Irrigation for crops in a sub-humid environment

Summary

A water balance model, incorporating the influence of water deficit during vegetative growth on depth of water extraction, was used to calculate stress days from soybean irrigation experiments. A stress day was defined as a day when soil water deficit exceeded 60% of the capacity of the current root zone. Using data from 17 experimental treatments the influence of water deficits on yield and leaf area were ascertained. Soybeans were most sensitive to water stress during flowering and pod filling, and stress just prior to and during flowering increased the effect of stress during pod filling. For example, in the cultivar Ruse, stress during pod filling reduced yield by 21 kg ha⁻¹ (stress day)⁻¹ if there was no previous stress. If there had been 10 days of stress during vegetative growth, yield in pod filling was reduced by 52 kg ha⁻¹ (stress day)⁻¹. Yield was expressed as a function of stress days. Using this function and past meteorological data in the water balance model, it was found by simulation that from an allocation of 960 thousand cubic meters of water, the greatest yields were obtained by growing 100 ha of soybeans and irrigating at 60% soil water deficit. With progressively larger areas the water allocation was exhausted before the end of the season in an increasing number of years. However, the larger area compensated for the reduced yield and net returns averaged over 20 years were greatest when 160 ha were grown. Increasing the deficit for irrigation in order to make the water last longer increased the yield in dry years when the allocation was exhausted prematurely, but when averaged over 20 years, this effect was outweighed by loss of yield in the other years.