Runoff Fluxes Research Articles

Testing a GCM land surface scheme against catchment-scale runoff data

A GCM land surface scheme was used, in off-line mode, to simulate the runoff, latent and sensible heat fluxes for two distinct Australian catchments using observed atmospheric forcing. The tropical Jardine River catchment is 2500 km2 and has an annual rainfall of 1700 mm y–1 while the Canning River catchment is 540 km2, has a Mediterranean climate (annual rainfall of 800 mm y–1) and is ephemeral for half the year. It was found that the standard version of a land surface scheme developed for a GCM, and initialised as for incorporation into a GCM, simulated similar latent and sensible heat fluxes compared to a basin-scale hydrological model (MODHYDROLOG) which was calibrated for each catchment. However, the standard version of the land surface scheme grossly overestimated the observed peak runoff in the wet Jardine River catchment at the expense of runoff later in the season. Increasing the soil water storage permitted the land surface scheme to simulate observed runoff quite well, but led to a different simulation of latent and sensible heat compared to MODHYDROLOG. It is concluded that this 2-layer land surface scheme was unable to simulate both catchments realistically. The land surface scheme was then extended to a three-layer model. In terms of runoff, the resulting control simulations with soil depths chosen as for the GCM were better than the best simulations obtained with the two-layer model. The three-layer model simulated similar latent and sensible heat for both catchments compared to MODHYDROLOG. Unfortunately, for the ephemeral Canning River catchment, the land surface scheme was unable to time the observed runoff peak correctly. A tentative conclusion would be that this GCM land surface scheme may be able to simulate the present day state of some larger and wetter catchments but not catchments with peaky hydrographs and zero flows for part of the year. This conclusion requires examination with a range of GCM land surface schemes against a range of catchments.

Development Impacts on Water Quality: A Case Study

The modified storage, treatment, overflow, runoff model (STORM) is applied to analyze water quality impacts of a proposed large-scale planned residential development in a New Jersey coastal community. Model adaptation to the area of interest is based on site-specific flow and water quality data collected during a severe storm. Due to the characteristics of the soils in the area, 95% of the runoff volume and pollutant fluxes are directed to subsurface receiving waters. The results of the analysis show that the planned development has no discernible impact on surface receiving water quality. Impacts on subsurface receiving waters are offset by the elimination of septic systems that served the area prior to development.

LOADING FUNCTIONS FOR PREDICTING NUTRIENT LOSSES FROM COMPLEX WATERSHEDS1

ABSTRACT: A loading function methodology is presented for predicting runoff, sediment, and nutrient losses from complex watersheds. Separate models are defined for cropland, forest, urban and barnyard sources, and procedures for estimating baseflow nutrients are provided. The loading functions are designed for use as a preliminary screening tool to isolate the major contributors in a watershed. Input data sources are readily available and the functions do not require costly calibrations. Data requirements include watershed land use and soil information, daily precipitation and temperature records and rainfall erosivities. Comparison of predicted and measured water, sediment, and nutrient runoff fluxes for the West Branch Deleware River in New York, indicated that runoff was underpredicted by about 14 percent while dissolved nutrients were within 30 percent of observed values. Sediment and solid‐phase nutrients were overpredicted by about 50 percent. An annual nutrient budget for the West Branch Delaware River showed that cornland was the major source of sediment, solid phase nutrients, and total phosphorus. Waste water treatment plants and ground water discharge contributed the most dissolved phosphorus and dissolved nitrogen, respectively.

Regional Surface Turbidity and Hydrographic Variability on the South Texas Continental Shelf

ABSTRACT Regional surface-water turbidity patterns and associated hydrography were monitored on the South Texas Continental Shelf during an 18-month period (November 1975-May 1977) as part of an environmental study. On six monitoring cruises, quasi-synoptic surface measurements were made of water transmissivity, suspended-sediment concentrations, temperature, salinity, and drifter trajectories. Time-sequence patterns of these parameters illustrate substantial spatial and temporal variability; temporal variations take place at both the seasonal and annual time scales. Turbidity and hydrographic patterns suggest a surface-sediment dispersal system regulated by a lateral shelf-water exchange process, driven by both wind and thermohaline forcing agents. Relatively turbid inner-shelf waters reflect the offshore and alongshore transport of sediment derived from coastal sources. Turbidity patterns along the tuner shelf are characterized by a regional gradient of shoreward-increasing turbidity with superimposed local gradients established at major tidal inlets that serve as prominent sediment point sources and dispersal centers. Turbidity variability along the inner shelf is jointly attributed to variations in coastal runoff, wind-driven currents, and relative flux from individual tidal inlets. Relatively nonturbid outer-shelf waters suggest the shelfward incursion of an open-ocean water mass regulated by deep-Gulf circulation; the extent of incursion appears to vary spatially and temporally, resulting in outer-shelf turbidity variations. The overall shelf turbidity patterns reflect the degree of lateral interchange between the gulfward movement of turbid inner-shelf waters and the shelfward incursion of clear open-ocean waters.

The strontium isotopic composition of seawater, and seawater-oceanic crust interaction

The 87Sr/ 86Sr ratio of seawater strontium (0.7091) is less than the 87Sr/ 86Sr ratio of dissolved strontium delivered to the oceans by continental run-off (∼0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current 87Sr/ 86Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 10 12 g/yr) against a hydrothermal recirculation flux of 3.6 × 10 12 g/yr, during which the 87Sr/ 86Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the 87Sr/ 86Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required 87Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus. The post-Upper Cretaceous increase in the strontium isotopic composition of seawater (∼0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the 87Sr/ 86Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.

The source of unusually low surface salinities in the gulf stream off Georgia

In September 1973, surface water salinities of the Gulf off Georgia were observed to be anomalously low. Salinity values of 35·5 to 36·4%0 are typical of thir area but values as low as 34.5%0 were observed. A conservative estimate of the onbserved volume flux of freshwater in the Gulf Stream was 1·2 km3 day−1. A possible source is runoff from the southeast United States; however, the total runoff flux is ca. 0·1 k;m3 day−1, which is not sufficient to supply the oberved flux. Therefore, it is hypothesized that the low water is of Mississippi River origin. This argument is further strengthened by observations of northern extension of the Gulf of Mexico Loop Current and Low salinity water is the Straits of Florida.