Path analysis technique for strategic irrigation management under adverse climatic conditions

Path analysis is a statistical technique widely used for modeling causation among a set of variables. A path diagram sets out the plausible causal relationships, and the path coefficients estimated by solving the relevant structural equations determine the strength of the relationships. The path diagram can be used to assess the change required in one variable to counteract the effect of another variable or several variables. In particular, a path diagram can be used to determine the timing, duration, and rate of irrigation to a crop to compensate for the effects of unfavourable climatic conditions. However, such an approach would assume other factors like soil fertility, microbial activities, and cultural practices to be time invariant. A path diagram has advantages over crop models in that a crop model cannot account for uncertainty, and the latter is restrictive to the particular field conditions in which it has been developed and has been criticised for its inability to be readily translated into regional parameters where the climatic variables are more stable.The study area in which such a path analysis technique has been applied is the Fitzroy river catchment in Central Queensland, Australia. The region has a semi-arid climate. The four major crops grown in the area are wheat, barley, cotton, and sunflower. The critical stages for the yield of these four crops, as reported in published literature, are planting time, flowering time, and harvesting time significantly influenced by the minimum, average, and maximum values of rainfall, temperature, and humidity. Water stress in wheat reduces the duration of grain filling. High temperature adversely affects germination and seedling emergence and reduces grain number/spike and grain weight of wheat. High humidity is known to cause stem rust and leaf rust problems in wheat plants. Climatic requirements for optimal yields of wheat and barley are different. Cotton also requires different climatic conditions for good yield. Cotton is a long-season crop which needs over 160 days with temperatures above 150C and adequate sunshine during the growing season. It can tolerate moderate salinity levels, and therefore, can be irrigated by brackish water or return flows. Sunflower production faces major problems with excess rainfall or moisture. Rust infection is a great concern for sunflower production which is favoured by heavy dews, wet weather, and temperatures above 200C. The path analysis technique applied to the Fitzroy region’s historic data yielded four different models for the four crops. Each model captures the range of optimal climatic variables and provides coefficients for the degree of adjustment required in the endogenous variable (irrigation supply) due to deviations in the exogenous variable(s). Each model can be used to plan the irrigation management strategy to offset the effects when one or more of the climatic variables deviate from ideal conditions. The climate information is available daily from the Bureau of Meteorology, Queensland.