Irrigation management technology is advancing in a pace faster than you can imagine. This contribution will provide you with a lot of insight.
Intentions of targeted irrigation
The main operational objectives to be achieved with precision irrigation are essentially these points:
- Controlled supplemental irrigation
- Nutrient utilization
- Quality and yield of harvested products
The factors influencing and the points associated with irrigation are many:
- Adjust watering to the needs of the plants to be irrigated.
- Align the amount of water with the water storage capacity of the soil
- Align application and timing with weather forecast
- Avoid evaporation losses as far as possible
- Prevent nutrient leaching
- Observe the quality of the irrigation water
- Observe concession conditions
The necessary watering depends on:
- Stage of development and condition of the plant
- Rooting and stress level of the plant
Soil moisture measurement
A soil is water-saturated when all pores of the soil are filled with water. Part of this soil water penetrates into deeper zones as seepage water, while another part, the adhesive water, which is held in the soil against gravity, forms the actual soil moisture. At the permanent wilting point, the water conductivity of the soil is so low that the transpiration losses of the plant can no longer be compensated. The pore volume of the soil is drained in the area of the coarse pores and medium pores. The remaining part of the soil moisture, which is no longer usable for the plants, is also called dead water.(Wikipedia)
What do soil moisture sensors measure?
Soil moisture sensors determine the water content (also moisture content) of the soil, i.e. the amount of water contained in a soil at a specific point in time. The water content can be specified both as gravimetric water content and volumetric water content, whereby the reference to the soil volume (volumetric) is usually preferred. The specification is made either proportionally (between 0 ... 1) or in volume %.
What is the purpose of determining soil moisture?
- Understanding of the behavior of soils and internal soil processes.
- Assessment of soil fertility
- Assessment of plant growth, plant water demand and consumption
- Control of irrigation measures
- Assessment of water storage capacity of soils
- Assessment of infiltration capacity, deep percolation and leaching of chemicals to groundwater
- Knowledge of other physical properties of soils (plasticity, consistency, bearing capacity, etc.)
- Measurement data for water balance studies
Current measurement methods Soil moisture
The measurement result is a negative pressure. This is caused by capillary forces sucking water out of a porous ceramic body embedded in the soil, which is connected to a water-filled and gas-tight tube. If the soil is dry, the capillary forces are greater and a higher negative pressure is generated accordingly. The suction force is also greater in the case of very fine-pored soil.
Capacitive sensor (capacitance measurement)
Measure the change in electrical capacitance by generating a high-frequency electric field around the sensor. Sensors detect the changes in the dielectric permeability (permittivity) of the soil.
(FDR - Frequency-Domain-Reflectometry) - Electric charges can flow more easily in a wet soil than in a dry one. However, electrical conductivity is also strongly dependent on salinity and soil temperature. In order to give a statement regarding soil moisture in a unit comparable to tensiometers, the conductivity is converted to a negative pressure under various assumptions.
Thermo measuring sensor
In this process, a small tip is heated by a heating resistor by approx. 1°C for a few seconds. After this heating phase, the cooling curve is recorded very precisely electronically and the time until a certain threshold value is reached is measured [cooling time after heating of the sensor (thermal conductivity)].
Water balance method
Geisenheim University of Applied Sciences works with the water balance method. Based on weather data and crop-specific factors, evapotranspiration is calculated and an irrigation recommendation is derived. With this method, largeﬂscale irrigation recommendations can be made for a wide variety of crops with relatively low material investments. In Switzerland, the recommendations have not yet been validated in the field and the calculations are not yet speciﬁcally adapted to Swiss climatic conditions.
Soil water content is calculated using weather data, crop- and stage-specific factors, a root growth model, a soil water model, and a single-rate model. From the weather data and factors, daily evapotranspiration is calculated using Penman-Monteith's formula and guidance from FAO Irrigation and Drainage Paper No. 56. From the root and soil water models, soil physical principles such as field capacity, irrigation threshold, and usable field capacity are derived. Reasonable amounts for irrigation applications are derived from the single application model. The recommendations were validated with trials conducted by LWK-Niedersachsen.
- Objective determination of the irrigation amount/date.
- Knowledge of the soil physical properties of the site
- Rooting depth/ stage of development
- Calculation of the daily water balance
- Daily water balance = (evaporation according to PENMAN x kc) - precipitation
- Calculate the time of irrigation
- Irrigate when the sum of the daily water deficits has reached the specified irrigation amount.
For the evaluation of irrigation measures, a distinction must be made between the terms need for irrigation and worthiness of irrigation (Seis et al., 2016, pp. 135) (Fig. 1). Irrigation is necessary if the production of sufficient yields and qualities of certain crops is not possible without irrigation. Irrigation worthiness considers the economic component of irrigation. It evaluates the economic viability of irrigation, taking into account producer prices, costs of irrigation and contractual obligations (FRICKE, 2015).
The aim is to irrigate the crops optimally and at the same time to protect the environment. Thus, it is important to administer the right amount at the right time. An intelligent automatic irrigation system carries out the irrigation fully automatically. The basis of the control is the recording of soil moisture and, if necessary, other factors such as soil temperature, precipitation, air temperature, humidity, etc. and transmits the measured values to a control center. This controls the irrigation process on the basis of the measured values in the field. It starts the irrigation, regulates the quantities and regularly adjusts them to the target situation.
© Alexandra Hildebrandt, Werner Landhäusser from: CSR and digitalization
Example from a manufacturer of a self-learning irrigation computer that intelligently evaluates the readings from soil moisture sensors and, in addition to the correct time for irrigation, also calculates the effective water requirement of the plants and automatically readjusts the irrigation duration according to the growth phases, as well as the seasonal temperature changes (Plantcare PlantControl CX).
Special knowledge for soil moisture
Usable field capacity
Water in different soil layers
Field capacity is the amount of water that an initially water-saturated soil can still hold against gravity after 2 to 3 days. Field capacity has a high practical importance for water supply to plants, plant availability of water-soluble nutrients, leaching of water-soluble substances, and irrigation. Also, the adhesive water stored in the soil is not completely available to plants. The portion of the field capacity (FC) that can be taken up by plants through the roots is the usable field capacity (nFC). Dead water (TOT) is held in pores smaller than 0.2 µm by adhesive forces to such an extent that plants cannot release it from them. (Wikipedia)
KC - value plant coefficient
The plant coefficient (Kc value) is used in agriculture to calculate the water consumption of a plant. The plant coefficient was first determined at the Geisenheim Research Institute using lysimeters. The Kc value differs from plant to plant and changes during the growth phase of a plant, whereby the value usually increases, i.e. the plant requires more water.
The Kc value thus represents a correction factor that, together with the reference evapotranspiration over an ideal grass surface (ETo, according to Penman), determines the water consumption of a given plant. If you subtract the precipitation from this, you also get an indication of how much water is still needed by the plant for optimal growth. (Wikipedia)
Plant coefficient (kc) * reference evapotranspiration (ETo) = water consumption of the plant
Water consumption of the plant - precipitation = water balance