HortResearch Publication - Irrigation of horticultural crops
In September 1996, Dr Caspari attended the Second International Symposium on Irrigation of Horticultural Crops in Crete, Greece where he presented a paper. The following is a summary of the conference taken from his travel report.
This successful symposium was attended by about 100 delegates from 26 countries around the world. The symposium included four days of oral presentations plus a one day field trip. The research areas covered six sessions: Crop Water Requirements, Plant Water Relations, Irrigation Scheduling, Irrigation Systems, Water Quality for Horticultural Crops, and Environmental Impact of Irrigation, plus two poster sessions.
Following the official opening, the chairman of the organising committee, Dr Chartzoulakis gave an overview of the importance of irrigation in Greece. In Greece, 1.35 million ha are irrigated, which is approximately 35% of the agricultural area. Of the various crops mentioned, about 20% of the total area planted in grapes was irrigated, 30% of orchards, 30% of row crops and 92% of vegetable crops. Eighty percent of the available water in Greece is used for agriculture, 15% is used for household purposes, and the remainder for industry. There is a sharp north-south decline in the available water. More than 50% of available water is found in northern Greece and only about 15% in southern Greece and its islands.
On the island of Crete, nearly 85% of the available water is used for agriculture, with 12% for household and the rest for industry purposes. The island has an average precipitation of 1,120 mm, with about 600 mm falling in the central plains, but more than 1,500 mm falling in the mountains. Evaporation measured by a class A evaporation pan can reach up to 310 mm in July. Currently less than 5% of irrigated area is still irrigated by flood or channel irrigation, with the remainder irrigated by sprinkler or drip. There are a number of large and medium sized irrigation networks which have been paid for by the government and local authorities. Farmers are charged for the water used. Prices are as low as 19-20 Drachmas per m3 but might reach up to 60 Drachmas in some private schemes (NZ$1 ~ 166 Drachmas). The average size of irrigated area per farm is about 0.5 ha. That means that complex or expensive systems for irrigation scheduling like neutron probe or time domain reflectometry are not used. For tree crops, crop coefficients are used for irrigation scheduling.
The following is a summary of some of the papers presented at this Conference:
R G Allen, USA.
Dr Allen presented an overview of the recent revisions of the FAO ID 24 publication (Crop Water Requirements) which utilises the FAO form of the Penman-Monteith equation for predicting the grass reference evapotranspiration. The revision includes modified procedures for estimating the crop coefficients. A new equation allows for consideration of the influence of soil type on the rate of soil surface drying. The revised book is probably being published in early 1997.
P G Mostert, South Africa
Mature mango trees were subjected to winter water stress by withholding water for a total of three months. When averaged over the six years of the trial, the winter water stress resulted in a 9% higher yield with similar fruit size. Water use was reduced by up to 20%. The total seasonal requirement of mature mango trees is approximately 1,200 mm per year. Water use in winter is as low as 2 mm per day, peaking at 4.4 mm per day in the summer.
J R Castel, Spain.
The design, calibration, and performance of a large weighing lysimeter (4 x 4 x 1.5 m) were described. The crop coefficient of citrus determined with the lysimeter was about 20% lower than those suggested by the FAO 24 publication. The evaporation from the bare soil surface might contribute up to 50% of total evapotranspiration if the soil surface is wet, otherwise it is generally around 20-30% of ET.
H G Jones, UK
The author reviewed significant advances in the understanding of plant water relations for the last 10 years. He addressed some of the criticism by a number of scientists regarding the validity of the concept of water potential as the driving force for water transport in plants. He suggested that some of the conclusions drawn from experiments with the xylem probe might be inaccurate due to errors associated with the xylem probe itself. He concluded that the cohesion theory for water transport in plants was still the best theory explaining water transport, and that the measurement of water potential in plants was still one of the best methods available for determining plant water status.
V O Mogensen
The reflectance index is calculated from incoming and reflected photosynthetically active radiation and from incoming and reflected near infra-red radiation. Comparing the reflectance index of a well-watered crop to that of a water stressed crop yields the so called relative reflectance index. The author showed that the relative reflectance index is very sensitive to the beginning of water stress. It should be noted, however, that so far this reflectance index has only been used and proven to work on field crops such as wheat, rape, and lupins, i.e. crops with very uniform canopies.
M Genard, France
A semi-mechanistic model of fresh matter, dry matter and sugar accumulation in peach fruit was presented. The model allows the flow of assimilates to be partitioned into sucrose, sorbitol, glucose, and fructose. Using the maximum daily shrinkage of the trunk as an indicator of tree water status was the main input for the model. Using well-watered and water-stressed peach trees growing in the field, a close agreement between the simulated and the field data was achieved.
V Novello, Italy
The performance of two Italian cultivars grafted to the roostocks 140 Ruggeri, 420A and Kober 5BB was compared. Rootstocks had a significant effect on the leaf water potential, the quantum yield, and the water use efficiency. The rootstock Kober 5BB proved to be the most drought sensitive of the rootstocks tested.
C D Economakis, Greece
Lettuce plants were grown using nutrient film technique (NFT) in an unheated glasshouse at three solution temperature regimes of 10, 15, and 20 °C. Increasing the solution temperature led to significantly higher production (nearly doubled) but the leaf water content was also higher. This higher leaf water content could lead to problems with a reduced shelf life. Increasing the solution temperature also increased the concentration of nitrate in the leaves. However, the nitrate concentration always remained below 1,000 mg NO3--N per Kg fresh weight, which is below the EU limit of 3,500 mg NO3--N.
J Girona, Spain
The seasonal growth of almond can be divided into two phases: the vegetative phase which ends in June (in Spain), followed by a fruit growth phase. The start of the fruit growth phase is the phase of dry weight gain of the kernel. Water stress imposed in the second half of the fruit growth phase led to increases in flower number in the next year. The higher flower number is due to the fact that the water stress coincided with the phase of floral initiation. Average fruit weight is not reduced by such water stress and yield can be increased due to the higher flower number.
Z Plaut, Israel
The author reviewed the current research into effects of irrigation with low quality water. Recent research in Israel showed that the early loading of salinity of the rootzone of tomato plants increased the tomato fruit glucose concentration. This increased glucose concentration remained elevated until harvest, even when the irrigation water quality was switched to high quality, ie low salinity late in the season. When irrigation with saline water early in the growing period is followed by irrigation with high quality water, the debilitating effects of the saline water on yield can be reduced while at the same time the glucose concentration can be elevated, ie the taste of tomato fruit can be improved.
D A Goldhamer, USA
Flood irrigation of pistachio orchards on slowly permeable soils can result in serious water ponding. The severity of Alternaria alternata, a fungal disease, is enhanced by the high humidity under such conditions. Changing from flood irrigation to sub-surface drip irrigation reduces the relative humidity in the orchard. Sub-surface drip irrigation also increased both the maximum and minimum temperatures in the orchard and reduced the time of dew formation periods. Leaf incidence of late blight was reduced nearly four fold and the fruit infection by 45%. Dr Goldhamer noted that this study was a very special case where sub-surface drip irrigation had led to significant improvements. However, he commented that there was a lot of claims about the general advantages of sub-surface drip irrigation in California which was not backed up by research results.
J Beltrao, Portugal
Lettuce plants were irrigated with saline water ranging from 1-8 dS/m electrical conductivity and received either low, medium, or high nitrogen fertilisation. Lettuce yields reduced drastically when the salinity of the water was raised from 1 to 2 dS/m. There was an interaction between salinity and nitrogen nutrition. The reduction in yield due to salinity is delayed at higher nitrogen nutrition.
The next symposium on irrigation of horticultural crops is scheduled to take place in 1999 in Portugal.
I am very grateful to the following organisations for funding my participation at the international symposium:
HortResearch, Marlborough Research Centre Trust, Marlborough Grape Growers Association, New Zealand Horticultural Science Advancement Trust.
I am also very grateful to my scientific colleagues for their time and hospitality.
Edited for HortNET by Helen Percy from HortResearch Internal Report 96/84 (Overseas Travel Report - USA, Germany, Italy, Switzerland. H.W. Caspari. November 1996).