Urban irrigation in the modeling of a semi-arid urban environment: Ballona Creek watershed,Los Angeles,California

ABSTRACT

Ballona Creek watershed in Los Angeles, California provides a unique combination of heterogeneous urban land cover, a semi-arid environment, and a large outdoor water-use flux that presents a challenge for physically-based models. We ran simulations using the Noah Land Surface Model and Parflow-Community Land Model and compared to observations of evapotranspiration (ET), runoff, and land surface temperature (LST) for the entire 11-year study period. Both models were systematically adjusted to test the impact of land cover and urban irrigation on simulation results. Monthly total runoff and ET results are greatly improved when compared to an in-situ stream gauge and meteorological tower data: from 0.64 to 0.81 for the Nash–Sutcliffe efficiency (NSE) for runoff and from a negative NSE to 0.82 for ET. The inclusion of urban irrigation in semi-arid urban environments is found to be vital, but not sufficient, for the accurate simulation of variables in the studied models.     

Resistive ballooning modes in line-tied coronal fields

The stability of coronal arcades to localized resistive interchange modes in the ballooning ordering, including photospheric line tying, is investigated. It is found that the anchoring of magnetic footpoints in the photosphere is not sufficient to stabilise ballooning modes, once resistivity is taken into account. All configurations with a pressure profile decreasing from the arcade axis at some point are unstable, a purely growing mode being excited. The dependence of the growth rate on the parameter Rm ^–1 k ² , where is the resistivity and k the wavenumber in a direction perpendicular to the equilibrium field, can be described by a power law with varying index: for small values of k ² and an ideally stable field one finds Rm ^–1. As k ² is increased or marginal stability is approached one finds Rm^–1/3. T implications of these localised instabilities to the temporal evolution and overall energy balance of arcade structures in the solar corona is discussed.     

A four-dimensional validation of a coupled physical–biological model of the Arabian Sea

In this paper, we use a coupled biological/physical model to synthesize and understand observations taken during the US JGOFS Arabian Sea Process Study (ASPS). Its physical component is a variable-density, -layer model; its biological component consists of a set of advective–diffusive equations in each layer that determine nitrogen concentrations in four compartments, namely, nutrients, phytoplankton, zooplankton, and detritus. Solutions are compared to time series and cruise sections from the ASPS data set, including observations of mixed-layer thickness, chlorophyll concentrations, inorganic nitrogen concentrations, particulate nitrogen export flux, zooplankton biomass, and primary production. Through these comparisons, we adjust model parameters to obtain a “best-fit” main-run solution, identify key biological and physical processes, and assess model strengths and weaknesses.Substantial improvements in the model/data comparison are obtained by: (1) adjusting the turbulence-production coefficients in the mixed-layer model to thin the mixed layer; (2) increasing the detrital sinking and remineralization rates to improve the timing and amplitude of the model's export flux; and (3) introducing a parameterization of particle aggregation to lower phytoplankton concentrations in coastal upwelling regions.With these adjustments, the model captures many key aspects of the observed physical and biogeochemical variability in offshore waters, including the near-surface DIN and phytoplankton P concentrations, mesozooplankton biomass, and primary production. Nevertheless, there are still significant model/data discrepancies of P for most of the cruises. Most of them can be attributed to forcing or process errors in the physical model: inaccurate mixed-layer thicknesses, lack of mesoscale eddies and filaments, and differences in the timing and spatial extent of coastal upwelling. Relatively few are clearly related to the simplicity of the biological model, the model's overestimation of coastal P being the most obvious example. Overall, we conclude that future efforts to improve biogeochemical models of the Arabian Sea should focus on improving their physical component, ensuring that it represents the ocean's physical state as closely as possible. We believe that this conclusion applies to coupled biogeochemical modeling efforts in other regions as well.     

Distributions of copepod nauplii and turbulence on the southern flank of Georges Bank: implications for feeding by larval cod (Gadus morhua)

The vertical distributions of copecod nauplii and water properties were sampled at well-mixed and stratified sites on Georges Bank using a pumping system, CTD and in vivo fluorometer during a four day period in late May 1992. At each stratified station at least one sample was taken within the thermocline and the fluorescence maximum, which usually co-occurred. Well-mixed sites had low average concentrations of nauplii, ca 41⁻¹, and showed little variation of abundance with depth. Stratified sites had from 4 to 16 times the integrated (0–50 m) abundance of nauplii compared to well-mixed sites and showed strong vertical patterns of distribution. Maximum concentrations of nauplii, up to 1601⁻¹, were associated with the thermocline at 7 of the 9 stratified stations. At the two remaining stratified sites the naupliar maximum was in the upper mixed layer, sampled at 5 m depth. The encounter rate between early feeding cod (Gadus morhua) larvae and their naupliar prey was calculated with and without turbulence. Turbulence was estimated from two sources: wind stress in the upper layer (calculated from wind observations during our cruise) and tidal shear in the lower layer (estimated initially from a tidal mixing equation). We ultimately replaced the lower layer estimates with turbulence data from a series of measurements made in 1995. The latter are more robust and had the advantage of providing dissipation rates for the pycnocline as well as the lower layer. Theory predicts an increase in encounters between a predator and its prey with the addition of turbulence parameters into standard models of encounter. We combined turbulence profiles with the vertical distribution of nauplii to examine the potential contribution of turbulent kinetic energy to predator-prey encounter rates at various depths in stratified and mixed water columns. Our calculations suggest the following increases due to turbulence at stratified sites on Georges Bank during our cruise: from 34 to 219% in the upper mixed layer, depending on wind speed and depth; approximately 8% in the pycnocline; and approximately 110% below the pycnocline. Mixed sites experience increases of at least 110% (tide only), but greater increases (118–192% in this study) occur when the wind blows because of the combined (spatially overlapped) effects of wind and tidal mixing at these sites. The absolute values for encounter rates and their modification by turbulence are sensitive to a number of assumptions in the models. We used a series of stated assumptions to generate estimates that range from 0.6 to 26.5 prey h⁻¹, depending on geographical location, physical forcing and depth.     

Carbon dioxide partial pressures in North Pacific surface waters. 2. General late summer distribution

Observations of the equilibrium partial pressure of carbon dioxide in the surface waters of the North Pacific Ocean and Bering Sea indicate conditions of local upwelling or vertical mixing near the Aleutian Island passes, seasonal depletion of CO₂ in the sea surface by photosynthesis, and conditions of CO₂ supersaturation in the surface waters off the mouths of large rivers. Horizontal mixing has a large effect on the P_CO2 distribution. The area distribution of carbon dioxide in the surface waters of the Pacific Ocean from 19°N to 55°N latitude and in the Bering Sea is presented.     

Operational snow modeling: Addressing the challenges of an energy balance model for National Weather Service forecasts

Prediction of snowmelt has become a critical issue in much of the western United States given the increasing demand for water supply, changing snow cover patterns, and the subsequent requirement of optimal reservoir operation. The increasing importance of hydrologic predictions necessitates that traditional forecasting systems be re-evaluated periodically to assure continued evolution of the operational systems given scientific advancements in hydrology. The National Weather Service (NWS) SNOW17, a conceptually based model used for operational prediction of snowmelt, has been relatively unchanged for decades. In this study, the Snow–Atmosphere–Soil Transfer (SAST) model, which employs the energy balance method, is evaluated against the SNOW17 for the simulation of seasonal snowpack (both accumulation and melt) and basin discharge. We investigate model performance over a 13-year period using data from two basins within the Reynolds Creek Experimental Watershed located in southwestern Idaho. Both models are coupled to the NWS runoff model [SACramento Soil Moisture Accounting model (SACSMA)] to simulate basin streamflow. Results indicate that while in many years simulated snowpack and streamflow are similar between the two modeling systems, the SAST more often overestimates SWE during the spring due to a lack of mid-winter melt in the model. The SAST also had more rapid spring melt rates than the SNOW17, leading to larger errors in the timing and amount of discharge on average. In general, the simpler SNOW17 performed consistently well, and in several years, better than, the SAST model. Input requirements and related uncertainties, and to a lesser extent calibration, are likely to be primary factors affecting the implementation of an energy balance model in operational streamflow prediction.     

Linking hydrology and stream geochemistry in urban fringe watersheds

Urbanization represents a dramatic example of human interference with the hydrological cycle. Changes to ground cover affect both the hydrological and geochemical characteristics in a watershed. Ecosystem degradation also occurs in undisturbed watersheds at the “urban fringe” due to regional atmospheric deposition. These urban fringe catchments can also serve as an upstream source of various chemical constituents into downstream (urban) river systems. The current study focuses on the impacts of regional urbanization in the upper Arroyo Seco watershed located on the eastern edge of the Los Angeles basin, where estimates of dry deposition are considered some of the highest in North America. Collected hydrologic, geochemical and atmospheric data were assessed at seasonal time scales to evaluate current hydrochemical dynamics. Stream water chemical composition in the upper Arroyo Seco watershed exhibits significant seasonal variability, particularly for . Almost all study solutes show dilution behavior. However, hydrologically enhanced behavior was observed for with increased concentrations during the wet season. Seasonal stream concentration–discharge relationships were developed using a hyperbolic dilution model. The developed model was then used to predict concentrations for observational gaps in stream water chemical composition, allowing for seasonal and annual mass loadings to be estimated for the downstream urban stream. The hydrological signal in the resultant chemical loads is extremely strong, especially during the wet season. Both observations and model predictions indicate the watershed is a sink for atmospheric nitrate and a source for various cations.     

Stream temperature response to variable glacier coverage in coastal watersheds of Southeast Alaska

We measured stream temperature continuously during the 2011 summer run‐off season (May through October) in nine watersheds of Southeast Alaska that provide spawning habitat for Pacific salmon. The nine watersheds have glacier coverage ranging from 0% to 63%. Our goal was to determine how air temperature and watershed land cover, particularly glacier coverage, influence stream temperature across the seasonal glacial meltwater hydrograph. Multiple linear regression models identified mean watershed elevation (related to glacier extent) and watershed lake coverage (%) as the strongest landscape controls on mean monthly stream temperature, with the weakest (May) and strongest (July) models explaining 86% and 97% of the temperature variability, respectively. Mean weekly stream temperature was significantly correlated with mean weekly air temperature in seven streams; however, the relationships were weak to non‐significant in the streams influenced by glacial run‐off. Streams with >30% glacier coverage showed decreasing stream temperatures with rising summer air temperatures, whereas those with <30% glacier coverage exhibited summertime warming. Glaciers also had a cooling effect on monthly mean stream temperature during the summer (July through September) equivalent to a decrease of 1.1 °C for each 10% increase in glacier coverage. The maximum weekly average temperature (an index of thermal suitability for salmon) in the six glacial streams was substantially below the lower threshold for optimum salmon growth. This finding suggests that although glaciers are important for moderating summer stream temperatures, future reductions in glacier run‐off may actually improve the thermal suitability of some glacially dominated streams in Southeast Alaska for salmon. Copyright © 2013 John Wiley & Sons, Ltd.     

Systematic variations in xenocryst mineral composition at the province scale, Buffalo Hills kimberlites, Alberta, Canada

The Buffalo Hills kimberlites define a province of kimberlite magmatism occurring within and adjacent to Proterozoic crystalline basement termed the Buffalo Head Terrane in north-central Alberta, Canada. The kimberlites are distinguished by a diverse xenocryst suite and most contain some quantity of diamond. The xenocryst assemblage in the province is atypical for diamondiferous kimberlite, including an overall paucity of mantle indicator minerals and the near-absence of compositionally subcalcic peridotitic garnet (G10). The most diamond-rich bodies are distinguished by the presence of slightly subcalcic, chromium-rich garnet and the general absence of picroilmenite, with the majority forming a small cluster in the northwestern part of the province. Barren and near-barren pipes tend to occur to the south, with increasing proximity to the basement structure known as the Peace River Arch. Niobian picroilmenite, compositionally restricted low-to moderate-Cr peridotitic garnet, and megacrystal titanian pyrope occur in kimberlites closest to the arch. Major element data for clinopyroxene and trace element data for garnet from diamond-rich and diamond-poor kimberlites suggests that metasomatism of lithospheric peridotite within the diamond stability field may have caused destruction of diamond, and diamond source rocks proximal to the arch were the most affected.     

Magnetic anomalies near Apollinaris Patera and the Medusae Fossae Formation in Lucus Planum, Mars

The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3 km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50 A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO₂) environment in the presence of a strong (?10 μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.