Todhia Proposal and technical Work

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Broad Aims

The overall broad aim of the isotope work to be completed at Todhia is to use the isotope measurements to provide additional information about the local hydrological cycle that standard measurements can not. The obvious area of interest is partitioning ET into component fluxes. The focus will be to determine the added value for the exchange of water between the crop ecosystem and the atmosphere. UAV’s/Quadcopter (surface temperature, leaf water content)?? Beyond this, I will be aiming to collect a dataset that can be used to test isotopic models and potential constrain other hydrological models suing the isotopic observations. Some of these models relate to:

  • leaf water transport

  • transport of the water vapour from the leaf, through the leaf boundary layer, canopy, roughness layer and into the turbulent atmosphere (C-G model and resistance parameterisation)

  • evaporation from the soil - resistance parameterisation and testing C-G model

To address these aims, data that provides information about the transport of energy, momentum and water between the crop and the atmosphere is required. In particular profile measurements made from a tower will provide the basis of our measurement campaign - using Eddy covariance measurements, and vertically resolved meteorology and isotopes. The profile measurements should be complemented by additional measurements of leaf water content, water input (and isotopes), leaf water isotopes, soil water isotopes (vertically resolved). A potential interesting application could be to use the UAV or Quadcopter to measure leaf water content and leaf temperature and determine how an flux weighted area average improves the representation of isotopes in C-G model and additional parameterisation.


  • Arid land agriculture

    • How much of the worlds agriculture

    • Problems - water, soil quality, heat

    • predicted productivity effects of declining water availability/quality and climate

    • what has been done investigating arid land agriculture

    • particular problems - efficient water use (ET partitioning), models not tested or developed

  • Water isotopes in agricultural hydrology

    • Why are hydrologists interested in water isotopes?

      • source partitioning when fluxes occur simultaneously

      • evaporation conditions

      • water transport through the hydrological cycle

    • How can isotopes help in agriculture?

      • ET partitioning

      • water transport in soils

      • constraining evaporation models

  • ET Partitioning

    • Why would we want to do it?

      • Important for land surface models - better constraint then total ET

      • for water use efficiency

      • water management

    • Classical assumptions and why it may work

    • Are the classical assumptions observed?

    • How has it been applied successfully? What holes these works have?

    • Why this would be important for arid land agriculture?

    Testing isotope models of ET

    Central to any isotope representation of ET is the C-G model (Craig 1965)

    \label{eq:1} \[\delta_E = \bigg[\frac{\alpha_e\delta_L - h\delta_a - \epsilon_e - \Delta\epsilon}{(1 - h)}\bigg]\]
    The evaporation model describes the isotopic fractionation between the liquid water surface and the water vapor transported to the free atmosphere. It includes equilibrium effects at the water surface (reflects the temperature at the evaporating surface) and the kinetic effects related to the diffusion of water vapour from the liquid surface to the atmosphere above. The kinetic fractionation term (\(\Delta\epsilon\)), includes the parameterisation of the relative importance of turbulence and molecular diffusion for the transport from the diffusive sub-layer to the free atmosphere above (turbulent transport is assumed to be non-fractionating and molecular diffusion is). For modeling the isotope ratios of ET fluxes, there are number areas of uncertainty in the parameterisation of the model:

    • temperature of the evaporating surface - hard to measure and can have a large effect on attempts to partition ET using isotopes (Dubbert 2013).

    • The height of \(\delta\)_a - normally just assumed to be the measurement height

    • The \(\delta\)_L definition - where is the evaporating surface in the soil? What is the isotopic composition of the evaporating leaf water?

    • the parameterisation of kinetic fractionation factor

    • steady state vs non-steady state plant transpiration

      • what time scales does the system begin to reflect steady state

      • how non-steady state transpiration should be modeled - parameterising leaf water turnover rate and measurement of leaf water content (UAV’s?)

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