Relationships between yield, crop water stress index (CWSI) and transpiration of cowpea (Vigna sinensis L)

This research was conducted to correlate the crop water stress index (CWSI) with evapotranspiration (ET), transpiration (T), and yield of cowpea (Vigna sinensis L) in order to apply these relationships to irrigation scheduling and yield production models for this crop. Furthermore, the influence of air and soil parameters in the application of canopy and air temperatures differential (Tc - Ta) for irrigation scheduling of cowpea were investigated. The lower baseline of cowpea during the growing season was (Tc - Ta) = 1.62 - 0.113 (VPD) in degrees C and mb. Its coefficients were smaller than those of most crops reported by other investigators. The upper baseline throughout the growing season was 5.0 degrees C which is comparable to that of sugarbeet in the same region. The k value of cowpea in equation Yt/T = k/(e*-e) (where Yt is the yield and e*, e are the saturated and actual vapor pressures) was 0.041 mb which is similar to that of soybean. It was concluded that the harvest index of cowpea (grain/aerial dry matter), HI, was dependent on transpiration (T, kg/ha) as HI = 2.37 x 10(-4) (T)(0.5). The quadratic production functions (yield and irrigation water and/or ET) for grain and aerial dry matter of cowpea were obtained. The water sensitivity factors for grain and aerial dry matter of cowpea were 1.01 and 0.53, respectively, which are less than the factors for soybean grain production. The CWSI was 1.31 x (1 - ET/ETm) due to the fact that CWSI was measured and averaged over a portion of the growing season. Therefore, it was concluded that by correcting the CWSI for total growing season the CWSI nearly equals 1 - ET/ETm. Irrigation timing of cowpea might be determined by a CWSI of 0.106. Furthermore, it was found that (Tc - Ta) = 0.26RH - 3.71 may be applicable for irrigation scheduling of cowpea by canopy and air temperature measurements between 13 h 00 and 14 h 00. Finally, the appropriate equations were given for ET estimation by using canopy and air temperature measurements between 13 h 00 and 14 h 00.