Medium-sized Catchment Research Articles

A regional model to assess the hydrological sensitivity of medium size catchments to climate variability

A regional model to assess the hydrological sensitivity of medium size catchments to climate variability

Estimating Salt Loads in High Water Table Areas. II: Regional Salt Loads

This paper describes the development of MIDASS, a lumped conceptual model for the prediction of streamflow and salt yield from irrigated catchments with high water tables. The model is suited for application to medium-sized catchments, though several models can be linked into a cascade arrangement for the simulation of catchments several hundred square kilometers in area. The model requires daily values of rainfall and irrigation deliveries as well as potential evaporation. As streamflow routing is not undertaken, model output is suited to weekly or monthly estimates of yield. The model was calibrated on a gauged 465 km2 subcatchment of the Tragowel Plains Irrigation Area in south eastern Australia. Considering differences in scale, the relative proportions of baseflow, exfiltration, and runoff were found to agree reasonably well with physically based model results from a 9 ha study site. The model was applied to the Tragowel Plains to test the impacts of a number of irrigation and drainage management options on downstream river salinity.

Water and salt balance modelling to predict the effects of land‐use changes in forested catchments. 1. Small catchment water balance model

A long-term water balance model has been developed to predict the hydrological effects of land-use change (especially forest clearing) in small experimental catchments in the south-west of Western Australia. This small catchment model has been used as the building block for the development of a large catchment-scale model, and has also formed the basis for a coupled water and salt balance model, developed to predict the changes in stream salinity resulting from land-use and climate change. The application of the coupled salt and water balance model to predict stream salinities in two small experimental catchments, and the application of the large catchment-scale model to predict changes in water yield in a medium-sized catchment that is being mined for bauxite, are presented in Parts 2 and 3, respectively, of this series of papers. The small catchment model has been designed as a simple, robust, conceptually based model of the basic daily water balance fluxes in forested catchments. The responses of the catchment to rainfall and pan evaporation are conceptualized in terms of three interdependent subsurface stores A, B and F. Store A depicts a near-stream perched aquifer system; B represents a deeper, permanent groundwater system; and F is an intermediate, unsaturated infiltration store. The responses of these stores are characterized by a set of constitutive relations which involves a number of conceptual parameters. These parameters are estimated by calibration by comparing observed and predicted runoff. The model has performed very well in simulations carried out on Salmon and Wights, two small experimental catchments in the Collie River basin in south-west Western Australia. The results from the application of the model to these small catchments are presented in this paper.

Water and salt balance modelling to predict the effects of land‐use changes in forested catchments. 3. The large catchment model

This paper presents an application of a long-term, large catchment-scale, water balance model developed to predict the effects of forest clearing in the south-west of Western Australia. The conceptual model simulates the basic daily water balance fluxes in forested catchments before and after clearing. The large catchment is divided into a number of sub-catchments (1–5 km2 in area), which are taken as the fundamental building blocks of the large catchment model. The responses of the individual subcatchments to rainfall and pan evaporation are conceptualized in terms of three inter-dependent subsurface stores A, B and F, which are considered to represent the moisture states of the subcatchments. Details of the subcatchment-scale water balance model have been presented earlier in Part 1 of this series of papers. The response of any subcatchment is a function of its local moisture state, as measured by the local values of the stores. The variations of the initial values of the stores among the subcatchments are described in the large catchment model through simple, linear equations involving a number of similarity indices representing topography, mean annual rainfall and level of forest clearing. The model is applied to the Conjurunup catchment, a medium-sized (39·6 km2) catchment in the south-west of Western Australia. The catchment has been heterogeneously (in space and time) cleared for bauxite mining and subsequently rehabilitated. For this application, the catchment is divided into 11 subcatchments. The model parameters are estimated by calibration, by comparing observed and predicted runoff values, over a 18 year period, for the large catchment and two of the subcatchments. Excellent fits are obtained.

Water and salt balance modelling to predict the effects of land-use changes in forested catchments. 3. The large catchment model

This paper presents an application of a long-term, large catchment-scale, water balance model developed to predict the effects of forest clearing in the south-west of Western Australia. The conceptual model simulates the basic daily water balance fluxes in forested catchments before and after clearing. The large catchment is divided into a number of sub-catchments (1–5 km2 in area), which are taken as the fundamental building blocks of the large catchment model. The responses of the individual subcatchments to rainfall and pan evaporation are conceptualized in terms of three inter-dependent subsurface stores A, B and F, which are considered to represent the moisture states of the subcatchments. Details of the subcatchment-scale water balance model have been presented earlier in Part 1 of this series of papers. The response of any subcatchment is a function of its local moisture state, as measured by the local values of the stores. The variations of the initial values of the stores among the subcatchments are described in the large catchment model through simple, linear equations involving a number of similarity indices representing topography, mean annual rainfall and level of forest clearing. The model is applied to the Conjurunup catchment, a medium-sized (39·6 km2) catchment in the south-west of Western Australia. The catchment has been heterogeneously (in space and time) cleared for bauxite mining and subsequently rehabilitated. For this application, the catchment is divided into 11 subcatchments. The model parameters are estimated by calibration, by comparing observed and predicted runoff values, over a 18 year period, for the large catchment and two of the subcatchments. Excellent fits are obtained.

The sensitivity of hydrological models to spatial rainfall patterns: an evaluation using observed data

Spatial variability of rainfall is often considered as a major source of temporal variability in the resulting basin hydrograph. Since direct experimental evidence is not available, this must be verified through a modelling approach, provided adequate data are available. A semidistributed version of TOPMODEL has therefore been applied to the Réal Collobrier experimental basin (71 km 2 in southeast France with 21 recording raingauges) using an hourly time step and a series of independent events. First, a set of reference results has been built under the assumption of spatial uniformity for the rainfall. Two different densities of network have been tested (including 5 or 21 gauges), showing a significant advantage for the dense network rainfall estimate. Next, the spatial variability of the rainfall field has been tested and confirmed, with commonly a factor of 3 between simultaneous average rainfall over subcatchment areas of 6–8 km 2. However, the model response reflects this spatial variability only in secondary peaks which are usually an order of magnitude smaller than the bulk of the hydrograph and not always present in the observed discharges. An extended discussion considers if these results may be dependent on the model or on the setting up of the numerical experiments. In fact, it seems that the spatial variability of rainfall, although important, is not sufficiently organized in time and space to overcome the effects of smoothing and dampening when running off through this rural medium-sized catchment. Such results may not hold for smaller urbanized areas or larger rural basins.

Hydrological response of a medium-sized mountainous catchment to climate changes

Abstract The long term hydrological response of a medium-sized mountainous catchment to climate changes has been examined, The climate changes were represented by a set of hypothetical scenarios of temperature increases coupled with precipitation and potential evapotranspiration changes. Snow accumulation and ablation, plus runoff from the study catchment (the Mesochora catchment in central Greece) were simulated under present (historical) and altered climate conditions using the US National Weather Service snowmelt and soil moisture accounting models. The results of this research obtained through alternative scenarios suggest strongly that all the hypothetical climate change scenarios would cause major decreases in winter snow accumulation and hence increases in winter runoff, as well as decreases in spring and summer runoff. The simulated changes in annual runoff were minor compared with the changes in the monthly distribution of runoff. Attendant changes in the monthly distribution of soil moisture and...

Holocene floodplain diachronism and inherited downstream variations in fluvial processes: A study of the river Perry, Shropshire, England

AbstractThis paper attempts to explain downstream variations in the rates and processes of Holocene floodplain formation in a small to medium‐sized, lowland catchment in Central England. Four macro‐reaches were identified and horizons from at least one stratigraphic cross‐section from each were radiocarbon dated. Considerable diachronism was revealed with the basal age and hence minimum residence time of the top two metres varying from 1000‐9000yrs. The trend, a decrease in minimum residence time downstream, is explained by the basin‐and‐gorge bedrock long‐profile inherited from the Devensian glaciation of the area. This is illustrated by a positive relationship between the mean floodplain accumulation rate and the SL index (slope‐stream length product). Pollen analysis of the organic sediments shows that floodplain clearance took place around 1 300 BP, but probably locally, pre‐dated and post‐dated accelerated alluviation associated with agricultural expansion in the catchment. This study concludes that (a) different macro‐reaches in this small to medium‐sized catchment have adjusted in different ways to long‐term catchment changes as part of autogenic floodplain evolution, (b) this has involved a change in the catchment's sediment conveyance characteristics during the Holocene, and (c) an important control on channel behaviour and floodplain sedimentation throughout the Holocene (as well as at present) has been the valley slopes inherited from the Quaternary glaciation of the area.

Spatial Variations in Water Quality in an Urbanizing Catchment

Spatial variations in water quality in a medium-sized complex catchment are described. The catchment is complex, both with respect to patterns of rock-type and land-use, and variations in water quality are related to the distribution of each. The 33 -03 km2 Monks' Brook catchment in Hampshire is described and the stream sampling procedure outlined. During a three year period over 1500 water samples were taken from numerous localities representative of the major rock-types and land-uses within the catchment. Each sample was analysed for a number of water quality determinands, twelve of which are considered here. The Kruskall-Wallis H (hereafter K-W H) and Mann-Whitney U (hereafter M-W U) non-parametric test statistics were employed to investigate the significance of spatial variations. The major rock-types, Chalk and Bracklesham Beds, have specific influence upon the concentrations of calcium (Ca2+) and bicarbonate (HCOi), and iron (Fe2+) and manganese (Mn2+) respectively. Urban baseflow drainage can be characterized mainly by elevated sodium (Na +) and chloride (Cl +) concentrations, and possibly by increased potassium (K+) concentrations.