J. D. Kalma

One-dimensional soil moisture profile retrieval by assimilation of near-surface observations: a comparison of retrieval algorithms

This paper investigates the ability to retrieve the true soil moisture and temperature profiles by assimilating near-surface soil moisture and surface temperature data into a soil moisture and heat transfer model. The direct insertion and Kalman filter assimilation schemes have been used most frequently in assimilation studies, but no comparisons of these schemes have been made. This study investigates which of these approaches is able to retrieve the soil moisture and temperature profiles the fastest, over what depth soil moisture observations are required, and the effect of update interval on profile retrieval. These questions are addressed by a desktop study using synthetic data. The study shows that the Kalman filter assimilation scheme is superior to the direct insertion assimilation scheme, with retrieval of the soil moisture profile being achieved in 12 h as compared to 8 days or more, depending on observation depth, for hourly observations. It was also found that profile retrieval could not be realised for direct insertion of the surface node alone, and that observation depth does not have a significant effect on profile retrieval time for the Kalman filter. The observation interval was found to be unimportant for profile retrieval with the Kalman filter when the forcing data is accurate, whilst for direct insertion the continuous Dirichlet boundary condition was required for an increasingly longer period of time. It was also found that the Kalman filter assimilation scheme was less susceptible to unstable updates if volumetric soil moisture was modelled as the dependent state rather than matric head, because the volumetric soil moisture state is more linear in the forecasting model.

One-Dimensional Soil Moisture Profile Retrieval by Assimilation of Near-Surface Measurements: A Simplified Soil Moisture Model and Field Application

The Kalman filter assimilation technique is applied to a simplified soil moisture model for retrieval of the soil moisture profile from near-surface soil moisture measurements. First, the simplified soil moisture model is developed, based on an approximation to the Buckingham‐Darcy equation. This model is then used in a 12month one-dimensional field application, with updating at 1-, 5-, 10-, and 20-day intervals. The data used are for the Nerrigundah field site, New South Wales, Australia. This study has identified (i) the importance of knowing the depth over which the near-surface soil moisture measurements are representative (i.e., observation depth), (ii) soil porosity and residual soil moisture content as the most important soil parameters for correct retrieval of the soil moisture profile, (iii) the importance of a soil moisture model that represents the dominant soil physical processes correctly, and (iv) an appropriate forecasting model as far more important than the temporal resolution of near-surface soil moisture measurements. Although the soil moisture model developed here is a good approximation to the Richards equation, it requires a root water uptake term or calibration to an extreme drying event to model extremely dry periods at the field site correctly.

Estimating evaporation from pasture using infrared thermometry: evaluation of a one-layer resistance model

A one-layer resistance model has been used with infrared thermometry to estimate sensible and latent heat flux in pastures near Goulburn, New South Wales. The model compares reasonably well with energy balance-Bowen ratio measurements. However, the relative error in the evaporation estimates becomes significant in very dry conditions and at low net radiation values. An aerodynamic surface temperature may be computed from independent sensible heat flux measurements, air temperature measurements and estimates of aerodynamic resistance. The differences between computed and observed surface temperatures show characteristic diurnal trends and vary between days of measurement. These differences may be caused by errors in measuring the sensible heat flux and the surface temperature and in estimating the aerodynamic resistance. The pasture data obtained in this study are used to assess such uncertainties. The major cause for differences between the computed and observed surface temperatures is considered to be the incompleteness of the vegetative cover during each of the experimental periods. The measured Ts values apply to both the vegetation and the soil surface, whereas the observed T0 values are based on a model which assumes complete cover and only considers foliage temperature. Finally, the two-layer model of Shuttleworth and Wallace is used to show that the relationship between the measured infrared surface temperature and the canopy air temperature depends on the aerodynamic resistances, the fractional vegetated area and the temperature differences between foliage, canopy air and soil surface.

A disaggregation scheme for soil moisture based on topography and soil depth

This paper reports on a new soil moisture disaggregation scheme based on topography and soil depth information. It is designed for low resolution remote sensing data assimilation into hydrological modelling. The scheme makes use of a simple Soil Vegetation Atmosphere Transfer model coupled to the TOPMODEL formalism. Water and energy balance are computed at the catchment scale, taking lateral flows due to topography into account. Lumped values of near-surface and deep soil water content are then disaggregated at local scale using simple relationship between mean quantities, local topography and soil depth information. Results for a small water catchment in South-eastern Australia show satisfactory reproduction of the local soil moisture patterns using a combination of topography and soil depth information. Due to subgrid variability and differences between the simulation and observation scale (the Digital Elevation Model pixel versus the point measurement), the point-to-point comparison between observations and simulations shows a poor correlation. Rescaling shows that a good correlation is obtained when averaging the simulated and observed soil moisture over a length of 100 m.

An evaluation of the benefits of source control measures at the regional scale

Source control measures include rainwater tanks, infiltration trenches, grassed swales, detention basins and constructed wetlands that can be used in housing allotments and subdivisions. A methodology for evaluating the regional economic benefits due to implementation of source control measures is presented and illustrated for two case studies in the Lower Hunter and Central Coast regions of New South Wales, Australia. It is demonstrated that use of rainwater tanks to supplement mains water supply for toilet, hot water and outdoor uses can very significantly reduce demand on mains water supply. Reductions in regional water demand will enable deferment of water supply headworks augmentation, while reductions in peak mains water demand will extend the life of water supply distribution infrastructure. In addition, substantial reduction of stormwater discharge from allotments can be realised. For the Lower Hunter region with an urban population of about 450,000 it is shown that construction of new water supply headworks infrastructure can be delayed by up to 34 years. Compared with the traditional provision of mains water and stormwater disposal, the use of rainwater tanks along with other source control measures can produce present worth savings to the Lower Hunter region conservatively estimated to be up to

Multi-parameter fingerprinting of sediment deposition in a small gullied catchment in SE Australia

67 million. Similar results were found for the Central Coast region.

Goulburn River experimental catchment data set

The determination of relative contributions of potential sediment sources is an important step in the development of management strategies to combat soil erosion. In a 1.2 km2 gullied catchment in southeastern New South Wales, multi-parameter fingerprinting of sediment deposited in successive downstream pools has identified gully walls as the dominant sediment source when the grazed pasture surface was the only other potential source. The median fractional contributions remained relatively steady in the successive downstream pools, with the gully walls responsible for between 90% and 98% of the pool sediment. This result was achieved despite the ratio of the source areas varying considerably between successive nested subareas. Reliability bounds on the predictions, accounting for limited sampling of sources, were well constrained and varied between 5.4% and 13.8%. Downstream of an unsealed road crossing, sediment from the road source dominated the pool sediments such that contributions from the pasture surface and gully sources could not be determined.

Importance of soil moisture measurements for inferring parameters in hydrologic models of low-yielding ephemeral catchments

(651 km 2 ) and Krui (562 km 2 ) subcatchments in the northern half of this experimental catchment with a few monitoring sites located in the south. The data set comprises soil temperature and moisture profile measurements from 26 locations; meteorological data from two automated weather stations (data from a further three stations are available from other sources) including precipitation, atmospheric pressure, air temperature and relative humidity, wind speed and direction, soil heat flux, and up- and down-welling shortand long-wave radiation; streamflow observations at five nested locations (data from a further three locations are available from other sources); a total of three surface soil moisture maps across a 40 km � 50 km region in the north from � 200 measurement locations during intensive field campaigns; and a high-resolution digital elevation model (DEM) of a 175-ha microcatchment in the Krui catchment. These data are available on the World Wide Web at http://www.sasmas.unimelb.edu.au.

The Nerrigundah Data Set: Soil moisture patterns, soil characteristics, and hydrological flux measurements

Low-yielding catchments with ephemeral streams provide a stern test of the capability of conceptual catchment models for predicting the hydrologic response of the natural landscape. Sustained periods of little or no flow mean that the information content of the streamflow time-series for parameter estimation is limited. During periods with no streamflow, such ephemeral catchments also offer no information on a catchment’s soil moisture status. As a result, parameters estimated solely from streamflow data are often poorly identified and span a wide range of the feasible parameter space. These general observations were confirmed by an application of the conceptual VIC model in a 6 ha experimental catchment in eastern Australia. Using a Monte Carlo style assessment of parameter uncertainty, it was shown that the simple three-parameter model was ill-posed when calibrated solely to the streamflow response. Failure of the calibration procedure to distinguish unique antecedent moisture storage conditions prior to large rainfall events meant that the observed streamflow response could be replicated from a large envelope of potential parameter combinations. The inclusion of an estimated time-series index of areal soil moisture status into the calibration procedure, however, significantly reduced the number of feasible parameter combinations, and resulted in predictions that confirmed Bowen ratio measurements of actual evapotranspiration. Attempts to further reduce parameter uncertainty by including the measured evapotranspiration data into the joint calibration procedure were unsuccessful. The shortness of the measurement record was seen as a major factor inhibiting improvement. The results of this study highlight the critical importance of antecedent moisture conditions on streamflow yields in ephemeral catchments and point to the desirability of spatio-temporal soil moisture accounting. Future research efforts are discussed in terms of establishing the appropriate spatial and temporal resolution of soil moisture measurements needed to extend the results observed for this small experimental study to larger catchments.  2002 Elsevier Science Ltd. All rights reserved.

Hydrological implications of the Southern Oscillation: variability of the rainfall-runoff relationship

This paper presents a data set that describes the spatial and temporal variability of soil moisture within the 6 ha Nerrigundah catchment, located in a temperate region of eastern Australia. The data set includes high-resolution elevation data; high- resolution (20 m) near-surface soil moisture maps; soil moisture profile measurements at 13 locations, with one being applicable for one-dimensional modeling; soil moisture measurements from four different measurement devices at a single location; soil temperature profile measurements; soil heat flux and supporting meteorological measurements, including data obtained with two pluviometers and four collecting rain gauges; surface roughness measurements; soil information for 19 locations, including field measurements of saturated hydraulic conductivity; and catchment runoff measurements. These data are available on the World Wide Web at http://www.civag.unimelb.edu.au/ jwalker/data/nerrigundah.