Development of a continuous soil moisture accounting procedure for curve number methodology and its behaviour with different evapotranspiration methods
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Abstract The curve number (CN) method is widely used for rainfall–runoff modelling in continuous hydrologic simulation models. A sound continuous soil moisture accounting procedure is necessary for models using the CN method. For shallow soils and soils with low storage, the existing methods have limitations in their ability to reproduce the observed runoff. Therefore, a simple one‐parameter model based on the Soil Conservation Society CN procedure is developed for use in continuous hydrologic simulation. The sensitivity of the model parameter to runoff predictions was also analysed. In addition, the behaviour of the procedure developed and the existing continuous soil moisture accounting procedure used in hydrologic models, in combination with Penman–Monteith and Hargreaves evapotranspiration (ET) methods was also analysed. The new CN methodology, its behaviour and the sensitivity of the depletion coefficient (model parameter) were tested in four United States Geological Survey defined eight‐digit watersheds in different water resources regions of the USA using the SWAT model. In addition to easy parameterization for calibration, the one‐parameter model developed performed adequately in predicting runoff. When tested for shallow soils, the parameter is found to be very sensitive to surface runoff and subsurface flow and less sensitive to ET. Copyright © 2007 John Wiley & Sons, Ltd.Cite
There are currently several electromagnetic sensors commercially available for determining in situ moisture content. All of these sensors work in a similar manner. A known electrical signal is sent into the soil. The signal measured back from the soil is expressed in the form of a complex dielectric constant that is directly related to the amount of water present in the soil. As the real component of the dielectric constant of water is four to eight times greater than most soils, changes in water content directly affect the sensor output. Soil calibration models specific to the various soil types are developed by plotting measurements of the real component versus gravimetric moisture contents. The subsequent linear regression line then becomes the soil-specific model. The focus of this study was to compare linear calibration models developed for various soil samples, with various geotechnical properties. The intent was to develop a generalized calibration model for the electromagnetic sensor. The generalized model will allow moisture content to be determined in situ, regardless of the soil type. The results of this study was the development of a generalized soil model that predicts the in situ moisture content for fine grain soils as well as the soil-specific models. In general, this study provides a framework for developing methodology to predict other in situ geotechnical parameters from measurements of the complex dielectric constant.
Gravimetric analysis
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An instrument for making rapid measurements of the soil moisture content in the root zone is an essential tool for many applications, including understanding of soil water dynamics, evaluation of agriculture water stress, and validation of soil moisture modeling. Studies have shown that electrical resistance measurements may be used to infer soil moisture content under special circumstances. In this paper, electrical resistivity (resistance multiplied by a geometric factor) measurements of the soil by the Geometrics Inc. OhmMapper instrument are compared with point measurements of soil moisture to a depth of 70 cm. It was found that the OhmMapper resistivity measurements could be used to infer soil moisture content with a coefficient of determination as high as 0.34 when a simple power law relationship was used. A more sophisticated analysis of the resistivity measurements could potentially lead to a greater coefficient of determination.
Pedotransfer function
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Summary Moisture content and bulk density largely characterize physical and mechanical soil status and behaviour. A nondestructive determination of these soil properties is essential. Time domain reflectometry (TDR), although widely accepted for determination of volumetric water content, θ , has its limitations, and recently a frequency domain (FD) sensor has been developed and tested. An equation relating relative permittivity, ɛ′, to gravimetric water content, w , and bulk density, p , was established for three soil types (sand, sandy loam and clay). If ɛ′ and w are known, our model can be used to calculate bulk density and associated volumetric water content, θ , keeping in mind that θ= pw. Utilization is found in long‐term monitoring of moisture fluctuations or short‐term detection of traffic‐induced soil compaction.
Gravimetric analysis
Reflectometry
Water retention curve
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Summary Time‐domain reflectometry (TDR) is being used increasingly for measuring the moisture content of porous media. However, successful application for measuring water in soil has been limited to non‐deformable soils, and it would be a valuable extension of the technique if it could be used for soils that shrink on drying. We have recently investigated its application to soils rich in clay and organic matter and peats. Here we propose a method for determining moisture content in deformable soils based on the relation between the dielectric constant, K , and the volumetric moisture content, Θ, measured by TDR. Parallel TDR probes with a length of 15 cm and a spacing of 2 cm were placed horizontally in soil cores with a diameter of 20 cm and height of 10 cm taken from a forest. The soil is very porous with large proportions of both silt and clay. The sample weight and travel time of the electromagnetic wave guided by parallel TDR probes were simultaneously measured as a function of time, from saturation to oven‐dryness during which the core samples shrank considerably. Vertical and horizontal components of shrinkage were also measured to take the air‐exposed region of TDR probe into account in the determination of K . The effect of deformation on volumetric moisture content was formulated for two different expressions, namely actual volumetric moisture content (AVMC) and fictitious (uncorrected) volumetric moisture content (FVMC). The effects of air‐exposure and expressions of volumetric moisture content on the relation between K and Θ were examined by fitting the observations with a third‐order polynomial. Neglecting the travel time in the air‐exposed part or use of the FVMC underestimated the Θ for a given K . The difference was more pronounced between AVMC and FVMC than between two different dielectric constants, i.e. accounting for air‐exposure, K ac , and not accounting for air‐exposure, K au . When the existing empirical models were compared with the fitted results, most underestimated the relation based on the AVMC. This indicates that published empirical models do not reflect the effect of deformation on the determination of Θ in our forest soil. Correct use of the Θ expression has more impact on determining moisture content of a deformable soil than the accommodation of travel time through the air‐exposed region of TDR probe.
Reflectometry
Silt
Shrinkage
Saturation (graph theory)
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Pedotransfer function
Permanent wilting point
Biometeorology
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Pedotransfer function
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Understanding soil moisture variability and its relationship with water content at various scales is a key issue in hydrological research. In this paper we predict this relationship by stochastic analysis of the unsaturated Brooks‐Corey flow in heterogeneous soils. Using sensitivity analysis, we show that parameters of the moisture retention characteristic and their spatial variability determine to a large extent the shape of the soil moisture variance‐mean water content function. We demonstrate that soil hydraulic properties and their variability can be inversely estimated from spatially distributed measurements of soil moisture content. Predicting this relationship for eleven textural classes we found that the standard deviation of soil moisture peaked between 0.17 and 0.23 for most textural classes. It was found that the β parameter, which describes the pore‐size distribution of soils, controls the maximum value of the soil moisture standard deviation.
Pedotransfer function
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