Multiple modes of storm runoff generation in a North Carolina coastal plain watershed

The results of field measurements conducted in a small (19·37 ha) agricultural watershed on the North Carolina coastal plain during the summer of 1996 are presented. The objective of the study was to develop a more complete understanding of basin response in the region with respect to stormflow generation and, in particular, to identify the processes that determine storm runoff and the conditions under which such processes occur. Twenty‐four storm events were monitored, including two tropical storm systems and two hurricanes. The data demonstrate considerable spatial and temporal heterogeneity in runoff generation within the watershed. Surface flowpaths, in the form of Hortonian overland flow and saturation overland flow, were found to be the dominant runoff processes during the storm events measured. The hillslope flowpaths had the same response time as the basin streamflow, but significantly shorter time of rise and lag times. The importance of Hortonian flow in a basin with sandy, permeable soils, as well as the rapid stormflow response in a low‐relief area with a humid climate, was contrary to expectations. This, coupled with the contingency of runoff response, suggests that it may be difficult to generalize about runoff generation mechanisms in broad terms, and that a synoptic approach may be more appropriate. Copyright © 2006 John Wiley & Sons, Ltd.     

Comparative assessment of two distributed watershed models with application to a small watershed

Distributed watershed models are beneficial tools for the assessment of management practices on runoff and water‐induced erosion. This paper evaluates, by application to an experimental watershed, two promising distributed watershed‐scale sediment models in detail: the Kinematic Runoff and Erosion (KINEROS‐2) model and the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model. The physics behind each model are to some extent similar, though they have different watershed conceptualizations. KINEROS‐2 was calibrated using three rainfall events and validated over four separate rainfall events. Parameters estimated by this calibration process were adapted to GSSHA. With these parameters, GSSHA generated larger and retarded flow hydrographs. A 30% reduction in both plane and channel roughness in GSSHA along with the assumption of Green‐Ampt conductivity K_G‐A = K_s, where K_s is the saturated conductivity, resulted in almost identical hydrographs. Sediment parameters not common in both models were calibrated independently of KINEROS‐2. A comparative discussion of simulation results is presented. Even though GSSHA's flow component slightly overperformed KINEROS‐2, the latter outperformed GSSHA in simulations for sediment transport. In spite of the fact that KINEROS‐2 is not geared toward continuous‐time simulations, simulations performed with both models over a 1 month period generated comparable results. Copyright © 2006 John Wiley & Sons, Ltd.     

Rainfall‐runoff modelling using artificial neural networks technique: a Blue Nile catchment case study

A rainfall‐runoff model based on an artificial neural network (ANN) is presented for the Blue Nile catchment. The best geometry of the ANN rainfall‐runoff model in terms of number of hidden layers and nodes is identified through a sensitivity analysis. The Blue Nile catchment (about 300 000 km²) in the Nile basin is selected here as a case study. The catchment is classified into seven subcatchments, and the mean areal precipitation over those subcatchments is computed as a main input to the ANN model. The available daily data (1992–99) are divided into two sets for model calibration (1992–96) and for validation (1997–99). The results of the ANN model are compared with one of physical distributed rainfall‐runoff models that apply hydraulic and hydrologic fundamental equations in a grid base. The results over the case study area and the comparative analysis with the physically based distributed model show that the ANN technique has great potential in simulating the rainfall‐runoff process adequately. Because the available record used in the calibration of the ANN model is too short, the ANN model is biased compared with the distributed model, especially for high flows. Copyright © 2005 John Wiley & Sons, Ltd.     

Analytical model for a geomorphological instantaneous unit hydrograph

The rainfall‐runoff modelling of a river basin can be divided into two processes: the production function and the transfer function. The production function determines the proportion of gross rainfall actually involved in the runoff. The transfer function spreads the net rainfall over time and space in the river basin. Such a transfer function can be modelled using the approach of the geomorphological instantaneous unit hydrograph (GIUH). The effectiveness of geomorphological models is actually revealed in rainfall‐runoff modelling, where hydrologic data are desperately lacking, just as in ungauged basins. These models make it possible to forecast the hydrograph shape and runoff variation versus time at the basin outlet. This article is an introduction to a new GIUH model that proves to be simple and analytical. Its geomorphological parameters are easily available on a map or from a digital elevation model. This model is based on general hypotheses on symmetry that provide it with multiscale versatile characteristics. After having validated the model in river basins of very different nature and size, we present an application of this model for rainfall‐runoff modelling. Since parameters are determined relying on real geomorphological data, no calibration is necessary, and it is then possible to carry out rainfall‐runoff simulations in ungauged river basins. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance of grass and eucalyptus riparian buffers in a pasture catchment,Western Australia,part 1: riparian hydrology

Rainfall takes many flowpaths to reach a stream, and the success of riparian buffers in water quality management is significantly influenced by riparian hydrology. This paper presents results from hydrometric monitoring of riparian buffer hydrology in a pasture catchment. Runoff processes and riparian flowpaths were investigated on two planar hillslopes with regenerating grass and E. globulus buffers. Surface runoff and subsurface flows (A‐ and B‐horizons) were measured for 3 years using surface runoff collectors, subsurface troughs and piezometers. Water volumes moving through the riparian buffers via the measured flowpaths were ranked B‐horizon ? surface runoff ≈ A‐horizon. Runoff volumes through the B‐horizon troughs were an order of magnitude greater than those recorded for the most productive surface runoff plots or the A‐horizon troughs. Subsurface runoff and saturation‐excess overland flow (SOF) were limited to the winter months, whereas infiltration‐excess overland flow (IEOF) can occur all year round during intense storms. Surface runoff was recorded on 33 occasions, mostly during winter (late May–early October), and total annual surface runoff volumes collected by the 20 unconfined (2 m wide) runoff plots varied between > 80 and < 20 m³. Subsurface flow only occurred in winter, and the 6 m wide B‐horizon subsurface troughs flowed above 1 l s^?1 continuously, whereas the A‐horizon troughs flowed infrequently (<6 days per year). In summer, surface runoff occurred as IEOF during intense storms in the E. globulus buffer, but not in the grass buffer. Observations suggest that surface crusting reduced the soil's infiltration capacity in the E. globulus buffer. During winter, SOF and seepage were observed in both buffers, but subsurface flow through the B‐horizon was the dominant flowpath. Key hydrologic differences between the grass and tree buffers are the generation of IEOF in the E. globulus buffer during intense summer storms, and the smaller subsurface runoff volumes and fewer flow days in the E. globulus buffer. Low surface runoff volumes are likely to limit the potential of these buffers to filter pollutants from surface runoff. High subsurface flow volumes and saturated conductivities are also likely to limit the residence time of water in the subsurface domain. Based on their hydrologic performance, the key roles of riparian buffers in this landscape are likely to be displacing sediment and nutrient‐generating activities away from streams and stabilizing channel morphology. Copyright © 2006 John Wiley & Sons, Ltd.     

Wisconsinan Late-glacial environmental change on the southeast Baffin shelf,southeast Baffin Island and northern Labrador

The importance of the Baffin-Labrador region is explained in terms of the channelling through this area of the mass flux of meltwater from the Laurentide Ice Sheet and the probable effects of this on the thermohaline circulation of the North Atlantic. The chronology of glaciation and deglaciation in the region is summarised. Reference is made to recent marine records from the Baffin Shelf. Isotopic changes recorded in foraminiferal assemblages obtained from marine sediments in the Baffin Shelf-Labrador Sea area indicate the timing and possible magnitude of meltwater fluxes during the period 14.0-11.0 ka BP.     

Soil frost effects on soil water and runoff dynamics along a boreal transect: 2. Simulations

A physically based SVAT‐model was tested with soil and snow physical measurements, as well as runoff data from an 8600 m² catchment in northern Sweden in order to quantify the influence of soil frost on spring snowmelt runoff in a moderately sloped, boreal forest. The model was run as an array of connected profiles cascading to the brook. For three winter seasons (1995–98) it was able to predict the onset and total accumulation of the runoff with satisfactory accuracy. Surface runoff was identified as only a minor fraction of the total runoff occurring during short periods in connection with ice blocking of the water‐conducting pores. Little surface runoff, though, does not mean that soil frost is unimportant for spring runoff. Simulations without frost routines systematically underestimated the total accumulated runoff. The possibility of major frost effects appearing in response to specific combinations of weather conditions were also tested. Different scenarios of critical initial conditions for the winter, e.g. high water saturation and delayed snow accumulation leading to an increased frost penetration, were tested. These showed that under special circumstances there is potential for increased spring runoff due to soil frost. Copyright © 2001 John Wiley & Sons, Ltd.     

A two‐component hydrograph separation for three high‐elevation catchments in the Sierra Nevada,California

Two‐component hydrograph separations were performed for three, nested, snowmelt‐dominated catchments in Sequoia National Park. The purpose of the hydrograph separations was to: (i) differentiate between the old and new water contributions to discharge during snowmelt using δ¹⁸O signatures; (ii) identify the fraction of snowmelt that travelled through the subsurface (reactive) compartment during the snowmelt period using silica or sodium; and (iii) investigate the impact of changing end‐member signatures on the separations. ‘Old’ water refers to water that was stored in the watershed during the previous year, whereas ‘new’ water is current snowmelt. Hydrograph separations were performed for both a high‐accumulation (1998, annual precipitation 2·4 m) and an average year (1999, 1·3 m). The proportion of old water contribution to discharge during the rising limb of the hydrograph was 10–20%, with 80–100% of snowmelt being reactive, i.e. passing through soil and talus. Estimates of old and new soil water and direct snowmelt entering the stream varied among the catchments in 1999. Differences between these components were minimal in 1998, regardless of varying topography and differing proportions of soil, rock and talus. Using time‐dependent rather than constant δ¹⁸O meltwater and silica soil‐water signatures made a meaningful impact on both new and old water, and reactive and unreactive, estimates. Copyright © 2004 John Wiley & Sons, Ltd.     

Hydrological modelling of a small watershed using generated rainfall in the soil and water assessment tool model

An adequately tested soil and water assessment tool (SWAT) model was applied to the runoff and sediment yield of a small agricultural watershed in eastern India using generated rainfall. The capability of the model for generating rainfall was evaluated for a period of 18 years (1981–1998). The watershed and subwatershed boundaries, drainage networks, slope, soil series and texture maps were generated using a geographical information system (GIS). A supervised classification method was used for land‐use/cover classification from satellite imageries. Model simulated monthly rainfall for the period of 18 years was compared with observations. Simulated monthly rainfall, runoff and sediment yield values for the monsoon season of 8 years (1991–1998) were also compared with their observed values. In general monthly average rainfall predicted by the model was in close agreement with the observed monthly average values. Also, simulated monthly average values of surface runoff and sediment yield using generated rainfall compared well with observed values during the monsoon season of the years 1991–1998. Results of this study revealed that the SWAT model can generate monthly average rainfall satisfactorily and thereby can produce monthly average values of surface runoff and sediment yield close to the observed values. Therefore, it can be concluded that the SWAT model could be used for developing a multiple year management plan for the critical erosion prone areas of a small watershed. Copyright © 2004 John Wiley & Sons, Ltd.