Development of irrigation guidelines for citrus under drip irrigation in Zimbabwe
Zirebwa, Farai Solomon
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This study was carried out to develop drip irrigation guidelines for citrus in Zimbabwe. In order to achieve this, the average ETo values for the study site were derived from climatic data. The study, which was carried out at Mazowe Citrus Estate, also involved the determination of the class A evaporation pan coefficient, Kpan to enable local estimations of ETo from the class A evaporation pan. The whole season crop coefficient, Kc, curve for citrus was developed for the estimation of crop water requirements. The ETo trend for the study site was established from the FAO Penman Monteith equation using observed and historical climatic data. Kpan was determined using two methods. The first method was empirical and Kpan was estimated from the mean relative humidity, mean wind speed at 2 m height and the fetch conditions. In the second method, Kpan was determined as the slope of the plot of the FAO Penman Monteith ETo against the pan evaporation, Epan. The Kc curve was developed after the empirical determination of Kc during the initial growth stage, the mid season stage and at the end of fruit development. The Kc value of the initial growth stage that was developed for the sub humid climates was adjusted to reflect the wetting frequency of the soil. The mid season Kc value developed for the sub humid climate was adjusted to the local climatic conditions using wind speed at 2 m height, the minimum relative humidity and the average tree height. During the crop development and the late season stages, Kc was estimated by considering that Kc varies linearly between the value at the end of the previous stage and that at the beginning of the next stage. Transpiration rate was monitored using a Stem Heat Balance sap flow gauge that was installed on a branch of the tree. The leaf area was used as the scaling up factor for the sap flow from the branch to the whole hectare. This was done to compare the crop water requirement to the irrigation depth prescribed by the developed irrigation guidelines. BUDGET (version 6.2), a soil water balance model was used to develop irrigation calendars using a fixed irrigation interval of one day. The model was also used to simulate the soil water status after using the irrigation guidelines. Four treatments were established to assess the effect of different irrigation strategies on fruit growth and abscission. These were the control (use of ETo data for irrigation scheduling), well watered (where both sides of the tree were irrigated but each side receiving half the amount of water as the control), the grower’s scheduling practice and the last one was a PRD 50 where one side of the tree was irrigated whilst the other one was drying out. The switching of the drip lines was done on a 10 day interval. The effects of the different irrigation strategies were evaluated in terms of fruit growth rate and fruit abscission during the fruit drop period. ETo was approximately 4 mm day-1 from January to April, lowest in winter (almost 3 mm day-1) and was highest in October (slightly above 5 mm day-1). The green fetch Kpan was high from January to April (almost 0.8). The value fell from May and the lowest Kpan value of 0.72 was established in October. Using the second method, a Kpan value of 0.78 was determined for October to March. The initial Kc value was 0.47 but it rose sharply during the crop development stage. Kc values for the mid season and at the end of the season were 0.84 and 0.76 respectively. There is a possibility that the grower was over irrigating based on simulated soil moisture conditions usually above the field capacity. High drainage losses of up to 1964 mm for the whole season were simulated compared to 381 mm for the irrigation scheduling guidelines. No significant differences (p < 0.05) were observed on fruit growth rates although the control treatment had larger fruit diameters than the other treatments. Fruit abscission was generally low for all treatments. A single Kpan value can not be used throughout the season and the variations along the season should be taken into account for better estimations of ETo using the evaporation pan. Kc fluctuations during the crop development stage should be noted to reduce errors in the estimation of citrus crop water requirements. Taking into account the atmosphere’s evaporative demand is an effective way of irrigation scheduling. Irrigating both sides of the tree at the same time is not effective if the irrigation depth is low. PRD 50 showed great potential in balancing fruit growth and water conservation. Zimbabwean citrus farmers are encouraged to adopt the developed irrigation guidelines as well as the Kpan and Kc values. PRD, if well timed, proves to be a better irrigation strategy.