*a*_{x} | Vein cross-sectional area (m^{2}) |

*C* | Molar concentration of liquid water [mol(H_{2}O) m^{–3}] |

*D* | Diffusivity of isotopologues in ‘normal’ water (m^{2} s^{–1}) |

*D*_{air} | Vapour pressure deficit (Pa) |

*D*_{lm} | Effective diffusivity in mesophyll in longitudinal direction, if different from *D*_{m} (m^{2} s^{–1}) |

*D*_{m} | Effective diffusivity in mesophyll (m^{2} s^{–1}) |

*D*_{x} | Effective diffusivity in xylem in radial direction (m^{2} s^{–1}) |

*E* | Transpiration rate [mol(H_{2}O) m^{2} s^{–1}] |

*Ē* | Longitudinal mean transpiration rate [mol(H_{2}O) m^{2} s^{–1}] |

*f*_{em} | Factor relating bulk mesophyll water enrichment with enrichment at the evaporating site |

*f*_{E} | Normalized function for non-uniform transpiration rate in longitudinal direction |

*f*_{min} | Minimum of normalized functions for non-uniform gas-exchange parameters if described with Gaussian profile |

*f*_{g} | Normalized function for non-uniform stomatal conductance in longitudinal direction |

*f*_{T} | Normalized function for non-uniform leaf temperature in longitudinal direction |

*f*_{xx} | Factor relating bulk xylem water enrichment with enrichment at xylem–mesophyll interface |

*g*_{b} | Boundary-layer conductance [mol(air) m^{−2} s^{–1}] |

*g*_{s} | Stomatal conductance [mol(air) m^{−2} s^{–1}] |

*ḡ*_{s} | Longitudinal mean stomatal conductance [mol(air) m^{−2} s^{–1}] |

*g*_{t} | Total conductance [mol(air) m^{−2} s^{–1}] |

*h* | Relative humidity corrected to leaf temperature |

*h*′ | Relative humidity corrected to leaf temperature and fractionations |

*I* | Cumulative transpiration rate from *l* to *l*_{max}[mol(H_{2}O) m^{–1} s^{–1}] |

*I*_{max} | Cumulative transpiration rate from petiole to *l*_{max}[mol(H_{2}O) m^{–1} s^{–1}] |

*k* | Exponent in desert river model including radial diffusion |

*l* | Longitudinal coordinate (m) |

*l*_{max} | Leaf length (m) |

*l*_{opt} | Length at which the mean of normalized functions for non-uniform gas exchange parameters occurs if described with Gaussian profile (m) |

*L*_{m} | Effective length in mesophyll (m) |

*P*_{air} | Air pressure (Pa) |

℘_{l} | Longitudinal Péclet number in xylem |

℘_{m} | Radial Péclet number in mesophyll |

℘_{x} | Radial Péclet number in xylem |

*r* | Radial coordinate (m) |

*r*_{e} | Radial coordinate evaporation sites (m) |

*r*_{m} | Mesophyll half-thickness (m) |

*r*_{max} | Maximum value of *r* (m) |

*r*_{x} | Xylem radius (m) |

| Radial coordinate *r* at xylem radius, in xylem (m) |

| Radial coordinate *r* at xylem radius, in mesophyll (m) |

*R* | Isotope ratio |

*R̄* | Longitudinal mean isotope ratio |

*R*_{C} | Craig–Gordon isotope ratio |

*R*_{e} | Isotope ratio at evaporative site |

*R̄*_{e} | Longitudinal mean isotope ratio at evaporative site |

*R*_{E} | Isotope ratio of transpiration |

*R̄*_{E} | Longitudinal mean isotope ratio of transpiration |

*R*_{lw} | Radial mean bulk isotope ratio (xylem plus mesophyll) |

*R̄*_{lw} | Longitudinal and radial mean bulk isotope ratio (xylem plus mesophyll) |

*R*_{m} | Radial mean mesophyll isotope ratio |

*R̄*_{m} | Longitudinal and radial mean mesophyll isotope ratio |

*R*_{M} | Maximum possible leaf water enrichment isotope ratio |

*R*_{s} | Isotope ratio of source water from the petiole |

*R*_{v} | Isotope ratio of ambient water vapour |

*R*_{x} | Radial mean xylem isotope ratio |

*R̄*_{x} | Longitudinal and radial mean xylem isotope ratio |

*s*_{x} | Distance between lateral veins (m) |

*t* | Time coordinate (s) |

*T*_{air} | Ambient air temperature (K) |

*T*_{leaf} | Leaf temperature (K) |

*T̄*_{leaf} | Longitudinal mean leaf temperature (K) |

**v** | Advection velocity vector (m s^{–1}) |

*v*_{l} | Advection velocity in longitudinal direction (m s^{–1}) |

*v*_{m} | Radial advection velocity in mesophyll (m s^{–1}) |

*v*_{r} | Advection velocity in radial direction (m s^{–1}) |

*v̄*_{r} | Longitudinal mean advection velocity in radial direction (m s^{–1}) |

*v*_{x} | Radial mean longitudinal advection velocity in xylem (m s^{–1}) |

*v*_{x0} | Longitudinal advection velocity in xylem at leaf base (m s^{–1}) |

*w*_{a} | Ambient humidity [mol(H_{2}O) mol(air)^{−1}] |

*w*_{i} | Humidity in the stomatal cavity [mol(H_{2}O) mol(air)^{−1}] |

*W* | Leaf water volume (m^{3}) |

*W*_{lw} | Leaf water volume per leaf area [mol(H_{2}O) m^{−2}] |

*α*^{+} | Equilibrium water-vapour fractionation factor |

*α*_{k} | Kinetic fractionation factor through stomata and boundary layer |

*δ*_{s} | Isotope ratio of source water from the petiole relative to VSMOW |

*δ*_{V} | Isotope ratio of ambient water vapour relative to VSMOW |

*δl* | Longitudinal dimension of elemental cylinder (m) |

*δr* | Radial dimension of elemental cylinder (m) |

*δt* | Infinitesimally small time step (s) |

Δ | Isotope ratio relative to source water |

| Longitudinal mean isotope ratio relative to source water |

Δ_{C} | Craig–Gordon isotope ratio relative to source water |

Δ_{e} | Isotope ratio at evaporative site relative to source water |

| Longitudinal mean isotope ratio at evaporative site relative to source water |

Δ_{E} | Isotope ratio of transpiration relative to source water |

| Longitudinal mean isotope ratio of transpiration relative to source water |

Δ_{lw} | Radial mean bulk isotope ratio relative to source water (xylem plus mesophyll) |

| Longitudinal and radial mean bulk isotope ratio relative to source water (xylem plus mesophyll) |

Δ_{m} | Radial mean mesophyll isotope ratio relative to source water |

| Longitudinal and radial mean mesophyll isotope ratio relative to source water |

Δ_{v} | Isotope ratio of ambient water vapour relative to source water |

Δ_{x} | Radial mean xylem isotope ratio relative to source water |

| Longitudinal and radial mean xylem isotope ratio relative to source water |

*ε*^{+} | Equilibrium water-vapour fractionation |

*ε*_{k} | Kinetic fractionation through stomata and boundary layer |

*ε*_{k0} | Kinetic fractionation through stomata |

φ_{x} | Volume fraction of leaf occupied by veins |

*κ*_{m} | Tortuosity factor in mesophyll |

*κ*_{x} | Radial tortuosity factor in xylem |

*σ*_{l} | SD of normalized functions for non-uniform gas-exchange parameters if described with Gaussian profile (m) |