Evapotranspiration (ET) represents the water consumption of a soil due to the combination of two processes: water is lost from the soil surface by evaporation on the one hand and from the crop by transpiration on the other. ET values are typically measured in millimetres per day (mm/day) and are higher in the summer months and relatively lower in spring and autumn. Furthermore, the main factors affecting its value are sunlight, wind, humidity and temperature.
Why should I wish to use ET to schedule irrigation?
ET information allows farmers to better adapt irrigation to the needs of growing crops, thus helping to achieve higher yields and improve irrigation efficiency. Monitoring ET values on a daily basis helps eliminate uncertainties in irrigation. A small but regular improvement in irrigation scheduling can significantly increase production and overall crop quality. Another advantage of using ET is that the values are relevant for an entire plot and not at a single point, as is the case with soil moisture sensors. This allows for accurate irrigation scheduling considering the variability of soil type, soil water retention capacity, water extraction rates and irrigation uniformity.
The animation below shows the variation of daily ET values in a soybean field, generated by the Irreo web platform.
ET values are estimated using various satellite data and are provided to the farmer daily with a spatial resolution of 9 square metres (each pixel in the image corresponds to an area of 9 square metres).
Without objective information, it can be difficult to determine the best time to schedule irrigations. It is especially important for farmers to pay attention to ET data during the summer period, when crop water demands are high and growth rates are more sensitive to delayed irrigation.
What are the basic principles of using ET for irrigation scheduling?
To match irrigation to the needs of vegetatively growing crops, irrigation must be timed in relation to the amount of water in the soil that is readily available to plants (also known as 'Readily Available Water' or 'RAW'). RAW is the component of soil moisture that can be readily extracted by plants before they suffer moisture stress and lower growth rates. The basic concept is that as plants extract water (the value of which can be estimated using daily ET data), the soil water content decreases and reaches a threshold value above which water becomes more strongly bound to the soil and more difficult to extract.
When the cumulative water extraction exceeds the RAW value, soil water can no longer be extracted quickly enough to meet the water needs of vegetatively growing crops. Furthermore, there is a risk of a 'green drought' if the watering interval is 'stretched' and the cumulative water extraction exceeds the estimated RAW value. If this happens, the plants may appear healthy, but growth is impaired. In the extreme case without irrigation or rain, eventually a point is reached where water uptake is zero and the plants wither and die. In general, RAW varies depending on the crop and soil type. For example, for maize there will generally be up to 60-70 mm of RAW, while for alfalfa RAW tends to be in the range of 70 mm to 100 mm.
Traditional methods for calculating RAW on a particular soil require the use of tensiometric probes and several manual calculations to be performed, resulting in a complicated and time-consuming process. Farmers can now use Irreo to know exactly the RAW value on a particular soil, without the need for any sensors to be installed and in a fully automatic manner.
This is possible thanks to our technology, which combines various satellite data and 'Machine Learning' algorithms, and is able to estimate the soil structure under the leaf cover by monitoring the water infiltration process. This allows our users to analyse several soil moisture variables, such as field capacity (FC), permanent wilting point (WP), readily available water content (RAW) and total available water (TAW).
How can I use ET values to schedule irrigation?
The optimal time to irrigate is determined by subtracting the daily ET values from the maximum estimated RAW value (e.g. 40 mm) until the value becomes close to zero. When the RAW value is close to zero, it is time to irrigate again. In addition to water absorption by plants, rainfall clearly also influences RAW values, as it causes an increase in the water content of the soil. Furthermore, precipitation that does not contribute to the available moisture reserve of crops ('ineffective' precipitation) should not be considered. For example, the amount of precipitation that is lost due to surface runoff or that through infiltration exceeds the root zone should be ignored.
This table shows an example of how the readily available water (RAW) depletion process works within a plot. The table shows how daily ET and precipitation are used to estimate RAW each day and determine the time of the next irrigation. To optimise irrigation efficiency, the next irrigation is scheduled when the RAW is exhausted or close to zero. It is estimated that the soil in this case can hold up to 40 mm of RAW and that the soil is freely drained at the end of 'Day 0'.
In this graph, the data in the table above are visually represented, and it can be seen that the RAW gradually decreases each day due to ET and recharges with rainfall.
Where can I start?
The real problem for optimal irrigation scheduling, as we have described above, is finding and monitoring the different variables needed.
Using Irreo you can monitor ET values, rainfall and RAW without needing any sensors to be installed. In addition, the information is used to advise you daily on the optimal time for your next irrigation and the exact amount of water needed in a fully automatic manner.
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