Sub-catchment Scale Research Articles

Predicted and measured levels of azinphosmethyl in the Lourens River, South Africa: Comparison of runoff and spray drift

Runoff and spray drift are important sources of nonpoint pesticide pollution in surface waters, but few studies have directly compared these routes of input in an exposure assessment scenario. To this end, a runoff formula suggested by the Organization for Economic and Cooperative Development (Paris, France) and basic drift values (95th percentiles) were integrated into a geographical information system (GIS) to predict runoff and spray drift-related loading of azinphosmethyl (AZP) in the Lourens River (LR), South Africa. The GIS-integrated calculations were first validated in the tributaries of the river, where measured loads were well predicted for both runoff (r(20 = 0.95; p < 0.0001; n = 9) and spray drift (r(2) = 0.96; p = 0.0006; n = 8). Through extrapolation to the catchment scale containing 400 ha of orchards, the GIS-integrated calculations predicted similar loads of AZP as measured in the Lourens River mainstream for six runoff (between a factor of 1.03 and 1.86 lower) and six spray drift (between a factor of 1.1 and 2.4 higher) events. Mean measured loads per event were significantly (p = 0.004) higher for runoff (27.8 +/- 19.1 g) than for spray drift (0.69 +/- 0.32 g). Based on long-term meteorological data and average application regimes, runoff leads to a higher annual load (47.6 g) than spray drift (5.5 g) in the Lourens River. Runoff is clearly a more important source of nonpoint pollution in the studied catchment, and mitigation strategies should focus first on addressing this aspect on a catchment scale and second on addressing problem areas on a subcatchment scale.

The influence of land use, soils and topography on the delivery of hillslope runoff to channels in SE Spain

AbstractIt is generally accepted that within particular physiographic and climatic regions catchments exhibit differences in their hydrological response. These differences result from the interaction of spatial variability in catchment characteristics, variability of rainfall inputs and surface and subsurface hydrological processes. These interactions are complex and difficult to unravel. Hydrologically similar surfaces (HYSS) have been used to identify catchment areas that have a similar response to rainfall and have been identified at a number of scales. HYSS have been identified at the subcatchment scale for the Rambla de Nogalte in SE Spain. Areas with similar at‐a‐point hydrological storages were distinguished by using a combination of geology, land use and topography. This mapping was compared with discharge estimates made throughout the catchment following a seven‐year return interval flood in September 1997. From this significant flood source areas were identified from reaches showing rapidly increasing channel discharge, and associated with HYSS that combined suitable internal characteristics with good connectivity to the main channel. This paper presents a simulation model that has been developed to investigate the way in which the hydrological response of areas within a HYSS respond to changes in source area, gradient, connectivity to the channel, storm size and intensity profile. This is one of the first studies using a hillslope model to investigate spatial patterns of runoff‐response in semi‐arid areas and results have implications for scaling up hydrological response, and on how the dynamics of runoff producing areas vary both under changing storm conditions and over time. It is implicit in our results that the nature of stream–slope coupling differs substantively between semi‐arid and humid areas. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Neighbourhood catchments: a new approach for achieving ownership and change in catchment and stream management

The Neighbourhood Catchment approach integrates land and stream management practices at a property and through to a local catchment scale, links production and environmental goals, and is a building block to achieve ownership and change at a sub-catchment scale and larger. Research conducted in two 'focus' Neighbourhood Catchments has shown that land management practices that retain >30% soil cover reduce sediment movement to streams. The Neighbourhood Catchment approach engages both early and cautious adopters, and enables continuous improvement of resource management to take place, and be recorded at an individual property and local catchment scale.

Impacts of water management options on flows in the Condamine River in Southern Queensland

This paper examines the implications for river flows of a number of water practices and potential management options in the alluvial plains of the Upper Condamine River. It is an intensively cultivated area where irrigation is limited by the availability of water resources. The practice of capturing overland flows was investigated by the development of a model that simulates the performance of clusters of offstream storages up to sub-catchment scale. Management options examined included improvement to on-farm water use efficiency, the suppression of evaporation from open water storages, increasing the depth of those storages, decreasing their number, and improved tailwater return from irrigated land. Impacts of management options were analysed using a catchment scale water allocation model.

Sediment yields and turbidity records from small upland subcatchments in the Warragamba Dam Catchment, southern New South Wales

Accelerated rates of soil erosion, following European settlement in Australia, pose a major threat to river ecosystems and water supply storages with agricultural catchments, such as Warragamba Dam (the main water supply dam for Sydney). Sediment yields from 1988 to 1999 have been determined by stream monitoring for 6 small grazing catchments located near Goulburn in southern New South Wales, in the outer Warragamba catchment. Average specific sediment yields were 0.07 t/ha.year from 3 small catchments of around 10 ha, where the drainage lines were not continuously gullied, and the drainage lines were undisturbed. Sediment yields from larger gullied catchments of 29, 52, and 510 ha were at least an order of magnitude higher than from the ungullied catchments at around 1 t/ha.year. Gullies continue to be the dominant source of sediment from these subcatchments despite indications that yields from gullies in the Southern Tablelands are much less than in the first few decades following settlement and are gradually declining as gullies move towards a stable vegetated state. Measured sediment yields from this study are comparable with yields for southern New South Wales determined using other methods, such as radionuclide and farm dam survey techniques. The impact of soil conservation works, such as sediment trapping and grade stabilisation works, on sediment yields can be mixed depending on the activity of the gully being treated and the mobility of the bed sediments. Turbidity levels from runoff events from the gullied catchments were an order of magnitude higher than from the ungullied catchments. For the Warragamba catchment turbidity levels are scale dependent; at the farm paddock scale turbidities are generally low where the land use is grazing, they increase dramatically at the subcatchment scale where active gullies dominate the drainage network, and then drop back again at the large river catchment scale. Turbidity is positively correlated with sediment concentration, so works that reduce sediment loads will also reduce the turbidity of water leaving the subcatchment. However, the link between high turbidity levels in small upland catchments and downstream water quality in large complex river basins is tenuous.

Soil and water salinity in Queensland: the prospect of ecological sustainability through the implementation of land clearing policy

In Queensland, legislation has recently been enacted that outlines minimum standards for the retention of native vegetation in bioregions and prescribes Performance Requirements for the maintenance of biodiversity and ecological processes, and the prevention of land degradation. It also details Acceptable Solutions to satisfy the Performance Requirements. It this paper we analyse the links between science and action to determine whether the legislation and associated policy are likely to lead to sound management decisions being made for prevention of land and water salinity. We also compared the requirements for biodiversity protection relative to those for salinity control. For salinity (and biodiversity) the minimum standards and Acceptable Solutions in the legislation have a sound technical basis. The main challenge in implementing the salinity Performance Requirements will be provision of information required to assess salinity risk at appropriate scales. This information is: (1) salinity hazard assessment at catchment/regional scale planning scale; (2) technical information to support a sub-catchment scale implementation; (3) guidelines, procedures and local expertise for interpretation of salinity hazard at the property scale. In the process of assessing clearing applications there is no formal role for planners or decision makers at the sub-catchment level who could play an important role in coordinating information gathering for land managers developing clearing applications. There are also limited pathways for accessing relevant scientific information and expertise at an appropriate level to provide support for land managers. We suggest that this situation is likely to limit the successful implementation of the policy for preventing land and water salinity. The minimum standard of 30% retained vegetation would provide similar protection for biodiversity and salinity control at the bioregional level. In smaller areas (e.g. an individual property) however, there could be contrasting requirements for the retention of vegetation.

Modelling the spatial and temporal variations of the water balance for the Weser catchment 1965–1994

This study describes the application of the physically based SVAT model PROMET (PRocess Oriented Model for EvapoTranspiration) in the mesoscale catchment of the Weser (Northern Germany, approx. 37,500 km 2) utilizing a 30 years time series of meteorological input data. To enable a representative analysis of the spatial and temporal variations of the water balance components, the modelization is performed continuously without re-initialization of the state variables or specific calibration. Therefore, PROMET is expanded with the one-layer snow model ESCIMO (Energy balance Snow Cover Integrated MOdel) to provide an integrated model structure for continuous simulations of the water cycle. All necessary input data fields are integrated in a four-dimensional GIS data structure with a raster grid spacing of 1 km: a DEM, soil texture information derived from digitized maps, landuse distribution computed by unmixing a time series of NOAA/AVHRR satellite images and meteorological input data fields which are spatially and temporally interpolated using data provided by the standard measurement network of the German Weather Service (DWD). Spatially non-distributed physical soil and plant parameters are either derived from measurements or taken from literature. The study presents the structure of ESCIMO and its validation at the point and the catchment scale. Then, the modelled mean annual evapotranspiration, aET, as obtained by application of the linked models PROMET/ESCIMO is compared with the corresponding term ET calculated by inserting the measured precipitation and gauged runoff into the water balance equation. It is started from the assumption that for the 30 years period, the overall underground storage change Δ S is negligible. The mean annual deviation aET–ET over the 30 years period is 10.9 mm, indicating that the results represent a valid long-term description of the water balance. The patterns of the simulated water balance components are discussed with respect to the physical–geographical properties of the environment and their interaction with the predominant meteorological conditions. The study then addresses smaller spatial and temporal scales: for the subcatchment scale, the variation of aET–ET and thus the model error is considerably larger; for shorter periods, aET–ET is used for the estimation of Δ S. The simulation accuracy as depending on the length of the aggregation period and the mean monthly evolution of the storage are discussed.

Distributed hydrological modelling of the Senegal River Basin — model construction and validation

A modified version of the physically-based distributed mike she model code was applied to the 375,000 km 2 Senegal River Basin. On the basis of conventional data from meteorological stations and readily accessible databases on topography, soil types, vegetation type, etc. three models with different levels of calibration were constructed and rigorous validation tests conducted. Calibration against one station and internal validation against eight additional stations revealed significant shortcomings for some of the upstream tributaries, particularly in the semi-arid zone of the river basin. Further calibration against additional discharge stations improved the performance levels of the validation for the different subcatchments. Although there may be good reasons to believe that the model operating on a model grid of 4×4 km 2 to a large extent reflects field conditions at a scale smaller than subcatchment scale, this could not be validated due to lack of spatial data.

Factors regulating the spatial and temporal distribution of solute concentrations in a major river system in NE Scotland

The River Dee in NE Scotland, an oligotrophic soft water system, has a catchment area of approx. 2100 km 2, its source in the Cairngorm mountains being approx. 140 km from its outlet to the North Sea at Aberdeen. A comprehensive sampling strategy and analytical programme, commensurate with the size and nature of the Dee system, have been established for major water quality determinands to identify the controls on, and origins of, dissolved species throughout the system at a range of catchment scales and over a range of flow regimes. Fifty-nine sites covering a range of catchment types and scales were therefore sampled bi-weekly for 1 year. At the basin scale, there is a general downstream increase in determinand concentrations. This produces strong linear relationships between many determinands which are unrelated in terms of a common terrestrial process or origin. At the sub-catchment scale, however, specific hydrochemical processes control streamwater chemistry. The Dee basin divides into two distinct geographic regions in terms of land use (upland and lowland) which produce clear differences in water chemistry. Individual sub-catchments can also be grouped in terms of temporal variations in streamwater chemistry. The strength of the relationship between weathering-derived ionic concentrations and flow in the upland sub-catchments has lead to the identification of specific concentration limits in sub-catchments which can be used as characteristics of soil water and groundwater end-members. This provides a basis for the prediction of upland weathering-derived component concentrations for each sub-catchment at a range of flows.

The influence of catchment land use on stream integrity across multiple spatial scales

1. Despite wide recognition of the need for catchment‐scale management to ensure the integrity of river ecosystems, the science and policy basis for joint management of land and water remains poorly understood. An interdisciplinary case study of a river basin in south‐eastern Michigan is presented.2. The River Raisin drains an area of 2776 km2, of which some 70% is agricultural land. The upper basin consists of till and outwash, and both topography and land use/cover are diverse. The lower basin consists of fine textured lake deposits, is of low relief, and land use is primarily agricultural.3. The River Raisin basin historically was a region of oak‐savannah and wetlands. It was deforested, drained and converted to farmland during the mid‐nineteenth century. Human population reached a plateau at about 1880, and then underwent a second period of growth after 1950, mainly in small urban areas. More recently, the amount of agricultural land has declined and forested land has increased, in accord with a general decline in farming activity.4. It could be suggested that the influence of land use on stream integrity is scale‐dependent. Instream habitat structure and organic matter inputs are determined primarily by local conditions such as vegetative cover at a site, whereas nutrient supply, sediment delivery, hydrology and channel characteristics are influenced by regional conditions, including landscape features and land use/cover at some distance upstream and lateral to stream sites.5. Sediment concentrations measured during low flows were higher in areas of greater agriculture. In a comparison of two subcatchments, sediment yields were up to ten times greater in the more agricultural location, in response to similar storm events. A distributed parameter model linked to a geographical information system predicted that an increase in forested land cover would result in dramatic declines in runoff and sediment and nutrient yields.6. Habitat quality and biotic integrity varied widely among individual stream sites in accord with patterns in land use/cover. Extent of agricultural land at the subcatchment scale was the best single predictor of local stream conditions. Local riparian vegetation was uncorrelated with overall land use and was a weak secondary predictor of habitat quality and biotic integrity.7. Investigation of the regulatory agencies involved in land and water management in the basin revealed a complex web of overlapping political jurisdictions. Most land‐use decision‐making occurs at the local level of township, city or village. Unfortunately, local decision‐making bodies typically lack the information and jurisdictional authority to influence up‐ and downstream events.

The effectiveness of management options for dryland salinity control at Wanilla, South Australia

A calibrated groundwater flow model was used to assess several options for management of dryland salinity at Wanilla on Lower Eyre Peninsula of South Australia. The management options were chosen as potentially able to rectify the hydrological imbalance produced by the clearing of native vegetation which will reverse the salinisation process. The model predicted that current agricultural practices will result in groundwater levels continuing to rise in saline areas over the next 20 years (although tending towards equilibrium) increasing soil salinisation. As groundwater levels rise seepage of groundwater to the local drainage (natural) line will also increase, flowing down-gradient towards the catchment outlet and a seasonally flooded lower lying basin. A 50–90% reduction in catchment recharge, achieved with improved management of existing crops and establishment of perennial pastures, is predicted to reduce groundwater levels across the catchment over a 10–20 year period. This will lead to recovery of saline areas and lower baseflow. However, the model predicts that management of recharge needs to be implemented at least at the catchment scale to be effective. Treatments at the sub-catchment scale (e.g. property) have limited impact. Short term benefits (less than 1 year) can be gained from pumping groundwater from a high transmissivity zone close to discharge area. Groundwater levels are lowered over a small area leading to less evaporative discharge and reduced salt accumulation in the root zone. This option needs to work in conjunction with longer term management such as recharge control.

A study on subcatchment scale for a distributed runoff model

Physically-based distributed runoff models are more advantageous than concentrated conceptual models for predicting the hydrologic response of ungauged basins and the changing response resulting from changes in land-use. In constructing distributed models, the river basin must be divided into reasonable smaller subcatchments corresponding to a characteristic scale for rainfall-runoff response. Thus, one of the still-unsolved problems in distributed models is the estimation of the appropriate subcatchment scale. This paper focuses only on the appropriate subcatchment scale considering the channel network geomorphology, although the effects of subcatchment scale on the overall river basin response are important. For this purpose, some new stochastic functions are proposed for discussing the hydrologic response of channel networks. By applying these functions into a simplified distributed runoff model, the relationship between the hydrologic response of the river basin and variation of the channel network geomorphology is shown.

Computing procedures for mapping soil features at sub-catchment scale

Procedures for detailed mapping of a large number of soil features in small landscape units displayed in either vertical cross section (i.e. soil toposequences) or horizontal plane (i.e. soil maps) are presented. Rom hand coloured drawings that display the soil-landscape features, an Apple Macintosh Computer, with Adobe Illustrator and Adobe Patterns and Textures programs, is used to produce high quality coloured maps ready for reproduction in any form (e.g. posters, publications, slides or overhead transparencies). The first step is to produce the detailed computer map with all soil features included. This detailed computer map is made up of the following three layers or bases: (i) a black linework and lettering base, (ii) a polygon base filled with black and white textures, and (iii) a polygon base filled with colours. The detailed computer map, saved as the master file, is very easily modified to generate more simplified and thematic maps by: (i) grouping soil features into larger soil components in order to display soil-landscapes in a more simplified form, (ii) deleting certain soil-landscape components in order to highlight specific soil features, and (iii) adding newly aquired information (e.g. chemical and hydrological data) to previous versions.