AbstractsBiology & Animal Science

Modelling deficit irrigation of wheat in Zimbabwe.

by John Findlay. MacRobert

Institution: University of KwaZulu-Natal
Department: Crop science
Degree: PhD
Year: 2015
Keywords: Crop science.
Record ID: 1476633
Full text PDF: http://hdl.handle.net/10413/11953


Wheat is grown in Zimbabwe during the relatively dry, cool winter with irrigation. On most large-scale farms, land resources exceed irrigation water resources. Consequently, the efficient use of water is of prime concern. This has led to the development and adoption of deficit irrigation techniques, with the aim of maximizing net financial returns per unit of applied water rather than per unit land area. This often requires that less water be applied than that required for maximum yields, which implies that water deficits are allowed to develop in the crop. Although the basic principles of deficit irrigation are known, there exists no systematic procedure for advising farmers on whether or not to, or how to, employ such a management option in Zimbabwe. This research was therefore undertaken to develop an interactive computer programme that would assist farmers in determining optimum irrigation strategies for wheat. The CERES-Wheat version 2.10 crop simulation model (WHV21) was chosen as the basis for this programme. In order to validate and modify, where necessary, WHV21, a series of field experiments were conducted at a number of wide-ranging locations in Zimbabwe during the period 1986 to 1992. These included sowing date x cultivar, sowing date x seeding rate, growth analysis and irrigation experiments. In all, 122 data sets were collected, of which 47 were used for model validation and 75 used for calibration and modification of WHV21. The initial validation of WHV21 showed that the model gave biased and imprecise predictions of phenological development, particularly under deficit-irrigated conditions. The simulation of tillering was poor and the model tended to over-predict dry matter accumulation and under-predict leaf area indices. The yield component and grain yield predictions were also generally imprecise. On the other hand, for most data sets, the simulated soil water contents were similar to measured soil water contents. These inconsistencies prompted a revision of the phenological and growth subroutines of the model. In the phenological subroutine, new thermal time durations and base temperatures (Tb ) for all growth phases were determined from regressions of the rate of phasic development on mean air temperatures. For growth phases one, two and three, a Tb of 4°C was established, whereas for growth phases four and five, a Tb of 3°C was used. The revised model included the prediction of leaf emergence (as apposed to leaf appearance) and first node appearance (Zadoks growth stage 31). In order to hasten plant development under conditions of soil water deficit stress, daily thermal time was made to increase whenever the actual root water uptake declined below 1.5 times the potential plant evaporation. These changes improved the prediction of crop phasic development: for example, the Index of Agreement for the prediction of physiological maturity was improved from 0.643 with WHV21 to 0.909 with the revised version. Many changes were made to the growth subroutine, inter alia: 1. the extinction coefficient in…