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Srikanthan S.,Environment and Research Division
Hydrology and Water Resources Symposium 2014, HWRS 2014 - Conference Proceedings | Year: 2014

Two methods are generally used to sample the original data for extreme events: Annual maximum series (AMS) and partial duration series (PDS). In AMS series the largest events from each year is selected, so that the series length equals the number of years of record. As the events have been sampled at fixed intervals, the return period of an event of magnitude greater or equal to a given value equals the inverse of its probability of exceedance. The PDS has all the values of the variable that exceed an a priori determined threshold. The main reason put forward for using PDS is that it uses the second, third ... largest values in a year while the AMS ignores these. Most of the literature supporting the use of PDS only considers the variance or the root mean square error of the quantile estimates and not the bias. Two simulation experiments are carried out to compare the bias and variance of the quantile estimates obtained from AMS and PDS. In simulation experiment 1, known population values of generalised Pareto distribution parameters are used and in the second simulation experiment long sequences of daily rainfall are generated. The generated sequences are then broken into sub-samples of different lengths and the quantiles for different average recurrence intervals are estimated using AMS and PDS. The bias and the standard deviation of the quantiles obtained from the replicates are compared. From the results, it is found that it is not possible to select a single value for the mean number of events per year for PDS. Since AMS gave the smallest bias for most of the cases in the second experiment and for negative k values in the first experiment, use of AMS in frequency analysis is preferred to PDS. Source


Perera K.C.,University of Melbourne | Western A.W.,University of Melbourne | Nawarathna B.,Environment and Research Division | George B.,University of Melbourne
Agricultural and Forest Meteorology | Year: 2014

Farmers and irrigation system operators make real-time irrigation decisions based on a range of factors including short-term weather forecasts of rainfall and temperature. The simplest and oldest statistical method for forecasting daily ET0 is to use the long-term monthly mean ET0 based on historical observations. Forecasts of reference crop evapotranspiration (ET0) can be calculated from Numerical Weather Prediction (NWP) outputs and ET0 has the advantage of being more directly relevant to crop water requirements than temperature. This paper evaluates forecasts of ET0 made using the Bureau of Meteorology's operational NWP forecasts derived from the Australian Community Climate and Earth System Simulator - Global (ACCESS-G). The forecast performance for ET0 was quantified using the root mean squared error (RMSE), coefficient of determination (R2), anomaly correlation coefficient (ACC) and mean square skill score (MSSS). Daily ET0 forecasts for lead times up to 9 days were compared against ET0 calculated using hourly observations from the 40 automatic weather stations across Australia.It was found that using NWP forecast daily ET0 was better than using the long-term monthly mean ET0 for lead times up to 6 days, beyond which the long-term monthly mean was better. The average MSSS for ET0 forecasts across all stations varied between 66% and 12% for lead times of 1-6 days, respectively. Further, it was found that forecast performance for daily ET0 was highest in autumn for tropical climates and lowest in spring for temperate climates. Errors in incoming solar radiation were the most important source of ET0 forecast error, followed by air temperature, dew point temperature and wind speed, for all lead times. Also, it was found that the forecast performances for incoming solar radiation and mean wind speed were relatively poor compared with the air and dew point temperatures. © 2014 Elsevier B.V. Source


Khan U.,University of New South Wales | Tuteja N.K.,Environment and Research Division | Ajami H.,University of New South Wales | Sharma A.,University of New South Wales
Water Resources Research | Year: 2014

The computational effort associated with physically based distributed hydrological models is one of their major limitations that restrict their application in soil moisture and land surface flux simulation problems for large catchments. In this work, a new approach for reducing the computational effort associated with such models is investigated. This approach involves the formation of equivalent cross sections, designed in a manner that ensures comparable accuracy in simulating the hydrological fluxes as a fully distributed simulation. Single or multiple equivalent cross sections are formulated in each Strahler's first-order subbasin on the basis of topographic and physiographic variables representing the entire or part of the subbasin. An unsaturated soil moisture movement model based on a two-dimensional solution of the Richards' equation is used for simulating the soil moisture and hydrologic fluxes. The equivalent cross-section approach and the model are validated against observed soil moisture data in a semiarid catchment and found consistent. The results indicate that the equivalent cross-section approach is an efficient alternative for reducing the computational time of distributed hydrological modeling while maintaining reasonable accuracy in simulating hydrologic fluxes, in particular dominant fluxes such as transpiration and soil evaporation in semiarid catchments. Key Points An approach to reduce the computational time in distributed hydrological model Validation is done against observed soil moisture data Soil moisture model is based on a 2-D solution of the Richards' equation © 2014. American Geophysical Union. All Rights Reserved. Source


Perera K.C.,University of Melbourne | Western A.W.,University of Melbourne | Nawarathna B.,Environment and Research Division | George B.,University of Melbourne | George B.,International Center for Agricultural Research in the Dry Areas
Agricultural Water Management | Year: 2015

Estimates from the FAO Penman-Monteith (FAO-PM) and the standardized ASCE Penman-Monteith (ASCE-PM) hourly and daily reference evapotranspiration (ET0) equations were compared at daily scale, based on the hourly climate data collected from forty (40) geographically and climatologically diverse Automatic Weather Stations (AWS) across the Australian continent. These locations represent 23 agricultural irrigation areas in tropical, arid and temperate climates. The aims of this paper are to: compare the effects of different methods of estimating Clear-sky-radiation-(Rso); compare sum-of-hourly and daily ET0; compare the results of aggregation of hourly ET0 over 24h compared with daylight hours; and examine the impact of seasonality and climate type. At selected AWS locations, the hourly ET0 was calculated using the hourly FAO-PM and the ASCE-PM equations and then summed to derive daily ET0 (reported as ET0,soh). This was compared against the daily ET0 values, calculated using the corresponding daily equation (reported as ET0,daily). Using Rso calculated following the "complex" approach improves the agreement between ET0,soh and ET0,daily of both hourly equations, compared with the "simple" approach. Better agreement between ET0,soh and ET0,daily estimates for the FAO-PM and ASCE-PM were found, when the hourly values are aggregated over 24h rather than over daylight hours. The average ratio between ET0,soh and ET0,daily for the FAO-PM and ASCE-PM equations is 0.95 and 1.00, respectively. The range of the former is 0.90-0.98 and that of the latter is 0.96-1.04. There was very strong correlation between the two hourly equations at the daily time step: on average 0.997, with a range of 0.993-0.998. The results imply that the ASCE-PM hourly equation's daily ET0 values are higher than those of FAO-PM, which can be explained by the difference in the treatment of surface resistances. Better agreements between ET0,soh and ET0,daily values for winter, spring and autumn were found for the FAO-PM version, while during summer, the ASCE-PM version showed better agreement. The best agreement between the hourly and daily results for the FAO-PM version was found in temperate climates and the ASCE-PM version showed best agreement in the tropical and arid climates. © 2014 Elsevier B.V. Source


Nair S.,University of Melbourne | George B.,University of Melbourne | Malano H.M.,University of Melbourne | Arora M.,University of Melbourne | Nawarathna B.,Environment and Research Division
Resources, Conservation and Recycling | Year: 2014

Water supply and wastewater services incur a large amount of energy and GHG emissions. It is therefore imperative to understand the link between water and energy as their availability and demand are closely interrelated. This paper presents a literature review and assessment of knowledge gaps related to water-energy-greenhouse gas (GHG) nexus studies in an urban context from an 'energy for water' perspective. The review comprehensively surveyed various studies undertaken in various regions of the world and focusing on individual or multiple subsystems of an urban water system. The paper also analyses the energy intensity of decentralized water systems and various water end-uses together with the major tools and models used. A major gap identified from this review is the lack of a holistic and systematic framework to capture the dynamics of multiple water-energy-GHG linkages in an integrated urban water system where centralized and decentralized water systems are combined to meet increased water demand. Other knowledge gaps identified are the absence of studies, peer reviewed papers, data and information on water-energy interactions while adopting a 'fit for purpose water strategy' for water supply. Finally, based on this review, we propose a water-energy nexus framework to investigate 'fit-for-purpose' water strategy. © 2014 Elsevier B.V. All rights reserved. Source

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