Jeffrey P Walker

Jeffrey P Walker
Monash University (Australia) · Department of Civil Engineering, Clayton

BSurv, BE(Civil), PhD(Water Resources)

About

533
Publications
103,761
Reads
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20,847
Citations
Citations since 2016
210 Research Items
14068 Citations
201620172018201920202021202205001,0001,5002,0002,500
201620172018201920202021202205001,0001,5002,0002,500
201620172018201920202021202205001,0001,5002,0002,500
201620172018201920202021202205001,0001,5002,0002,500
Additional affiliations
March 2010 - present
Monash University (Australia)
Position
  • Professor
March 2010 - September 2016
Monash University (Australia)
Position
  • Professor; Head of Department
July 2001 - March 2010
University of Melbourne
Position
  • Professor (Associate)

Publications

Publications (533)
Chapter
Capturing evolution of density or void ratio during the compaction of geomaterials (soils and unbound granular materials) is essential for improved performance. This study developed a framework where the density evolution during compaction can be estimated using advanced instrumentation. The framework’s suitability was validated using a simulated l...
Article
The moisture content level during compaction plays a crucial role in the performance of road pavements. Current state-of-the-art methods involve manually taking small samples from a few isolated locations, making it difficult to monitor the spatial variation in moisture content along the entire construction corridor during compaction in an economic...
Article
Full-text available
L-band brightness temperature (TB) has been shown to provide the best sensitivity to soil moisture (SM) although C- and X-band based products offer a longer time-series from satellite-based measurements. Currently, global coverage SM is routinely produced from spaceborne measurements using all three frequency bands, but despite continued validation...
Article
Floods are the most commonly occurring natural disaster, with the Centre for Research on the Epidemiology of Disasters 2021 report on “The Non-COVID Year in Disasters” estimating economic losses worth over USD 51 million and over 6000 fatalities in 2020. The hydrodynamic models which are used for flood forecasting need to be evaluated and constrain...
Article
The measurement of density or void ratio during the compaction of geomaterials (soils and unbound granular materials) in the field during road construction is essential for superior performance. The specifications adopted by the road authorities worldwide are exclusively based on density. However, estimating density evolution proximally or non-dest...
Preprint
Full-text available
Floods are the most commonly occurring natural disaster, with the Centre for Research on the Epidemiology of Disasters 2021 report on “The Non-COVID Year in Disasters” estimating economic losses worth over USD 51 million and over 6000 fatalities in 2020. The hydrodynamic models which are used for flood forecasting need to be evaluated and constrain...
Article
Full-text available
A capability for mapping meter-level resolution soil moisture with frequent temporal sampling over large regions is essential for quantifying local-scale environmental heterogeneity and eco-hydrologic behavior. However, available surface soil moisture (SSM) products generally involve much coarser grain sizes ranging from 30 m to several 10 s of kil...
Article
Full-text available
This paper introduces a Theory-Guided Machine Learning (TGML) framework, which combines a theoretical model (TM) and a machine learning (ML) algorithm to predict compaction density under cyclic loading. Several 1-D tests were conducted on uniformly-graded fine sand compacted at varying moisture contents (w), stress levels (σ_v) and loading frequenc...
Article
Context. Accurate prediction of within-field crop yield in response to spatial and temporal variability provides essential information for farm managers to improve productivity and ensure optimal use of inputs. Understanding yield spatial and temporal variability cannot be solely addressed by crop modelling or remote sensing but by integrating the...
Article
Full-text available
This paper presents a soil moisture retrieval scheme from Cyclone Global Navigation Satellite System (CYGNSS) delay-Doppler maps (DDMs) over land. The proposed inversion method consists of a hybrid global and local optimization method and a physics-based bistatic scattering forward model. The forward model was developed for bare-to-densely vegetate...
Article
Full-text available
Abstract: The feasibility of soil moisture retrieval from C-band Sentinel-1 data has been widely acknowledged, with pre-operational 1-km products currently available at regional and/or continental scale using the long-term (LTCD) or short-term change detection (STCD) methods. Both algorithms share the same assumptions of time-invariant roughness an...
Article
Glacier surface velocity is an important variable for glacier dynamics studies. Estimation of accurate surface velocity from remote sensing is a challenge, especially for glaciers with no in-situ observations. To overcome this challenge, a new method for glacier feature tracking named as Spatially varying WIndow based maximum likelihood Feature Tra...
Article
Full-text available
It has been over ten years since the successful launch of the first-ever dedicated satellite for global soil moisture monitoring; Soil Moisture and Ocean Salinity (SMOS). Looking towards the future, P-band (0.3–1 GHz) is a promising technique to replace or enhance the L-band (1.4 GHz) SMOS and SMAP (Soil Moisture Active Passive) missions because of...
Article
Full-text available
L-band passive microwave remote sensing is currently considered a robust technique for global monitoring of soil moisture. However, soil roughness complicates the relationship between brightness temperature and soil moisture, with current soil moisture retrieval algorithms typically assuming a constant roughness parameter globally, leading to a pot...
Article
Full-text available
Precision agriculture is experiencing substantial development through the improved availability of cost-effective instruments for data collection. This includes ground-based proximal sensing technologies that are able to compete with satellite and aircraft observation systems, due to low operational costs, high operational flexibility, and high spa...
Article
Full-text available
This paper extends the application of an existing change-detection-based, time-series soil moisture retrieval algorithm to non-concurrent active and passive measurements from WindSat/AMSR2 and the Soil Moisture Active Passive radar, which was active from late April until mid-July on 2015. A time-series of L-band radar backscatter observations was u...
Article
Measuring the microwave brightness temperature (TB) using a radiometer is important for estimating soil moisture (SM). However, no study has demonstrated the effect of environmental conditions on these measurements. With this technology being explored for use in precision agriculture, indoor application and/or utilization in environments close to b...
Article
Full-text available
Optimised farm crop productivity requires careful management in response to the spatial and temporal variability of yield. Accordingly, combination of crop simulation models and remote sensing data provides a pathway for providing the spatially variable information needed on current crop status and the expected yield. An ensemble Kalman filter (EnK...
Poster
Full-text available
Abstract: The climate emergency has exacerbated catastrophic flooding and put an unprecedented number of people at risk. Accurate information on flood inundation is therefore critical to bolster preparedness and reduce flood related losses. Satellite-based Synthetic Aperture Radar (SAR) sensors with all-day/all-weather imaging capabilities, provide...
Article
Remote estimation of leaf nitrogen content is a critical requirement for precision farm management. Precise knowledge of nitrogen distribution in the crop enables farmers to decide the fertilisation amount required at specific locations on the farm. Importantly, nitrogen related molecules in plants are transported using water molecules, and water m...
Article
Vegetation canopy height is one of the important remote sensing parameters related to forests’ structure, and it can be related to the biomass and the carbon stock. Global Navigation Satellite System - Reflectometry (GNSS-R) has proven capable to retrieve vegetation information at a moderate resolution from space (20-65 km) using L1 C/A signals. In...
Article
Full-text available
In 2009, the International Soil Moisture Network (ISMN) was initiated as a community effort, funded by the European Space Agency, to serve as a centralised data hosting facility for globally available in situ soil moisture measurements (Dorigo et al., 2011b, a). The ISMN brings together in situ soil moisture measurements collected and freely shared...
Article
Full-text available
NASAs Soil Moisture Active Passive (SMAP) mission has been validating its soil moisture (SM) products since the start of data production on March 31, 2015. Prior to launch, the mission defined a set of criteria for core validation sites (CVS) that enable the testing of the key mission SM accuracy requirement (unbiased root-mean-square error <0.04 m...
Article
Increasing urban green spaces and canopy cover requires careful planning of irrigation strategies, especially in arid and semiarid areas. This study investigates how vegetation cover and irrigation affect the water balance and vegetation productivity of a small urban reserve in the Melbourne metropolitan area, Australia. Using a mechanistic ecohydr...
Article
Full-text available
In August 2020, SMAP released a new version of its soil moisture (SM) and vegetation optical depth (VOD) retrieval products. In this work, we review the methodology followed by the SMAP regularized dual-channel (DCA) retrieval algorithm. We show that the new implementation generated SM retrievals that not only satisfy the SMAP accuracy requirements...
Article
Altimetric performance of Global Navigation Satellite System - Reflectometry (GNSS-R) instruments depends on receiver’s bandwidth and signal-to-noise ratio (SNR). The altimetric delay is usually computed from the time difference between the peak of the direct signal waveform and the maximum of the derivative of the reflected signal waveform. Dual-f...
Article
Full-text available
Early-stage vegetation water stress Pure-pixel narrowband water-sensitive vegetation indices Drone-based hyperspectral imaging Leaf water content A B S T R A C T Remotely sensed estimation of leaf water content (LWC) using optical data at early crop growth stage is important for identification of water-stressed plants. However, its accurate estimat...
Article
This study presents an assessment of a pre-operational soil moisture product at 1 km resolution derived from satellite data acquired by the European Radar Observatory Sentinel-1 (S-1), representing the first space component of the Copernicus program. The product consists of an estimate of surface soil volumetric water content Θ [m 3 /m 3 ] and its...
Article
This study presents the first evaluation of using commercial microwave link (CML) data for rainfall measurements in Australia, with the test site being the greater Melbourne Metropolitan area. More than 100 CMLs with microwave frequency ranging between 10 and 40 GHz have been used for the rainfall retrieval. The 15-minute received signal levels (RS...
Article
Full-text available
The moisture retrieval depth is commonly held to be the approximately top 5 cm at L-band (~21-cm wavelength/1.41 GHz), which is seen as a limitation for hydrological applications. A widely held view is that this moisture retrieval depth increases with wavelength, ranging approximately from one-tenth to one-fourth of the wavelength. Accordingly, P-b...
Article
Fresh water is considered to be one of the most precious natural resources in many countries. Given that soil moisture plays a salient role in vegetation growth, the continuous and timely measurement of soil moisture content is critical to manage water shortages, especially in the agriculture sector [1] , [2] , which is the largest water use se...
Preprint
Full-text available
NASA’s Soil Moisture Active Passive (SMAP) mission has been validating its soil moisture (SM) products since the start of data production on March 31, 2015. Prior to launch, the mission defined a set of criteria for core validation sites (CVS) that enable the testing of the key mission SM accuracy requirement (unbiased root-mean-square error <0.04...
Preprint
Full-text available
NASA’s Soil Moisture Active Passive (SMAP) mission has been validating its soil moisture (SM) products since the start of data production on March 31, 2015. Prior to launch, the mission defined a set of criteria for core validation sites (CVS) that enable the testing of the key mission SM accuracy requirement (unbiased root-mean-square error <0.04...
Conference Paper
Full-text available
A novel proposal to create probabilistic attenuation nowcasting as a by-product from ensembles of rainfall forecasts is presented in this study. These attenuation nowcasts may eventually be used by mobile network operators to dynamically adjust their wireless network operations in advance and during heavy and extreme rainfall events. It may also fa...
Article
Full-text available
Microwave radiometry has provided valuable spaceborne observations of Earth's geophysical properties for decades. The recent SMOS, Aquarius, and SMAP satellites have demonstrated the value of measurements at 1400 MHz for observing surface soil moisture, sea surface salinity, sea ice thickness, soil freeze/thaw state, and other geophysical variables...
Article
Full-text available
The dynamics of Leaf Area Index (LAI) from space is key to identify crop types and their phenology over large areas, and to characterize spatial variations within growers' fields. However, for years remote-sensing applications have been constrained by a trade-off between the spatial and temporal resolutions. This study resolves this limitation. Ove...
Article
Full-text available
Leaf Area Index (LAI) is one of the most important biophysical properties of a crop, used in detecting long-term water stress, estimating biomass, and identifying crop growth stage. Remote sensing based LAI estimation techniques perform well for early growth stages but tend to produce high error during the crop reproductive stage due to canopy clos...
Article
In this paper, we propose a novel approach to produce attenuation forecasts for microwave links using a probabilistic approach. It uses ensembles of forecast rainfall fields to easily derive attenuation forecasts for specific frequencies. The proposed approach uses the Short Term Ensemble Prediction System (STEPS) to generate ensembles of high, spa...
Article
Full-text available
The monitoring of wildfire smoke is important to help mitigate impacts on people such as by sending early warnings to affected areas. Received signal levels (RSLs) from radio links have been used as an opportunistic way to accurately measure rainfall and humidity. Radio links provide integrated measurements along their paths and are an exceptional...
Article
Full-text available
p>The following authors were omitted from the original version of this Data Descriptor: Markus Reichstein and Nicolas Vuichard. Both contributed to the code development and N. Vuichard contributed to the processing of the ERA-Interim data downscaling. Furthermore, the contribution of the co-author Frank Tiedemann was re-evaluated relative to the co...
Article
Full-text available
Global Navigation Satellite System—Reflectometry (GNSS-R) has already proven its potential for retrieving a number of geophysical parameters, including soil moisture. However, single-pass GNSS-R soil moisture retrieval is still a challenge. This study presents a comparison of two different data sets acquired with the Microwave Interferometer Reflec...
Article
Full-text available
In this paper, an eight‐way substrate integrated waveguide unequal power divider with an enhanced co‐phase bandwidth is presented, and demonstrated a potential application for the Ku‐band radiometer antennas. The proposed divider design is based on the −35 dB Chebyshev beam‐shaping algorithm to achieve at minimum −25 dB of antenna side‐lobe level (...
Article
Full-text available
Accurate flood inundation forecasts have the potential to minimize socioeconomic losses, but uncertainties in inflows propagated from the precipitation forecasts result in large prediction errors. Recent studies suggest that by assimilating independent flood observations, inherent uncertainty in hydraulic flood inundation modelling can be mitigated...
Article
Full-text available
Satellite observations of flood inundation have the potential to increase the reliability and accuracy of flood forecasts, thereby contributing to improved flood resilience of vulnerable populations. However, new generation high-resolution satellites can only observe small portions of large river systems river during a flood. Since, the model-data...
Article
Soil moisture information is important for a wide range of applications including hydrologic modelling, climatic modelling and agriculture. L-band passive microwave satellite remote sensing is the most feasible option to estimate near-surface soil moisture (~0-5 cm soil depth) over large extents, but its coarse resolution (~10s of km) means that it...
Preprint
Full-text available
In 2009, the International Soil Moisture Network (ISMN) was initiated as a community effort, funded by the European Space Agency, to serve as a centralised data hosting facility for globally available in situ soil moisture measurements (Dorigo et al., 2011a, b). The ISMN brings together in situ soil moisture measurements collected and freely shared...
Chapter
Accurately simulating floodplain inundation is absolutely vital to minimize damage to life and property. The giant strides made in advanced computing, now allow running increasingly complex models at reasonable resolutions over large areas, with the promise of further improvement in the near future. However, the uncertainty contributed by input, bo...
Preprint
Full-text available
Soil moisture impacts exchanges of water, energy and carbon fluxes between the land surface and the atmosphere. Passive microwave remote sensing at L-band can capture spatial and temporal patterns of soil moisture in the landscape. Both ESA and NASA have launched L-band radiometers, in the form of the SMOS and SMAP satellites respectively, to monit...
Article
Full-text available
Soil moisture observations are of broad scientific interest and practical value for a wide range of applications. The scientific community has made significant progress in estimating soil moisture from satellite-based Earth observation data, particularly in operationalizing coarse-resolution (25-50 km) soil moisture products. This review summarizes...