Parameters for Use of BEWAB+ Programme to Schedule Irrigation of Pea (Pisum sativum L.)

A large portion of irrigation farmers make use of subjective (intuition) irrigation scheduling methods as supposed to objective or scientific irrigation scheduling methods, which need to be changed. The BEsproeiingsWAterbestuursprogram (BEWAB+) irrigation scheduling programme is based on the water balance equation and needs: (1) a crop production function; (2) a relative consumptive water demand curve and (3) an allowable depletion subroutine. The objective of this paper was to describe research aimed at obtaining information on (1) and (2) for pea and also to describe the effect of water application on yield and water use of pea. BEWAB+ uses this information to estimate the daily irrigation water requirements for a particular soil-crop-atmosphere system under irrigation. A field experiment, based on published line-source irrigation methodology, was conducted on a 3 m deep loamy fine sand Bainsvlei or Ustic Quartzipsamment soil near Bloemfontein (26°08'S; 29°01'E) in South Africa. Results showed that there is a linear relationship of the form Ys = 8.07ET - 249 (r 2 = 0.91), where Ys is the seed yield of pea (kg/ha) and ET is evapotranspiration for the growing season (mm). The relative consumptive water demand curve is represented by the following third order polynomial function that describes the relationship between time and relative ET for a pea growing season of 120 days: ETrelx = 0.09419646 - 0.01302413x + 0.00059008x 2 - 0.00000371x 3 . ETrelx denotes relative ET and x denotes time in days. A workable balance between practical problem solving and advanced irrigation science has been established with BEWAB+. Pre-plant irrigation schedules can be made for semi-arid areas with the BEWAB+ programme using easily obtainable inputs, like target yield, soil depth and soil particle size distribution information.