The effect of soil surface temperature on the crop water stress index

Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T_s) due to the drying of the soil surface, on the vegetation temperature (T_v). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T_v was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T_v predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T_s on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T_s. The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols C Sensible heat flux from the soil-vegetation system (W m^–2) - c_{l shade} Mean stomatal conductance of the shaded leaf area (m s^–1) - c_{l sun} Mean stomatal conductance of the sunlit leaf area (m s^–1) - c_max Maximum stomatal conductance (m s^–1) - c₀ Minimum stomatal conductance (m s^–1) - C_p Specific heat at constant pressure (J kg^–1 °C^–1) - C_s Sensible heat flux from the soil (W m^–2) - C_v Sensible heat flux from the vegetation (W m^–2) - c_v Bulk stomatal conductance of the vegetation (m s^–1) - CWSI Crop Water Stress Index (dimensionless) - e_a Vapor pressure at the reference height (kPa) - e_b Vapor pressure at the virtual source/sink height of heat exchange (kPa) - e₀^* Saturated vapor pressure at T₀ (kPa) - e_s Vapor pressure at the soil surface (kPa) - e_v^* Saturated vapor pressure at T_v (kPa) - G Soil heat flux (Wm^–2) - GLAI Green leaf area index (dimensionless) - GLAI_shade Green shaded leaf area index (dimensionless) - GLAI_sun Green sunlit leaf area index (dimensionless) - k Extinction coefficient for photosynthetically active radiation (dimensionless) - k₁ Damping exponent for Eq. A 5 (m² W^–1) - LAI Leaf area index (dimensionless) - LE Latent heat flux from the soil-vegetation system (W m^–2) - LE_s Latent heat flux from the soil (W m^–2) - LE_v Latent heat flux from the vegetation (W m^–2) - p_a Density of air (kg m^–3) - PAR_a Photosynthetically active radiation above the canopy (W m^–2) - PAR_u Photosynthetically active radiation under the canopy (W m^–2) - r_a Aerodynamic resistance (s m^–1) - r_b Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m^–1) - r_c Bulk stomatal resistance of the vegetation (s m^–1) - R_n Net radiation above the canopy (W m^–2) - R_s Net radiation flux at the soil surface (W m^–2) - r_st Mean stomatal resistance of leaves in the canopy (s m^–1) - R_v Net radiation absorbed by the vegetation (W m^–2) - r_w Heat exchange resistance between the soil surface and the boundary layer (s m^–1) - S Photosynthetically active radiation on the shaded leaves (W m^–2) - S_d Diffuse photosynthetically active radiation (W m ^–2) - S₀ Photosynthetically active radiation on a surface perpendicular to the beams (W m^–2) - T_a Air temperature at the reference height (°C) - T_b Temperature at the virtual source/sink height of heat exchange (°C) - T₀ Aerodynamic temperature (°C) - T_s Soil surface temperature (°C) - T_v Vegetation temperature (°C) - w₀ Single scattering albedo (dimensionless) - Psychrometric constant (kPa °C) - ₀ Cosine of solar zenith angle (dimensionless)