Translating wind measurements from weather stations to agricultural crops

Wind-speed profiles within the internal boundary layer (IBL) are determined by the roughness of the underlying surface and the general velocity of the air mass. Most evapotranspiration (ET) estimation equations, especially the Penman or Penman-Monteith types, employ this principle by measuring weather data at an elevation z above the ground surface. Aerodynamic resistance algorithms used in the Penman-Monteith equation assume that wind speeds measured at height z are characteristic of the crop being evaluated and can be extrapolated to the vegetation surface using logarithmic profiles characteristic of the crop roughness and height. In many studies, however, meteorological data are from standard types of weather stations situated over clipped grass or bare soil, rather than the surface in question. Differences in aerodynamic roughness affect measured wind velocities and the subsequent predictions of aerodynamic resistance (ra). Magnitudes of ra, and bulk surface (stomatal) resistance for agricultural crops are often similar, averaging about 40-60 s/m-1 when wind speeds 2 m above the crops range from 2 to 3 m/s-1. Although surface resistance is generally known with less certainty than ra, it still may be important to translate wind-speed data to better predict ra, and ET using the Penman-Monteith equation for a specific crop