Application of a dynamic greenhouse climate model for irrigation scheduling in a greenhouse rose crop in Zimbabwe
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An assessment on the potential of the Gembloux Greenhouse Climate Model (GDGCM) as a tool for irrigation scheduling was done on a rose (Rosa Hybrida) crop grown in an Azrom type greenhouse, located in Harare, Zimbabwe. The transpiration sub-model of the GDGCM, consisting of a canopy resistance model within it was mainly considered in this study. The canopy resistance model and the transpiration sub model were calibrated and validated. Field measurements were done for climatic and physiological parameters required for the canopy resistance model and transpiration sub model input parameters. Climatic data was continuously measured inside and outside the greenhouse throughout the research. Historical data for Whole Plant Transpiration (WPT) measured by stem heat balance sap flow gauges obtained from Floraline (Pvt) Ltd for the period December 2007 and January 2008 was used for calibration and validation of the transpiration sub model. The canopy resistance model was fitted to experimental data of canopy resistances and coefficients a, b and c of 788.38 ± 82.51, 85.78 ±16.14 and -0.146 ± 0.080 respectively were determined. The validation results showed a strong fit between the measured and simulated values (R2=0.91). Several input parameters were determined, including the canopy resistances from the canopy resistance model, to calibrate the transpiration sub model. The transpiration sub-model was fitted to experimental WPT data and the results showed a good fit between the simulated and measured values (R2=0.64). Simulations of crop transpiration were carried out for a whole year: winter (May to August 2007) and summer (September 2007 to April 2008). The GDGCM uses outside weather data to simulate the internal greenhouse microclimate, as well as crop transpiration rates. The crop water requirements (CWR) were calculated as the amount of water requirement to replenish the water lost by transpiration. The results showed that the rose crop transpired more in summer than in winter, as expected; and there was also transpiration at night but it was very small. Daily and seasonal CWR were determined. Daily CWR fluctuated everyday depending on the weather conditions, and seasonal CWR showed that the CWR was less in winter than in summer. June and July had the lowest CWR in winter; while December and January had the least CWR in summer. September and October had the highest CWR for that year. The CWR of the rose crop for the whole year was compared with the actual amount of water that was supplied by the existing irrigation system. The existing irrigation system was automated, applying water for 4 minutes whenever the cumulative solar radiation outside the greenhouse reached 1600 kJ/m2. The results showed that the CWR was lower than the actual water applied by the irrigation system throughout the year. The total CWR for the year was 1.45 Ml/year and the actual water applied was 2.74 Ml hence the existing irrigation system was over-irrigating by almost half the CWR by the crop.