Spatial variability of surface properties and estimation of surface fluxes of a savannah

doi:10.1016/S0168-1923(97)00061-0

Agricultural and Forest Meteorology

Volume 89, Issue 1, January 1998, Pages 15-44

https://doi.org/10.1016/S0168-1923(97)00061-0 Get rights and content

Abstract

During the HAPEX-Sahel experiment in 1992, a data set including atmospheric forcing, soil temperature at several depths, surface fluxes and surface soil moisture from 0 to 15 cm was collected on a degraded fallow savannah over a period of 18 days. The SiSPAT SVAT model was calibrated using this data set to provide a realistic reference set of parameters. The sensitivity of surface fluxes to the specification of surface properties based on this reference set of parameters was quantified focusing on runoff, evapotranspiration and soil moisture at the field scale. Runoff and latent heat flux, predominantly bare soil evaporation, were found to be the most sensitive processes in relation to soil parameters. For transpiration, even with such a sparse vegetation, leaf area index was the most sensitive factor. A stochastic approach was used to analyze the sensitivity of surface fluxes to the spatial variability of surface parameters. For a variation of ±50% of the parameters, no significant bias was obtained between the mean of the stochastic simulations and the 1-D simulation performed with the median values of the parameters. The analysis indicates that the variations of the components of the water budget are linearly related. For larger variations of the parameters, the bias is significant; therefore, simple aggregation rules fail to capture the nonlinearities induced by water transfer into the soil. When diurnal cycles are considered, the standard deviation of bare soil evaporation and surface soil moisture was found to be maximum for the intermediate wetting range. For this period, it would be valuable to parameterize the spatial variability of surface properties into larger scale models. Finally, the SiSPAT model, which solves equations derived from the Richards equation [Richards, L.A., 1931. Capillary conduction of liquids through porous mediums. J. Phys. 1, pp. 318–333.]. for soil water distribution is shown to be very sensitive to the specification of soil parameters. This result would hold for similar models, showing that the Richards equation should be used with caution within large-scale models if a robust estimation of the long- term water budget is to be obtained.

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A series of numerical experiments has been designed to investigate how effective satellite estimates of radiometric surface temperatures and soil surface moisture are for calibrating a Soil–Vegetation–Atmosphere Transfer (SVAT) model. Multi–objective calibration based on error minimization of temperature and soil moisture model outputs is performed in a semi–arid environment. Model accuracy when calibrated using in situ versus satellite objectives is explored in detail. Observational meteorological datasets from the African Monsoon Multidisciplinary Analysis (AMMA) were used to force a column model during a growing season in Mali. Fourier Amplitude Sensitivity Test (FAST) revealed the most sensitive parameters to model outputs. Parameters found sensitive were subsequently optimized in a series of model calibrations to reveal trade-offs between model objectives. Our main findings are (1) the SVAT model performs well in the semi–arid environment, but underestimates peak growing season evapotranspiration and overestimates soil moisture, (2) most of the parameters important for flux estimates can be constrained using surface temperature and soil surface moisture with the three exceptions: root depth, the extinction coefficient and unstressed stomatal resistance, (3) flux simulations are improved when the model is calibrated using in situ surface temperature and soil surface moisture versus satellite estimates.
Assessing the ability of mechanistic volatilization models to simulate soil surface conditions. A study with the Volt'Air model
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Citation Excerpt :
SiSPAT describes in detail the physics of soil surface heat and water transfers: it particularly solves the coupled heat and water transfer equations and describes the water vapor transfers. It has also shown its ability to simulate water transfers under a wide range of soil water contents, including very dry ones (Boulet et al., 1997, 1999; Braud, 1998). This comparison was performed on a reference scenario built from a dataset obtained in a specific field experiment.
Ammonia and pesticide volatilization in the field is a surface phenomenon involving physical and chemical processes that depend on the soil surface temperature and water content. The water transfer, heat transfer and energy budget sub models of volatilization models are adapted from the most commonly accepted formalisms and parameterizations. They are less detailed than the dedicated models describing water and heat transfers and surface status. The aim of this work was to assess the ability of one of the available mechanistic volatilization models, Volt'Air, to accurately describe the pedo-climatic conditions of a soil surface at the required time and space resolution. The assessment involves: (i) a sensitivity analysis, (ii) an evaluation of Volt'Air outputs in the light of outputs from a reference Soil–Vegetation–Atmosphere Transfer model (SiSPAT) and three experimental datasets, and (iii) the study of three tests based on modifications of SiSPAT to establish the potential impact of the simplifying assumptions used in Volt'Air. The analysis confirmed that a 5 mm surface layer was well suited, and that Volt'Air surface temperature correlated well with the experimental measurements as well as with SiSPAT outputs. In terms of liquid water transfers, Volt'Air was overall consistent with SiSPAT, with discrepancies only during major rainfall events and dry weather conditions. The tests enabled us to identify the main source of the discrepancies between Volt'Air and SiSPAT: the lack of gaseous water transfer description in Volt'Air. They also helped to explain why neither Volt'Air nor SiSPAT was able to represent lower values of surface water content: current classical water retention and hydraulic conductivity models are not yet adapted to cases of very dry conditions. Given the outcomes of this study, we discuss to what extent the volatilization models can be improved and the questions they pose for current research in water transfer modeling and parameterization.
Towards an understanding of coupled physical and biological processes in the cultivated Sahel - 1. Energy and water
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This paper presents an analysis of the coupled cycling of energy and water by semi-arid Sahelian surfaces, based on two years of continuous vertical flux measurements from two homogeneous recording stations in the Wankama catchment, in the West Niger meso-site of the AMMA project. The two stations, sited in a millet field and in a semi-natural fallow savanna plot, sample the two dominant land cover types in this area typical of the cultivated Sahel. The 2-year study period enables an analysis of seasonal variations over two full wet–dry seasons cycles, characterized by two contrasted rain seasons that allow capturing a part of the interannual variability. All components of the surface energy budget (four-component radiation budget, soil heat flux and temperature, eddy fluxes) are measured independently, allowing for a quality check through analysis of the energy balance closure. Water cycle monitoring includes rainfall, evapotranspiration (from vapour eddy flux), and soil moisture at six depths.
The main modes of observed variability are described, for the various energy and hydrological variables investigated. Results point to the dominant role of water in the energy cycle variability, be it seasonal, interannual, or between land cover types. Rainfall is responsible for nearly as much seasonal variations of most energy-related variables as solar forcing. Depending on water availability and plant requirements, evapotranspiration pre-empts the energy available from surface forcing radiation, over the other dependent processes (sensible and ground heat, outgoing long wave radiation). In the water budget, pre-emption by evapotranspiration leads to very large variability in soil moisture and in deep percolation, seasonally, interannually, and between vegetation types.
The wetter 2006 season produced more evapotranspiration than 2005 from the fallow but not from the millet site, reflecting differences in plant development. Rain-season evapotranspiration is nearly always lower at the millet site. Higher soil moisture at this site suggests that this difference arises from lower vegetation requirements rather than from lower infiltration/higher runoff. This difference is partly compensated for during the next dry season. Effects of water and vegetation on the energy budget appear to occur more through latent heat than through albedo. A large part of albedo variability comes from soil wetting and drying. Prior to the onset of monsoon rain, the change in air mass temperature and wind produces, through modulation of sensible heat, a marked chilling effect on the components of the surface energy budget.
Development and assessment of an efficient vadose zone module solving the 1D Richards' equation and including root extraction by plants
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From the non iterative numerical method proposed by [Ross, P.J., 2003. Modeling soil water and solute transport—fast, simplified numerical solutions. Agronomy Journal 95, 1352–1361] for solving the 1D Richards' equation, an unsaturated zone module for large scale hydrological model is developed by the inclusion of a root extraction module and a formulation of interception. Two root water uptake modules, first proposed by [Lai, C.-T. and Katul, G., 2000. The dynamic role of rott-water uptake in coupling potential to actual transpiration. Adv. Water Res. 23: 427–439; Li, K.Y., De Jong, R. and Boisvert, J.B., 2001. An exponential root-water-uptake model with water stress compensation. J. Hydrol. 252: 189–204], were included as the sink term in the Richards' equation. They express root extraction as a linear function of potential transpiration and take into account water stress and compensation mechanism allowing water to be extracted in wetter layers.
The vadose zone module is tested in a systematic way with synthetic data sets covering a wide range of soil characteristics, climate forcing, and vegetation cover. A detailed SVAT model providing an accurate solution of the coupled heat and water transfer in the soil and the surface energy balance is used as a reference. The accuracy of the numerical solution using only the SVAT soil module, and the loss of accuracy when using a potential evapotranspiration instead of solving the energy budget are both investigated.
The vadose zone module is very accurate with errors of less than a few percent for cumulative transpiration. Soil evaporation is less accurately simulated as it leads to a systematic underestimation of soil evaporation amounts. The [Lai, C.-T. and Katul, G., 2000. The dynamic role of rott-water uptake in coupling potential to actual transpiration. Adv. Water Res. 23: 427–439] module is not adapted for sandy soils, due to a weakness in the compensation term formulation. When using a potential evapotranspiration instead of the surface energy balance, we evidenced a difference in partitioning the energy between the soil and the vegetation. A Beer-Lambert law is not able to take into account the complex interactions at the soil-vegetation-atmopshere interface. However, under field conditions, the accuracy of the vadose zone module is satisfactory provided that a correct crop coefficient could be defined.
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Spatial variability of surface properties and estimation of surface fluxes of a savannah

Abstract

J. Hydrol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

Agric. For. Meteorol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

J. Hydrol.

Soil Technol.

Conceptual aspects of a statistical-dynamical approach to represent landscape subgrid-scale heterogeneities in atmospheric models

J. Geophys. Res.

Observations of leaf stomatal conductance at the canopy scale: an atmospheric modeling perspective

Bound. Layer Meteorol.

Scaling of the land-atmosphere interactions: an atmospheric modelling perspective

Modélisation de la conductance stomatique de converts végétaux sahéliens-Site Central Ouest, Hapex-Sahel, 1992

Analysis of surface moisture variations within large field sites

Water Resour. Res.

A stochastic approach to studying the influence of the spatial variability of soil hydraulic properties on surface fluxes, temperature and humidity

J. Hydrol.

A Simple Soil-Plant-Atmosphere Transfer model (SiSPAT): development and field verification

J. Hydrol.

Hydraulic Properties of Porous Media

Relative permeability calculation from size distributions data

Trans. AIME

Modélisation des transferts de masse et de chaleur dans le système sol-plante-atmosphère. Influence de la variabilité spatiale des caractéristiques hydrodynamiques du sol

Efficient prediction of ground surface temperature and moisture with inclusion of a layer of vegetation

J. Geophys. Res.

Thermal properties of soils

Heat transfer in soils

Land surface hydrology parameterization for atmospheric general circulation models including subgrid scale spatial variability

J. Clim.

Evapotranspiration and runoff from large land areas: land surface hydrology for atmospheric General Circulations Models