Introduction to Crop Evapotranspiration and Crop Coefficients

Crop ET (ETc) is calculated by multiplying the reference crop ET (ET0) by a dimen-sionless crop coefficient Kc:

The reference crop corresponds to a living, agricultural crop (clipped grass) and it incorporates the majority of the effects of variable weather into the ET0 estimate. Therefore, because ET0 represents an index of climatic demand on evaporation, the Kc varies predominately with the specific crop characteristics and only somewhat with climate. This enables the transfer of standard values for Kc between locations and between climates.

The crop coefficient Kc is the ratio of the crop ETc to the reference ET0, and it represents an integration of the effects of three primary characteristics that distinguish the crop from the reference ET0. These characteristics are crop height (affecting roughness and aerodynamic resistance); crop-soil surface resistance (affected by leaf area, the fraction of ground covered by vegetation, leaf age and condition, the degree of stomatal control, and soil surface wetness); and albedo (reflectance) of the crop-soil surface (affected by the fraction of ground covered by vegetation and by the soil surface wetness).

The crop height h influences the aerodynamic resistance ra [Eqs. (5.4) and (5.5)] and the turbulent transfer of vapor from the crop into the atmosphere. The combined crop-soil surface resistance influences the bulk surface resistance rs [Eq. (5.6)], which represents the resistance to vapor flow from within plant leaves and from beneath the soil surface. The albedo of the crop-soil surface influences the net radiation of the surface Rn, which is the primary source of the energy exchange for the evaporation process.

Two Kc approaches are considered. The first uses a time-averaged Kc to estimate ETc, which includes time-averaged (multiday) effects of evaporation from the soil surface. The second approach uses the basal crop coefficient, and a separate, daily calculation is made to estimate evaporation from the soil surface.

The time-averaged Kc is used for planning studies and irrigation system design where averaged effects of soil wetting are acceptable and relevant. For typical irrigation management, the time-averaged Kc is valid and currently is utilized in many irrigation-scheduling simulation models. The basal Kc approach, which requires more numerical calculations, is best for irrigation scheduling, soil water-balance computations, and for research studies where effects of day-to-day variation in soil surface wetness and the resulting impacts on daily ETc, the soil moisture profile, and deep percolation fluxes are important.

The crop coefficient curve (Fig. 5.3) represents the changes in Kc over the length of the growing season. The shape of the curve represents the changes in the vegetation and ground cover during plant development and maturation that affect the ratio of ETc to

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