Soil moisture-based irrigation is a tool to reduce the demand for irrigation water by giving the user insight into the current soil moisture condition. There is an increasing demand for water to irrigate agricultural crops, as well as for irrigating pastures, arboriculture, gardens, parks, sports fields and golf courses, especially in warm, dry periods. At the same time the demand increases in other sectors, such as for maintaining water levels, cooling water, cold drinks and swimming pools. In addition, the discharge of the large rivers is low which also requires more fresh water in order to combat salinity and for maintaining sufficient water-depth for navigation.
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Soil moisture-based irrigation is a tool to reduce the demand for irrigation water by giving the user insight into the current soil moisture condition. There is an increasing demand for water to irrigate agricultural crops, as well as for irrigating pastures, arboriculture, gardens, parks, sports fields and golf courses, especially in warm, dry periods.
At the same time the demand increases in other sectors, such as for maintaining water levels, cooling water, cold drinks and swimming pools. In addition, the discharge of the large rivers is low which also requires more fresh water in order to combat salinity and for maintaining sufficient water-depth for navigation. This scarcity of water is already a fact during dry summers and the frequency of this occurring will increase due to climate change.
It has been shown that more water is used for agricultural irrigation when the user does not have any insight into the actual moisture condition of the soil. Soil moisture-based irrigation therefore is a means to reduce the demand for irrigation water. Because the water gift can be better adjusted to the actual water requirements of the crop, there is also a reduced discharge or run-off of insecticides and nutrients into ground and surface water.
Topics: water shortage and fresh water supply, drought damage, salt damage Delta facts: pricing of water for agriculture , soil as a buffer , effectiveness of water intake , Controlled drainage. Soil moisture-based irrigation is a strategy of optimal water supply.
As a strategy for farmers: yield maximization by creating optimal soil moisture conditions for crops and limiting irrigation costs. As a strategy for water managers: reducing the demand for irrigation water , preventing water waste and the decline of water quality through excessive irrigation. In the simple home version figure 1 the supply switch receives a signal from the soil moisture sensor when the moisture in the soil drops below a certain pre-determined value.
This triggers the supply of water to the sprinkler, which has been set for an adjustable time. In the advanced version figure 2 the soil moisture is measured at several depth levels and the information on current weather conditions are sent through to a central computer. Aside from the soil moisture measurements, the control of the irrigation system also allows for measuring and taking other criteria into consideration.
A large number of methods are used in irrigation practice for measuring or estimating the moisture conditions in the soil. The most reliable but also the most labour-intensive method is by means of soil sampling, at various depths with a ground drill. The moisture content is determined by weighing samples before and after drying in an oven. Commercial companies often use tension meters figure 3 to measure suction tension, from which moisture content is derived. Figure 3. Determining irrigation schedule with tensiometers.
Explanation Two questions have to be asked when irrigating crops: 1. When should irrigation take place? How much water has to be administered? This results in a lower crop yield due to conditions being too wet or too dry in conjunction with suboptimal water productivity. Field experience has shown that too much water is usually given if there is a lack of information about the actual irrigation need. This leads to water waste, subsiding soil or surface water levels and nutrient as well as insecticide run-off into the ground and surface water.
In soil moisture-based irrigation the time and quantity of irrigation is ascertained by means of the actual moisture condition of the soil.
The soil moisture is the determining factor in this method. This method of working distinguishes itself from two other methods often applied in irrigation practice:. The soil moisture content is optimal for crop growth when the crop can access both sufficient water and sufficient oxygen in the root zone. With crop growth on full ground in the open air , the soil moisture content is continually under the influence of precipitation and evaporation.
After heavy rainfall, the top layer of the soil is saturated with water. After a few hours, depending on the type of soil, the excess water sinks to the deeper soil layers of the ground water. The volume percentage soil moisture at field capacity depends on the soil structure and the soil composition.
Evaporation reduces the moisture content in the soil. As the soil dehydrates further , it becomes more difficult for the crop to absorb moisture from the root zone. At the point of wilting, the moisture content of the soil is so low that most crops are unable to absorb water from the soil. The volume percentage soil moisture at wilting point, just like at field capacity, depends on the soil structure and composition figure 5 and table 1.
Figure 4. Components of the water balance that are the determining factors for the actual moisture content of the soil. Figure 5. Moisture characteristics for four different types of soil.
Source: Cultuurtechnisch vademecum Table 1. Moisture holding capacity for different soil types Source: Dr. Soil moisture-based irrigation means that the actual moisture content in the soil is the determining factor for both the irrigation schedule and the water quantity. The time schedule depends on the chosen strategy. At sufficient water availability, the choice is for maximizing crop yield.
Irrigation is scheduled as soon as the soil moisture content has reached a level where the actual evaporation becomes less than the potential evaporation. As much water as required is given to bring the root zone back to field capacity. The irrigation can be postponed or skipped, based on one or more of the following considerations:.
In an automated system where the soil moisture sensor directly controls the irrigation system, the abovementioned considerations do not play a role: both the time and quantity of irrigation are then only dependent on the previously provided settings of the soil moisture content levels that trigger activation of the irrigation system.
In more advanced systems, these considerations can be taken into account. In addition, the system then also provides support for making these decisions. Other water related matters could also play a role, such as preventing or combatting plant disease. The most advanced systems use sensors for soil moisture as well as weather data from a meteostation on the premises, a nearby location or a weather satellite. Another tool that is used are Crop growth simulation models, with which the effects of whether to irrigate or not are estimated.
These crop growth models can be adjusted based on the results that were achieved at the same location in previous years. There is insufficient data available for Dutch conditions as yet to indicate what the savings could be in terms of water use with soil moisture-based irrigation compared to conventional irrigation.
Under controlled conditions, it is easy to show the effect of a soil moisture-based water gift on production see Figure 6. In the execution of field test in the Netherlands, significant yield differences through irrigation depend on the precipitation pattern.
The cost reducing aspect of soil moisturebased irrigation becomes more clearly visible in the decreased use of pesticides and crop protection products. More examples can be found on www. From the above, it appears that soil moisture-based irrigation only becomes viable if irrigation is economically rewarding.
Pre-conditions are:. Figure 7. In homogeneous soil, the groundwater level can drop to the stated levels below effective root zone before the soil moisture delivery stagnates. If irrigation is financially viable, the following considerations apply for the promising application of soil moisture sensors:.
Table 2. Uniformity of irrigation. In the WaterSense project more than sensors were placed on a surface of 20, hectares with the intention of determining the effect of soil moisture-based irrigation on both production as well as the quality of surface and groundwater.
On balance, was not successful, the differences are far too small. It is evident that under these conditions, it is difficult to make up for the investment in soil moisture sensors around 2, Euro each. There are far more inexpensive sensors on the market that are already being applied on large-scale in agriculture in North America and Australia, and also in southern Europe see www.
The presence of a large number of commercial companies that offer soil moisture sensors indicates that soil moisture-based irrigation is worthwhile in these areas.
One thing that should be taken into consideration however , is the demand and spending power of the recreation industry especially golf courses. As yet, there is insufficient information available to indicate what the savings are in terms of water use under Dutch conditions. In final report of WaterSense the abovementioned conclusion remains: the research has shown that the added value of irrigation for agricultural yield was only shown for one of the two potato varieties. The added value was not very large: better use of fertilizer and a slightly higher yield.
This does not make clear whether irrigation is worthwhile in the current climatological conditions in the Netherlands. For Dutch conditions, it can be made plausible that moisture-based irrigation leads to a reduction in crop protection products. There is insufficient data available to quantify the effect on water use. There is a striking discrepancy between the trouble taken by the researchers and companies to accurately determine the irrigation requirements on the one hand, while on the other hand, their in accuracy at water supply on plot level.
Also see the text on uniformity under pre-conditions. The use of soil sensors to ascertain the irrigation requirements — as well as the application of mathematical models — is based on the assumptions that soils are homogeneous and that crop growth is determined by physical and chemical soil conditions. In actual fact, the soil is often highly heterogeneous and biological factors are co-determining in crop growth. Home Deltafacts: altijd de nieuwste feiten over klimaat, waterbeheer en waterkwaliteit Zoetwatervoorziening Delta facts, English versions Soil moisture-based irrigation Soil moisture-based irrigation Soil moisture-based irrigation is a tool to reduce the demand for irrigation water by giving the user insight into the current soil moisture condition.
Thema Zoetwatervoorziening, Delta facts, English versions Tags bodembeheer droogte zoetwatervoorziening Downloads ga direct naar de downloads. Figure 1. Simple home and garden version. Figure 2. This method of working distinguishes itself from two other methods often applied in irrigation practice: Moisture Accounting. The current moisture levels in the soil are not actually measured, but determined from measurement or estimate of at least the precipitation and the present crop evaporation.
In actual fact, the moisture stock levels actually depend on many other components as well Figure 4. In practice, it is impossible to make an exact determination of all these components. Observation or measurement of the current moisture condition of the plant.
One objection against this method is the fact that once dehydration conditions become visible, the crop is already experiencing a shortage.
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