Article by Dr. Klaus Spohrer, University of Hohenheim, Institute of Agricultural Engineering
Kassel, Germany — This year, articles dealing with low water reserves for land used for agricultural purposes, the high prevailing temperatures or climate change in general often carried the title "Drought in Germany". Hand-in-hand with this, the necessity of irrigation is also being discussed with increasing frequency. The consensus is that irrigation is becoming ever more important to safeguard agricultural yields.
Many climate model calculations are forecasting an increase in temperatures and dry spells for Germany. Recent studies show that irrigation-worthiness, that is the economically justified use of irrigation, is increasing along with these dry spells. It is therefore to be assumed that, in the medium to long-term, irrigation will also be extended to crops and areas where it has previously been deemed unprofitable.
Planning Security is Decreasing
Planning security for farmers, authorities and associations is declining along with this extension of irrigation to new crops and areas. All stakeholders require knowledge regarding future location-specific and crop-dependent irrigation requirements in order to be able to plan and steer irrigation infrastructure in the long-term. Models for forecasting future, small-scale irrigation requirements estimated on the basis of climate projections, such as those developed for Lower Saxony (LBEG, GeoBerichte 20) or Baden-Württemberg (LUBW, BeProBW), may prove helpful in accomplishing this.
Efficient Use of Water is a Key Trend
Farmers in many locations are already finding themselves confronted with water shortages. Together with increasing water requirements, efficient use of water is therefore becoming ever more important in irrigation in order to avoid reductions in crop yield and quality. Developments and innovations in irrigation are gearing up to meet this challenge. The fundamental trend towards continuously reducing water consumption is clear, for instance.
Irrigation efficiency can essentially be achieved in two ways: if the potentially possible water efficiency of the irrigation system that is used is not achieved, optimized, location-specific irrigation planning can be implemented to move the actual water efficiency closer to its potential. In addition, the potentially possible water efficiency itself can be further improved through technical and functional modifications or developments.
Irrigation Planning is Becoming Simpler
Adapted irrigation planning can be undertaken on the basis of soil moisture data. The development of dielectric measurement approaches in the 1980s laid the foundations for the development of very good and easily operated sensors for measuring soil water content. A wide variety of these sensors are now available on the market at low cost. A few professional matrix potential sensors are also available; these supplement the tensiometers, which require maintenance, that measure the tension or suction that plants' roots must exert to extract water from the soil. However, matrix potential sensors with good measurement quality are still relatively expensive, which is due to their functional principle. As matrix potential sensors are able to determine the optimum start of irrigation very well, the development of inexpensive matrix potential sensors with good measurement quality is highly desirable.
An alternative to soil moisture sensors is the climatic water balance, in which the irrigation time and irrigation level are derived from measured weather data (precipitation, temperature, humidity, wind velocity and radiation). The climatic weather balance necessitates a nearby weather station to measure the required data. Suppliers that have access to corresponding measurement networks and data, and are therefore able to offer location-specific planning, have become established. Increasing digitalization and the possibility of networking sensors have led to the development of new options. Individual sensors (for example rain sensors) can be incorporated in the Internet of Things (IoT), for instance, thus enabling optimized water balancing. The measurement density has also improved due to the integration of private weather stations into publicly accessible weather databases.
Remote Sensing and Irrigation Planning
Data has to be acquired and processed, and action instructions regarding the irrigation time and level ultimately have to be issued for irrigation planning. Digitization has made it possible to acquire large volumes of data and quickly process measurement data relevant to planning. On the basis of current soil moisture measurements, key soil parameters (for example field capacity), characteristic plant properties and the weather forecast, modern systems already enable optimum irrigation planning to be implemented easily and quickly, output on mobile terminals and also controlled and administered with these devices. The trend in this development is shifting towards small-scale planning areas and therefore towards sub-area-specific (precision) irrigation.
However, spatially high-resolution, up-to-the-minute data regarding the crops' need for irrigation are required for this. The use of soil moisture sensors or the climatic water balance can be limited though, for example in the case of very heterogeneous soils and a very high number of sub-areas. Conversely, the derivation of irrigation requirements from image information enables a very high spatial resolution. Work is currently being undertaken on corresponding determination approaches in many locations, and small-scale irrigation planning by means of satellite images will be possible in the longer-term. As the spatial resolution of satellite images is often still too low at present, drones are being used in current research to obtain the required image information.
Optimization in Irrigation Technology
It must be possible to implement small-scale irrigation planning through small-scale water output. Corresponding control systems with decentralized operation that can be used for this purpose are already available on the market. In stationary systems, these can be flexibly usable radio nodes that can simultaneously activate sensors (for example, soil moisture) and actuators (for example, solenoid valves). Conversely, sub-area-specific irrigation in mobile systems is implemented using individually actuated solenoid valves that are installed upstream of the spray nozzles or small sprinklers and are activated depending on their current position.
A clear trend towards the reduction of operating water pressures, and thus the reduction of energy requirements during irrigation, can also be seen. In drip irrigation, pressure-compensating drippers, that implement identical flow rates with different water pressures, ensure spatially homogeneous water output. Modern pressure-compensating drippers can be operated with pressures as low as 0.4 bar. Similar progress has been achieved in spray nozzles for use in mobile irrigation systems. For instance, nozzles that can also be used with water pressures as low as about 0.4 bar are already available.
Lower water pressures when using spray nozzles necessitate smaller spaces between the spray nozzles on mobile irrigation systems. In turn, water efficiency and distribution accuracy can be further improved thanks to this close nozzle arrangement. This is particularly achieved through shorter spraying heights and distances, but also by means of larger droplets, resulting in less water loss due to wind drift and evaporation.
Low energy precision application (LEPA) aims to provide fundamental improvement of water efficiency in mobile irrigation by means of spatially limited water output with low water pressures. The objective of irrigation in this case is no longer the output of water across the entire crop, but directly onto the soil between the plants. In technical terms, this can be achieved through very low-hanging nozzles with short spraying distances and low water pressures. The resulting optimized water efficiency arises due first to the avoidance of interception, that is the trapping and evaporation of water on the leaves. Second, wind drift is negligible and evaporation losses across the surface of the soil are comparatively lower.
In summary, it can be stated that the developments and innovations in irrigation take account of the present and future problems concerning resource wastage and water shortage. They are also specifically supported by subsidy programs.
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