<p> In ephemeral streams, infrequent surface flow can be the main source of water that sustains plants throughout long dry periods. The objectives of this research are to: (1) explore seasonality of rainfall runoff in different channel types and (2) examine how runoff thresholds vary by channel type. The study area was two watersheds with areas of 188 km² and 323 km² on the Yuma Proving Grounds (YPG) in the Sonoran Desert near Yuma, Arizona. Eight tipping bucket rain gauges were installed to measure precipitation. Runoff was measured with 18 pressure transducers in five different channel types with different channel morphologies and contributing areas ranging from 0.002 km² to 225 km². Over approximately two years there were 11 to 48 rain events at the different rain gauges. Stream types with bedrock channels and small watershed areas between 0.005 km² and 0.015 km² produced runoff when the peak 60-minute precipitation intensity (I60) exceeded 4-6 mm hr^-1. At these sites, 17-25 percent of the rain storms generated runoff. I60 values of 5-9 mm hr^-1 produced runoff in streams with contributing areas of 0.021-0.061 km² on mid-Pleistocene piedmont surfaces covered by desert pavement. At these sites, 31-36 percent of rain events produced runoff. Streams incised into bedrock with some alluvium fill produced runoff at larger I60's of 13-18 mm hr^-1. Contributing areas for these sites were 0.8 km² to 2.2 km², and up to 10 percent of precipitation events at these sites produced flow. Precipitation thresholds for runoff generation in streams with contributing areas >3 km² were not clearly defined due to the influences of variable precipitation in upstream tributaries and transmission losses of streamflow through channel bed alluvium. For watersheds with &lt;3km², rain intensity thresholds increased with the log of catchment area, and as a result flow frequency tended to decrease with increasing catchment area.</p>

In urban areas with high amounts of impervious land cover, stormwater basins provide a valuable service by trapping and slowly infiltrating excess runoff which would otherwise impair streams. Stormwater basins also trap fine sediment suspended by runoff, which can lead to clogging of pore spaces needed for proper infiltration and cause the basin to fail. Basins require maintenance, but a thorough understanding of basin behavior is needed to find appropriate maintenance strategies. The goal of this study was to use multiple techniques – electrical resistivity tomography (ERT), ground-penetrating radar (GPR), sediment sampling, soil moisture sensors, and water level loggers – to assess stormwater basin failure and aid in the creation of targeted basin maintenance strategies. Three basins were selected for monitoring based on size and age: the Upper Moreland Middle School Basin (or “Betz”), the Ethel Jordan Memorial Park Basin, and the Lukens Park Basin. Five to six sediment cores were collected from each basin from depths down to 40 cm. Sediment data did not provide evidence for a layer of fine sediment at the basin surface. Basin sediments did not show consistent patterns with depth: Betz coarsened with depth, Ethel Jordan was relatively homogeneous, and Lukens fined with depth. Sediment at Betz and Lukens fined across the basin from inlet to outlet, but sediment from Ethel Jordan was more uniform across the basin. ERT was able to demonstrate differences in resistivity between the basins, as well as structure beneath the basins. The technique was most useful at Lukens, where buried stormwater chambers were identified, and Ethel Jordan, where rocky, high-resistivity portions of the basins were mapped. GPR used at Ethel Jordan showed strong reflected signals from the same region that displayed high resistivity values, helping to confirm the location of the subsurface features. Neither technique was able to demonstrate sediment heterogeneity. Time-lapse ERT surveying was employed at Betz to detect infiltration during storms. However, changing water levels and conductivity values made data inversion difficult and obfuscated any signal from infiltration. Furthermore, it was not apparent that any infiltration was occurring at Betz. Recession rates from water ponded in the basin were more rapid in the summer than in the winter, and were higher in Betz than in Ethel Jordan. Recession at Lukens was not calculated because the basin did not store water due to a broken riser pipe. Soil moisture sensors placed within basin were used to calculate infiltration rates. Rates could only be calculated at Lukens Park, as the subsurface was saturated at Betz and Ethel Jordan. The sensors demonstrated heterogeneity in infiltration rates throughout the basin and the adjacent land surface, as well as lower infiltration rates caused by vegetation. Basins showed no signs of failure based on the results of the monitoring techniques used, but did demonstrate variations in infiltration and recession behavior. While a concise maintenance plan could not be created, this study demonstrated the advantages of multiple monitoring techniques that can be used to improve maintenance strategies. / Geology

No description available.

<p> In order to make informed decisions in response to future climate change, researchers, policy-makers, and the public need climate projections at the scale of few kilometers, rather than the scales provided by Global Climate Models. The North American Regional Climate Change Assessment Program (NARCCAP) is such a recent effort that addresses this necessity. As the climate models contain various levels of uncertainty, it is essential to evaluate the performance of such models and their representativeness of regional climate characteristics. When assessing climate change impacts, precipitation is a crucial variable, due to its direct influence on many aspects of our natural-human ecosystems such as freshwater resources, agriculture and energy production, and health and infrastructure. The current study performs an evaluation analysis of precipitation simulations produced by a set of dynamically downscaled climate models provided by the NARCCAP program. The Assessment analysis is implemented for a period that covers 20 to 30 years (1970-1999), depending on joint availability of both the observational and the NARCCAP datasets. In addition to direct comparison versus observations, the hindcast NARCCAP simulations are used within a hydrologic modeling analysis for a regional ecosystem in coastal Louisiana (Chenier Plain). The study concludes the NARCCAP simulations have systematic biases in representing average precipitation amounts, but are successful at capturing some of the characteristics on spatial and temporal variability. The study also reveals the effect of precipitation on salinity concentrations in the Chenier Plain as a result of using different precipitation forcing fields. In the future, special efforts should be made to reduce biases in the NARCCAP simulations, which can then lead to a better presentation of regional climate scenarios for use by decision makers and resource managers.</p>

<p> Streams in the Kenai River watershed are characterized by a fish-rich environment, with competing interests between commercial industry and recreational users. Resource managers strive to balance the needs of both these user groups while maintaining the sustainability of the resource. The ability to estimate future river conditions could help maintain the resource, and a strong, sustainable economy on the Kenai Peninsula. </p><p> This research used the Army Corps of Engineers Hydrological Modeling System (HMS), which transforms rainfall to river discharge. The main goal was to define a set of parameters that were calibrated using an event based strategy, and concurrent rainfall and discharge data. The model was calibrated and validated in three sub-basins located in different environmental settings (i.e. lowlands, mid, and high elevation). In addition, the Kenai River watershed, as a whole, was modeled. </p><p> Due to limited concurrent datasets, a combination of current and historic rainfall and discharge data was used in the calibration. Over the period of time between the historic data and the current data, no major changes in the watershed were detected. </p><p> Model results at the sub-basin and watershed scale provided reasonable results over the modeling period. Each sub-basin maintained errors below 10% for the calibration and only slight increase in the error for the verification trials. It was found that during an extreme precipitation event, the model did not perform within reasonable bounds.</p>

<p> Hydrogeologically, Rapa Nui (Easter Island, Chile) is one of the least understood islands in Polynesia. There are no surface streams, the soils are poor in productivity and highly permeable, and the water table sits far below the surface of the island. One of the many mysteries of Rapa Nui is how the ancient inhabitants survived with so few sources of freshwater. </p><p> Fieldwork was conducted to identify terrestrial sources of freshwater and to evaluate the occurrence of submarine groundwater discharge (SGD). We documented observable surface water features located in the interior and coast of the island and made field measurements of water temperature and salinity to identify areas of SGD along the coast. The limited number of interior surface water features, periods of drought, permeable aquifers, and existence of <i> puna</i> (dug wells) along the coast of Rapa Nui lead us to conclude that coastal seeps were an important prehistoric source of freshwater.</p>

This thesis seeks to extend the work of Thornes and Scoging on hillslope processes in south east Spain by paying particular attention to the relative roles of surface and sub-surface water movement in gully head change on two contrasting soil types. Gully growth is a major agent of erosion in semi-arid environments, yet studies to date have assumed the dominance of surface wash, and only speculated on the role of subsurface water movement. The sampling design was based on the hillslope hydrological cycle and incorporates measures of precipitation, infiltration, and runoff and subsurface flow on catchment areas above gully heads. Additional data on vegetation and some soil properties were collected. The field work was undertaken on three occasions in the summer, autumn and spring of 1982/3 to examine seasonal variations. The analysis of surface flow was hindered by the drought which meant there were only 10 rain days in 1982, and only one storm occurred during the field sessions on 26 November 1982. Despite this several observations can be made. Wash volumes were twice as high on the marl on 26 November. Both lithologies are susceptible to relatively high erosion rates by surface wash although rates tend to be higher on the marl, and there is considerable variation on both lithologies. However for neither lithology is the amount of sediment transported sufficient to fill in the gullies in the medium term. The analysis of the subsurface hydrology shows that saturated conditions were not monitored anywhere, and maximum soil moisture values reached between 50--60% saturation on the marl and conglomerate soils. There are marked seasonal variations in soil moisture and most of the variation occurs in the upper horizons. Flux rates are negligble on the marl and dominantly in the vertical plane. On the conglomerate rates are much faster, and throughflow may well occur on occasions, and at rates exceeding e'apotranspiration. This will contribute to wetter conditions around and in gullies on the conglomerate.

<p> In Louisiana, human actions, such as levee construction for flood protection, have disrupted the connectivity between many rivers and their surrounding wetlands, leading to marsh degradation and land loss. River sediment diversions from the Mississippi River into adjacent wetlands are currently proposed as a large-scale wetland restoration technique. The reintroduction of sediment-rich waters into adjacent wetlands will also come with altered hydrology and nutrient loads. Despite hydrology and nutrient loading being central to many previous studies, many uncertainties remain regarding the marsh response to this type of large-scale environmental alteration. </p><p> To better understand the effect of river sediment diversion related increases in nutrient supply and altered hydrology on tidal freshwater marsh species specific responses and soil physico-chemical properties, we conducted a greenhouse experiment consisting of three common tidal freshwater and brackish marsh species (<i>Panicum hemitomon, Sagittaria lancifolia</i>, and <i> Spartina patens</i>). To mimic diversion specific conditions, we tested high and low nitrate loading rates (35 g N m² y^&ndash;1 and 0.25 g N m² y^&ndash;1, respectively) and two hydrologic regimes to compare diversion and non-diversion. Units were also set at two different soil surface elevations within each treatment, to simulate healthy and degraded marsh types. </p><p> Growth responses measured throughout the study differed between species, but were not influenced by the treatments. After 3 months, aboveground and belowground biomass was not impacted by the treatment for any of the species. Soil redox potential was influenced by changes in hydrology but recovered to pre-study conditions by harvest. This study provides an insight into how proposed river diversion conditions and subsequent recovery may influence the productivity of freshwater marshes. We also further highlight the need for a more complete understanding of how changes in flooding depth, duration, and nitrate load from proposed river sediment diversions will influence the productivity of marshes in receiving basins.</p><p>

<p> Managed Aquifer Recharge (MAR) systems have become an increasingly important approach to the management of groundwater in Southern California in recent years. This thesis describes an experimental investigation of the vadose zone dynamics beneath a recharge basin used by the Orange County Water District. Soil moisture probes, pressure transducers, and fiber optic distributed temperature sensing (FODTS) at multiple depths beneath the basin were used to monitor infiltration. The purpose was to measure the diurnal temperature flux using heat as a tracer of infiltrating water to gain insight on the influence of basin stage (i.e., water level) on infiltration rate. To increase the temperature resolution from the standard 1 m, we installed a wrapped fiber optic cable at two locations using direct push technology. The wrapped FODTS cable was spliced to a trenched cable that ran laterally across the basin at depths of 30 cm and 1 m, installed during a previous experiment. The wrapped cable was then installed vertically at two locations to observe both the spatial distribution and vertical dynamics of fluid flow at 10 cm intervals. Propagation of the diurnal heat flux at the surface was related to water velocity. The infiltration behavior was affected by subtle changes in stratigraphy below the basin. The heat tracer suggests strong components of horizontal flow due to the presence of thin fine-grained hydrostratigraphic units. Water movement during initial saturation was particularly complex and suggested that simple one-dimensional vertical flow models will not accurately predict infiltration rates. The FODTS system provided high-resolution dynamic imaging of percolation that is not possible using a multi-level transducer system.</p><p>

<p> The purpose of this study was to investigate multiple methods used to locate and identify sources of submarine groundwater discharge (SGD) along the coastline of Santa Catalina Island (Catalina) in Los Angeles, California. The driving hypothesis is that SGD may be identified by higher temperature/lower salinity plumes. To test this hypothesis, temperature and conductivity measurements were made along the shoreline of Catalina and compared with previously collected airborne thermal infrared images. In some locations where potential zones of SGD were identified, samples were collected from the surface water within the plume and analyzed for the presence of radon. Radon in ocean water can be used as a tracer to identify the presence of terrestrial groundwater. The analysis was complicated by dynamic variables including tides, weather, and sample collection. In spite of this uncertainty, one potential zone of SGD was identified using sea surface temperature, electrical conductivity, and radon at Toyon Bay. This zone of SGD is consistent with the location of warm water shown in airborne infrared imagery. These results suggest that high-resolution thermal imagery may be useful for identifying diffuse SGD in southern California, but further field tests will be necessary to confirm this conclusion.</p><p>