Irrigation Best Practice

Water Management of Soil and Substrate-Grown Crops

A Guide for Top and Soft Fruit Growers

TOP TIPS

• Invest in a water meter so you know how much water is being applied

• Get your water supply analysed annually so that feeds can be adjusted as necessary

• The weather can affect a crop’s daily water requirement by a factor of 3, so do take this into account

• Monitor the water status of your crops – choose a method that suits you and persevere with it to gain maximum benefit

This booklet may be sent to you from Defra (Census & Surveys) Division, using an address compiled from information collected as part of the agricultural census. No disclosure of the identities or addresses of farmers or growers has been made outside of the Division.

Contents

Section Page

1 Introduction 1

2

Water Quality

2

3

Seasonal Water

Requirements and

Storage

4

4 Irrigation Scheduling and Crop 6 Response Periods

5 Water Monitoring Techniques 10

6 Legislation 16

7 Sources of Information 18

ADDENDUM – February 2012 Please note that information regarding relevant legislation, both in the text and in referenced publications, although accurate at the time of publication, may now be out of date.

1 Introduction

Water management is an important factor in the production of fruit crops. An adequate supply of water is essential to maximise yield and quality. It is also important for water to be applied at the correct time, in sufficient quantities, but without excessive waste.

This best practice guide aims to address the key aspects of water and irrigation management that are appropriate to top and soft fruit production. Particular emphasis has been given to soft fruit crops grown in soil-less substrates. It draws attention to the principles of irrigating fruit crops, and identifies the critical factors for measuring crop requirements and the control of irrigation, both to optimise cropping and minimise waste.

We know that water supplies are frequently stretched and action has to be taken to safeguard this essential resource. Proposed legislation will change the basis of licensing water abstraction and will for the first time embrace water for trickle irrigation, which has previously been excluded from these controls. It is probable that the demand for, as well as the cost of, water will increase. This will mean that all users will need to increase water and irrigation management efficiency to make the most of this basic resource.

Environmental considerations are increasingly playing a part in the growing and marketing of crops. Careful and effective crop monitoring and irrigation control will be critical to profitable cropping in the future.

Horticulture and Potatoes Division

Department for Environment, Food and Rural Affairs

February 2003

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2 Water Quality

An understanding of the quality of the water used for irrigation is important. All water is not the same. Mains water is more consistent and does not have the potential problems caused by biological or herbicide contamination or by soil particles. For growing in soil-less substrate, understanding the basic chemical and biological content of irrigation water helps in identifying crop problems, the need for fertiliser use, dilution and the use of acid. Growers should know the analysis results of their water source before planning any irrigation, so that problems can be avoided.

The two main aspects of water quality are its chemical and microbiological properties. Other factors can occasionally affect quality, especially from river or reservoir sources, such as the presence of suspended peat or silt particles or other specific pollutants (e.g. herbicides). The chemical parameters that are of importance for irrigation water include:

• pH • Alkalinity or bicarbonate content • Electrical conductivity (measure of the total salts content) • Chloride level • Sulphate level • Iron and other trace element levels

(Note that some trace elements such as iron and manganese, although often present in water, are not in a form that can be readily taken up by plants.)

pH and Alkalinity

pH is the measure of how acid or alkaline water is. However, it is the level of bicarbonates (principally calcium, but also magnesium and sodium) in the water that determines whether it is termed either ‘hard’ or ‘soft’. Water may, for example, have a high pH but a low calcium bicarbonate content.

Water with a high bicarbonate content is termed ‘hard’. Hard water occurs in areas where the underlying rocks are calcareous and may be either mains, well or borehole in origin. A high bicarbonate level can cause the formation of insoluble phosphate salts which cause clogging of trickle irrigation lines, and in the case of soil-less substrate grown crops, a gradual rise in the pH of the growing medium, and serious blockages of irrigation nozzles. High pH can also reduce the availability of certain nutrients such as iron. Whilst it is not practical or economic to consider acidification for soil-grown crops, it may be worth considering the use of acidifying fertiliser mixes when fertigating with hard water.

For soil-less substrate grown crops, acidification is recommended if the bicarbonate level exceeds 150 mg/l. In areas of very soft water, there may be the opposite problem of a very low level of calcium, when additional calcium must be applied in the feed for fruit firmness.

Electrical Conductivity and Trace Elements

The electrical conductivity (E.C. or sometimes referred to as cF) is a measure of the total salts content of the water, reported in micro-siemens per cm. (mS/cm).

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The main contributors to high conductivity levels are nitrate, chloride and sulphate ions. Water with a naturally high E.C. could give rise to a build up of salts in the soil or soil-less substrate that could lead to root damage. A typical E.C. of mains water is around 500 mS/cm; water with an E.C. of 1000 mS/cm or higher should be avoided or diluted with water from other sources if at all possible.

Rainwater is generally pure, it has a low conductivity and pH and a very low calcium level. Borehole water in coastal and Fenland areas may have a high conductivity due to salt water contamination. A high conductivity due to high chloride levels restricts the amount of feed that can be added to the water and may cause crop damage due to accumulation in the substrate, especially for strawberry plants.

Some plants are also known to be sensitive to high fluoride levels; therefore irrigation water with less than 0.25 ppm fluoride is desirable. Boron toxicity is also occasionally found where water (e.g. river water) contains high boron levels originating from detergents.

Suggested maximum levels for irrigation water for substrate grown strawberries

Parameter Maximum level

Electrical conductivity Sodium Chloride Iron Zinc Boron Sulphate

850 mS/cm (measured at 20˚C) 35 mg/l 52 mg/l 1.0 mg/l 0.35 mg/l 0.33 mg/l 144 mg/l

Microbiological Contamination

The main microbiological contaminants of water are fungal spores (e.g. the mobile spores of Phytophthora, the cause of red core disease in strawberries and root rot in raspberries) and bacteria. The presence of bacteria such as E. coli would be particularly significant if irrigation water is to be applied overhead. Irrigation water should be checked to ensure it has an acceptably low level of microbial contaminants before use. There are several chemical, biological and physical techniques which can be used to purify the water if such contaminants are present, for example UV light treatment.

Water Analysis

Hand-held equipment is available for monitoring E.C. and pH during the production season, but a full laboratory water analysis is recommended at least every 12 months for substrate- grown crops.

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3 Seasonal Water Requirements and Storage

The April - September water requirement will vary with soil type, climate, the crops to be irrigated and the growing system. Where annual storage is required, it is normal to design a scheme around the demand in the 15th driest year in 20. This has been accepted as the normal basis for water allocation by the Environment Agency and local planners. The requirement for a business to store reasonable quantities of water is likely to become increasingly important as water costs rise and supplies are increasingly restricted.

The need for water storage facilities will depend on the availability and type of water source. These can be summarised as:

• Summer surface water directly abstracted for irrigation • Winter surface water for reservoirs • Boreholes, including wells and well points • Public mains supply

These days it is unlikely that licences would be granted for summer surface water abstraction. Similarly, there are many areas where new licences for borehole abstraction are unlikely to be granted. The remaining options require storage facilities. For winter abstraction of surface water, a reservoir would be required to store the predicted annual requirement. Mains water systems are more appropriate for smaller scale production areas and production in soil-less substrates. Here sufficient water should be stored to meet one or two day’s use at maximum demand.

In the future it is possible that in some areas, water from the mains will be available at “Off Peak Rates”. It is also possible that availability from the mains will be restricted at times of peak demand. To cater for such eventualities, additional storage may be necessary and cost- effective.

For planning purposes it is possible to predict a crop’s annual water requirement for a given geographical area using past meteorological data with the aid of a planning tool, such as the ADAS meteorological unit programme “Irriplan”.

• For many soil-grown soft fruit crops, a working figure of up to 300mm over the irrigated bed

area (typically 50% of the field area) per season can be used. • For less intensively irrigated crops, such as established tree fruit, 125mm can be adopted. • For substrate-grown strawberries, it is more important to consider the maximum daily water

requirement per plant. This has been estimated at 0.5 litres per plant per day.

Reservoirs

Reservoirs may be created in two ways. Firstly, by damming an existing watercourse, though this option can be expensive, requiring overflow and bypass arrangements. Alternatively, an off-stream, earth embankment storage reservoir can be constructed, which is filled by gravity or pumping. This second option avoids the need to make provisions for flood flows and any surplus water can be returned via an overflow pipe. This type of reservoir is usually easier to site and less expensive.

4

Depending on scale, site considerations and the need for lining (which can itself account for 40% of the total cost), installation costs can be high for large reservoirs, approximately £2000 - £3000 per 1,000m3. However, subsequent cost savings on mains water can reach several thousand pounds each year for a medium sized fruit farm.

Storage Tanks

Water for immediate use should be stored in a covered tank to prevent contamination. The storage tank can be an elevated sectional metal tank, a ground level reinforced concrete tank, or a ground level corrugated metal tank. The latter is the most cost-effective. Such storage tanks are available in a range of sizes and dimensions. The size of the tank installed should be appropriate for the individual unit, calculated on crop requirements at peak demand.

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4 Irrigation Scheduling and Crop Response Periods

It is useful to think of the soil as a holding reservoir for water. In late winter the soil is generally at field capacity (full point). The soil is fully saturated and any excess has drained off. As the crop grows, water is extracted from the soil and lost by transpiration from the leaves and evaporation from the soil and leaf surfaces, (the combined effect is known as evapo- transpiration). The accumulated amount of water lost from the soil is referred to as the soil moisture deficit (SMD). Any water added either by rain or irrigation is deducted from the SMD to determine the current moisture status.

In the case of crops grown in soil-less substrates, all the water removed by the plant through transpiration needs to be replaced by irrigation.

Uncovered soil-grown crops may of course benefit from rainfall, when irrigation can then be correspondingly reduced.

The type of soil or soil-less substrate in which the fruit crops are growing influences the amount of water which is available to the plant, with heavier soils normally holding more water.

In both soil and soil-less substrates, a proportion of the total water present is not available to the plant. This is particularly apparent in clay soils which can have a high water content, but a relatively high proportion is not readily available. As the water reserve is depleted so the plant has increasing difficulty obtaining the remaining water to the point where water is unavailable to the crop, though still present in the soil.

During periods of moisture stress, plant shoots are more affected than the roots. When moisture levels are restored, the roots will continue to function normally, but the shoot growth may have been damaged, resulting in a yield reduction.

In the case of soil-grown crops, rooting depth and soil type are both important factors that affect water availability for the crop. To assess suitable irrigation regimes, soils can be divided into 3 classes, each having different levels of water availability for a given depth:

Class A Low soil water availability: Not more than 60 mm of water per 500mm of depth

Coarse Sand; Loamy Coarse Sand; Coarse Loamy Sand.

Class B Medium soil water availability: Between 60mm - 100mm of water per

500 mm of depth.

Loamy Sand; Loam; Silty clay loam; Clay etc.

Class C High soil water availability: Above 100mm per 500mm of depth.

Very Fine Sand; Peat; Peaty Loam.

To enable crops to access the maximum moisture reserves, rooting depth is important, especially for perennial fruit crops, which remain in-situ for several years. Good soil preparation is essential prior to planting, thus allowing the plant roots to penetrate to depth, and so enabling the crop to have full access to available moisture reserves.

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Where crops are grown in soil-less substrates, it is important that the growing medium has the following characteristics:

✦ High water holding capability

✦ Ability to let water move laterally

✦ Sufficient pore space to give adequate aeration

These characteristics are necessary because the water available to plants at any one time is much reduced as the substrate volume for each plant is relatively small. Regular irrigation is therefore vital for optimum crop performance and profitability.

It is also important that water is applied before the crop comes under stress. Neither should water be applied to excess, as this will simply drain from the substrate and go to waste. Where soil-less substrates are used, accurate control of irrigation to satisfy crop requirements is essential.

To ensure a healthy root system it is important that the substrate is not saturated for any significant length of time or anaerobic conditions and root death will result. This can happen where the drainage is not adequate, particularly during the winter period. Where the soil-less substrate is too porous, water availability is limited. The consequent need for frequent watering means that the crop is likely to suffer stress and yield loss if peak demand is not always satisfied. Soil-less substrates therefore need to achieve a balance between holding the maximum amount of water whilst retaining sufficient aeration. It is important that the supplier formulates the soil-less substrate to meet the requirements that you specify.

Water uptake is affected by the root’s proximity to the soil or soil-less substrate. Young plants when first planted need sufficient water during the establishment stage to grow away rapidly. Careful planting is essential to ensure that the soil or soil-less substrate is settled closely around the roots, without being over-firmed, thus causing root damage. Where possible, watering after planting settles the soil around the roots and fosters rapid plant establishment.

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Response Periods and Critical Application Times

Soil Grown Crops Sprinkler or Trickle Irrigated

Crop

Apples Heavy Crop

Light Crop

Pears

Plums and Cherries

Blackcurrants Redcurrants Whitecurrants

Response Period Plan for 3 Soil Types (mm of water at mm SMD) A B C

July to September When SMD is more than 50mm apply 50mm of water for 2 weeks. Continue irrigation (Irri + Rain) equal to 50mm per 2 weeks for remainder of July, 40mm per 2 weeks in August and 25mm per 2 weeks in September

Nil Nil Nil

July to August As for apples

May to July 50 at 50mm 50 at 75mm 50 at 75mm

May to August 50 at 50mm 50 at 75mm 50 at 75mm

Soil Grown Crops Trickle Irrigated

Crop

Raspberries (Summer and Autumn)

Loganberries

Blackberries

Strawberries June bearers

Everbearers

Response Period Plan for 3 Soil Types (Maximum SMD mm)

Class A Class B Class C

Mid May to late August 10mm 20mm 35mm

June and July 10mm 25mm 40mm

July and August, 10mm 25mm 40mm September for cane growth

Start at petal fall to end 10mm 15mm 25mm of fruiting or late September for crops to be overwintered

Start of petal fall to end 10mm 15mm 25mm of fruiting

Substrate Crops Dripper or Trickle Irrigated

Strawberries

Raspberries (Mainly Protected)

Maintain near full wetness through the year.

Maintain near full wetness mid February to late October.

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Calculating Water Use for Strawberries Grown in Soil or Soil-less Substrate

Unless soil and soil-less substrate monitoring devices directly control the amount of water being applied to meet daily crop requirements, it will be necessary to check the amount of water which has been applied during a given time, ideally on a daily basis. This will confirm whether adequate water has been applied for the crop’s needs without excess being lost to drainage.

To carry out a check the following are required:

• An in-line water meter • Number of plants being irrigated • Quantity of water required for the crop stage

On bright sunny days plants should be near peak water demand. Under these conditions, strawberries will require about 0.5 litres per day for each plant, equivalent to a cubic metre of water for every 2,000 plants or 17.3 cubic metres each day per ha of plants at 34,600 plants per ha. When the plants are smaller, their requirements and, therefore, the amount of water applied are reduced. Ideally a calculation should be made for each block of strawberries which are at different growth stages, taking account of plant numbers and the stage of development.

Using an in-line meter, it is possible to check the amount of water being applied, which can then be related to the plants irrigated for a specific area. It is very easy to construct a simple chart with the plant numbers for each block, which takes account of the maximum water demand, with appropriate reductions according to crop stage and weather conditions.

Research has shown that yields are quickly affected once plants are subject to moisture stress. This is especially so with strawberries where yield and quality are reduced with the slightest stress, even though the plants may not be showing any visual symptoms of water shortage.

When plants are grown in soil, the frequency of irrigation is not so critical because of the greater moisture reserves and potentially greater rooting depth in most soils. In all but the most sandy soils it is reasonable to apply the quantities needed daily or two days requirements on alternate days during periods of low moisture stress. During periods of high demand and particularly during the defined crop response period, irrigation must be applied regularly to prevent stress occurring. However, when plants are grown in substrate, then water must be applied regularly through the day, in sufficient quantities to meet the crop’s requirements and so maximise yields and quality. Because the substrate can only hold a limited amount of water, its reserves are low and efforts to increase applications beyond short-term needs will only result in the surplus draining from the substrate. In substrate production it is much better to have frequent applications of short duration rather than fewer, longer applications. By following such an approach, plants experience less stress and are more likely to achieve maximum yields.

When applying water to substrate grown crops, it is worth remembering that the plant’s demand for water does not occur evenly through the day, but follows the increase in solar radiation received by the plant.

Where possible the plant’s demands should be matched by adjusting the frequency of waterings to increase during the brightest part of the day. The use of a solarimeter to provide information or control will help match a crop’s requirements.

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5 Water Monitoring Techniques

It is possible to have the best irrigation system but still get application wrong. Any system, even an automatic one, still depends on someone making a decision as to when to irrigate and how much water to apply. Irrigation is critical when trying to maximise yield and profitability. It is important that staff are sufficiently trained to identify when crops need water and when they do not. Watering application is still largely based on experience, but equipment and services are now available to measure soil or substrate water content to help in making these important decisions.

Several water monitoring methods can be used, based on different technologies. There are three broad categories: indirect methods, which rely on environmental measurements to estimate crop water loss; direct methods, using various techniques for measuring the soil water content or tension; and plant based methods, which are still largely at the experimental stage and rely on measurements of the crop plant water status or water stress. There are advantages and disadvantages to each method.

Indirect Methods

Balance Sheets

By using known models of evapo-transpiration, together with weather station data on rainfall, temperature, relative humidity, wind speed and solar energy, it is possible to estimate the rate of water loss from a crop, given the crop cover. A balance sheet can then be constructed to schedule irrigation applications to match water that has been lost via evapo-transpiration. In the simplest form these can be manual sheets filled in with details of farm rainfall and irrigation, with evapo-transpiration from weekly published Meteorological Office data.

Example of average daily evapo-transpiration rates for an agroclimatic area in south-east England

Month mm/day

April 1.9 May 2.7 June 3.1 July 3.1 August 2.3 September 1.5

The example of average daily evapo-transpiration rates relates to a full grass crop cover. For most crops, a conversion adjustment must be made to the published figure. In the case of strawberries in full leaf and crop, water usage is around 50-70% of the full grass evapo- transpiration figure.

However, these figures are averages. Actual daily evapo-transpiration during the summer can vary from 1.5 mm/day on a cool damp day to 5 mm/day on a hot windy day. It is highly desirable to take some account of weather conditions and potential water loss when deciding how much water to apply over the coming week. It is important to take into account the weather forecast for the coming days.

On-farm measurements of evapo-transpiration are also possible using specifically designed equipment such as the etGuage. There are also computerised balance sheet methods available

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such as ADAS Irriguide, which takes away the need for tedious calculations but still require the grower to provide rainfall and crop cover data on a regular basis.

The information provided by balance sheets is only as good as the information fed in, so any inaccuracy in measuring rainfall or irrigation will lead to accumulated errors.

The assumptions used for crop cover and rooting depth can be another source of error. However, the advantages of this method are that relatively little equipment is required, forward predictions are possible and an average result is given that is not dependent on a direct soil water measurement at a point that might not be representative of the whole field.

Direct Methods

Measuring Substrate Tension - Tensiometers and Resistance Blocks

The physical characteristics of a soil or soil-less substrate and its water content determine the tension or pressure at which water is held. The availability of water to the crop and the ease with which this can be extracted from the soil or soil-less substrate is known as moisture tension and can be measured by Tensiometers. Of all the methods used for irrigation scheduling, measurement of soil or soil-less substrate tension most closely relates to the availability of water to the crop at that point.

Tensiometers contain water in a porous cup mounted on a column which is inserted into the soil or soil-less substrate. As the growing medium becomes dry, water moves out of the cup and this creates a vacuum elsewhere in the instrument which is read from a dial gauge or, in the more recent devices, transduced via a sensor to give a digital readout. Digital tensiometers can be connected to data loggers for continuous logging of water tension, or can be connected to the irrigation controller to trigger a pre-determined burst of irrigation once a critical tension level has been reached. This is a useful facility for soil-less substrate grown crops.

For soil-grown crops, tensiometers are normally sited in pairs. One is placed to take readings at about one third of the crop plant’s total rooting depth, the other is located to measure moisture deficits in the lowest third of the rooting depth. Irrigation is applied when a certain critical tension is reached in the upper zone. The reading in the lower zone indicates when the whole rooting zone has been restored to a moist state, thus avoiding over-watering.

Most tensiometers have a scale of readings from 0 - 1000 hPa, (hPa = hectoPascal. 1hPa = 1 millibar) with a working range of 0 - 750 hPa. A zero reading indicates saturated soil. Readings of 100 hPa indicate field capacity in very light soils, whereas readings of around 300 hPa indicate field capacity for heavier soils. The upper limit of 750 hPa indicates 90% depletion in light soils, but only 30% depletion in clay or silt soils. Tensiometers therefore cannot be used in heavier soils where the soil might be expected to dry out by more than 30%.

Siting of the instruments is critical and sufficient are required to cover variations of soil and irrigation application in the field. Tensiometers are quite well suited for use in trickle-irrigated crops. The main problems are breakage in the field and unexpected loss of tension in the water columns if the soil becomes exceptionally dry. A number of growers find tensiometers quite satisfactory in strawberry bed systems. Once purchased, the running costs are low as readings can be taken by the grower.

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Porous Blocks

Another method of measuring soil water tension is with the use of porous blocks. These have to be buried in the soil at the desired depth and allowed to equilibrate with the surrounding soil water tension. The water content of the block therefore varies according to the availability of water in the soil. Various types are available, but the most common are gypsum resistance blocks where the electrical resistance of the block is measured by a meter and can be calibrated to the soil water tension. The Watermark block is available in the UK, which is a synthetic block, longer lasting than the gypsum blocks, which tend to dissolve. However, in comparative trials, the Watermark block was slower to respond to changes in soil water tension than tensiometers or ‘TDR’ probes (see below). They are, however, relatively inexpensive.

Measuring Dielectric Constant - Time Domain Reflectometry (TDR) and Capacitance Probes

It has been shown experimentally that the measurement of the dielectric constant (the ease with which electromagnetic waves can be transmitted) of the soil or soil-less substrate can be directly related to the volumetric water content.

There are two approaches to measuring the dielectric constant of the soil and hence the water content: Time Domain Reflectometry and radio frequency capacitance.

Time Domain Reflectometry (TDR) has become an accepted technique for soil moisture measurements. In the last few years the cost of the equipment has reduced and a number of portable and data logging devices have become available in the UK offering the advantages of easy probe installation, independence of soil type or growing media, and the option of instantaneous readout or data logging.

The standard TDR technique uses 2 or 3 parallel steel rods attached to a handle connected to the unit via a coaxial cable. Pulsed transverse electromagnetic waves are transmitted along the probes and the return lines of the pulses reflected by the end of the probes are measured. This signal is determined by the dielectric constant of the surrounding soil.

When installed vertically, the probes give an average reading over the length of the probe. To obtain readings at different depth zones, probes would need to be inserted at each depth. This is much less convenient than with the neutron or capacitance probes where point readings can be taken down the profile through one access tube.

With the TDR equipment it is relatively easy to take readings of soil moisture in the 0 - 15cm zone and the results will be more accurate than with a neutron probe. The TDR probes are also ideally suited for taking measurements in soil-less substrate and a number of devices are marketed specifically for this purpose.

TDR equipment can be used for direct, roving, measurements in the field but it is probably best used permanently sited, with a data logger, so that variations in water content at one point can be continuously monitored, eliminating site to site variation. With all TDR devices it is important that the probe is inserted tightly into the soil or soil- less substrate, as air pockets will lead to erroneous readings. This is easier to achieve with devices with relatively short probe lengths (<10cm).

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Radio frequency capacitance devices (commonly known as Capacitance probes) have also become more widely available in recent years. These devices use high frequency radio waves to measure soil capacitance, which is related to the dielectric constant of the soil, and hence the volumetric water content. They employ a permanently sited access tube similar to that used with a neutron probe. The probe is lowered into the access tube and readings are taken at each required depth.

Examples of this manually operated probe are the “Diviner” from Sentec and the “Theta probe” from Delta T devices. There are also capacitance probes designed for continuous data logging either through cable connections or radio links.

Some care is needed to ensure the access tube is installed tightly into the soil with no air gaps, which would result in inaccurate, low readings. When correctly installed and calibrated, capacitance probes will give accurate readings with many of the advantages of the neutron probe system, repeatable measurements at the same location

and depth profiles. Unlike the neutron probe, near surface measurements are possible, and there are no radiological hazards. The size of the equipment means, however, that it is not practical to use in soil-less substrate grown crops, and it is also more expensive than TDR equipment.

Neutron Probes

The neutron probe offers another way of determining the volumetric water content of the soil. The soil moisture is measured by lowering a source of ‘fast’ neutrons and a detector tube down an aluminium access tube set into the soil for the life of the crop. The detector tube detects ‘slow’ neutrons which are created when ‘fast’ neutrons from the source in the probe collide with the hydrogen atoms of water. The quantity of ‘slow’ neutrons is proportional to the amount of water present.

Because of the radiological hazards, only licensed operators can use this equipment so specialist contractors are normally employed. Commercial operators generally offer a service whereby 2 or 3 x 1.2m tubes are inserted in a field and readings are taken by the contractor weekly or as the grower requires. The results are converted into mm water at 100 - 200 mm zones down to 1.2m and a deficit calculated. Daily crop water use is estimated by deducting any rainfall or irrigation from the water loss.

Correction factors or calibration curves are required for each soil type to convert the readings into volumetric water content. In practice, commercial operators generally select a standard curve for the soil type on the site.

As with TDR and capacitance probes, the neutron probe gives a reading of volumetric water content. To convert volumetric water content into a soil moisture deficit (which is essential to interpret the results) it is necessary to know what the water content was at field capacity (“full point”). This requires readings to be taken at an appropriate time during the winter or at any time when the soil is thought to be at field capacity.

Normally the access tubes are sited in pairs in one location in the field, near the headland for convenience of reading. However, there is a risk that they may not be representative of the field as a whole. Another important consideration for shallow-rooted crops is that the neutron probe is less accurate for readings within 15cm of the soil surface, due to the escape of ‘slow’ neutrons from the soil surface.

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Notwithstanding these limitations, neutron probe readings have proved accurate in estimating the amount of water lost from a soil profile at a site between one reading and the next. This is still a very useful facility when trying to maintain soil moisture at a set level.

The ability to measure water content at different depths through the profile is also useful in interpreting the results and can lead to a better understanding of rooting depth and soil compaction within a profile.

The need to employ contractors to take readings at each site on a regular basis is unfortunately becoming increasingly expensive. Although there are advantages of subcontracting this work to knowledgeable specialists, some growers prefer the flexibility of taking readings using their own equipment such as TDR or capacitance probes.

Advantages and disadvantages of currently available methods of soil moisture monitoring : a grower’s perspective

Scores are qualitative; *****is most favourable, * is least favourable

Type Typical products

Initial cost

Ease of use

Running costs

Accuracy Use in soil-less substrate

Continuous data logging

possible

Water content by depth profiling

Manual sheet balance

*****

*

*****

*

No

No

No

Computerised balance sheet

Irriguide

*****

**

***

***

No

No

No

Tensiometer Irrimeter **** ** ***** **** No No No Digital electronic tensiometer

Skye elec.

tensiometer

***

***

*****

****

Yes

Yes

No

Synthetic block Watermark **** ** ***** *** Yes No No TDR

Theta probe Grodan meter ISUF (Netafim)

***

***

*****

*****

Yes

Yes

No

Capacitance probe with data logging

Enviroscan Delta T profile

probe

*

*****

*****

*****

No

Yes

Yes

Capacitance probe

Diviner

**

*****

*****

*****

No

No

Yes

Neutron Probe service

Various

***

*****

*

****

No

No

Yes

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Plant Based Methods

Remote Sensing

A number of techniques have been used experimentally to measure plant water status directly. Of these, the use of infra-red sensors is now the closest to commercial exploitation. The technique involves taking measurements of leaf temperature using an infra-red thermometer or camera and comparing it with a reference reading of a standard wet and dry surface. The degree to which the leaf is able to maintain a temperature below that of the ambient air temperature is used to generate a measure of plant water stress.

At present the potential application of infra-red thermography is being assessed in a limited number of field crops within the UK.

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6 Legislation

The European Union has identified water as a key natural resource, which must be managed in a sound and environmentally responsible manner. EU Directives are implemented in the UK under various Acts of Parliament and Regulations, relating to both water pollution and management of water resources. The new European Water Framework Directive and the draft Water Bill will impact on growers during the coming years.

Enforcement

The enforcement of most water-related legislation is the responsibility of the Environment Agency (England and Wales) and the Scottish Environmental Protection Agency (Scotland). Most concerns over legal compliance should be routed through these agencies in the first instance.

Water Pollution and Contamination

The Water Resources Act (WRA) 1991 allows for individuals responsible for causing pollution to be prosecuted. Under the WRA it is an offence to cause or knowingly permit a discharge of poisonous, noxious or polluting matter or any solid waste to enter ground or surface waters without proper authorisation. The Groundwater Regulations 1998 make it an offence to dispose of listed substances (including pesticides and some elements in fertilisers) to land where this may lead to a direct or indirect discharge to groundwater. Growers need to apply to the Environment Agency for authorisation to dispose of listed substances, which include pesticide tank washings.

Water pollution offences can carry a penalty of up to £20,000 in a Magistrates Court and an unlimited fine in the Crown Court. It may also be necessary to pay for any damage caused by pollution and clean-up costs.

The Code of Good Agricultural Practice for the Protection of Water (‘Water Code’) 1998 contains a section on ‘Specialised Horticulture’ which is relevant to soil-less substrate grown crops. The Code suggests that irrigation and feed system runoff should be monitored and operated in a way that minimises nutrient run-off.

Businesses using the public mains water supply come under the Water Supply (Water Fittings) Regulations 1999 (Water Byelaws 2000 in Scotland). These Regulations replace the former Water Supply Byelaws. They relate to the design, installation and maintenance of plumbing systems (including irrigation), water fittings and water using appliances. Their purpose is to prevent misuse, waste, undue consumption or erroneous measurement of water and, most importantly, to prevent contamination of drinking water. Businesses are required to carry out a contamination risk assessment for each water fitting or appliance that contains water, or other liquids, and is connected to the plumbing system. Only approved fittings should be used to prevent backflow and water suppliers should be provided with advance notification of any water system (e.g. irrigation) installation. Approved plumbers can provide certificates to demonstrate that systems are compliant.

Regulations under the Water Industry Act 1991 implement drinking water standards set under EC Directives 80/778/EEC and 98/83/EC (the 'Drinking Water Directives'). These set maximum admissible concentrations (MACs) for 57 listed parameters. The MAC for nitrate in drinking water is 50 mg/l NO3. The MAC for total pesticides is 0.5 µg/l, and 0.1 µg/l for a single pesticide. It is quite likely that the immediate run-off from soil-less substrate grown strawberries would exceed the limit for nitrate. Restrictions could be imposed on holdings found to be causing contamination of the source waters of public drinking water supplies.

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The EU Water Framework Directive (WFD) (2000/60/EC) came into force in December 2000 and should be incorporated into UK law by 22 December 2003. The Directive covers both pollution and water resource management and aims to bring together various EU Directives relating to water. The Directive will require the development of Catchment Area Management Strategies (CAMS) across the country, which will regulate water use. Each area will develop its own strategies, depending on the regional pressures faced. Growers may be affected in terms of the range of pesticides and fertilisers that can be used within certain catchments as well as measures to minimise impacts on local ecology and habitats.

Water Resource Management

Dry summers during the 1990s highlighted regional water shortages in various parts of the country. The Government is in the process of preparing a Draft Water Bill to enable water resources to be managed efficiently and effectively now and in the future. The draft Bill was widely publicised during the late 1990s and will undoubtedly impact on water abstraction licence holders. Under present legislation, all new abstraction licences are time limited. Existing non-time limited licences will become time limited under the proposed Bill. The Environment Agency (England and Wales) will assess licences and will probably tie time limits in with the six yearly cycles developed for CAMS. In the majority of cases, licenses will be granted for 12 years. This will be increased for schemes with reservoirs and reduced in areas where demand is high or habitats are threatened.

Where abstractions are causing adverse impact on the environment, then action will have to be taken to reduce the levels of abstraction. This may involve providing compensation to the abstractor involved. This compensation would be financed through increased license fees. In some areas of ‘water stress’, licence holders will be required to carry out environmental monitoring which may make water abstraction too costly for some abstractors. The efficiency of water use will also be assessed when licences are reviewed.

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7 Sources of Information

Useful References

Legislation guides and codes of practice

‘Code of Good Agricultural Practice for the Protection of Water’ (The Water Code). 1998. Defra Publications.

‘Taking Water Responsibly – Government decisions following consultation on changes to the water abstraction licensing system in England and Wales’. 1999. Defra Publications.

‘Tuning Water Taking – Government decisions following consultation on the use of economic instruments in relation to water abstraction’. 2001. Defra Publications.

‘The Water Supply (Water Fittings) Regulations 1999 and The Water Byelaws 2000 (Scotland) – What are they and how do they affect you?’. 2001. The Water Regulations Advisory Scheme.

‘Water Supply Systems: Prevention of Contamination and Waste of Drinking Water Supplies – Agricultural Premises’. 2001. The Water Regulations Advisory Scheme.

‘Guidelines for Farmers in NVZs – Nitrate Vulnerable Zones’. 1998. Defra Publications.

Technical notes, guides and other publications

Winter Storage Reservoirs – Getting control over your water resource (PB2512) Reprinted May 1996 (revised version in preparation)

Good Irrigation Practice – Making every drop count when there is not enough to go around (PB 2513) Reprinted May 1996

Irrigation Water Assessment and Management Plan – Efficient use of irrigation water (PB3935) Printed February 1999

Irrigation Scheduling – Uses and Techniques (PB 2511) Printed 1996

All from: Defra Publications, ADMAIL 6000, London SW1A 2XX Tel: 08459 556000 Fax: 020 8957 5012

Relevant Organisations and Contact Details

Environment Agency, Head Office, Rivers House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS12 4UD. Tel: 01454 624 400 Tel: 08459 333 111. Emergency Hotline: 0800 80 70 60 Internet: http://www.environment-agency.gov.uk http://www.environment-agency.wales.gov.uk

The Met Office, London Road, Bracknell, Berkshire, RG12 2SZ. Tel: 0845 300 0300 Internet: http://www.metoffice.com

UK Irrigation Association Executive Secretary: Melvyn Kay. Tel: 01427 717627 Fax: 01427 717624 Internet: http://www.ukia.org

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‘Irrigation Best Practice - A Guide for Top and Soft Fruit Growers’ was written by ADAS Consulting Ltd. with funding from the Department for Environment, Food and Rural Affairs (Horticulture and Potatoes Division)

Principal authors Michael Perry John Atwood

For further copies contact: ADAS Horticulture

Boxworth Cambridge CB3 8NN

Tel: 01954 267666 Fax: 01954 267659 Email: horticulture@adas.co.uk Website: www.adas.co.uk/horticulture

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