Soil moisture monitoring has as its main objective to determine when and how much water to irrigate. This monitoring can be accomplished by different techniques, including sensors in the soil, like a C-Probe, or a water budget modeling approach using climate parameters, such as evapotranspiration and rainfall. All of these techniques have been tested and validated for different crops around the world, showing promising results.
By adopting irrigation monitoring techniques growers will be able to irrigate rationally, improving crop yield and quality, increasing the effectiveness of fertilizers and encouraging the proper balance of micro-organisms. In addition, the proper management of irrigation will reduce soil salinity and increase soil oxygen content, improving root activity. Irrigation monitoring allows for improved crop performance and can reduce the grower’s water bill directly in maximizing water usage along with extending the lifetime usage of the irrigation equipment.
A C-Probe records volumetric soil moisture data by using capacitance sensors. The sensors are mounted on a column that is inserted into a PVC access tube in the ground (Fig. 1). A standard C-Probe may have up to six sensors on the column. The sensors can be placed at any depth required by the grower. Typically, most sensors are placed near the portion of the plant’s root zone that draws the most water from the soil. WIN often uses a C-Probe with moisture sensors at 10 cm (4 in), 30 cm (1 ft) and 50 cm (2 ft).
Fig. 1 - C-Probe installed in a vineyard.
The data from each sensor is processed by an electronic circuit on the C-Probe and is transmitted to a telemetry communication unit. The unit stores the data then relays it to a central base station, where they are processed. Chatham, Ontario is WIN’s central processing location.
The soil moisture data is sent to growers through a password protected program provided by Weather INnovations Incorporated (WIN) available at www.weatherinnovations.com.
An example of a graph showing the soil moisture levels at different soil depths is presented in Fig. 2, as follows:
Fig. 2 - Soil moisture levels at different soil depths.
A discussion on how best to read and understand the C-Probe graph is found later under the heading How to read a C-Probe graph. However before that, one needs to have some information about Soil Moisture Characteristics and Evapotranspiration.
Field Capacity (FC) is the maximum amount of water a soil can hold after drainage. Irrigation Lower Limit (ILL) is the point at which the plant has used all water that is readily available. Beyond ILL, as the soil dries out, the plants will extract water under stress on the crop. The interval between FC and ILL is called Readily Available Water (RAW) – water stored in the soil that is easily extracted by the plants. Unless a grower is trying to stress his crop (e.g. deficit irrigated wine grapes), he should aim to maintain soil moisture above ILL, through irrigation, which means under RAW at all times. The amount of RAW available to a crop will vary with soil type, crop type, crop rooting depth and irrigation system.
Fig. 3 shows a representative scheme of the soil moisture characteristics, demonstrating the soil saturation point (SAT), field capacity (FC) and permanent wilting point (PWP).
The intervals define residual water (RW), which can not be taken up by plant roots, gravitational water (GW), which will drain, and available water (AW) for plants. Irrigation lower point (ILL) is also presented in the picture and represents the bottom limit of soil moisture for plants extract water without stress. For grapes and peaches this level should be around 60% available water. The irrigation upper limit (IUL) is the maximum moisture that irrigators should aim for after an irrigation event. Ideally plant growth is maximized if water levels range between the IUL – irrigation upper limit and the ILL – irrigation lower limit. The interval between FC and ILL defines readily available water (RAW), whereas the interval between IUL and ILL defines the readily available water for irrigation purposes (RAWi). Keeping soil moisture under RAWi helps avoiding occurrence of saturation, during heavy rainfall events.
Fig. 3 - Representative scheme of the soil moisture characteristics.
Evapotranspiration is defined as the simultaneous soil evaporation and plant transpiration processes, since there is no easy way of distinguishing between them. Apart from the water availability in the topsoil, the evaporation from a cropped soil is mainly determined by the fraction of the solar radiation reaching the soil surface. This fraction decreases over the growing period as the crop develops and the crop canopy shades more and more of the ground area. When the crop is small, water is predominately lost by soil evaporation, but once the crop is well developed and completely covers the soil, transpiration becomes the main process.
The amount of water required to compensate the evapotranspiration loss from the cropped field is defined as crop water requirement. Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost through evapotranspiration. The irrigation water requirement basically represents the difference between the crop water requirement and effective precipitation.
The crop type, variety and development stage should be considered when assessing the evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics result in different ET levels in different types of crops under identical environmental conditions. To estimate crop evapotranspiration (ETc), reference evapotranspiration (ETo), which is the evapotranspiration rate from a reference surface not short of water and determined with weather data, must be multiplied by a crop coefficient (Kc), which represents empirically the effects of crop type and characteristics.
The C-Probe sensor offers the possibility of site-specific soil moisture monitoring. The graphs below show an example of how soil moisture varies with time. Soil moisture is a result of the water balance, which means the balance between rainfall and crop evapotranspiration (ETc). During dry days or when rainfall is less than ETc, the soil moisture will decrease. While the soil water content is high, the decrease will be fast, but as the soil gets drier the slope of moisture line starts to diminish and when it becomes flat it means that plants are not extracting enough water from soil for their metabolism. On the other hand, when rainfall is greater than ETc or irrigation is applied, soil moisture will increase. When a heavy rainfall happens, soil moisture can increase quickly and if sufficient rainfall occurs the soil moisture rises above the field capacity (FC), into the saturation point (SAT), resulting in water drainage and run-off.
For irrigation management purposes, a C-probe should be used to keep soil water under RAWi (ready available water for irrigation purposes), or between IUL and ILL. When soil moisture decreases to ILL it is time to irrigate with enough water to increase soil moisture up to IUL. This is why growers should know the soil characteristics of their irrigated fields. Fig. 4A shows the conditions for an irrigated crop, where soil moisture was always between ILL and IUL.
On the other hand, Fig. 4B shows the conditions of a non-irrigated crop, where soil moisture is close or below ILL, the majority of time. Under these conditions, the crop is under water stress, which will reduce crop growth and yield.
So, one of the most important steps, when monitoring irrigation using a C-Probe, is to define ILL and IUL. As stated earlier, the allowable soil water depletion for grapes and peaches is around 40%, which represents that ILL is 60% of AW. As an example, if a soil has AW = 70 mm, RAW will be 28 mm. In other words, growers should start to irrigate when the C-Probe reading is at 42 mm (ILL), stopping when the soil water achieves 60 mm (IUL = 0.85*AW), or with a net irrigation of 18 mm.
Fig. 4 - Examples of soil moisture monitoring using C-Probes at 10 and 30 cm depth, for an irrigated crop (A) and a non-irrigated crop (B).
For a period of time, irrigation experts would use one or the other of these soil monitoring techniques. Nowadays the technologies complement one another, with soil moisture sensors used to fine tune ETo. Growers on the other hand use the C-probe graphs as trend lines helping them "see into the soil" providing visually guidance when and how much to irrigate.
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Allen, R.G., L.S. Pereira, D. Raes, and M. Smith, 1998: Crop Evapotranspiration - Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. Food and Agriculture Organization. Rome.
Doria, R., C.A. Madramootoo, P.Eng., B.B. Mehdi, A. Suchorski 2007: Irrigation Scheduling Technology for Peach and Wine Grape Production in Southern Ontario. Brace Centre for Water Resources Management, and Faculty of Agricultural and Environmental Sciences, McGill University.
Ontario Ministry of Agriculture and Food, 2004: Best Management Practices - Irrigation Management.
Van der Gulik, T.W. 1999: B.C. Trickle Irrigation Manual. B.C. Ministry of Agriculture and Food.