Apr 7, 2007

Scheduling Achieves Agronomic Benefits Beyond Production

South Tyrol’s apple growing area extends to more than 44,400 acres, with virtually all this area being equipped with irrigation. Irrigation is in fact one of the pre-requisites for commercial apple production, although the amount and distribution of average annual rainfall apparently closely match actual orchard needs, with the exception of the notably drier Vinschgau-Valley. However, dry spells of up to several weeks occur quite frequently. In addition, the low-lying production areas, such as the valley floors, are prone to late spring frosts and therefore rely on overhead sprinklers for frost-protection.

The total area equipped with anti-frost irrigation amounts to about 29,640 acres. This implies that the larger part of South Tyrol’s apple plantations will remain under dual purpose (anti-frost and drought) overhead irrigation, whereas in some areas of the hills drip irrigation has replaced overhead irrigation. There are several reasons for this continuing shift: the often-scarce water supplies in the hilly areas; the lower installation costs of micro-irrigation systems; and the recent appearance of fire blight.

Agriculture is currently the sector of the local economy accounting by far for the greatest water consumption. With water demand from sectors other than agriculture still growing, future water availability for agriculture is likely to be reduced.

Irrigation water in the hilly areas is mainly derived from mountain streams or springs, whereas in the valley floors most of the water is pumped from boreholes tapping the relatively shallow water table. In this latter case growers normally have their individual pumping stations and can schedule irrigation individually and independently. On the contrary, in large areas of the hills, water is supplied through cooperative irrigation schemes, whereby the single farmer gets his share of irrigation water according to pre-established timetables with little or no possibility of individual control. This is a particular characteristic of the Vinschgau valley, where most of the around 9,880 acres of apple orchards are included in one of the many cooperative irrigation schemes, which range in size from 74.1 acres to 741 acres. If water availability is sufficient, weekly irrigation cycles of 1.2 inches to 1.6 inches are commonly applied.

This rather peculiar situation, with ample and cheap groundwater supplies at shallow depth in some areas, and centralized scheduling of sometimes limited water supplies in others, has led to the fact that the improvement of the agronomic and economic efficiency of water use has largely been neglected over previous decades and that irrigation management is lagging behind the many other technical and agronomic achievements of the local fruit industry. The adoption or development of easily applicable guidelines and tools for irrigation management in horticulture should therefore be one of the main objectives of research and extension workers in the future.

Irrigation management

The most general objective of irrigation has always been to safeguard the productivity of the apple orchards. Research has shown, however, that water relations of apple trees are extremely complex and that they can affect a number of plant physiological processes. With an increasing understanding of these relations it has become possible to use irrigation as a more sophisticated tool for orchard management, achieving agronomic objectives besides just mere productivity, such as control of tree vigour and improvement of fruit quality. At the same time conspicuous amounts of water and energy can be saved by better adapting irrigation to actual environmental conditions and to plants’ physiological requirements.

At present, irrigation is scheduled more often according to growers’ feelings rather than on the basis of objective parameters and precise strategies. This is partly due also to a lack of knowledge by many fruit growers about suitable methods of assessing the water status of their orchards. The methods proposed by the scientific community often are too expensive, complicated or time-consuming and sometimes even lacking the required accuracy.

An ideal moisture monitoring system should present the following characteristics: reliability, affordability, simplicity of use and easy interpretation of data. There still is some controversy within the scientific community itself as to which parameters most significantly indicate the water status of an orchard. In particular there seems to be an argument between those in favor of monitoring soil water and others preferring plant physiological parameters, such as plant water potential in its various forms or the variations of trunk and fruit diameter. Without going into lengthy details about the pros and cons of the various approaches, there is one basic aspect that should be underlined: the only component of the soil-plant-atmosphere continuum on which the fruit grower can exert his influence in terms of water relations remains the soil. In other words, even if the grower was able to detect conditions of water stress on his plants, the only thing he could normally do is to apply water to the soil, provided that soil moisture actually constitutes a limiting factor.

It is well known that the evaporative demand of the atmosphere on warm summer days can easily exceed the water absorbing capacity of the roots even under optimal soil moisture conditions and thus determine plant water stress. In such a situation, a mere physiological indicator is not sufficient for deciding whether irrigation is needed or not. Plant physiological indicators need, therefore, to be crosschecked with soil moisture data. It follows that the monitoring of soil moisture still constitutes the most immediate basis for decision-making in irrigation management.

Monitoring and interpretation

Various instruments and techniques have been proposed over many decades for measuring or estimating soil moisture. In accordance with the requirements of an ideal monitoring system defined earlier, the actual choice is restricted few options. Among those, the most suitable instrument for intensive apple orchards seems to be the tensiometer, due to a number of distinct advantages: low cost, easy handling, measurement of water availability rather than quantity and reference values that are little dependent on soil characteristics.

Correct placing of tensiometers in the orchard:

The tip of a tensiometer should be placed in the area of highest root density, which in intensive orchards on dwarfing rootstocks generally corresponds to a depth of 8 inches to 12 inches. The tensiometer should be placed at a distance of about 8 inches to 12 inches from the next tree and not right below an emitter in the case of a drip irrigation system. In the case of overhead irrigation, tensiometers should be placed at representative sites of the orchard in order to minimize the effects of the uneven water distribution. It is in any case advisable to place more than one tensiometer in an orchard in order to take account of local soil variability.

Reference values for irrigation management:

Irrigation scheduling can best be adapted to plant needs and soil characteristics by comparing tensiometer readings with defined reference values. The reference value at the wet end of the ideal soil moisture range lies at about 100 mbar water suction, which approximates the field capacity of most soils and should be considered as the endpoint of any irrigation cycle. Values close to zero indicate water saturation of the soil with possible negative side effects due to lack of oxygen in the root zone. It should be noted that the equilibration of water in soil and tensiometer may take several hours and the endpoint of irrigation should therefore be determined by an approach of trial and error, i.e. by testing various lengths of irrigation and assessing final tensiometer readings after due periods of equilibration.

The threshold for tensiometer readings at the dry end of the soil moisture range is not a single well-defined value, but varies according to orchard factors and, to a much lesser extent, with soil conditions. In general, lower threshold values of soil water suction should be adopted during the early stages of fruit development (growth by cell division), whereas later in summer less abundant water supplies with concomitant higher values of soil water suction may exert beneficial effects by reducing shoot growth and improving fruit quality. This strategy corresponds basically to what is referred to in literature as the concept of “regulated deficit irrigation”.

One of our field trials in the Vinschgau valley has confirmed that regulated deficit irrigation can reduce tree vigor without significantly affecting fruit size.

Results reported in literature on the effect of water supplies on fruit quality often are contradictory and also our own trials have not shown consistent effects in terms of sugar content or fruit firmness. A clear correlation, however, has been observed between the amount of water supplied and fruit nitrogen content, similarly as reported by others. In some years, blush color was more developed in the plots with lower water supplies.

What can be considered to be the “dry” limit of the soil moisture range in intensive apple orchards? Based on the limited data available from literature and on our own experience, the upper limit of soil water suction could be around 300 hPa (1hPa=1mbar) early in the season, shifting to values of up to 500 hPa to 600 hPa in summer and returning to lower values with approaching fruit maturity. Higher values may be adopted for varieties of large fruit size or in orchards with low crop load. Lower values, on the other hand, will be suitable for small-sized varieties or in conditions of high crop load. The lower threshold values should be adopted also on very light soils.

Water distribution

The application of the above mentioned guidelines in the farmers’ fields poses no substantial problem, provided soil moisture can be monitored and water distribution in the orchard is sufficiently uniform. This latter condition is normally met in the case of micro-irrigation systems, which supply similar amounts of water to each single tree. In contrast, the situation can be much different in the case of overhead sprinkler irrigation. Water uniformity coefficients (CU) of about 80 are commonly still regarded as satisfactory. If irrigation is being scheduled to apply the minimum required amount of water to the low watering zones, then an overall waste of irrigation water and frequently also of energy will result. If instead the timing of the irrigation cycle is adapted to satisfy the areas receiving the average application rate, then the plants in the lower watering zones are likely to suffer from water shortage.

The degree of uniformity of water distribution also greatly affects the water use efficiency during frost irrigation. Proper irrigation design could therefore help to reduce the enormous water demand during late spring frosts, when vast areas need to be irrigated at the same time. As large parts of the apple orchards will continue to rely on overhead irrigation, much more emphasis should be placed on improving the technical characteristics of the irrigation plants. The uniformity of water distribution can generally be improved by replacing medium- and long-range sprinklers with higher numbers of short-range sprinklers. A promising approach could be the installation of localized micro-irrigators above the tree rows, allowing not only water savings due to the high distribution uniformity along the tree rows, but also by reducing the amount of unproductive water reaching the interspace between tree rows.

Conclusions:

Irrigation management in South Tyrol’s fruit industry needs to address some important challenges lying ahead, notably increasing pressure on available water resources from other economic sectors and the need to safeguard the ecological integrity of some natural habitats such as the mountain streams.

Scheduling irrigation according to actual orchard needs by monitoring soil water status not only provides ample opportunities for saving water, but also may constitute a valuable tool for achieving agronomic benefits besides just crop productivity.

A further important aspect is the improvement of the technical standards of many of today’s irrigation plants. In particular, water-use efficiency could be significantly improved by adopting drip irrigation in areas not prone to late spring frosts and by improving the uniformity of water distribution where overhead irrigation remains essential for frost protection.

Martin Thalheimer is with the Research Centre for Agriculture and Forestry, located in Laimburg, Ora, Italy.