basin (TLB) located across the Southern Sierra Nevada Mountains. Three cal...
Accurate assessments of soil organic carbon (SOC) stocks are needed at multiple scales given thei... more Accurate assessments of soil organic carbon (SOC) stocks are needed at multiple scales given their importance to both local soil health and global C cycles. Rangelands cover 54% of California, representing a large stock of SOC, but existing SOC estimates are uncertain. To improve understanding of fine-resolution SOC stocks in complex terrain and provide guidance to rangeland SOC inventories, we grid-sampled 105 locations (21-m grid cells) at two depths (0-10 and 10-30 cm) in a 10-ha annual grassland catchment in California's Central Coast Range. Soils were analyzed for bulk density, coarse fragments, SOC and texture. Monthly aerial imagery was acquired by an unmanned aerial vehicle to compare surface reflectance during two contrasting years (wet vs. dry) to SOC stocks. We found that the 0-30 cm soil thickness held 3.64 ± 0.71 kg SOC m −2 (mean ± SD) with a range of 1.97-5.49 kg SOC m −2. The 0-10 cm soil thickness stored 47% of the 0-30 cm SOC stock with SOC concentrations twice as high in the 0-10 cm layer (1.40 ± 0.38%) as in the 10-30 cm layer (0.71 ± 0.15% SOC). Multiple linear regression (MLR) models explained 50-57% of SOC variability at 0-30 and 10-30 cm, but only 25% of variability at 0-10 cm. Based on cross-validation tests, MLR outperformed spatial interpolation methods and Random Forest models, best explaining SOC stocks with five environmental covariates: wet-year greenness, mean curvature, elevation, insolation, and slope. Lower hillslope positions, concave landforms, and enhanced wet-year greenness were associated with more SOC, and explained 11%, 24%, and 31% of variability in 0-30 cm SOC stocks, respectively. This study demonstrates that the accuracy of regional-scale SOC mapping of California rangelands benefits from considering microclimatic and topographic controls at the catchment-scale, in addition to broader scale mineralogical and macroclimatic controls identified in previous SOC studies.
In this short communication, we report on dissolved organic and inorganic carbon concentrations f... more In this short communication, we report on dissolved organic and inorganic carbon concentrations from a summer stream monitoring campaign at the main hydrological catchment of the Tarfala Research Station in northern Sweden. Further, we place these unique high-alpine observations in the context of a relevant subset of Sweden's national monitoring programme. Our analysis shows that while the monitoring programme (at least for total organic carbon) may have relatively good representativeness across a range of forest coverages, alpine/tundra environments are potentially underrepresented. As for dissolved inorganic carbon, there is currently no national monitoring in Sweden. Since the selection of stream water monitoring locations and monitored constituents at the national scale can be motivated by any number of goals (or limitations), monitoring at the Tarfala Research Station along with other research catchment sites across Fennoscandia becomes increasingly important and can offer potential complementary data necessary for improving process understanding. Research catchment sites (typically not included in national monitoring programmes) can help cover small-scale landscape features and thus complement national monitoring thereby improving the ability to capture hot spots and hot moments of biogeochemical export. This provides a valuable baseline of current conditions in high-alpine environments against which to gauge future changes in response to potential climatic and land cover shifts.
Given the complex topography of California rangelands, contrasting microclimates affect forage gr... more Given the complex topography of California rangelands, contrasting microclimates affect forage growth at catchment scales. However, documentation of microclimate-forage growth associations is limited, especially in Mediterranean regions experiencing pronounced climate change impacts. To better understand microclimate-forage growth linkages, we monitored forage productivity and rootzone soil temperature and moisture (0-15 and 15-30 cm) in 16 topographic positions in a 10-ha annual grassland catchment in California's Central Coast Range. Data were collected through two strongly contrasting growing seasons, a wet year (2016-17) with 287-mm precipitation and a dry year (2017-18) with 123-mm precipitation. Plant-available soil water storage (0-30 cm) was more than half full for most of the wet year; mean peak standing forage was 2790 kg ha −1 (range: 1597-4570 kg ha −1). The dry year had restricted plant-available water and mean peak standing forage was reduced to 970 kg ha −1 (range: 462-1496 kg ha −1). In the wet year, forage growth appeared energy limited (light and temperature): warmer sites produced more forage across a 3-4°C soil temperature gradient but late season growth was associated with moister sites spanning this energy gradient. In the dry year, the warmest topographic positions produced limited forage across a 10°C soil temperature gradient until late season rainfall in March. Linear models accounting for interactions between soil moisture and temperature explained about half of rapid, springtime forage growth variance. These findings reveal dynamic but clear microclimate-forage growth linkages in complex terrain, and thus, have implications for rangeland drought monitoring and dryland ecosystems modeling under climate change.
In California's semi-arid climate, replenishment of groundwater aquifers relies on precipitation ... more In California's semi-arid climate, replenishment of groundwater aquifers relies on precipitation and runoff during the winter season. However, climate projections suggest more frequent droughts and fewer years with above-normal precipitation, which may increase demand on groundwater resources and the need to recharge groundwater basins. Using historical daily streamflow data, we developed a spatial index and rating system of high-magnitude streamflow availability for groundwater recharge, STARR, in the Central Valley. We found that watersheds with excellent and good availability of excess surface water are primarily in the Sacramento River Basin and northern San Joaquin Valley. STARR is available as a web tool and can guide water managers on where and when excess surface water is available and, with other web tools, help sustainable groundwater agencies develop plans to balance water demand and aquifer recharge. However, infrastructure is needed to transport the water, and also changes to the current legal restrictions on use of such water. UC Davis researchers developed a web tool that can be used by water managers to identify whenand in which watersheds-excess surface water is available for groundwater recharge. C alifornia's Central Valley produces more than 400 commodities and 17% of the U.S. total agricultural production (valued at nearly $54 billion in 2014) on just 1% of the land in the contiguous United States (CDFA 2015). The massive agricultural production in the Central Valley has resulted in critical groundwater overdraft, triggering state legislation in 2014 to require sustainable management of groundwater basins. There is growing interest in flooding fields during the winter with surplus surface water to recharge underlying groundwater basins; to make this happen, farmers first need to understand the physical distribution and occurrence of excess surface water, particularly the most promising source-high-magnitude streamflows. Agriculture in the Central Valley consumes nearly 40% of California's annual water supply (surface water, groundwater and reused water developed for agricultural, environmental and urban uses), much of it during summer, when surface water supplies are relatively limited (Hanak et al. 2011). Across urban, environmental and agricultural sectors, groundwater accounts for 38% of the state's water supply during a normal year, reaching upward of 48% during a dry year (DWR 2015). The constant use of groundwater over the past century has led to a groundwater overdraft in the Central Valley of over 150 million acre-feet (Faunt 2009). During the 2012-2016 severe drought, groundwater depletion averaged 8.1 million acre-feet per year (Xiao et al. 2017). With the passage of the Sustainable Groundwater Management Act (SGMA) in 2014, landowners are now required to implement groundwater sustainability plans by 2040 (SWRCB 2014). One increasingly considered approach to achieve groundwater sustainability is managed aquifer recharge, which places more water in groundwater aquifers than would otherwise naturally occur (
Groundwater aquifers provide an important "insurance" against climate variability. Due to prolong... more Groundwater aquifers provide an important "insurance" against climate variability. Due to prolonged droughts and/or irrigation demands, groundwater exploitation results in significant groundwater storage depletion. Managed aquifer recharge (MAR) is a promising management practice that intentionally places or retains more water in groundwater aquifers than would otherwise naturally occur. In this study, we examine the possibility of using large irrigated agricultural areas as potential MAR locations (Ag-MAR). Using the California Central Valley Groundwater-Surface Water Simulation Model we tested four different agricultural recharge land distributions, two streamflow diversion locations, eight recharge target amounts, and five recharge timings. These scenarios allowed a systematic evaluation of Ag-MAR on changes in regional, long-term groundwater storage, streamflow, and groundwater levels. The results show that overall availability of stream water for recharge is critical for Ag-MAR systems. If stream water availability is limited, longer recharge periods at lower diversion rates allow diverting larger volumes and more efficient recharge compared to shorter diversion periods with higher rates. The recharged stream water increases both groundwater storage and net groundwater contributions to streamflow. During the first decades of Ag-MAR operation, the diverted water contributed mainly to groundwater storage. After 80 years of Ag-MAR operation about 34% of the overall diverted water remained in groundwater storage while 66% discharged back to streams, enhancing base flow during months with no recharge diversions. Groundwater level rise is shown to vary with the spatial and temporal distribution of Ag-MAR. Overall, Ag-MAR is shown to provide long-term benefits for water availability, in groundwater and in streams.
This study examines the question of how much information one can extract from a tracer-based hydr... more This study examines the question of how much information one can extract from a tracer-based hydrograph separation in a remote and minimally gaged alpine catchment in Chile. We combine PCA-based endmember mixing analysis to identify the sources of flow contribution to the Diguillín River with a hierarchical Bayesian mixing model to integrate spatial and temporal variability in endmember concentration and quantify the source contributions to streamflow over time. The PCA-analysis shows that precipitation isotopes do not vary by elevation (e.g. snow and rainfall had identical signatures) but vary significantly by season, and that a third endmember is necessary to bound streamflow variability at the basin outlet, which was not captured by our field sampling. One of the main advantages of Bayesian methods is the quasi-machine learning capabilities, where we treated the third endmember as a parameter from which the mixing model could both estimate proportional contributions as well as posterior estimates for the tracer concentrations. The two tracer, three endmember hydrograph separation revealed groundwater to be the largest and precipitation (rain and snow) to be the smallest contributor, on average, to streamflow with the third unknown endmember contributing around 40% of streamflow during the Winter wet season. We hypothesize that interflow is occurring as the third endmember in the Alto Diguillín subwatershed, based on inferred tracer values and the presence of alluvium atop impermeable bedrock along certain reaches. More work is necessary to observe and sample these flowpaths, which was not possible during this study. The results of this work have implications for water resource management, since groundwater sustains the majority of streamflow in the Diguillín, and climate change will impact the timing and quantity of baseflow and interflow. Overall, we demonstrate the utility of combining PCA with Bayesian statistical modeling and inference to extract maximum information from a limited field dataset in a remote alpine catchment. The findings of this work can guide future water management in the Diguillín, but also provide clear questions for future research.
Fluvial flood events have substantial impacts on humans, both socially and economically, as well ... more Fluvial flood events have substantial impacts on humans, both socially and economically, as well as on ecosystems (e.g., hydroecology and pollutant transport). Concurrent with climate change, the seasonality of flooding in cold environments is expected to shift from a snowmelt-dominated to a rainfall-dominated flow regime. This would have profound impacts on water management strategies, that is, flood risk mitigation, drinking water supply, and hydro power. In addition, cold climate hydrological systems exhibit complex interactions with catchment properties and largescale climate fluctuations making the manifestation of changes difficult to detect and predict. Understanding a possible change in flood seasonality and defining related key drivers therefore is essential to mitigate risk and to keep management strategies viable under a changing climate. This study explores changes in flood seasonality across near-natural catchments in Scandinavia using circular statistics and trend tests. Results indicate strong seasonality in flooding for snowmelt-dominated catchments with a single peak occurring in spring and early summer (March through June), whereas flood peaks are more equally distributed throughout the year for catchments located close to the Atlantic coast and in the south of the study area. Flood seasonality has changed over the past century seen as decreasing trends in summer maximum daily flows and increasing winter and spring maximum daily flows with 5-35% of the catchments showing significant changes at the 5% significance level. Seasonal mean daily flows corroborate those findings with higher percentages (5-60%) of the catchments showing statistically significant changes. Alterations in annual flood occurrence also point towards a shift in flow regime from snowmelt-dominated to rainfall-dominated with consistent changes towards earlier timing of the flood peak (significant for 25% of the catchments). Regionally consistent patterns suggest a first-order climate control as well as a local second-order catchment control, which causes inter-seasonal variability in the streamflow response.
Advances in Chemical Pollution, Environmental Management and Protection, 2018
A growing population and an increased demand for water resources have resulted in a global trend ... more A growing population and an increased demand for water resources have resulted in a global trend of groundwater depletion. Arid and semi-arid climates are particularly susceptible, often relying on groundwater to support large population centers or irrigated agriculture in the absence of sufficient surface water resources. In an effort to increase the security of groundwater resources, managed aquifer recharge (MAR) programs have been developed and implemented globally. MAR is the approach of intentionally harvesting and infiltrating water to recharge depleted aquifer storage. California is a prime example of this growing problem, with three cities that have over a million residents and an agricultural industry that was valued at 47 billion dollars in 2015. The present-day groundwater overdraft of over 100 km3 (since 1962) indicates a clear disparity between surface water supply and water demand within the state. In the face of groundwater overdraft and the anticipated effects of climate change, many new MAR projects are being constructed or investigated throughout California, adding to those that have existed for decades. Some common MAR types utilized in California include injection wells, infiltration basins (also known as spreading basins, percolation basins, or recharge basins), and low-impact development. An emerging MAR type that is actively being investigated is the winter flooding of agricultural fields using existing irrigation infrastructure and excess surface water resources, known as agricultural MAR. California therefore provides an excellent case study to look at the historical use and performance of MAR, ongoing and emerging challenges, novel MAR applications, and the potential for expansion of MAR. Effective MAR projects are an essential tool for increasing groundwater security, both in California and on a global scale. This chapter aims to provide an overview of the most common MAR types and applications within the State of California and neighboring semi-arid regions.
Journal of colloid and interface science, Jan 15, 2018
Poly(lactic-co-glycolic acid) (PLGA) particle carriers of synthetic DNA have recently received in... more Poly(lactic-co-glycolic acid) (PLGA) particle carriers of synthetic DNA have recently received increased attention for environmental applications due to their biodegradability, customizability, and nearly limitless number of uniquely identifiable "labels". In this paper, we present methodologies for the preparation of DNA-labeled particles, control of particle size, extraction of DNA-labels, and analysis via quantitative polymerase chain reaction (qPCR). Characterization and analysis of the DNA-labeled particles reveal spherical particles of diameters ranging from 60 to 1000 nm, with consistent zeta potentials around -45 mV, that are stable to aggregation, even in the presence of concentrated mono- and divalent cations. A highly correlated and consistent relationship between particle concentration and DNA-label count was observed, with a detection range spanning 7 orders of magnitude, from 0.01 to 10,000 mg/L (10-10 particles/μL). The results of two environmental applicati...
Escherichia coli (E. coli) level in streams is a public health indicator. Therefore, being able t... more Escherichia coli (E. coli) level in streams is a public health indicator. Therefore, being able to explain why E. coli levels are sometimes high and sometimes low is important. Using citizen science data from Fall Creek in central NY we found that complementarily using principal component analysis (PCA) and partial least squares (PLS) regression provided insights into the drivers of E. coli and a mechanism for predicting E. coli levels, respectively. We found that stormwater, temperature/season and shallow subsurface flow are the three dominant processes driving the fate and transport of E. coli. PLS regression modeling provided very good predictions under stormwater conditions (R = 0.85 for log (E. coli concentration) and R = 0.90 for log (E. coli loading)); predictions under baseflow conditions were less robust. But, in our case, both E. coli concentration and E. coli loading were significantly higher under stormwater condition, so it is probably more important to predict high-flo...
California's climate is characterized by the largest precipitation and streamflow variability obs... more California's climate is characterized by the largest precipitation and streamflow variability observed within the conterminous US This, combined with chronic groundwater overdraft of 0.6-3.5 km 3 yr À1 , creates the need to identify additional surface water sources available for groundwater recharge using methods such as agricultural groundwater banking, aquifer storage and recovery, and spreading basins. High-magnitude streamflow, i.e. flow above the 90th percentile, that exceeds environmental flow requirements and current surface water allocations under California water rights, could be a viable source of surface water for groundwater banking. Here, we present a comprehensive analysis of the magnitude, frequency, duration and timing of high-magnitude streamflow (HMF) for 93 stream gauges covering the Sacramento, San Joaquin and Tulare basins in California. The results show that in an average year with HMF approximately 3.2 km 3 of high-magnitude flow is exported from the entire Central Valley to the Sacramento-San Joaquin Delta often at times when environmental flow requirements of the Delta and major rivers are exceeded. High-magnitude flow occurs, on average, during 7 and 4.7 out of 10 years in the Sacramento River and the San Joaquin-Tulare Basins, respectively, from just a few storm events (5-7 1-day peak events) lasting for 25-30 days between November and April. The results suggest that there is sufficient unmanaged surface water physically available to mitigate long-term groundwater overdraft in the Central Valley.
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Papers by H. Dahlke