Best Practice in Row Crop Irrigation

Transcription

1 2004 G R D C F O R I R R I G A T I O N C R O P P E R S Best Practice in Row Crop Irrigation An introduction to WATERpak a guide for irrigation management in Cotton Paper prepared by DAVID WILLIAMS NSW Department of Primary Industries Dubbo Tel: david.williams@agric.nsw.gov.au IREC C/- CSIRO Land and Water, Griffith Private mail bag 3 Griffith NSW 2680 Tel: Fax: irec@irec.org.au

2 Introduction Best practice in row crop irrigation is not often achieved for many reasons. However the desire to strive for best practice has been on the increase in recent years due to both internal factors on farm and external pressures essentially from the wider community. The internal factors include maximising the economic return from a set irrigation allocation or maintaining production in time of reduced allocations. The external pressures relate to increasing competition for water from irrigation, community requirements and the environment. The means to assist row crop irrigators to improve their operations have not always been available. A new publication titled An introduction to WATERpak a guide for irrigation management in cotton. is currently being printed for release in August at the 12 th Australian Cotton Conference. What is WATERpak? WATERpak has been in development for the past 18 months as a co-operative effort between researchers, consultants, extension staff and growers in the Australian cotton industry from New South Wales, Australian Capital Territory and Queensland. The WATERpak manual has been designed to provide technical and practical information to cottongrowers who are seeking to meet the requirements of the Cotton Industry s Best Management Practices program. WATERpak, while aimed specifically at irrigated cottongrowers, has over 75% of its content relevant to all row cropping irrigators. What does WATERpak aim to achieve? WATERpak aims to empower irrigators to achieve higher levels of productivity and irrigation efficiency. The challenge for irrigators is to find the balance between the higher costs of improved water use efficiency and environmental stewardship and the maintenance of farm profits. Irrigation efficiency has not been high as it should have been on the cotton agenda until quite recently but is certainly a stand out issue at the moment. This situation will be the norm for some time to come. While most irrigated cottongrowers are looking to meet the accepted industry standard of one bale of cotton per megalitre, the leading growers are already achieving two bales per Ml. However some researchers have raised the bar higher claiming that five and more bales per Ml is an achievable potential. The beauty of this efficiency index is that it combines both irrigation and agronomic performance. Both areas are required to perform well to get good results. The easiest gains to improve farm water use efficiency are within the field: minimisation of tailwater losses, drainage and the potential improvement in yield through the reduction of waterlogging effects. Harder to achieve but very significant in terms of water use efficiency, gains exist in the control of evaporative and seepage losses from storages and channels. Page No 1

3 WATERpak content and chapter key points Each of the WATERpak sections is listed below along with the associated chapters and their relevant key points. Self assessment checklist for irrigation efficiency. This first section aims to allow irrigators to identify the areas of their operation where irrigation efficiency improvements can be made. In this checklist the terminology has a cotton flavour, but the general principles should transfer to other crops readily. Once areas of potential improvement have been identified, information on how to make the required change can be found by using the WATERpak link for each question. Farm planning. This is a small section that looks at the links between state irrigation farm plans and the industry s Best Management Practice program. Efficient irrigation. o Assessing whole farm use efficiency Water use efficiency describes a relationship between system inputs and outputs. Relating production outputs (such as $ or bales) to water input (Ml) results in a water use index (WUI). Relating water output (Ml) to water input (Ml) results in a dimensionless (%) irrigation system efficiency. o Water use efficiency in the Australian cotton industry The industry average whole farm irrigation efficiency was 59% but all studies have observed a large variability in efficiency between farms. The greatest losses identified were likely to be from farm storages, via evaporation losses, and seepage from unlined distribution channels. The industry average crop water use index was 2.79kg/mm/ha (1.32 bales per Megalitre). Water movement and amounts need to be documented. Producers at the lower end of the ranges have significant scope to increase the efficiency with which they use water. Improving whole farm irrigation efficiency by just 1% could produce an additional $6,500 per hectare per 1000Ml allocation, when the price is at $500 per bale. o Water balance and deep drainage under irrigated cotton Deep drainage below the rootzone causes rising watertables and salinity and can be significant even in heavy clays. Drainage occurs when more rain or irrigation is added to the soil than there is empty storage capacity to hold it. Drainage risk can be reduced by maintaining sufficient empty storage (soil water deficit) as a buffer. o Deep drainage under irrigated cotton in Australia: a review Deep drainage varies considerably depending on soil properties and irrigation management, and is not necessarily very small. Drainage of 100 to 200 mm/yr (1 to 2 Ml/ha) is typical, although 3 to 900 mm/yr ( 0.03 to 9 Ml/ha) has been observed. Soils used for irrigated cotton have much more diverse properties and management requirements than the simple description clay soil suggests. Some drainage is needed to avoid salt build up in the soil profile. Page No 2

4 The consequences of deep drainage are distinctly different where underlying groundwater can be used for pumping (fresh water, high flow rate) and where it cannot (saline water, low flow rate). Increased stream salinity is a threat to the irrigated industry. o Developing a surface irrigation system Selecting an irrigation system. Upgrading a surface irrigation system. Soil types for storages and channels. The perfect layout for an irrigation system. Storage and distribution efficiency. o Assessing the efficiency of storages, channels and reticulation systems Don t assume that evaporation and seepage are the greatest losses from your storages, channels and reticulation system. Take measurements to find out what your greatest loss is. Determine if these losses are a problem by carrying out an economic assessment of the benefit that could arise from addressing these losses. o Managing evaporation and seepage in storages and channels Modifying the effect that wind speed and surface area have on evaporation losses from storages and channels. Seepage issues are most commonly caused by unplanned or poor construction, use of suitable soil type, poor soil compaction and poor maintenance. Prevention is better than cure! Maintain and monitor storages and channels to save expense and losses in the long term. To choose how or whether to mitigate evaporative or seepage losses, balance the cost of the repairs against the short or long term benefit, and the value given to the water being lost and the crop being produced. o Metering irrigation water Water meters can be used to establish pump efficiencies and benchmark irrigation system performance. Correct installation of a water meter is as important as the choice of water meter. There are three main types of water flow meters electromagnetic, ultrasonic and propeller actuated. In comparing water meters, irrigators should consider their accuracy, repeatability, ability to handle trash and irrigation water, the effect of wear on their performance and cost. Application efficiency and irrigation scheduling. o Assessing field-scale water efficiency Relative small design or management changes at a field level can greatly increase the water use efficiency of a farming system. Measuring water use at a field level is an extremely useful management tool. There are techniques available that either irrigators or irrigation consultants can use. Water use efficiency at a field level is affected by the volume, uniformity and timing of irrigations and rainfall as well as crop performance. Page No 3

5 Measurement of water volumes can be undertaken by monitoring bulk flows onto a field (any irrigation system) or by monitoring individual furrows and extrapolating data across the field (furrow systems). Accurate measurement of soil moisture is important in determining accurate water use efficiencies. Commercial services can provide detailed measurements of water use and modelling to assess and optimise irrigation performance of individual fields. o Irrigation scheduling of cotton Irrigation scheduling improves water use efficiency, reduces waterlogging, quantifies the effectiveness of rain and allows better management of soil structure problems. A decline in the crop daily water use indicates the crop needs irrigating. Regular and careful monitoring is needed to detect this decline in crop water use. Extending the irrigation interval once regular irrigation has started without monitoring soil water levels can result in yield loss. Don t stress the crop during peak flowering and boll filling. Every cotton field is different. Soil structure and management have a dramatic impact on soil water availability and the irrigation interval. Do not assume the deficit or readily available water capacity is the same for neighbouring fields. Look after the cotton plants near your soil moisture device. If the data seems suspect, check the measuring site. o Calibrating soil water monitoring devices Set-up, calibration and operation of neutron and capacitance soil water probes are discussed. Different software-based corrections are used to convert raw data into soil water content levels, resulting in discrepancies between devices. Calibration of soil water monitoring equipment is not always required to undertake irrigation scheduling of crops. o Evapotranspiration Evapotranspiration is the combined loss of water to the atmosphere from evaporation from soil and plant surfaces, and transpiration through plants. Many factors affect the rate of water loss by evapotranspiration the weather, the crop, the environment and management. Crop evapotranspiration (Et c ) can be estimated using a crop coefficient (k c ) and a reference crop evapotranspiration (Et o ). The Penman-Monteith approach is the preferred method to estimating Et o. Evapotranspiration is difficult to measure directly. It can be estimated using meteorological data or the Class A Pan. The Class A Pan must be correctly sited and maintained for meaningful estimates of Et o to be made. It should only be used for estimates greater than 10 days duration. o Using automatic weather stations Automatic weather stations (AWS) provide site-specific atmospheric information that irrigators can use to assist irrigation scheduling decisions. There are a range of factors to consider when purchasing and AWS: sensor availability, accuracy, robustness, method of calculating Et o, maintenance issues and availability of technical support. The siting of the AWS is critical to the accuracy of climatic data recorded. Regular and proper maintenance of the AWS is necessary to obtain accurate data. Page No 4

6 Irrigation management of cotton. o Cotton growth responses to water stress Cotton plant responses to water stress vary depending on the stage of growth at which the stress occurs, the degree of stress, and the length of time the stress is imposed. The plant aims to establish a balance between carbohydrate supply and demand. Water stress at any stage of growth will affect both the production and distribution of carbohydrates throughout the plant. Carbohydrate demands on the plant, primarily made by developing bolls, restrict excessive vegetative growth. Through adaptation, the cotton plant survives during periods of water stress by prioritising the maintenance of different physiological processes to ensure the production of viable seed and therefore cotton fibre. The impact of water stress on final yield depends on the degree to which each physiological process is affected. o Managing irrigated cotton agronomy Crop rotation, mepiquat chloride and nitrogen rate can affect the irrigation requirements and scheduling during the season. Several tools exist to help growers manage their farm and irrigation water. Climatic risk and rainfall probabilities can be determined for any location in the Australian cotton industry. o Waterlogging: its impact on cotton Waterlogging soils reduce the access of the roots to oxygen, impairing root growth and function and ultimately nutrient uptake. Toxic gases in the waterlogged soil can also increase. Waterlogging reduces cotton yields by reducing the number of bolls on the plant. The risk of waterlogging can be reduced by optimising field design, bed formation and irrigation scheduling. The application of some foliar fertilisers may also assist in fields known to waterlog. o Managing irrigation with limited water Do not risk low crop yields by spreading water too thin. A positive gross margin on a smaller area of crop is better than a negative one on a large area! Consciously decide how much risk you are prepared to accept. Calculate the area you are able to fully irrigate with the supply available. Select high priority fields on an efficient supply and good yield history. Choose a cultivar suited to your production region. Avoid excessive nitrogen, which encourages rank vegetation growth and wastes irrigation water. Reduced fibre length is the main quality concern with limited water. Varieties with inherently long fibre buffer the risk of penalties. Maintain your normal irrigation strategy and only increase the irrigation interval in extreme cases. Delay the first irrigation interval in extreme cases. Delay the first irrigation rather than stress the crop during flowering. Approach defoliation as normal, deciding on the last harvestable boll, and monitor plant maturity to determine the defoliation date. o HydroLOGIC furrow irrigation water management software HydroLOGIC is a software tool designed to help growers manage furrow irrigation in cotton. Predictions of crop growth are made using OZCOT crop growth model and historical climate information. Page No 5

7 HydroLOGIC has maximised yield and water use efficiency under trial conditions, in both full and limited water situations. HydroLOGIC complements existing soil moisture and weather station technology. o Irrigation and cotton disease interactions Irrigation practices have contributed significantly to the Fusarium wilt, black root rot and Verticillium wilt problems of the Australian cotton industry. Irrigation practices can and should be modified to reduce the rate of increase of plant disease problems. o Measuring plant water status Plant based measurements are effective at monitoring the water status of a crop. Plant based measurements are not practical to schedule irrigations due to the high frequency of clouds at solar noon in most cotton areas in Australia. Plant based measurements work best in sunny conditions when air is dry (that is, relative humidity is low). Plant based measurements may be useful for on-farm trials where researchers are involved. Irrigation systems. Whatever system you have you must think appropriately and your management and your operations will change and should change as you review what return you are making on every millimetre of water you have whether it be rain or pumped. o Furrow irrigation systems Adequate management and maintenance of all components between the head ditch and the tail drain is important for furrow irrigation systems. Improving furrow irrigation performance involves careful management of flow rates and irrigation duration and appropriate timing (scheduling) of irrigation events. The relationship between head and flow determines the amount of water applied to a field. Flow through siphons and culverts increases as head increases and decreases as head decreases, hence variations in head cause variations in volume of water applied. Optimal furrow irrigation performance requires understanding of application efficiency and distribution uniformity and the methods for improving both. o Drip irrigation systems - The inclusion of this section and the following one on centre pivot and lateral move irrigation systems is an indication that best practice row crop irrigation does indeed include drip and spray irrigation systems as an alternative to surface irrigation. A system designed by a row crop engineer who is experienced, preferably in cotton, is critical to achieve the potential water savings and flexibility in crop management that drip irrigation can offer. A well-planned maintenance program is essential to maintain proper system operation. It is important to monitor and control the quality of water used with the drip system, which determines the frequency of flushing required. Drip allows accurate application of water and fertiliser to suit crops; requirements and flexibility in field operations, but the management requirement is higher than conventional surface systems. o Centre pivot and lateral move irrigation systems Ensure the system capacity of centre pivots and lateral moves (CP&LMs) is large enough when managed correctly, to keep up with peak crop water requirements. Page No 6

8 Using larger diameter pipe spans cost more, but lifetime running costs are dramatically reduced. All CP&LMs will operate with sprinklers to germinate cotton crops, including those machines that operate LEPA irrigation throughout the main growing season. Sprinkler packages represent a small part of the overall performance of the machine more than any other aspect. New systems have problems with wheel tracks and wheel ruts, but these become less as levelled land compacts. Simple equipment alterations can help: reducing nozzle flow rates around towers; relocating LEPA outlets and sprinklers to keep wheel tracks dry; and reducing tower water interception from sprinklers. Consider larger tyres, or using third or fourth incline wheels and gearboxes on electrically powered towers. Support and assist local manufacturers who are prepared to resize jigs and build 48 metre spans for guidance systems, popular in the Australian cotton industry. Guidance systems can now operate in circles for centre pivots, and swath widths can be adjusted under spans that are not 48 metres. Ensure that all water drains from span pipes, so that pipe insides remain dusty dry between irrigation. Test irrigation water quality before you buy a system, to ensure compatibility of irrigation waters and pipe coatings. Continue irrigation long enough after fertigation has finished to ensure machine is fully flushed. Managing soil and water. o Managing soils for irrigated cotton production Farm management affects soil structure, which in turn affects plant available water. Good soil structure is essential in maximising water use efficiency Soil pit observations, chemical testing and visual inspection will help soil management decisions. Irrigation system construction efficiency will be influenced by soil type. Applying water-run fertiliser Water-run urea is an effective means of applying N to cotton up to boll filling. Use urea, not anhydrous ammonia, to reduce N loss. Other mineral fertilisers are not well suited to application in irrigation water. o Assessing and managing irrigation salinity: including EM surveying Irrigation salinity is a significant but often hidden issue. Especially in the case of salinity, prevention is much better than cure. Irrigators need to understand the relationship between irrigation and the causes of irrigation salinity. Measure soil and water salinity levels on a regular basis to observe trends and identify problems. Irrigation salinity can be managed and quite often reversed. While cotton is quite tolerant of saline conditions, steps should be taken to minimise saline impacts on other crops and the landscape in general. EM surveys need to be groundtruthed with soil tests. o Using poor quality water to irrigate cotton Sources of poor quality water are usually bores or treated sewage effluent. Poor quality water is usually dominated by sodium and chloride which cause both salinisation and sodification, with some water having excessive bicarbonates, which cause very high alkalinity. Page No 7

9 Immediate consequences of salinisation are nutritional and osmotic stress on the crop, whereas sodification causes soil structural destabilisation leading to waterlogging than others. Poor quality water can be used for irrigation if appropriate management practices are put into place. Using salt-tolerant cotton varieties. Some cotton varieties are more tolerant of salinity during their seedling stage than others. Catchment scale impacts. o Catchment water quality and cotton: northern NSW case study o Water quality in the Gwydir Valley watercourses o Water quality in Queensland catchments and the cotton industry Investigative irrigation research. o Using PAM in irrigated cotton o Regulated deficit irrigation and partial rootzone drying Conclusion WATERpak will be a valuable tool for row crop irrigators who are seeking to improve their operation and seek to achieve industry best practice. Irrigators of crops other than cotton are fortunate that a good proportion of the content of WATERpak is readily transferable to other irrigated row crops. Note: WATERpak is available on CD from the Cotton Technology Resource Centre in Narrabri on Reference Dugdale, H, Harris, G, Neilson, J, Richards, D, Roth, G and Williams, D 2004, WATERpak A guide for irrigation management in cotton, Cotton Research and Development Corporation, Narrabri, Australia. Page No 8