Yeboah Gyasi-Agyei

Identification of regional parameters of a stochastic model for rainfall disaggregation

Abstract This paper demonstrates how the Gyasi-Agyei–Willgoose hybrid model for point processes could be regionalised for daily rainfall disaggregation using limited high resolution data within a region of interest. Their model is a product of the binary non-randomised Bartlett–Lewis rectangular pulse model and a lognormal autoregressive model used as a jitter. The computationally efficient multinormal approximation to parameter uncertainty is used to group the monthly parameter values of the binary model. For central Queensland, Australia, it has been established that the parameters of cell origins and the duration of the rectangular pulse of the binary model could have constant values for all months. Second harmonic Fourier series is used to represent the seasonal variation of the parameter governing the storm lifetime. The storm arrival rate is a function of the daily dry probability and the other parameters. Additive properties of random variables with finite variances were used to scale down the daily mean and variance of the historical data to the simulation timescale, values required by the jitter model. The results of using observed daily rainfall statistics to capture sub-daily statistics by the regionalised model are very encouraging. The model is therefore a valuable tool for disaggregating daily rainfall data.

A dynamic hillslope response model in a geomorphology based rainfall-runoff model

This paper presents a technique for the determination of a dynamic hillslope instantaneous unit hydrograph (IUH) in concert with a variable saturation excess runoff production model using a grid-based digital elevation model (DEM). The total channel network responsible for routing the runoff produced on the catchment is divided into two parts: the main channel network and the hillslope channel network. The hillslope IUH, which routes water from the hillslopes to the main channel network, is essentially the solution of the linear advection-dispersion routing model weighted by the hillslope travel distance distribution of saturated pixels to the main channel network. The shape of the hillslope travel distance is found to consist of an initial spike, representing saturated pixels on the main channel network, and an exponential decay function for those pixels on the hillslope. However, the proportion of saturated pixels on the main channel network varies with total saturated pixels, causing an inverse change of scale of the spike and the exponential decay part. As the number of saturated pixels changes during a storm event, the hillslope IUH is dynamic. The main channel network IUH is also modelled by the linear advection-dispersion model weighted by the normalized width function of the main channel network. Convolution of the hillslope and main channel network IUHs gives the catchment IUH, which is also dynamically changing with the degree of saturation. It is demonstrated that the direct runoff hydrograph is sensitive to the variation of the degree of saturation within and between storm events.

Assessment of radar-based locally varying anisotropy on daily rainfall interpolation

ABSTRACT Spatial variability of rainfall has been recognised as an important factor controlling the hydrological response of catchments. However, gauged daily rainfall data are often available at scattered locations over the catchments. This paper looks into how to capitalise on the spatial structure of radar rainfall data for improving kriging interpolation of limited gauge data over catchments at the 1-km2 grid scale, using for the case study 117 gauged stations within the 128 km × 128 km region of the Mt Stapylton weather radar field (near Brisbane, Australia). Correlograms were developed using a Fast Fourier Transform method on the Gaussianised radar and gauged data. It is observed that the correlograms vary from day to day and display significant anisotropy. For the radar data, locally varying anisotropy (LVA) was examined by developing the correlogram centred on each pixel and for different radial distances. Cross-validation was carried out using the empirical correlogram tables, as well as different fitting strategies of a two-dimensional exponential distribution for both the gauged and the radar data. The results indicate that the correlograms based on the radar data outperform the gauged ones as judged by statistical measures including root mean square error, mean bias, mean absolute bias, mean standard deviation and mean inter-quartile range. While the radar data display significant LVA, it was observed that LVA did not significantly improve the estimates compared with the global anisotropy. This was also confirmed by conditional simulation of 120 rainfields using different options of correlogram development. EDITOR M.C. Acreman; ASSOCIATE EDITOR Q. Zhang

Rainfall–runoff modelling of railway embankment steep slopes

Abstract A distributed 1D rainfall–runoff model is presented. It consists of the Saint Venant continuity and momentum equations for overland flow and a modified Green-Ampt model for the infiltration on railway embankment steep slopes. The model is applied to adjacent 10-m-wide erosion control experimental plots with different percentages of grass cover. A relationship between the 2-day antecedent rainfall and initial moisture content was established and used to predict the saturated hydraulic conductivity (Ks). Average values of Ks for 0, 50 and 100% grass cover were found to be 0.1, 1.19 and 2.56 mm/h, respectively. For the majority of cases, the model simulated runoff with acceptable accuracy, 68% having Nash-Sutcliffe efficiency (NSE) values above 0.50. The average NSE value varied between 0.60 and 0.80, with 0% grass-covered plots yielding the highest values. As expected, the runoff volume decreased with increasing percentage of grass cover. Citation Sajjan, A.K., Gyasi-Agyei, Y., and Sharma, R.H., 2013. Rainfall–runoff modelling of railway embankment steep slopes. Hydrological Sciences Journal, 58 (5), 1162–1176. Editor D. Koutsoyiannis

Compatibility Assessment of Drip Irrigation Laterals

Emitters inserted in drip irrigation laterals cause local head loss, generally estimated as a product of a coefficient and the velocity head. This local head loss coefficient and the emitter discharge curve hydraulic parameters may exhibit considerable variability attributable to the manufacturing process. This paper provides a framework for assessing whether the variability in the hydraulic parameters could lead to significant differences in the performance of rolls of drip irrigation laterals from the same manufacturing batch. A system approach with inlet pressure as input, pressure distribution along the drip lateral and inlet discharge as outputs (or responses), and a drip lateral hydraulic model as the transfer function is explored. Within a Bayesian statistical framework of parameter uncertainty based on the Metropolis algorithm, the hydraulic parameters of pressure-compensating drip lateral rolls from the same manufacturing batch were inferred (calibrated). Overlapping of the space (region) of the ...

Estimation of downstream hydraulic geometry exponents with emphasis on channel flow velocity

By using the continuity equation for water transport, the Manning equation for hydraulic friction, the simplified Einstein-Brown sediment transport equation and the balance between the uniform energy expenditure per unit area and the minimum total energy expenditure in a channel network as a whole, the explicit relationships between velocity-, depth-, width- and roughness- discharge scaling exponents in the Leopold-Maddock hydraulic geometry equations and the slope-area scaling exponent are established. The slope-area scaling exponent is determined through the map data analysis of digital elevation. These relationships indicate that a catchment with a higher concave profile (lower slope-area scaling exponent) has a higher spatial rate of change in velocity and depth, and a lower rate of change in width for a unit change in discharge. The rate of change in roughness is mild. It is also established that the assumption of constant channel flow velocity throughout a catchment during a flood event may be true for catchments with slope-area scaling exponent larger than −0.614.