Richard P. Ibbitt

Evaluation of optimal channel network and river basin heterogeneity concepts using measured flow and channel properties

Abstract The optimal channel network (OCN) concept provides a physical argument for the form of river networks based on testable hypotheses relating to the exponents of the downstream hydraulic geometry relationships and the variation of longitudinal slope along the channel. Measurements made over 5-day periods of steady flow in two catchments of areas 121 and 158 km 2 are described. At each site in each basin measurements of the flow, channel cross-section and elevation have been made. A total of 340 measurements were made in one basin and 308 in the other and provide one measurement for about every 0.5 km of channel. The data are used to test the OCN concept. Less extensive data from two larger catchments are also used to help with the evaluation. The results do not support the depth, velocity and slope exponent values derived in the OCN concept. Analysis of the cumulative distributions of the spatially distributed flows shows agreement at flows above 6 l s −1 with results derived for aggregation systems with constant injection of mass of which the OCN concept is a specific example. Reasons for the differences from the OCN-derived exponents are discussed in terms of the heterogeneity of basin properties. ©1997 Elsevier Science B.V.

Channel network simulation models compared with data from the Ashley River, New Zealand

This paper compares the ability of two channel network simulation models to simulate the channel network properties of the Ashley River in the foothills of the Southern Alps of the South Island of New Zealand. The basin was chosen because it contains a large-scale topographic feature, a central ridge, the simulation of which would provide a measure of each models ability to handle spatial nonuniformities. The two models assessed were the optimal channel network (OCN) model and a catchment evolution model (SIBERIA) catchment evolution model. The ability of these models to replicate observed geomorphic statistics and relationships was assessed. The models were also compared against each other to assess their relative suitability for simulating the observed geomorphology. Model performance was gauged using the single-valued measures of catchment convergence, hypsometric integral, and energy expenditure by the network; and plots of the width function, the slope of the cumulative area diagram, and the hypsometric curve of basin topography. The effects of different forcings were examined. The basic forcing was one in which the climate and the geological and tectonic properties of the basin were assumed to be unchanging in both space and time. The first variant on the basic forcing looked at what happens when there is a permanent spatial gradient in the rainfall over a basin with spatially uniform geological and tectonic properties. The second variant considered the effects of constant, spatially uniform rainfall on a basin in which spatially variable tectonic uplift is occurring. Neither model adequately simulated the observed geomorphology when the spatially nonuniform tectonic forcing was ignored. When spatially nonuniform tectonic effects at length scales of tens of kilometers were simulated, SIBERIA performed more satisfactorily. The effect of nonuniform rainfall was found to be small for both models. The performance of the OCN as gauged by single-valued measures improved markedly when energy expenditure calculations were consistent with the geometric length of flow paths.

Data for Ashley River to test channel network and river basin heterogeneity concepts

To provide data to investigate hypotheses about the evolution of channel networks, specifically the optimal channel network concept, discharge and channel properties were measured at 336 sites in a 121 km2 basin over a 5-day period of reasonably steady flows. The data are also suitable for investigating how discharge increases down river channels. The data collection was a major logistical exercise which involved 80 person-days in remote field locations. In the expectation that the data will be of use to other researchers, this paper describes how the data were measured, checked and archived. The archive is available at http://www.niwa.cri.nz/hydrology/ashpage.htm and includes the associated time series of streamflow at the basin outlet and the channel network as plotted on 1:50,000 scale maps.

Model application to assess effects of urbanisation and distributed flow controls on erosion potential and baseflow hydraulic habitat

The catchment model SWMM was modified to include on-site flow-control devices, and then linked with models of hydraulic habitat suitability and erosive potential for specific reaches in a study catchment. Urbanisation decreased the modelled baseflow by up to 33% and reduced the area of suitable hydraulic habitat by 0.5–13.3%, depending on the reach and species. Hypothetical infiltration devices increased the baseflow to pre-development levels, with an associated increase in habitat, but other measures were not effective. Urbanisation increased the erosion potential index by a factor of 1.58–9.32, depending on the reach. Erosion-control ponds decreased the erosion potential to pre-development level in some reaches. Detention tanks reduced the erosion potential significantly, but not to pre-development levels. The poor predictive ability of the models for baseflow compromised the utility of the hydraulic habitat assessment. Predictions of the effects of urbanisation on baseflow habitat should therefore be treated cautiously.

Taieri river data to test channel network and river basin heterogeneity concepts

The paper describes data collected in a 158-km2 basin for testing hypotheses underpinning the optimal channel network concept. Over a 4-day period, discharge, channel cross sections, and longitudinal slope were measured at 300 sites. Measured channel widths range from 0.050 to 16.60 m, mean depths range from 0.010 to 0.546 m, and mean water velocities range from 0.004 to 0.575 m/s. Bias in the site selection was checked by comparing measurements made at predefined distances upstream or downstream of the measurement section. Slope measurements were based on channel reach lengths typically of the order of 0.5 km. Since the data may be of use to other researchers, we have archived them on the World Wide Web (site http://www.cri.nz/hydrology/taieripage.htm) along with a 1:50,000 scale map and the associated time series of streamflow at the basin outlet during the measurement interval.

ON CHANNEL NETWORK FRACTAL PROPERTIES : A CASE OF STUDY OF THE HUTT RIVER BASIN, NEW ZEALAND

The paper considers river networks as three-dimensional self-affine fractal objects. The Hutt River basin (New Zealand∥ was selected for detailed analysis on the basis of a digital elevation model (DEM). To characterize network properties quantitatively we used three scaling exponents in the relationships l ∝ ℒυl, wℒυw, and h ∝ ℒυh where l, w, h are some characteristic longitudinal, transversal, and vertical scales of a channel network; ℒ is the total length of channel network in three-dimensional space; and υl, υw, and υh are the self-affine scaling exponents. We determined υl, υw, and υh using Lp ∝ Aβ, ℒp ∝ Aϵ, and S ∝ A−θ, where Lp is the length of the projection of the longest river channel on the horizontal plane, ℒp is the total length of channel network projection on the horizontal plane, A is the catchment area, and S is the local slope. An approximate relationship υh ≈ υl − θ(υl + υw) is derived which connects the main scaling exponents. For two New Zealand rivers, we found υl=0.60 and υw=0.40. On the basis of simple considerations, we estimated a range of possible values of υh from 0.1 to 0.5 with 0.2 for the case study. The slope-area-elevation relation introduced by Willgoose [1994] was applied to interpret data concerning υh. The influence of threshold area (TA) values on the scaling properties of channel networks is shown to be small, and double scaling relationships are suggested for connecting the physical scaling of channel networks with scaling caused by threshold effect.

Assessment of irrigation shortfall using WATHNET in the Otago region of New Zealand

Knowledge of the likely rainfall and river flow for a coming season can improve management of an overall water resources system without unduly compromising either the environmental or productive behaviour of the system. The objective of this study has been to assess the probability of irrigation demand shortfalls, i.e. soil moisture deficits, for a typical “run of the river” irrigation scheme so as to identify the duration and severity of potential shortfalls. In this study a multi-objective linear programming tool WATHNET has been used to build and run a model of the irrigation scheme. The focus of this study has been on how to use predictions of 3-monthly rainfall and temperature to estimate potential daily water available for irrigation. The method uses Monte-Carlo simulations, to produce multiple replicates of equally likely sequences of river flows, rainfall and potential evaporation values. A sub-set of the equally likely sequences is then selected using prediction information of the likely seasonal climate outlook from NIWA’s Climate Update. The selected sequences, which are biased towards the seasonal climate prediction, are then used as inputs to multiple model runs. By using the output from all the “biased” model runs a probability distribution can be made of water availability for irrigation. The methodology has been demonstrated using the Shag River Irrigation Scheme located in the Otago region of New Zealand. The results compare the predicted soil moisture variation over two three-month periods with retrospective simulations based on the observed rainfalls, river flows and potential evaporation values. Results suggest that WATHNET can simulate 3-month soil moisture dynamics. In order to develop WATHNET as a tool to assess probabilities of irrigation shortfall it needs to be validated using measurements of soil moisture variations over an irrigation season at sites with different soil types.

Application of GIS in Analysing Spatial Patterns of Multiple Runoff Events

Abstract Rainfall, topography and soil characteristics are considered to control spatial redistribution of phosphorus in catchments. To advance this research, a hydrology model is needed to describe spatial variation of soil moisture dynamics and runoff source areas in multiple runoff events. This study examined whether the spatially distributed, topographically based rainfall-runoff model, TOPMODEL provides superior performance for rainfall/runoff events in maritime climate and pastoral hill lands like New Zealand. Unlike previous efforts, we evaluate the hydrological model to identify runoff source areas for each individual runoff event. Geographical information system was used to analyse the model sensitivity on pattern dynamics of runoff, water tables and soil moistures of three major runoff events (low, medium and high). The model was tested for two catchments at Waipawa in Palmerston north, New Zealand. The study confirmed that TOPMODEL give high quality results (R2 of 84%) when validated against flow observations. Visual analysis on GIS systems showed that the predicted dynamics of variable source area and the component hydrological processes is realistic in the study area of pastoral farmlands. The TOPMODEL can be used to reflect both long-term evolutionary soil moisture content patterns and the short term forcing of flow dynamics during storm events in typical New Zealand mountainous and high rainfall volume (1200mm/year) regions.